Locomotion in restricted space: Kinematic and electromyographic analysis of

stoopwalking and crawling§

§ Mention of specific products in this article does not constitute endorsement by

NIOSH. The findings and conclusion in this report are those of the authors and do

not necessarily represent the views of NIOSH.

Sean Gallagher *,

* Corresponding author at: National Institute for Occupational Safety and Health,

Office of Mine Safety and Health Research, PO Box 18070, Pittsburgh, PA 15236­

0070, USA. Tel.: +1 412 386 6445; fax: +1 412 386 6764.

E-mail addresses: sgallagher@cdc.gov, sfg9@cdc.gov (S. Gallagher).

Jonisha Pollard, William L. Porter

National Institute for Occupational Safety and Health, Office of Mine Safety and Health Research, PO Box 18070, Pittsburgh, PA 15236-0070, USA

ABSTRACT

Stoopwalking and crawling are compulsory gait techniques in some occupational settings, as in lowseam coal mines (where vertical space may be less than 122 cm). Nine participants, six males and three

females (mean = 35 years + 17 SD), participated in a study examining kinematic and electromyographic

(EMG) responses to natural cadence stoopwalking, four-point crawling (all fours), and two-point

crawling (knees only). EMG data were collected from knee extensors and flexors, and a motion analysis

system was used to obtain kinematic data. The average gait velocity for stoopwalking was 1.01

(±0.32) m/s with an average cadence of 112.8 steps/min and stride length of 1.04 m. Four-point crawling

velocity averaged 0.50 (±0.20) m/s, with average cadence of 86.3 steps/min and stride length of 0.69 m. Twopoint crawling exhibited the slowest velocity (0.32 m/s) and shortest stride length (0.40 m); however,

cadence was greater than four-point crawling (96.8 steps/min). EMG findings included prolonged

contraction of both knee extensors and flexors (compared to normative data on normal walking), increased

relative activity SD of the flexors (versus extensors) in two-point crawling, and decreased thigh muscle

activity in four-point crawling. Interlimb coordination in four-point crawling trials indicated trot-like, no

limb pairing, and near pace-like limb contact patterns. Presence or absence of kneepads had no impact on

kinematic or EMG measures (p > 0.05); however, subjects complained of discomfort without kneepads

(especially in two-point crawling). Results of this study have implications for work performed in

underground coal mines, as well as emergency or evacuation considerations.

1. Introduction

Successful performance of occupational activities often requires

workers to efficiently (and safely) transport themselves from one

workplace location to another. This is normally accomplished via

upright walking. However, certain occupational environments,

such as low-seam coal mines (where vertical height is less than

122 cm), do not permit upright walking. The constrained vertical

space compels mine workers to stoopwalk or crawl (either on all

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

Previous studies of stoopwalking and crawling have disclosed

higher metabolic demands for gait in vertically constrained space

[1–5]. In fact, metabolic costs appear to rise as stooping postures

becomes more severe [4,5]. In addition to higher metabolic costs,

maximum gait velocity may be reduced as space restrictions

become more extreme [4,5]. Crawling speed may also be affected

by gender and body composition, with overweight individuals and

females exhibiting reduced speed [6]. Crawling speed also impacts

interlimb coordination patterns [7].

However, our understanding of locomotion demands in

confined space remains incomplete. For example, two-point

crawling (crawling on knees alone) had not been investigated,

and influence of kneepads on crawling remains unclear. Furthermore, studies have not examined muscle function of the lower

limbs in confined space. Accordingly, the following hypotheses

were tested in this study: (1) use of stoopwalking and crawling

techniques in confined space will impact gait parameters,

kinematics, and electromyographic (EMG) activity of knee

extensors and flexors, (2) use of kneepads will affect gait and

EMG activity, and (3) that the above factors will interact to affect

these measures.

2. Methods

2.1. Subjects

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

study. The average age was 35 years ± 17 (mean ± SD), the average
mass was 69.7 kg ± 10.6 and average stature was 168.0 cm ± 7.6. The

mean body mass index (BMI) for these subjects was 24.2 ± 4.0. All

subjects operated under terms of informed consent.

