NASA/TM–2006–213718 

Biomechanical Evaluation of Locomotion on the Russian 

BD-1 Treadmill in a Weightless Environment (KC-135) 

John DeWitt1

Grant Schaffner2 

Jason Bentley2

Eugenia Yarmanova3

Inessa Kozlovskaya3

Donald Hagan4

1

Bergaila Engineering Services, Houston, TX, USA 

2

Wyle Life Sciences, Houston, TX, USA 

3

Institute for Biomedical Problems, Moscow, Russia 

4

NASA Johnson Space Center, Houston, TX, USA 

National Aeronautics and 

Space Administration 

Houston, Texas 77058-3696 

March 2006 

Johnson Space Center 
THE NASA STI PROGRAM OFFICE . . . IN PROFILE 

Since its founding, NASA has been dedicated to 

the advancement of aeronautics and space 

science. The NASA Scientific and Technical 

Information (STI) Program Office plays a key 

part in helping NASA maintain this important 

role. 

The NASA STI Program Office is operated by 

Langley Research Center, the lead center for 

NASA’s scientific and technical information. 

The NASA STI Program Office provides access 

to the NASA STI Database, the largest 

collection of aeronautical and space science STI 

in the world. The Program Office is also 

NASA’s institutional mechanism for 

disseminating the results of its research and 

development activities. These results are 

published by NASA in the NASA STI Report 

Series, which includes the following report 

types: 

• TECHNICAL PUBLICATION. Reports of 

completed research or a major significant 

phase of research that present the results of 

NASA programs and include extensive data 

or theoretical analysis. Includes 

compilations of significant scientific and 

technical data and information deemed to be 

of continuing reference value. NASA’s 

counterpart of peer-reviewed formal 

professional papers but has less stringent 

limitations on manuscript length and extent 

of graphic presentations. 

• TECHNICAL MEMORANDUM. Scientific 

and technical findings that are preliminary 

or of specialized interest, e.g., quick release 

reports, working papers, and bibliographies 

that contain minimal annotation. Does not 

contain extensive analysis. 

• CONTRACTOR REPORT. Scientific and 

technical findings by NASA-sponsored 

contractors and grantees. 

• CONFERENCE PUBLICATION. Collected 

papers from scientific and technical 

conferences, symposia, seminars, or other 

meetings sponsored or cosponsored by 

NASA. 

• SPECIAL PUBLICATION. Scientific, 

technical, or historical information from 

NASA programs, projects, and mission, 

often concerned with subjects having 

substantial public interest. 

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific 

and technical material pertinent to NASA’s 

mission. 

Specialized services that complement the STI 

Program Office’s diverse offerings include 

creating custom thesauri, building customized 

databases, organizing and publishing research 

results . . . even providing videos. 

For more information about the NASA STI 

Program Office, see the following: 

• Access the NASA STI Program Home Page 

at http://www.sti.nasa.gov 

• E-mail your question via the Internet to 

help@sti.nasa.gov 

• Fax your question to the NASA Access Help 

Desk at (301) 621-0134 

• Telephone the NASA Access Help Desk at 

(301) 621-0390 

• Write to: 

NASA Access Help Desk 

NASA Center for AeroSpace Information 

7121 Standard 

Hanover, MD 21076-1320 
NASA/TM–2006–213718 

Biomechanical Evaluation of Locomotion on the Russian 

BD-1 Treadmill in a Weightless Environment (KC-135) 

John DeWitt1

Grant Schaffner2 

Jason Bentley2

Eugenia Yarmanova3

Inessa Kozlovskaya3

Donald Hagan4

1

Bergaila Engineering Services, Houston, TX, USA 

2

Wyle Life Sciences, Houston, TX, USA 

3

Institute for Biomedical Problems, Moscow, Russia 

4

NASA Johnson Space Center, Houston, TX, USA 

National Aeronautics and 

Space Administration 

Johnson Space Center 

Houston, Texas 77058-3696 

March 2006 
Available from: 

NASA Center for AeroSpace Information National Technical Information Service 

7121 Standard 5285 Port Royal Road 

Hanover, MD 21076-1320 Springfield, VA 22161 

This report is also available in electronic form at http://techreports.larc.nasa.gov/cgi-bin/TRS 
 i 

Acronyms

0G Zero Gravity (i.e. weightlessness; acceleration field = 0 m/s2) 

1G Earth Gravity (i.e. acceleration field = g =9.807m/s2

) 

EL External Load 

GRF Ground Reaction Force 

GTL Heel to Greater Trochanter Length 

HE Harness Extender Strap Length Adjustment 

ISS International Space Station 

PC Personal Computer 

RS Restraint Strap 

ROM Range of Motion 

SLD Subject Loading Device 

SPD Subject Positioning Device 
 ii 

 Table of Contents

Abstract and Operational Relevance............................................................................................... 1 

GOAL ............................................................................................................................................. 2 

INTRODUCTION .......................................................................................................................... 2 

Hypotheses...................................................................................................................................... 4 

METHODS AND MATERIALS.................................................................................................... 4 

External Load, Bungee Extension and Treadmill Speed Measurement...................................... 6 

Ground Reaction Force ............................................................................................................... 7 

Gait Parameters ........................................................................................................................... 7 

Kinematics................................................................................................................................... 7 

Outcome Variables and Statistical Analysis ............................................................................... 9 

RESULTS ....................................................................................................................................... 9 

Locomotion Speed....................................................................................................................... 9 

Load and Displacement............................................................................................................. 10 

Mean Total External Load..................................................................................................... 10

Mean Total External Load..................................................................................................... 10

Gait Parameters and Ground Reaction Force............................................................................ 12 

Trial Analysis ........................................................................................................................ 13 

Contact Time and Cadence.................................................................................................... 13 

Typical GRF Trajectories ...................................................................................................... 13 

Peak Ground Reaction Force and Impulse ............................................................................ 15 

Kinematics................................................................................................................................. 16 

Qualitative Comments about Locomotion............................................................................. 16 

Joint Kinematics .................................................................................................................... 16 

Forward Lean and Joint Displacements................................................................................. 20 

Joint Velocities ...................................................................................................................... 21 

DISCUSSION............................................................................................................................... 22 

CONCLUSIONS........................................................................................................................... 25 

REFERENCES ............................................................................................................................. 26 

Appendix....................................................................................................................................... 28 
1 

ABSTRACT AND OPERATIONAL RELEVANCE 

PURPOSE: We evaluated locomotion biomechanics on the BD-1 treadmill during exercise in 

weightlessness (0G) without a horizontal restraint strap (RS), and in normal gravity (1G) with 

and without a RS. METHODS: Eight subjects (5 men, 3 women, 33.3 ± 6.0 years, 174.3 ± 8.6 

cm; 75.1 ± 11.7 kg) ran at 5, 8, and 14 km·h-1 during 0G onboard the KC-135 aircraft and in 1G. 

During 0G trials, the subjects were loaded using the BD-1 loading system and ran while gripping 

a stationary bar in front of their bodies with both hands. During 1G trials, subjects completed 

two conditions: gripping a stationary bar and using a RS. Treadmill speed, external load (EL; 0G 

only), gait timing parameters, vertical ground reaction forces (GRF), and kinematics data were 

collected. All statistical analyses were completed within each speed. 

Results and Operational Relevance: 

1) Subjects are able to attain speeds of 5 km·h-1 and 8 km·h-1 in 0G. This suggests that they 

may have difficulty obtaining sprinting speeds near and above 13 km·h-1 in 0G. It is 

unclear if the difficulty is due to individual subject characteristics, such as anaerobic 

fitness or leg strength, power, or endurance, or if it is due to the gravitational 

environment. Recommendation: Speeds greater than 13 km·h-1 should be prescribed 

with the advance knowledge that they may be difficult to attain and/or maintain. 

2) Mean total dynamic EL is approximately 520 N (116.9 lbs) and does not vary with 

locomotion speed. The EL for an individual is related to their heel to greater trochanter 

length (GTL) and the harness extender strap setting (HE). Recommendation: The 

approximate EL can be found as: EL (in kg) = 24.43 + 0.45 × GTL (in cm) – 0.38×HE 

(in cm).

3) Contact time and cadence do not differ between 0G and in 1G with or without the RS. 

