Characteristics of Gait in Restricted Vertical Space Versus Unrestricted 

Walking 

Sean Gallagher, Ph.D. 

Jonisha Pollard, M.S.
 

William L. Porter, M.S. 

National Institute for Occupational Safety and Health, Pittsburgh, Penna. 

Upright walking is not a viable gait option in work environments that have restricted vertical space, such as 

underground low-seam coal mines (< 1.2 m vertical height). In such circumstances, stoopwalking and 

crawling must be used. The objective of the current manuscript is to assess the difference between free 

cadence walking versus stoopwalking (under a 1.2 m ceiling), four-point crawling (hands and knees), and 

two-point crawling (knees only), both with and without kneepads. Compared to upright walking, 

stoopwalking resulted in a 24% reduction in gait velocity and exhibited reduced stride length (1.04 versus 

1.51 meters). Four-point crawling further slowed gait (to 0.50 m/s) and showed evidence of both trot-like 

and pace-like interlimb coordination patterns. Gait speed for two-point crawling was only 0.32 m/s. 

Introduction 

Workers are often required to move around from place to 

place at their worksite in order to perform their daily tasks. In 

most work environments, this can be easily accomplished via 

normal upright walking. However, there are occupational 

environments where upright walking is not possible. One such 

environment is a low-seam coal mine (defined as a mine with 

less than 1.2 m vertical height). The restricted space compels 

mine workers to stoopwalk (bipedal walking while severely 

bent at the waist with increased knee flexion) or crawl (either 

on all fours or on two knees) to fulfill their daily work duties. 

Early research efforts on restricted space work environments 

indicated that restricted space locomotion was associated with 

higher metabolic costs [Moss 1934, Bedford and Warner 

1955, Humphreys et al. 1962, Morrissey et al. 1985]. In 

addition, studies have found that the maximum speed 

attainable is decreased in severe stoopwalking (< 70% stature) 

and crawling activities [Morrissey et al. 1985]. However, to 

date no studies have compared stoopwalking, and crawling to 

existing data on natural cadence upright walking, either in 

terms of gait velocity or in comparing knee extensor and 

flexor electromyographic (EMG) activity to knee kinematics. 

The current manuscript will assess the differences between 

natural cadence upright walking versus stoopwalking (under a 

1.2 m simulated mine roof), four-point crawling (hands and 

knees), and two-point crawling (knees only), both with and 

without kneepads. 

Method 

Subjects 

Nine subjects (six males, three females) participated in this 

study. The average age was 35 ± 17 (mean ± SD) years, while 

the average mass was 69.7 kg ± 10.6 and the average stature 

was 168.0 cm ± 7.6. The average body mass index (BMI) for 

these subjects was 24.2 ± 4.0. Prior to participating in the 

study, each subject read and signed an informed consent form 

approved by the National Institute for Occupational Safety and 

Health (NIOSH) Human Subjects Review Board. One subject 

had been previously diagnosed with bursitis, which did not 

require any intervention, and a second subject had slight nerve 

damage due to a motorcycle accident. However, neither 

subject was symptomatic at the time of testing, and both were 

approved for testing. 

Restricted Space Locomotion Data 

Independent variables in this study were kneepad condition 

(two levels) and locomotion style (three levels). Kneepad 

conditions included: (1) no kneepad, and (2) wearing an 

articulated kneepad with a hard contoured outer shell. 

Locomotion modes included stoopwalking (bipedal walking 

with a fully flexed torso in a 122 cm vertical space), crawling 

on two knees (two-point crawling) in a 122 cm vertical space, 

and crawling on hands and knees (four-point crawling) in a 97 

cm vertical space. Dependent variables consisted of the mean 

and maximum normalized EMG data of 10 thigh muscles (to 

ascertain muscle function) and motion analysis data (used to 

measure knee kinematics and to calculate gait velocity for 

each locomotion mode/kneepad combination). EMG activity 

was collected from left and right pairs of muscles including 

the vastus lateralis (VL), rectus femoris (RF), vastus medialis 

(VM), biceps femoris (BF), and semitendinosus (ST). 

Motion Analysis 

A motion capture system, the Eagle Digital System from 

Motion Analysis Corporation, Santa Rosa, Calif., was used to 

determine the location of body segments as the subject 

performed the crawling and stoopwalking tasks. A modified 

version of the Cleveland Clinic marker set was used. 