2.2. Experimental design

Independent variables consisted of kneepad condition (two

levels) and locomotion technique (three levels). Kneepad conditions included: (1) not wearing a kneepad and (2) wearing an

articulated kneepad (seen in Fig. 1).


 

Fig. 1. Subject performing (A) stoopwalk, (B) four-point crawling and (C) two-point

crawling with articulated kneepads.

Locomotion modalities

included stoopwalking (bipedal walking with a fully flexed torso),

crawling on two knees (two-point crawling), and crawling on

hands and knees (four-point crawling) as seen in Fig. 1. Dependent

variables included normalized EMG data of knee flexors and

extensors and motion analysis data (to determine knee kinematics,

interlimb coordination, and gait parameters). EMG activity was

collected from left (L) and right (R) pairs of the vastus lateralis (VL),

rectus femoris (RF), vastus medialis (VM), biceps femoris (BF), and

semitendinosus (ST). The two kneepad conditions (no kneepad vs.

articulated kneepad) were tested in random order. Within each

kneepad condition, a restricted randomization determined the

order of the three locomotion trials. Based on this randomization

scheme, a split–plot analysis of variance (ANOVA) was used to

evaluate EMG responses and gait parameters using Dunn–Sidak

post hoc tests. As an exploratory investigation, Type I error rates

were set at a per contrast 0.05 level.

2.3. Motion analysis

A motion capture system (Eagle Digital System by Motion

Analysis Corporation; Santa Rosa, CA) was used to ascertain body

segment kinematics during crawling and stoopwalking tasks. A

modified version of the Cleveland Clinic marker set was employed.

2.4. EMG preparation

Electrode locations for the thigh muscles were derived from a

previous study [8]. Disposable self-adhesive Ag/AgCl dual snap

surface electrodes (Noraxon USA Inc.; Scottsdale, AZ) with

electrode spacing of 2 cm center-to-center were used. Electrode

sites were shaved and cleaned with an EMG skin prep pad

(Dynarex Corp.; Orangeburg, NY). Electrodes were placed over the

muscle belly, distal to the motor point [8]. Reference electrodes for

each of two wireless transmitters were placed at remote sites.

Maximum voluntary contractions (MVCs) were obtained for the

thigh muscles of both right and left legs [9], and used to normalize

gait EMG [10–14]. The subject was instructed to lie in a supine

position in a Biodex1 chair with knee and hips at approximately

908 angles. Hips and ankles were secured via Velcro1 straps. The

subject then performed knee extension or flexion with maximal

effort for at least 5 s while verbal encouragement was provided.

EMG measurements were made using a Noraxon Telemyo 2400Rworldwide telemetry system with 16 channels (Noraxon USA Inc.,

Scottsdale, AZ). Several hardware filters were used: 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 >100 dB and EMG sampling

rate was 1020 Hz.

2.5. Procedure

Procedures were approved by the National Institute for

Occupational Safety and Health (NIOSH) Human Subject Review

Board. After informed consent was obtained, the subject donned a

T-shirt, athletic shorts, socks, and shoes appropriate to the needs of

the experiment. The motion analysis markers were applied, and

the subject performed motions necessary to develop a body

template. EMG electrodes were then applied and MVCs obtained.

Depending on the experimental conditions, the subject would don

kneepads or remain without kneepads. The subjectthen performed

the three locomotion tasks (in random order) within the specified

kneepad condition. Subjects were instructed to stoopwalk or crawl

using a natural (free) cadence for each condition [15], and were

provided a brief rest period (1–2 min) between trials.

2.6. Data conditioning and analysis

Crawling cycles were defined as starting and ending by the

position ofthe left shank marker as the left knee contacted the floor

(as determined via motion analysis), while starting and ending

times of the stoopwalking cycle were defined by the position of the

left ankle marker when the heel contacted the floor. In four-point

crawling, the time at which stance and swing phases were initiated

for each limb were expressed as a function of the left leg cycle.