Contact times were similar to those found during earlier motorized treadmill evaluations, 

but cadences appeared to be greater. Peak vertical GRF and impulse also do not differ 

within speeds between 0G and in 1G with or without the RS. Vertical GRF trajectories 

reveal the absence of an impact peak, suggesting that the foot has already begun to move 

rearward prior to heel strike. Peak GRF appears to be greater at each speed as compared 

to similar speeds on a motorized treadmill, but impulses are similar. Recommendation: 

Subjects will generate greater peak forces on the BD-1, probably due to the need to 

propel the treadmill. Exercise prescription should take increased use of the hip and knee 

musculature into account during BD-1 exercise. 

4) Subjects appear to perform gait with an increased forward lean, minimal hip extension, 

50-90 degrees of knee flexion, and varying ranges of ankle flexion. Range of motion 

(ROM) at each joint was not affected by gravitational or RS condition, but there were 

differences in trends between subjects. Hip ROM remains similar but knee ROM appears 

to increase and ankle ROM appears to decrease as locomotion speed increases. 

Recommendation: When prescribing exercise onboard ISS, hip extensor range of motion 

should be addressed. In addition, when utilizing locomotor exercise at higher speeds, 

ankle flexor/extensor exercises should be used to supplement the possible reduced use of 

this musculature. 
 2 

GOAL 

The purpose of this investigation was to evaluate locomotion on the Russian BD-1 nonmotorized treadmill under three conditions: weightlessness (0G) without a simulated subject 

positioning device, or horizontal restraint strap (RS), normal gravity (1G) without a RS, and 1G 

with a RS. Various outcome measures were evaluated, including the applied external load (EL), 

vertical ground reaction force (GRF), the ability to achieve specific locomotion speeds, and joint 

kinematics. The goal of this research was to better understand how locomotion and the related 

forces experienced by the musculoskeletal system during non-motorized treadmill locomotion in 

0G compare to locomotion on the same device in 1G. 

INTRODUCTION 

Long-term exposure to microgravity has been shown to result in physiological detriments to the 

musculoskeletal system, including bone loss in weight-bearing areas including the calcaneus, 

tibia, proximal femur, and pelvis (Schneider et al., 1995; LeBlanc et al., 2000). While the 

specific stimulus for bone maintenance remains unclear, it has been shown that bone responds to 

mechanical loading (Cullen et al., 2000; Mosley & Lanyon, 1998; Mosley et al., 1997). 

Research has shown that exercise which results in mechanical loading may be of benefit in 

stimulating bone growth (Judex & Zernicke, 2000; Heinonen et al., 1996). 

A treadmill is used onboard the International Space Station (ISS) to allow impact-loading 

exercise. During locomotion in weightlessness, crewmembers wear a harness and tether in order 

to maintain contact with the treadmill. The tether provides an external load (EL) that pulls the 

crewmember back towards the treadmill, creating impact-loading between the treadmill and foot. 

The amount of EL is adjustable and can be determined by the crewmember. Due to possible 

breakdowns of the treadmill, and owing to the criticality of this mode of exercise, the Russian 

designed BD-1 treadmill is a contingency device. 

The BD-1 treadmill (see Figure 1) is a non-motorized device and was the focus of this 

investigation. The device consists of a vinyl belt that moves over a series of metal rollers. The 

tread surface area is 82 cm long by 37 cm wide. The treadmill is non-motorized, requiring the 

subject to provide the power necessary to move the tread. The requirement for the subject to 

propel the belt may result in an increased amount of forward lean in order to aid in the generation 

of rearward forces by the foot and leg (Gamble, 1988; Lakomy, 1987). A subject positioning 

device (SPD) may aid in reducing forward lean because of the rearward force it places at the 

subject’s waist. The EL applied to the crewmembers during exercise on the BD-1 is only 

slightly adjustable because the bungee system built into the device provides a low relative 

extension during locomotion. Past investigations have demonstrated that the EL provided by 

other loading systems varies substantially during motorized treadmill exercise in 0G (Schaffner 

et al., 2005). It is unknown how the non-motorized treadmill will affect EL and locomotion 

kinetics and kinematics. 
 3 

Figure 1. The BD-1 treadmill. The padded frame surrounding the subject was 

used for safety purposes and to provide a handrail that subjects could push 

against; it is not a part of the treadmill. 

We have collected some data on locomotion kinematics and kinetics in weightlessness (DeWitt 

et al., 2004; Schaffner et al., 2004; DeWitt et al., 2003). These studies were conducted using a 

motorized treadmill on which the required gait patterns may be different than on a non-motorized 

treadmill. Cavanagh et al, (1987) and Thornton et al., (1997) presented kinematic data collected 

during non-motorized treadmill locomotion in weightlessness during STS-7 and STS-8. 

Although their dataset is small, joint kinematics are presented that serve as a reference for this 

study. There have been no reported data on the forces necessary to drive a non-motorized 

treadmill. 

Because of the lack of quantitative data concerning locomotion biomechanics on a nonmotorized treadmill, the purpose of this study was to quantify EL levels and locomotion 

parameters on the BD-1 in 0G and in 1G. The secondary purpose was to assess the use of the RS 

upon locomotion biomechanics during 1G exercise. Quantification of these parameters are 

beneficial to understanding how long term use of the device may affect the human body, and will 

help to create more suitable exercise prescriptions for crewmembers during extended spaceflight. 
 4 

HYPOTHESES 

1) The ability to achieve specific locomotion speeds will not be affected by gravitational or 

RS condition. 

2) The dynamic EL during locomotion in 0G will be affected by subject anthropometrics. 

3) The peak vertical GRF produced will be different in 0G compared to 1G for either RS 

configuration at the same absolute speeds. 

4) Gait parameters and locomotion kinematics, including contact time, cadence, joint 

displacements and velocities, and forward lean will be different in 0G compared to 1G 

for either RS configuration at the same absolute speeds. 

METHODS AND MATERIALS 

Eight subjects (5 men, 3 women, 33.3 ± 6.0 years, 174.3 ± 8.6 cm; 75.1 ± 11.7 kg, mean±SD) ran 

at 5, 8, and 14 km·h-1 during weightlessness (0G) onboard the KC-135 aircraft (see Figure 2) and 

in normal gravity on the ground (1G). Subjects were free from injuries and pathological 

conditions which could potentially affect the results of the study. Prior to participation, all 

subjects signed an informed consent document. This study was approved by the Johnson Space 

Center Committee for the Protection of Human Subjects. 

Figure 2. BD-1 treadmill evaluation experiment being performed on NASA KC135 Research Aircraft. 

All 0G trials occurred onboard the NASA KC-135 aircraft. The KC-135 flies in a parabolic 

trajectory that allows for approximately 20 seconds of reduced gravity, followed by 

approximately 50 seconds of non-zero gravity. Each flight consists of approximately 40 

parabolas performed in multiple sets. During this investigation, data were collected over four 
 5 

separate flights, with two subjects tested during each flight. All 1G trials occurred in the 

Exercise Physiology Laboratory at Johnson Space Center. For both airborne and ground trials, 

the treadmill was surrounded in the front and sides by a padded steel safety frame. The frame 

offered protection against the subject falling after a stumble and also provided a handrail for the 

subjects to push against in the non-RS trials. 

Prior to any data collection, each subject participated in a familiarization session in the 

laboratory. Each subject ran and walked at freely chosen speeds to become accustomed to the 

treadmill. Next, each subject completed a series of seven running trials to allow familiarization 

with the approximate time of exercise and rest onboard the aircraft. During these trials, the 

subjects ran for 20 seconds and rested for 50 seconds during multiple 5, 8, and 14 km·h-1 

locomotive bouts. These times correspond to the 0G and non-0G times on the KC-135 as found 

through past experience. 

Belt motion and locomotion on the BD-1 occurs solely due to the forces provided by the subject. 

Within the BD-1, there is an internal resistance to the motion of the belt that can be set by the 

user. For this investigation, the belt resistance was set at the lowest setting possible. Pilot 

testing revealed that the belt resistance was approximately 137 newtons (N). 

Because the internal resistance of the treadmill belt was not zero, a forward-directed reaction 

force will be applied to the subject by the treadmill. A rearward-directed force must also be 

applied to the subject to counteract the fore-aft ground reaction force, otherwise the subject will 

be propelled forward. During 0G and 1G trials, the subjects placed their hands on the front 

upper bar of the treadmill safety frame to create this rearward-directed reaction force. In 

addition, the subjects completed an additional condition during 1G trials using a horizontal 

restraint strap (RS). During the RS trials, the rearward-directed force was applied to the subjects 

using extender straps attached horizontally between the rear of the safety frame and the Russian 

harness worn by the subjects. These trials were used to simulate the SPD onboard ISS. 