Reflective markers could not be placed on medial and lateral 

aspects of the knee when wearing kneepads. Therefore, a full 

marker set was first developed that included these knee 

markers followed by a second marker set, where the knee 

markers were removed from the subject. The second marker 

set was then used during testing. 
EMG Preparation 

Preferred locations of the electrodes for each of the thigh 

muscles were derived from Ericson et al. [1985]. Disposable 

self-adhesive Ag/AgCl dual snap surface electrodes (Noraxon 

USA Inc.; Scottsdale, Ariz.) with electrode spacing of 2 cm 

center-to-center were employed. Each electrode site was 

shaved (if necessary) and cleaned using a skin prep pad 

consisting of 70% isopropyl alcohol and pumice (Dynarex 

Corp.; Orangeburg, N.Y.). Electrodes were placed over the 

belly of the muscles, distal to the motor point regions [Ericson 

et al. 1985]. Two reference electrodes were required (one for 

each wireless transmitter) and were placed at remote sites. 

Maximum voluntary contractions (MVCs) were obtained for 

the thigh muscles of both right and left legs [NIOSH 1992], 

and they were used to normalize the gait EMG [Dubo et al. 

1976; Ericson et al. 1986; Knudson and Johnston 1993]. The 

subject was instructed to lie in a supine position in a Biodex 

chair with the included knee angle at approximately 90 

degrees with the hips and ankle secured via Velcro straps. The 

subject was then instructed to perform knee extension with 

maximal effort for at least 5 seconds while a researcher 

provided verbal encouragement. The subject was then 

instructed to flex the knee for that leg with maximal effort for 

at least 5 seconds again with verbal encouragement. All EMG 

measurements were made using a Noraxon TeleMyo 2400R— 

worldwide telemetry system with 16 channels (Noraxon USA 

Inc.; Scottsdale, Ariz.). The gain was set to 10 for all channels. 

Several hardware filters were in place: first-order high-pass 

filters set to 10 Hz ± 10% cut-off, and eighth-order 

Butterworth/Bessels low-pass anti-alias filters set to 

500 Hz ± 2% cut-off. The common mode rejection was 

> 100dB. The sampling rate for all EMG data was 1020 Hz. 

Procedure 

After informed consent was obtained from the subject, the 

subject donned a T-shirt, athletic shorts, socks, and shoes; then 

the initial marker set was applied. Subsequently, the subject 

stood upright and assumed a standing “T-pose” (with arms 

outstretched to the right and left and parallel with the floor) in 

order to calculate the positions of the markers that were 

removed during testing. Selected markers were then removed, 

EMG electrodes were positioned on the subject, and MVCs 

were obtained. 

Depending on the experimental conditions, the subject was 

instructed to either don the kneepads or participate without 

kneepads. The subject then performed all three of the 

locomotion tasks (in random order) within the specified 

kneepad condition. Subjects were instructed to stoopwalk or 

crawl using a natural (free) cadence for each kneepad 

condition [Winter 1991] and were provided a brief rest (1–2 

minutes) between trials. When the trials were completed for a 

kneepad condition, the subject adopted the opposite kneepad 

condition and performed all locomotion trials, in random 

order. 

Data Conditioning and Analysis 

Locomotion cycles were defined for crawling and 

stoopwalking trials as follows: crawling cycles were defined 

as starting and ending by the position of markers on the left 

shank as the left knee contacted the floor (as determined via 

motion analysis); stoopwalking cycle starting and ending 

times were defined by the position of the left ankle marker 

when the heel contacted the ground. At least two complete 

cycles of data were collected for all crawling trials; however, 

for some subjects with long strides only one full cycle of 

motion analysis data was available for stoopwalking trials. For 

this reason, the data reported in the results represents one 

cycle per subject for stoopwalking and the average of two 

cycles per subject for crawling tests. 

Electromyography data was low-pass filtered to 500 Hz and 

high-pass filtered to 20 Hz using a fourth-order Butterworth 

filter (MATLAB®; The MathWorks, Inc.; Natick, Mass.) The 

signal was then rectified and normalized by dividing by the 

maximum voluntary contraction for each muscle. Mean 

absolute values (MAVs) of the normalized EMG were then 

calculated by determining the running mean of every 102 

samples, or 10% of the sampling rate [NIOSH 1992]. Trials 

containing evidence of artifacts were eliminated from the 

analysis. Ensemble averages for EMG data were generated by 

normalizing the muscle activity for all subjects in terms of the 

percentage of the gait cycle. 

Comparison Data for Upright Walking 

Comparison data for natural cadence upright walking was 

obtained from Winter [1991]. This reference provides 

normative data on joint angle histories, EMG activity, and gait 

velocity during walking. 

Results 

Temporal and Stride Measures 

Table 1 provides average values for cadence, stride period and 

length, and gait velocity for upright walking, stoopwalking, 

and crawling. Presence or absence of kneepads did not have 

any impact on speed of locomotion (F1,8 = 3.01, p > 0.05). 