EL during 0G trials was applied to each subject by the bungee loading system built into the 

treadmill. The BD-1 was designed with a long bungee that winds around pulleys on the lower 

front and rear of the device. The bungee applied load to the subjects through a waist and 

shoulder harness, and was attached to the right and left side of the belt with adjustable extender 

straps that clipped to the metal terminations at each end of the bungee. The applied EL can be 

distributed between the waist and shoulders by adjusting the length of the harness shoulder 

straps. The EL can also be varied slightly by adjusting the length of the extender straps. Each 

subject was outfitted prior to data collection with each harness in a configuration that was 

deemed comfortable. The load provided to the subjects was not standardized as a percentage of 

body weight because the range of adjustability of the bungee and extender straps could not 

accommodate the differences in subject anthropometrics. Also, the load distribution between the 

shoulders and waist was not standardized because there were no means to measure these load 

components independently. The harness was worn during 0G and 1G trials with the RS; during 

1G trials without the PH, no harness was worn. 

Data were collected using specialized instrumentation systems during each trial. EL was 

measured using load cells interfaced with a PC. Treadmill speed was recorded with a digital 

speedometer interfaced with a PC. Ground reaction forces were measured with pressure insoles. 

Running kinematics were evaluated with a motion capture system. Each system and its 

parameters will be discussed in greater detail in the following sections. The treadmill speed, 
 6 

load cells, and ground reaction force data were synchronized during collection. The motion 

capture system was not synchronized with the other devices. 

External Load, Bungee Extension and Treadmill Speed Measurement 

The EL applied to each subject was measured at 120 Hz using load cells (Entran, Inc., Fairfield, 

NJ). The load cells were placed between the harness extender strap attachment and the loading 

bungee on both sides of the subject to allow for external load measurement on both sides of the 

subject (see Figure 3). Displacement transducers mounted on the right and left side of the 

subject (Ergotest Technology, Langesund, Norway and Ametek Rayelco PV-80A, Costa Mesa, 

CA, respectively) were used to measure the linear displacement of the load attachment points at 

the subjects harness (bungee system extension). Two loading configurations were tested, a low 

load and high load. The low load condition consisted of the load cells inline with the harness 

extender straps at their longest setting. This provided the minimum bungee extension and thus 

the lowest loading for a given subject. In the high load condition, the load cells were removed 

(since the length they take up in the attachment scheme reduces the bungee extension) and the 

extender straps were moved to their shortest lengths. Unfortunately, this meant that the bungee 

load could not be measured directly during locomotion. To make up for this, a bungee loading 

curve characterization was performed prior to any exercise sessions. The two ends of the bungee 

were extended simultaneously, starting at 10 cm and increasing to 55 cm in increments of 5 cm. 

At each extension, the bungee tension was measured at each end using the load cell system. The 

total load was calculated by adding the two load cell measurements together. Two trials were 

performed and the results were averaged. 

Figure 3. Close-up of connection between harness and loading bungee 

illustrating load cell orientation during data collection. 

The BD-1 was built with an internal digital speed sensor. When the belt was in motion, the 

speed sensor output was a 3-volt peak-to-peak square wave signal where the frequency of the 

square wave was a function of belt speed. The speed was recorded at approximately 4 Hz and 

displayed on a notebook computer (Compaq, Houston, TX) placed in front of the subject. 
 7 

A data acquisition system, equipped with LabVIEW programming software (National 

Instruments, Austin, TX), was interfaced to the speed sensor and load cells. The customized 

LabVIEW software was used to capture and condition load cell data at a cutoff frequency of 4 

Hz with a Butterworth low-pass filter and to convert the frequency of the speed sensor signal to a 

numeric speed. 

Ground Reaction Force 

Vertical GRF data were measured during all trials using pressure sensing insoles (Tekscan, Inc., 

S. Boston, MA). Each subject had their own pair of custom-fitted insoles that were placed 

between the insole of the shoes and their feet. During each condition, the insoles were calibrated 

to the subject’s body weight prior to any data collection. Pressure data from the sensors within 

each insole were recorded by a data collection PC. A spotter ensured that the sensor wires that 

connected each insole to the PC did not interfere with the normal gait of the subject. Data were 

collected at 250 Hz for 15 sec. Data collection for the pressure insoles, treadmill speed, and load 

cells were initiated simultaneously with the same button-press trigger. GRF parameters (peak 

force, and impulse) were determined using custom software. Multiple footfalls from each foot 

were analyzed individually to ensure that outcome variables were computed accurately. Any 

partial footfall measurements were eliminated from the analysis. Trial means were then 

computed for each outcome variable using data from all of the footsteps for the entire trial. 

Gait Parameters 

Gait parameter outcome variables of interest included contact time and cadence and were 

obtained using the timing measures from the pressure insoles. Contact time was defined as the 

duration, in seconds, that the foot was in contact with the treadmill during a footfall. Initial 

contact was defined as the instant that the GRF rose above 10 N; toe-off, or the instance the foot 

left the ground, was defined as the instant that the GRF fell below 10 N. Stride time was defined 

as the duration between successive heel strikes of the same foot. Cadence was defined as the 

amount of strides that the subject took per second, and was computed as the inverse of stride 

time. 

Kinematics 

Kinematic data were collected with a six-camera infra-red video motion capture system 

(eMotion, Inc., Milan, Italy). Reflective markers were placed on the right side of the subject 

during 0G trials and left side of the subject during 1G trials with the assumption that leg motion 

was symmetrical. Markers were placed to approximate the joint centers of the shoulder, hip, 

knee, ankle, and top of the foot at the base of the third metatarsal. An additional marker was 

placed on the anterior-lateral thigh arbitrarily so it did not lie on the axis connecting the knee 

center to the hip center (see Figure 4). During 0G trials, the hip joint center was often obscured 

by the harness and EL connection. Therefore, an additional marker was placed on the lateralrear of the waist belt. 3-D position data were collected at 120 Hz. Prior to any data collection, 

the activity volume was calibrated to within 1 mm of re-prediction accuracy. After calibration, 

but prior to any running trials, a trial was collected while the subject stood still to define the 

marker orientation associated with each joint in the neutral position. 
 8 

Figure 4. Marker positions and joint angle definitions used during the 

kinematics analysis. 

Multiple strides from two trials at each speed were analyzed for each subject. Marker position 

data were filtered using a 4th order Butterworth low pass filter with cutoff frequencies ranging 

from 2-8 Hz as determined after a power analysis of the raw signal data. The markers allowed 

the body to be modeled as 4 rigid segments: trunk, thigh, shank and foot. Each segment was 

approximated as the vector connecting the distal to proximal joint centers at the segment 

endpoints. However, for 0G trials, the thigh was modeled as the vector connecting the knee to 

the thigh marker due to the absence of the hip marker. Forward lean was defined as the angle 

separating the trunk from the vertical axis of the lab reference frame using the average position 

of the shoulder as the endpoint of the trunk vector. Using the shoulder average position was 

required because some shoulder motion occurred due to rotation of the trunk, which would not 

be indicative of forward lean. Hip angle was defined as the angle separating the trunk and thigh. 

Knee angle was quantified as the angle separating thigh and shank. Ankle angle was defined as 

the angle between the shank and foot. 

It was assumed that any motion outside of the sagittal plane was negligible. Therefore, the 

angles of interest were computed as absolute angles separating the adjacent segments. Once 

joint angles were computed, they were adjusted using data obtained during the quiet standing 

trial to account for the offsets related to the neutral positions of each joint. Positive knee and hip 

angle values reflect flexion and positive ankle angles reflect plantarflexion. Angular velocity 

was calculated using finite central differences. Positive angular velocities indicate flexor 

movement, while negative angular velocities indicate extensor movement. 

Vertical (lab Y axis)

Knee Angle

Hip Angle

Forward Lean

Ankle Angle

Shoulder

Hip

Knee

Ankle

Toe

Trunk

Segment

Thigh

Segment

Shank

Segment

Foot

Segment

Vertical (lab Y axis)

Knee Angle

Hip Angle

Ankle Angle

Shoulder

Hip

Knee

Ankle

Toe

Trunk

Segment

Shank

Segment

Foot

Segment

Shoulder

Hip

Knee

Ankle

Toe

Trunk

Segment

Shank

Segment

Foot

Segment

Thigh 

Marker
 9 

Outcome Variables and Statistical Analysis 

During 1G data collection, subjects completed two trials at each speed at each condition (RS vs. 

no PH) resulting in 12 (2 × 3 × 2) trials per subject. During 0G trials, subjects completed 2-4 

trials at each speed. The numbers of trials per subject used during the 0G analysis varied and are 

addressed in the results. There were several outcome variables of interest in this investigation. 

Each variable and the statistical analysis utilized are described in the following paragraphs. For 

all statistical tests, an alpha of p<.05 was used to determine significance. 

1. Treadmill Speed: The mean and standard deviation of the treadmill speed for all trials 

were computed to determine if the subjects were able to achieve the desired speeds of 5, 

8, and 14 km·h-1. An ANOVA with trial type (0G, 1G without the PH, and 1G with the 

PH) was used for each speed to determine if there were any differences in locomotion 

speeds between conditions. 

2. External Load: The mean and standard deviation of the EL experienced by each subject 

was computed for each 0G trial. An ANOVA was used to determine if there were effects 

of running speed upon the EL. In addition, subject by subject results were examined 

using forward stepwise multiple regression to determine if EL was related to the subject 

heel to greater trochanter length and body weight. 

3. Kinetic and Gait Parameter Variables: Peak force and impulse were computed for each 

footfall of each trial. In addition, contact time and cadence were also computed using the 

footfall data. Each of the outcome variables were then examined within each speed using 

an ANOVA with repeated measures to determine any significant differences between 

conditions. Any significant interactions were examined with Tukey-Kramer post hoc ttests. 

4. Kinematic Variables: For each joint angle, the maximum, minimum, and range of motion 

were computed for multiple strides of each trial. In addition, the peak extension 

(positive) and flexion (negative) velocities were computed for each stride. The mean of 

each of these measures was then computed for each trial. An ANOVA with repeated 

measures was used to compare the locomotion conditions within each speed for each 

variable except forward lean. The loss of the hip marker during the 0G trials resulted in 

data only being available for the 1G condition. Therefore, a paired t-test was used to 

compare RS conditions within speeds. 

RESULTS 

Locomotion Speed 

Subjects completed multiple trials at the target speeds of 5, 8, and 14 km·h-1. One subject was 

unable to complete the 1G trials at 14 km·h-1, so that subject was eliminated from the analysis for 

that speed. Table 1 reveals that the subjects ran similarly during all conditions except for the 14 

km·h-1 conditions. Examination of the data suggests that at 5 km·h-1 and 8 km·h-1, the subjects 

were able to attain and maintain the desired speeds. At the fastest condition in 0G, however, the 

subjects had difficulty attaining a mean speed of 14 km·h-1. The 14 km·h-1 trials in 0G were 

different than those in 1G using the simulated PH. 
 10 

Table 1. Mean (SD) velocities obtained on the BD-1 during locomotion in 0G and 1G. 

 0G 1G 

 Desired Speed (km·h-1) No RS No RS RS 

5 5.05 (0.16) 5.06 (0.18) 5.27 (0.25) 

8 7.81 (0.33) 8.16 (0.13) 8.28 (0.14) 

14 13.03 (0.60) 13.73 (0.72) 14.43 (0.54)* 

* Significantly different than 0G (p<.05) 

Load and Displacement 

Mean Total External Load 

The bungee load versus extension calibration curve is shown in Figure 5. The two lower curves 

depict the load and extension measured from each side. The close correspondence of the two 

curves indicates that the load is well balanced on each side, a feature inherent in the single 

bungee design (as opposed to using a separate bungee on each side). The total load curve is 

highly linear, as indicated by the high r-squared value (0.99). The slope of the regression line, 

3.717 N/cm, represents the stiffness of the bungee loading system. 

y = 0.3793x + 43.47

R2

 = 0.9926

0

10

20

30

40

50

60

70

0 10 20 30 40 50

Extension (cm)

Load (kgf)

Left (kg)

Right (kg)

Total (kg)

Linear (Total (kg))

Figure 5. Bungee load versus extension calibration curve. The lower curves 

represent the load and extension measured on each side. The upper curve is the 

sum of these two curves, representing the overall bungee system load versus 

extension curve. 

Mean Total External Load 

EL was measured during 64 trials across seven subjects during 0G locomotion (for one subject 

the load cells failed). The mean total EL was computed as the sum of the mean load for the 

entire trial of the right and left load cells. Means are expressed in kilograms and in % BW (see 
 11 

Table 2). There were no significant differences found between speed conditions, suggesting that 

locomotion speed does not affect the mean EL experienced by each subject. 

Table 2. Mean (SD) total load expressed in kg and %BW for each speed during BD-1 

locomotion in 0G 

 Speed (km·h-1) 

 5 8 14 

Total Load (kg) 53.44 (2.20) 52.77 (2.14) 52.35 (2.06) 

Total Load (%BW) 0.71 (0.09) 0.70 (0.09) 0.70 (0.09) 

While the relatively small standard deviation for each measure indicates that the mean EL 

remains consistent across subjects, it was also of interest to see if stature affects the mean EL. 

Table 3 lists the subject by subject mean total loads in kg and %BW for all trials. In addition, 

each subject’s BW and heel to greater trochanter length (GTL) are shown. Pearson product 

moment correlations revealed that mean EL expressed in % BW was significantly correlated 

with GTL and BW (see Table 4). 

Table 3. Mean (SD) total load and anthropometric measurements for each subject 

Subject BW (kg) GTL (cm) Mean EL (kg) Mean EL (%BW) 

3 60.00 84 49.34 (0.87) 0.82 (0.01) 

1 63.64 85 51.65 (0.32) 0.81 (0.01) 

4 76.36 91 51.20 (1.03) 0.67 (0.01) 

6 81.82 92 53.15 (0.38) 0.65 (0.01) 

7 75.00 92 54.35 (1.03) 0.72 (0.01) 

5 95.00 93 54.16 (0.57) 0.57 (0.01) 

2 80.91 95.5 55.52 (0.45) 0.69 (0.01) 

Table 4. Correlation coefficients relating subject anthropometrics to mean external 

load 

 Mean Load (kg) 

BW 0.70* 

GTL 0.85* 

* Significantly correlated (p<.05) 

A post hoc stepwise multiple regression using body weight and GTL as inputs to predict mean 

total load (low load configuration) in kg was executed and it was found that the mean EL could 

be predicted using GTL (See Figure 6). Adding BW to the equation did not improve the 

prediction level. The EL for the high load configuration could not be measured directly since the 

load cells were not connected (to avoid decreasing the load). However the bungee load 

calibration curve (Figure 5) provides a means of estimating the high load based on the low load 

measurement. The change in bungee extension provided by the load cell removal and shortening 

of the harness extender straps was 32.4 cm. Multiplying this by the bungee system stiffness of 

3.72 N/cm gives a load change of 12.3 kg. Since the additional bungee extension was the same 
 12 

regardless of subject size, the change in load was the same for all subjects. By adding this load 

change to the linear regression line for the low load, the high load can be estimated (Figure 6). 

Thus, the overall range of loading available to all subjects was approximately 49.9 kg to 67.4 kg. 

The equation provided in the figure caption represents the case where the load cell is placed in 

line with the harness and the harness extender strap is on its longest setting. Removing the load 

cell and setting the harness extender strap on its shortest setting would increase the EL by 12.3 

kg. Also, if we account for harness extender strap length adjustment (HE), the equation becomes: 

EL (kg) = 24.43 + 0.45 × GTL (cm) - 0.38 × HE (cm), 

where HE = 0 cm (shortest adjustment) to 14.9 cm (longest adjustment). 

40

45

50

55

60

65

70

80 85 90 95 100

GTR Length (cm)

Mean Total Load (kgf)

Low Load

High Load

Linear

(Low Load)

 

Figure 6. Scatter plot of external load vs. greater trochanter length. 

EL (kg) = 12.13 +.45 × GTL (cm); r2

=.72 (low load) 

Gait Parameters and Ground Reaction Force 

Prior to any statistical analyses and after data processing, the dataset was examined subject by 

subject to determine if any outliers existed. A total of 5111 footfalls were analyzed from 168 

trials. There were trials during which data were not collected due to hardware malfunction. The 

complete subject trial means for contact time and peak GRF are shown in the Appendix. 

It was found that for one subject (5), no data were collected during the 0G trials. The dataset for 

that subject was therefore eliminated from the analysis. For another subject (6), the 0G peak 

GRF values were consistent, but much lower than those exhibited by the other subjects. It was 

assumed, therefore, that some calibration error may have occurred that resulted in erroneous data 

for that subject, so that subject was also eliminated from the analysis. Finally, a subject (1) could 

not complete the 14 km·h-1 trials in 1G, and the 0G data for the 8 km·h-1 trial were not recorded. 