Knee Joint Kinematics 

Figure 1 illustrates joint angle histories for the knee joints for 

natural cadence upright walking, stoopwalking under a 122 cm 

ceiling, four-point crawling under a 97 cm ceiling, and twopoint crawling under a 122 cm ceiling. It can be seen from this 

figure that the overall range of angles was similar between 

walking and stoopwalking (approximately 65 degrees); 

however, stoopwalking resulted in included knee angles that 

were generally 25 degrees less (i.e., more flexed) than normal 

walking. The range of motion of the knee joint was observed 

to be decreased in four-point crawling and especially in twopoint crawling compared to stoopwalking. The total range of 
motion for the four-point crawling was approximately 50 

degrees (55 to 105), while a very limited range of knee joint 

angles was observed in two-point crawling (50 to 70 degrees 

included knee angle). 

Electromyography 

Figure 2 provides ensemble averages for the EMG data 

provided by Winter [1991] for upright walking and from the 

current study for stoopwalking and crawling. EMG 

normalization procedures varied between Winter [1991] and 

the current study, thus direct comparisons of EMG magnitude 

are not possible. In addition, the Winter [1991] EMG data 

appears to be heavily smoothed. 

Discussion 

As postures change for the purpose of locomotion in restricted 

space, so do the physical demands on specific regions of the 

body. This study specifically examined the demands on the 

lower extremities associated with crawling and stoopwalking 

activities and compared EMG and kinematic results to those 

observed in natural cadence upright walking. Figure 2 

provides comparisons between gait conditions in terms of 

EMG activity. For example, in stoopwalking there appears to 

be a more prolonged period of vastus medialis and vastus 

lateralis activation. Another notable difference is that the 

major burst from the rectus femoris during normal walking 

(associated with the full knee extension in mid-stance) is 

totally absent in stoopwalking. This may be because the knee 

never fully extends in mid-stance during stoopwalking (per the 

kinematic data discussed above). Further, it can be seen that 

the knee extensor/flexor activation is generally lower in fourpoint crawling compared to other locomotion techniques. This 

is likely the result of a decreased burden on the legs to provide 

forces necessary for locomotion (owing to the assistance of the 

arms in this type of movement). 

In this study, comparisons between existing literature on 

unrestricted walking and stoopwalking disclosed several 

differences in terms of both kinematics and EMG activity of 

the thigh muscles. In kinematic terms, it appears that the range 

of included knee angles are somewhat reduced in 

stoopwalking compared to upright walking. The range of the 

included knee angles reported by Winter [1991] in normal gait 

went from about 180 degrees (full extension) to approximately 

115 degrees. The current study found that full extension of the 

knee never occurred in stoopwalking. In fact, the maximum 

included knee angle during stoopwalking was approximately 

155 degrees as opposed to the 180 degrees observed in normal 

walking. The lowest included knee angle observed in normal 

walking by Winter [1991] was approximately 115 degrees, 

while the knee was flexed almost 90 degrees during 

stoopwalking. Thus, stoopwalking resulted in included knee 

angles that were generally 25 degrees less than during normal 

walking. The reduced included knee angles would appear to 

be a consequence of the space restrictions present. As space 

becomes more confined, joints (such as the knee joint) need to 

maintain a flexed position so that the body can maintain 

adequate clearance in the reduced vertical space. 

The total knee joint range of motion for four-point crawling 

was approximately 50 degrees (55 to 105 included angle). 

Examining the pattern of motion (Figure 1) indicates that 

approximately 60% of the cycle time was spent in extending 

the knee (mostly during the contact phase) while the flexion 

phase (as the leg was brought forward) occurred more quickly. 

The magnitude of EMG for four-point crawling was lower in 

general (and periods of significant activity less prolonged) 

than for standing or stoopwalking. This may be due in part to 

less body weight being supported in four-point crawling due to 

the support of the upper body by the muscles of the arms and 

shoulders (which could not be quantified in the current 

investigation). As with stoopwalking, higher activity was 

observed for the VL and VM with lesser coactivity of the 

medial and lateral hamstrings. Activity of the RF was minimal 

in both crawling postures. 

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Table 1. Average values for temporal and stride measures for upright walking [Winter 1991], stooping, and crawling. 

Cadence (steps/min) Stride Period (s) Stride Length (m) Velocity 

(m/s) 

Upright walking (n=53) 105.3 1.14 1.51 1.33 

Stoopwalking (n=9) 112.8 1.06 1.04 1.01 

Four-point Crawling (n=9) 86.3 1.48 0.69 0.50 

Two-point Crawling (n=9) 96.8 1.24 0.40 0.32 
Walkin

g Stoopwalking Four-pt. Crawl Twopt. Crawl 

Figure 2. Thigh muscle EMG ensemble averages for upright walking [Winter 1991] in the top row 

versus stoopwalking (second row), four-point crawling (third row), and two-point crawling (fourth 

row). Note that normalization procedures by Winter (1991) differ from the current study. MVC = maximum voluntary contraction. Dotted lines represent ± SE.