This subject was only kept for the 5 km·h-1 trials. This resulted in a total of 3963 footfalls from 
 13 

133 trials being used from 6 subjects in the 5 km·h-1 analysis, and 5 subjects in the 8 and 14 

km·h-1 analysis. 

Trial Analysis 

Student’s t-tests were executed within each speed comparing each outcome variable during 0G 

between trials with and without the load cell placed inline with the bungees. No significant 

differences were found between the load cell conditions. Therefore, all data were grouped, 

regardless of load cell condition, for the subsequent analyses of gait and GRF parameters. 

Contact Time and Cadence 

There were no significant differences in contact time or cadences between locomotion conditions 

within each speed (see Table 5). It appears that contact times tended to decrease while cadences 

tended to increase as speed increased during BD-1 locomotion. 

Table 5. Mean (SD) contact times and cadences during locomotion on the BD-1 in 0G and 

1G 

 0G 1G 

Desired Speed (km·h-1) No RS No RS RS 

5 Contact Time (s) 0.57 (0.04) 0.57 (0.03) 0.56 (0.06) 

 Cadence (Strides·s-1) 1.08 (0.06) 1.07 (0.06) 1.11 (0.10) 

8 Contact Time (s) 0.35 (0.05) 0.33 (0.01) 0.33 (0.03) 

 Cadence (Strides·s-1) 1.42 (0.05) 1.50 (0.04) 1.49 (0.11) 

14 Contact Time (s) 0.24 (0.03) 0.23 (0.01) 0.23 (0.02) 

 Cadence (Strides·s-1) 1.61 (0.12) 1.71 (0.11) 1.71 (0.17) 

Typical GRF Trajectories 

Typical GRF trajectories are illustrated in Figure 7 – Figure 9. The trajectories for 5 km·h-1 are 

very similar to normal overground walking (Munro et al., 1987). The characteristic shape of two 

peaks are apparent, with the first peak occurring as the weight is applied to the heel during footground contact, and the second peak occurring as the subject propels the belt backward. 
 14 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 10 20 30 40 50 60 70 80 90

% Contact

Vertical GRF (BW) 

0G No RS

1G No RS

1G w ith RS

Figure 7. Typical vertical GRF trajectory during 5 km·h-1 locomotion in 0G and 

1G on BD-1. 

The GRF trajectories during 8 and 14 km·h-1 speeds are similar to each other in that a single peak 

is exhibited. The lack of an initial peak suggests that the heel strike that normally occurs during 

overground and motorized treadmill locomotion is not apparent during running on the BD-1. 

The single peak represents the peak force generated to propel the treadmill belt during the 

footfall. It should be noted that the GRF measured was in the vertical direction. It was not 

possible to measure any forces parallel with the treadmill surface. 
 15 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 10 20 30 40 50 60 70 80 90

% Contact

Vertical GRF (BW) 

0G No RS

1G No RS

1G w ith RS

Figure 8. Typical vertical GRF trajectory during 8 km·h-1 locomotion in 0G and 

1G on BD-1. 

0

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50 60 70 80 90

% Contact

Vertical GRF (BW) 

0G No RS

1G No RS

1G with RS

Figure 9. Typical vertical GRF trajectory during 14 km·h-1 locomotion in 0G and 1G on 

BD-1. 

Peak Ground Reaction Force and Impulse 

There were no significant differences in peak GRF or impulse between locomotion conditions 

(See Table 6). There was a significant interaction between condition and subjects during the 5 

and 8 km·h-1 speeds in peak GRF, suggesting that the variation between conditions was different 
 16 

between subjects. This suggests that although the mean vertical forces developed during 

locomotion on the BD-1 remain similar regardless of gravitational or SLD condition, peak GRF 

for individual subjects may vary differently between the conditions. 

Table 6. Mean (SD) peak GRF and impulse during locomotion on the BD-1 in 0G and 1G. 

 0G 1G 

Desired Speed (km·h-1) No RS No RS RS 

5 Peak GRF (BW) 1.44 (0.14) 1.45 (0.15) 1.39 (0.13) 

Impulse (BW·s) 0.45 (0.05) 0.45 (0.03) 0.44 (0.05) 

8 Peak GRF (BW) 1.84 (0.13) 1.87 (0.16) 1.88 (0.20) 

Impulse (BW·s) 0.33 (0.03) 0.32 (0.01) 0.32 (0.03) 

14 Peak GRF (BW) 2.30 (0.15) 2.24 (0.10) 2.30 (0.01) 

Impulse (BW·s) 0.28 (0.02) 0.27 (0.02) 0.28 (0.04) 

Kinematics 

Five of the original eight subjects were used in the kinematic analysis. The hip marker on one 

subject was occluded during the 1G RS trials, causing the data to be unusable. Data from two 

other subjects were missing due to hardware malfunctions. Rather than using a partial dataset in 

which these subjects had missing data, it was decided to complete the repeated measures design 

using the data from the remaining five subjects. For all trials, mean peak flexion and extension 

and range of motion values were found for the ankle, knee and hip joints. In addition, minimum, 

maximum, and mean values were found for the amount of forward lean. All range of motion 

values were computed as the difference of the mean minimum and maximum values of the joint 

angle for each trial. 

Qualitative Comments about Locomotion 

During this investigation, it was apparent that the subjects needed to provide considerable force 

in order to propel the tread belt. However, it did not appear that they needed to modify their gait 

in 0G. Some of the subjects appeared to have difficulty achieving and maintaining the desired 

speed for a trial. This may have been caused by the subjects not using the entire surface of the 

treadmill belt. The BD-1 design is such that if the subject were to step too far forward, they 

could step off the front of the treadmill and tumble forward. In order to maximize the activity 

surface of the belt, it was necessary to land with the front foot so the heel landed on the belt, but 

with the forefoot extending past the front edge of the tread belt. Subjects may not have done this 

in fear of falling, which could have modified gait and limited their performance at high speeds. 

Joint Kinematics 

Figure 10-Figure 12 show typical hip, knee and ankle joint trajectories for all three conditions at 

each speed. The gait cycle is depicted from heel strike (0%) to the following heel strike (100%) 

and is the mean of a single representative trial for each condition and speed from a single subject. 

The general shapes of the curves are similar, suggesting that gait patterns do not differ between 

0G, 1G without the RS and 1G with the RS. It appears that the general motion of the hip and 

ankle are similar for all speeds. However, during the 8 and 14 km·h-1 trials, after a brief period 

of extension, there is a reversal of knee motion followed by a return to extension. 
 17 

Hip

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90 100

% Gait Cycle

Hip Angle (degrees) 

0G No RS

1G No RS

1G w ith RS

Knee

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90 100

% Gait Cycle

Knee Angle (degrees) 

0G No RS

1G No RS

1G w ith RS

Ankle

-25

-20

-15

-10

-5

0

5

10

15

20

25

0 10 20 30 40 50 60 70 80 90 100

% Gait Cycle

Ankle Angle (degrees) 

0G No RS

1G No RS

1G w ith RS

TO HS

HS TO

HS TO

Figure 10. Typical lower extremity joint kinematics for locomotion on the BD-1 at 

5 km·h-1. HS = Heel Strike and TO = Toe Off. 
 18 

Knee

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90 100

% Gait Cycle

Knee Angle (degrees)

0G No RS

1G No RS

1G w ith RS

Ankle

-25

-20

-15

-10

-5

0

5

10

15

20

25

0 10 20 30 40 50 60 70 80 90 100

% Gait Cycle

Ankle Angle (degrees)

0G No RS

1G No RS

1G w ith RS

Hip

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90 100

% Gait Cycle

Hip Angle (degrees)

0G No RS

1G No RS

1G w ith RS

HS TO

HS TO

HS TO

Figure 11. Typical lower extremity joint kinematics for locomotion on the BD-1 at 

8 km·h-1. HS = Heel Strike and TO = Toe Off. Note slight reversal of knee motion 

from flexion to extension at approximately 20% of the gait cycle. 
 19 

Knee

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

% Gait Cycle

Knee Angle (degrees) 

0G No RS

1G No RS

1G w ith RS

HS TO

Hip

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90 100

% Gait Cycle

Hip Angle (degrees) 

0G No RS

1G No RS

1G w ith RS

HS TO

Ankle

-40

-30

-20

-10

0

10

20

30

0 10 20 30 40 50 60 70 80 90 100

% Gait Cycle

Ankle Angle (degrees) 

0G No RS

1G No RS

1G w ith RS

HS TO

Figure 12. Typical lower extremity joint kinematics for locomotion on the BD-1 at 

14 km·h-1. HS = Heel Strike and TO = Toe Off. Note slight reversal of knee motion 

from flexion to extension at approximately 20% of the gait cycle. 
 20 

Forward Lean and Joint Displacements 

Table 7-Table 10 list the means and standard deviations for forward lean and each joint angle of 

interest. Forward lean was not computed during 0G trials due to the loss of the hip marker 

during that condition. The paired analysis between RS conditions for forward lean revealed that 

trials with the RS had less lean than without the RS for all speeds. Repeated measures ANOVAS 

executed within each speed for the hip, knee and ankle kinematic variables revealed no 

significant differences between any testing conditions for any variables. However, there were 

some subject interactions identified. The subject interaction suggests that although group means 

remained consistent, individuals may have altered their kinematics differently from condition to 

condition. 

Table 7. Mean (SD) forward lean kinematics during locomotion on the BD-1 at all speeds. 

 0G 1G 

Desired Speed (km·h-1) No RS No RS RS 

5 Max Forward Lean (deg) 20.91 (2.77) 19.05 (3.3)* 

Min Forward Lean (deg) 14.21 (2.53) 13.12 (3.4) 

Mean Forward Lean (deg) 18.15 (2.48) 16.71 (3.14)* 

8 Max Forward Lean (deg) 27.74 (6.29) 20.43 (3.79)* 

Min Forward Lean (deg) 22.25 (5.94) 17.29 (3.76)* 

Mean Forward Lean (deg) 24.77 (6.15) 18.89 (3.69)* 

14 Max Forward Lean (deg) 35.2 (7.59) 25.36 (5.02)* 

Min Forward Lean (deg) 29.39 (7.67) 20.86 (5.32)* 

Mean Forward Lean (deg) 32.43 (7.62) 23.06 (5.18)* 

* Significantly different (p<.05) 

Table 8. Mean (SD) hip kinematics during locomotion on the BD-1 at all speeds. 

 0G 1G 

Desired Speed (km·h-1) No RS No RS RS 

5 Peak Hip Flexion (deg) 37.71 (10.37) 41.93 (3.45) 37.15 (1.64) 

Peak Hip Extension (deg) 0.77 (7.43) 3.3 (4.58) 2.88 (5.91) 

Mean Hip ROM (deg) 36.94 (4.81) 38.63 (4.79) 34.27 (4.38) 

8 Peak Hip Flexion (deg) 41.93 (11.3) 46.4 (2.38) 38.78 (4.37) 

Peak Hip Extension (deg) 1.29 (8.03) 10.42 (9.50) 8.4 (4.74) 

Mean Hip ROM (deg) 40.64 (5.65) 35.98 (8.77) 30.38 (6.58) 

14 Peak Hip Flexion (deg) 47.37 (9.27) 59.96 (4.11) 51.54 (6.86) 

Peak Hip Extension (deg) -0.07 (8.58) 11.12 (9.50) 6.41 (4.74) 

Mean Hip ROM (deg)* 47.44 (3.48) 48.84 (11.14) 45.13 (10.04) 

* Significant subject interaction (p<.05) 
 21 

Table 9. Mean (SD) knee kinematics during locomotion on the BD-1 at all speeds. 

 0G 1G 

Desired Speed (km·h-1) No RS No RS RS 

5 Peak Knee Flexion (deg) 57.56 (7.9) 48.77 (7.33) 53.36 (7.33) 

Peak Knee Extension (deg)* 6.57 (4.31) 0.54 (5.8) 0.85 (7.16) 

Mean Knee ROM (deg) 50.99 (6.74) 48.23 (3.64) 52.51 (3.47) 

8 Peak Knee Flexion (deg)* 74.36 (10.88) 64.99 (6.67) 67.24 (6.29) 

Peak Knee Extension (deg)* 14.97 (7.46) 17.01 (4.96) 18.54 (5.01) 

Mean Knee ROM (deg)* 59.39 (15.04) 47.98 (10.47) 48.7 (9.69) 

14 Peak Knee Flexion (deg)* 84.88 (11.89) 82.16 (11.3) 87.95 (11.98) 

Peak Knee Extension (deg)* 16.64 (4.78) 19.61 (7.77) 20.29 (7.83) 

Mean Knee ROM (deg)* 68.24 (13.32) 62.55 (16.4) 67.66 (18.51) 

* Significant subject interaction (p<.05) 

Table 10. Mean (SD) ankle kinematics (degrees) during locomotion on the BD-1 at all speeds. 

 0G 1G 

Desired Speed (km·h-1) No RS No RS RS 

5 Peak Dorsiflexion (deg) -12.8 (9.51) -10.4 (3.62) -11.77 (2.2) 

Peak Plantarflexion (deg) 21.49 (5.81) 15.54 (3.76) 14.45 (2.62) 

Mean Ankle ROM (deg) 34.29 (6.97) 25.94 (1.28) 26.22 (1.09) 

8 Peak Dorsiflexion (deg)* -22.2 (6.49) -23.11 (4.57) -23.06 (4.27) 

Peak Plantarflexion (deg)* 20.71 (7.06) 17.95 (5.47) 16.31 (1.95) 

Mean Ankle ROM (deg) 42.91 (6.8) 41.06 (4.31) 39.37 (4.36) 

14 Peak Dorsiflexion (deg)* -24.41 (6.17) -24.6 (2.73) -24.11 (3.98) 

Peak Plantarflexion (deg) 18.53 (6.12) 20.99 (5.01) 20.21 (4.76) 

Mean Ankle ROM (deg)* 18.53 (6.12) 20.99 (5.01) 20.21 (4.76) 

* Significant subject interaction (p<.05) 

Joint Velocities 

The peak velocities of the knee and hip did not differ between conditions (Table 11). However, 

there were subject interactions identified that varied depending upon the speed. This suggests 

that the subjects may have produced different peak velocity trends from condition to condition at 

the different speeds. Furthermore, the difference in trends was not consistent for a specific 

motion (i.e. hip flexion), but rather were different for the different speeds. 
 22 

Table 11. Mean (SD) peak joint angular velocities (deg•s-1) during locomotion on the BD-1. 

 0G 1G 

Desired Speed 

(km·h-1) No RS No RS RS 

5 Hip Flexion* 156.92 (19.39) 154.07 (21.3) 164.14 (14.23) 

 Hip Extension -131.67 (17.1) -125.57 (22.68) -113.57 (15.45) 

 Knee Flexion 339.86 (65) 324.35 (21.31) 364.61 (14.66) 

 Knee Extension -221.54 (44.72) -222.14 (29.15) -263.76 (28.37) 

8 Hip Flexion* 218.34 (25.23) 210.46 (44.85) 186.54 (31.11) 

 Hip Extension* -175.07 (27.6) -182.89 (26.96) -174.5 (27.04) 

 Knee Flexion* 415 (85.33) 373.4 (31.71) 395.92 (58.27) 

 Knee Extension* -379.47 (89.57) -389.66 (48.35) -390.9 (63.23) 

14 Hip Flexion 306.8 (46.8) 339.67 (61.28) 308.83 (64.1) 

 Hip Extension -265.54 (48.67) -302.24 (38.24) -290.97 (41.72) 

 Knee Flexion* 527.96 (118.23) 565.64 (133.31) 621.06 (112.22) 

 Knee Extension* -504.78 (60.46) -527.09 (74.96) -543.27 (75.76) 

* Significant subject interaction (p<.05) 

DISCUSSION 

The primary purpose of this investigation was to examine locomotion on the BD-1 during 0G 

and 1G to quantify kinematic, kinetic, and external loading parameters. A secondary purpose 

was to compare locomotion with and without a RS to determine if locomotive kinetics and 

kinematics are affected. It was found that subjects were able to complete trials at 5 km·h-1 and 8 

km·h-1 under all conditions, and were able to obtain running speeds of 14 km·h-1 in 1G. 

However, subjects had difficulty obtaining 14 km·h-1 in 0G. Mean EL varied slightly with 

subject leg length, but was not affected by locomotion speed. Contact time, cadence, peak 

ground reaction force, and impulse were not different between gravitational conditions. Group 

locomotion kinematics were also not different between 0G and 1G, but subject differences were 

identified. 

One of the questions of interest in this investigation concerned the ability of the subjects to run at 

various speeds on the non-motorized treadmill. While the subjects could obtain and maintain 

gait during walking (5 km·h-1) and slow jogging (8 km·h-1), it was difficult for some subjects to 

attain sprinting speeds of 14 km·h-1. There are many possible reasons for this. It is possible that 

during the 0G trials, there was not enough time for the subjects to attain the desired speed. It is 

also possible that a learning effect occurred, as all subjects completed their trials in the 

predefined order of 0G, 1G without the PH, and 1G with the PH. The subjects may have become 

better at running on the BD-1 as they completed more and more trials. Finally, it is possible that 

the gravitational condition had an effect upon the ability to generate high treadmill velocities. 

During locomotion on the current ISS treadmill, the crewmembers may place themselves under a 

wide range of EL using elastic bungees, the Subject Loading Device (SLD), and the SLD in 

combination with bungees. The EL magnitude is typically part of the exercise prescription and 
 23 

varies according to the crewmember and the duration and time in their mission. On the BD-1, 

the load was shown to be consistent at different speeds, which suggests that either the bungee 

length was not affected by the body’s motion during locomotion, or the displacements that 

occurred during locomotion were not different enough between 5 km·h-1 and 14 km·h-1 to 

produce a difference. It is unclear how much variability in total load would occur if the bungee 

were attached at different points on the harness. Because the mean EL was not affected by gait 

speed, it may be that changing the attachment location on the harness may also not affect the EL 

appreciably. These results suggest that crewmembers may not be able to modify the EL 

appreciably during their mission while using BD-1, and that the EL experienced by each 

crewmember can be approximated by their leg length. This result has important implications for 

exercise prescription, as the EL is taken into account during periodization schemes. 

It was surprising to find that temporal and kinetic parameters did not differ between 0G and 1G 

locomotion (see Table 12). McCrory et al., (2002) reported overground peak GRF at 3 mph (4.8 

km·h-1) and 6 mph (9.7 km·h-1) to be 1.24 and 2.40 BW respectively (see Table 12). Our 

approximate mean peak GRF for 0G and 1G of 1.4 BW at 5 km·h-1 appears to be greater than 

those obtained during overground locomotion. However, the peak GRF of 2.4 BW at 14 km·h-1 

appears to be similar or less than those obtained at similar speeds during overground and 

motorized treadmill locomotion. Impulses were less during our investigation at 5 km·h-1 and 14 

km·h-1 than McCrory et al. found (0.61 & 0.41 BW·s). 

Our findings were much different than Davis et al., (1996), who reported motorized and nonmotorized running on a horizontal treadmill while suspended to create peak GRF forces of 688 N 

and 1002 N and impulses of 335 N·s and 201 N·s at speeds of 3.6 km·h-1 and 14.4 km·h-1, 

respectively. Using the mean body mass of 76.2 kg from their study, these values correspond to 

peak GRF of 0.97 BW and 1.34 BW and impulses of 0.45 BW·s and 0.23 BW·s. It is interesting 

to note that while the peak GRF in our study were greater, the impulses were similar between 0G 

and 1G. 

Table 12. Mean peak GRF found during BD-1 locomotion as compared to that reported in 

other studies. 

Study Speed 

 Approximate Load Walking Running 

BD-1 (non-motorized) 0.70 BW @ 0G 1.40 BW @ 5 km·h-1 2.40 BW @ 14 km·h-1

McCrory et al. (2002) 1G 1.24 BW @ 4.8 km·h-1 2.40 BW @ 9.7 km·h-1

Davis et al. (1996)* 0.60 BW @ horizontal suspension 0.92 BW @ 3.6 km·h-1 1.34 BW @ 14.4 km·h-1

* Approximate values for a 76.2 kg subject 

Our GRF values appear to be more comparable to that of overground locomotion in 1G than to 

non-motorized locomotion during horizontal suspension. It is possible that the horizontal 

orientation of the subject affected the necessary biomechanics to propel the treadmill. Our study 

measured GRF during upright locomotion in 1G while Davis et al. studied locomotion while 

subjects were suspended in a supine position. During horizontal suspension, subjects probably do 

not need to be concerned with balance, as it is impossible to stumble or fall. However, during 

upright locomotion, this must be taken into account. It is possible that gait biomechanics are 

altered, and are fundamentally different between horizontal suspension and upright locomotion. 
 24 

A limitation to our study was that only vertical GRF could be measured. It is quite probable that 

significant horizontal forces are necessary to create the motion of the treadmill belt. Contact 

times may explain the differences in GRF and impulses between our study and McCrory et al. 

Those recorded in our study were consistent for each condition, but were less than those found 

by the other investigators. It is possible that upright running may result in different temporal 

patterns than suspended running. 

As BD-1 belt speed increased, peak GRF increased while contact time and impulse decreased. 

Because impulse is dependent upon peak GRF and contact time, this result implies that the 

decrease in contact time was greater than the increase in peak GRF as speed increased. It is 

possible that the actual GRF was slightly greater than the reported vertical GRF since the fore-aft 

GRF may increase as treadmill speed increases. It should also be noted that the mean speeds 

attained during the fastest trials in 0G were less than those attained in 1G. It may be that greater 

forces would have occurred in 0G had the subjects attained a higher treadmill speed. 

Although no data were collected during overground or motorized treadmill locomotion, results 

suggest that non-motorized treadmill locomotion is markedly different than motorized treadmill 

locomotion, probably due to the differences in task requirements. These differences may lead to 

different training effects when compared to long-term treadmill training on a motorized 

treadmill. GRF trajectories were absent of the initial impact peak reported in studies examining 

GRF during motorized treadmill exercise (Schaffner et al., 2005; McCrory et al., 2004). The 

initial peak, also known as the impact peak, occurs as the heel contacts the ground and is thought 

to be important for bone density maintenance because it provides a higher initial rate of change 

of force (Turner, 1998). The absence of this peak suggests that non-motorized treadmill 

locomotion may not create as great a stimulus for bone remodeling as motorized treadmill 

exercise. However, the increased GRF may compensate for this loss. 

With regard to kinematics, it is not surprising that 1G trials without the RS resulted in greater 

forward lean values than those with the PH. However, it appears that lower body kinematics 

were not affected because there were no differences found in the hip, knee or ankle motion 

between these two conditions during the repeated measures analysis. Thornton et al., (1997) do 

not present quantitative data, but mentioned that during 0G locomotion on a non-motorized 

treadmill, subjects exhibited greater amounts of forward lean as speed increased, which agrees 

with our data. It appears that there was little difference in mean forward lean between speeds 

during the RS trials. Future research is necessary to determine how the approximate mean 

forward lean of 20 degrees compares to overground and treadmill running. However, because 

there were no differences in lower body kinematics between RS conditions, it also appears that 

the RS had little effect upon lower body motion during non-motorized treadmill running. 

While it is difficult to assess the meaning of the range of motion and actual extremes of hip, knee 

and ankle flexion/extension, it is possible to make a few general statements. The mean hip ROM 

of approximately 40-45 degrees occurred as a flexor movement; the hip rarely exhibited any type 

of extension, probably due to the extreme forward lean of the subject. Novacek (1998) presented 

data during sprinting that showed typical hip range of motion of approximately 10 degrees. 

These data suggest that considerably more hip flexion occurs during non-motorized running, 

possibly due to the increased necessity to generate propulsive forces. The increased hip motion 
 25 

could result in overuse injuries of the quadriceps and other hip flexor/extensor musculature after 

prolonged exercise and should be noted by medical and training personnel. 

Knee ROM appeared to increase with locomotion speed. The ROM at 14 km·h-1 of about 70 

degrees was less than Novacek’s (1998) values of about 95 degrees during overground sprinting. 

The overground data suggest that near toe-off and heel-strike, the leg is near full extension. 

However, the data in this investigation suggest that the knee does not extend past approximately 

15 degrees of flexion. This may have occurred in order to place the foot under the body during 

foot-ground contact. By placing the foot under the body as opposed to in front of the body, the 

braking forces associated with foot-ground contact are reduced. The reduced ROM of the knee 

should also be noted by medical and training personnel, as it is possible that decreased hamstring 

flexibility could occur over time. Ankle motion is similar to that presented by Novacek (1998), 

suggesting that the primary differences between non-motorized treadmill and motorized or 

overground locomotion occur at the knee and hip joint. 

Although flexion and extension velocities in each joint were not compared, it appears that at the 

knee and ankle, similar peak velocities were attained during both forward and rearward 

movements of each joint. This is especially apparent during the 14 km·h-1 trials. This reflects 

the importance of not only a forceful extension of the hip and knee to propel the treadmill belt, 

but also a forceful recovery of those joints to prepare for the next stride. 

CONCLUSIONS 

We found that locomotion on the BD-1 treadmill, while different than motorized treadmill gait, 

is not markedly different in 0G or 1G. The use of a RS reduces the amount of forward lean, but 

does not affect joint excursion or vertical GRF measures. Because of the non-motorized nature 

of the treadmill, the subjects must provide considerably greater hip, knee and ankle extensor 

forces during the propulsive motion. The differences in joint excursions and required forces to 

run on the BD-1 may require an adjustment in the overall exercise prescription, including 

prescriptions for other exercise modalities, provided to crewmembers to accommodate for the 

long term physiological effects of training upon the device, which are not yet understood. 
 26 

REFERENCES 

Cavanagh PR, Buczek FL, Milliron MJ. (1987).The kinematics of locomotion in space: A final 

report to Krug International. 

Cullen DM, Smith RT, Akhter MP. (2000). Time course for bone formation with long-term 

external mechanical loading. Journal of Applied Physiology, 88:1943-8. 

Davis BL, Cavanagh PR, Sommer III HJ, Wu, G. (1996). Ground reaction forces during 

locomotion in simulated microgravity. Aviation, Space, and Environmental Medicine, 67(3): 

235-42. 

DeWitt JK, Schaffner G, Blazine K, Bentley JR, Laughlin MS, Loehr JA, Hagan RD. (2003). 

Loading configurations and ground reaction forces during treadmill running in weightlessness. 

Presented at the 2003 Annual Meeting of the American Society of Biomechanics, Toledo, OH. 

DeWitt JK, Schaffner G, Laughlin MS, Loehr JA, Hagan, RD. (2004). External load affects 

ground reaction force parameters non-uniformly during running in weightlessness. Presented at 

the 2004 Annual Meeting of the American Society of Biomechanics, Portland, OR. 

Gamble DJ, Bartlett RM, Jakeman PM. (1988). A comparison of non-motorized treadmill, 

motorized treadmill and overground running pattern, Biomechanics of Sport, 25-32. 

Heinonen A, Kannus P, Sievanan H, Oja P, Pasanen M, Rinne M, Uusi-Rasi K, Vuori I. (1996). 

Randomised controlled trial of effect of high-impact exercise on selected risk factors for 

osteoporotic fractures. The Lancet, 348:1343-7. 

Judex S, Zernicke RF. (2000). High-impact exercise and growing bone: relation between high 

strain rates and enhanced bone formation. Journal of Applied Physiology, 88:2183-91. 

Lakomy HKA. (1987). The use of a non-motorised treadmill for analysing sprint performance. 

Ergonomics, 30:627-37. 

LeBlanc A, Lin C, Shackelford L, Sinitsyn V, Evans H, Belichenko O, Schenkman B, 

Kozlovskaya I, Oganov V, Bakulin A, Hedrick T, Feeback D. (2000). Muscle volume, MRI 

relaxation times (T2), and body composition after spaceflight. Journal of Applied Physiology, 

89:2158-64. 

McCrory JL, Baron HA, Balkin S, Cavanagh PR. (2002). Locomotion in simulated microgravity: 

Gravity replacement loads. Aviation, Space and Environmental Medicine, 76:625-31. 

McCrory JL, Derr J, Cavanagh PR. (2004). Locomotion in simulated zero gravity: 

ground reaction forces. Aviation, Space and Environmental Medicine, 75:203–10. 

Mosley JR & Lanyon LE. (1998). Strain rate as a controlling influence on adaptive modeling in 

response to dynamic loading of the ulna in growing rats. Bone, 23(4):313-8. 
 27 

Mosley JR, March BM, Lynch J, Lanyon LE. (1997). Strain magnitude related changes in whole 

bone architecture in growing rats. Bone, 20(3):191-8. 

Munro CF, Miller DI, Fuglevand AJ. (1987). Ground reaction forces in running: a 

reexamination. Journal of Biomechanics, 20(2):147-55. 

Novacek TF. (1998). The biomechanics of running, Gait and Posture, 7:77-95. 

Schaffner G, DeWittJK, Bentley JR, Yarmanova E, Kozlovskaya I, Hagan RD. (2005). Effect of 

Subject Loading Device Load Levels on Gait, Ground Reaction Force and Kinematics during 

Human Treadmill Locomotion in a Weightless Environment. NASA Technical Report 

NASA/TP-2005-213196. 

Schaffner G, DeWitt JK, Laughlin MS, Bentley JR, Hagan RD. (2004). Effect of bungee loading 

on ground reaction force during treadmill locomotion in weightlessness. Presented at the 75th

Annual Scientific Meeting of the Aerospace Medical Association, Anchorage, Alaska. 

Schneider V, Oganov V, LeBlanc A, Rakmanov A, Taggart L, Bakulin A, Huntoon C, Grigoriev 

A, Varonin L. (1995). Bone and body mass changes during space flight. Acta Astronautica, 36(8-

12):463-6. 

Thornton WE, Cavanagh PR, Buczek FL, Burgess-Milliron MJ, Davis BL. (1997). The 

kinematics of treadmill locomotion in space. In: Allard P, Cappozzo A, Lundberg A, Vaughan 

CL, Eds. Three-dimensional analysis of human locomotion. 1st ed. New York: John Wiley & 

Sons; 1997:375-88. 

Turner CH. (1998). Three rules for bone adaptation to mechanical stimuli, Bone, 23(5):399-407. 
 28 

APPENDIX 

Mean contact times for all subjects - all trials

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

5 8 14 5 8 14 5 8 14 5 8 14 5 8 14 5 8 14 5 8 14 5 8 14

12345678

Speed (km/hr)

Subject

Contact time (s)

PH - 1G No PH - 1G - 0G

Mean Peak Ground Reaction Force for all subjects - all trials

0

0.5

1

1.5

2

2.5

3

5 8 14 5 8 14 5 8 14 5 8 14 5 8 14 5 8 14 5 8 14 5 8 14

12345678

Speed (km/hr)

Subject

Peak GRF (BW)

PH - 1G No PH - 1G - 0G

REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188 

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and 

maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including 

suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302, 

and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503. 

1. AGENCY USE ONLY (Leave Blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

 March 2006 NASA Technical Memorandum 

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Biomechanical Evaluation of Locomotion on the Russian BD-1 Treadmill in a Weightless 

Environment (KC-135) 

 

6. AUTHOR(S) 

J. DeWitt, G. Schaffner, J. Bentley, E. Yarmanova, I. Kozlovskaya, D. Hagan, 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION 

REPORT NUMBERS

Johnson Space Center S-975 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

National Aeronautics and Space Administration 

Washington, DC 20546-0001 

TM-2006-213718 

11. SUPPLEMENTARY NOTES

 

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE 

Unclassified/Unlimited 

Available from the NASA Center for AeroSpace Information (CASI) 

7121 Standard 

Hanover, MD 21076-1320 Category: 52

 

13. ABSTRACT (Maximum 200 words)

The purpose of this investigation was to evaluate locomotion on the Russian BD-1 non-motorized treadmill under three conditions

weightlessness (0G) without a simulated subject positioning device, or horizontal restraint strap (RS), normal gravity (1G) without a 

RS, and 1G with a RS. Various outcome measures were evaluated, including the applied external load (EL), vertical ground reaction 

force (GRF), the ability to achieve specific locomotion speeds, and joint kinematics. The goal of this research was to better 

understand how locomotion and the related forces experienced by the musculoskeletal system during non-motorized treadmill 

locomotion in 0G compare to locomotion on the same device in 1G. 

14. SUBJECT TERMS 15. NUMBER OF 

 PAGES

16. PRICE CODE 

exercise physiology, locomotion, physiological effects, Aerospace medicine, astronaut 

performance, bone demineralization, gravitational effects. 38 

17. SECURITY CLASSIFICATION 

OF REPORT

18. SECURITY CLASSIFICATION 

 OF THIS PAGE

19. SECURITY CLASSIFICATION 

 OF ABSTRACT

20. LIMITATION OF ABSTRACT 

Unclassified Unclassified Unlimited Unlimited 

Standard Form 298 (Rev Feb 89) (MS Word Mar 97) 

Prescribed by ANSI Std. 239-18 

298-102 

NSN 7540-01-280-5500