Interlimb coordination patterns in four-point crawling tasks
were assessed by calculating the ipsilateral phase lag (IPL) [7]—the

delay between the stance phase of the left arm and stance phase of

the left leg. IPLs in the 50% range indicate a trot-like gait, where the

diagonal limbs enter stance around the same time. IPLs close to 0%

or 100% indicate a pace-like gait, where ipsilateral limbs contactthe

ground around the same time. Intermediate values indicate no

pairing of limbs, with limbs entering stance approximately equally

spaced in time.

The EMG signal was rectified and normalized using the MVC for

each muscle. Mean amplitude values (MAVs) of the normalized

signal were calculated by determining the running mean of every

102 samples, or 10% of the sampling rate [9]. EMG data containing

evidence of artifacts were omitted from analysis. Data were

analyzed using Statistix software (Analytical Software; Tallahassee, FL).

3. Results

3.1. Temporal and stride measures

Table 1 provides cadence, stride period and length, and gait

velocity for stoopwalking and crawling techniques.

Table 1

Temporal and stride measures (mean ± SD) for the crawling and stoopwalking methods (n = 9).


 

Cadence (steps/min) Stride period (s) Stride length (m) Velocity (m/s)

Stoopwalking

Four-point crawling

Two-point crawling

112.8 (±40.0)

86.3 (±24.6)

96.8 (±39.5)

1.06 (±0.16)

1.48 (±0.39)

1.24 (±0.19)

1.04 (±0.24)

0.69 (±0.16)

0.40 (±0.09)

1.01 (±0.32)

0.50 (±0.20)

0.32 (±0.13)

Gait velocities

between methods were significantly different (F2,32 = 96.39,

p < 0.001). For stoopwalking, the forward movement speed

averaged 1.01 m/s, four-point crawling averaged 0.50 m/s, and

two-point crawling averaged 0.32 m/s. Post hoc tests indicated

significant differences in gait velocities between each of the

methods, with a critical value of comparison for the Dunn–Sidak

test of 0.12 m/s. Presence or absence of kneepads did not impact

speed of locomotion (F1,8 = 3.01, p > 0.05). However, an interaction

between locomotion technique and kneepad condition for stride

length (F2,32 = 45.17, p < 0.001) and cadence (F2,32 = 11.1,

p < 0.001) was observed. Simple effects tests indicated this

interaction was due to a decrease in cadence and increased stride

length when wearing kneepads in the two-point crawl. Stride

parameters in other postures were not affected by wearing

kneepads.

3.2. Knee joint kinematics

Fig. 2 presents ensemble averages of included knee angles for

the three locomotion methods.

Included Knee Angle (deg) Included Knee Angle (deg) Included Knee Angle (deg)

180 

160 

140 

120 

100 

80 

60 

40 

20 

0 

120 

100 

80 

60 

40 

20 

0 

80 

70 

60 

50 

40 

30 

20 

10 

0 

Stoopwalk 

0 10 20 30 40 50 60 70 80 90 100 

Four-Point Crawl 

0 10 20 30 40 50 60 70 80 90 100 

Two-point Crawl 

0 10 20 30 40 50 60 70 80 90 100 

Percentage of Gait Cycle 

Fig. 2. Ensemble averages for right (solid line) and left (dashed line) knee included

angles for stoopwalking, four-point crawling and two-point crawling (n = 9).

The overall angular range of motion

of the knee was similar in both stoopwalking and four-point

crawling (range of approximately 50–558 in both cases); however,

in stoopwalking the included knee angle was more extended (95–

1508), while the four-point crawl range involved greater flexion

(approximately 55–1058). The range of motion for the knee joint in

two-point crawling was limited to about 208.

Table 2 contains data on swing and stance phases during fourpoint crawling.

Table 2

Percentages of time in stance and swing for each limb during four-point crawling trials (n = 9).

Lshank Lwrist Rshank Rwrist

% Cycle in stance % Cycle in swing % Cycle in stance % Cycle in swing % Cycle in stance % Cycle in swing % Cycle in stance % Cycle in swing

Mean

SD

56.1

7.9

43.9

7.9

69.3

7.5

30.7

7.5

58.0

7.8

42.0

7.8

69.3

7.6

30.7

7.6

The arms spent a somewhat larger portion of the

cycle in stance compared to the leg (shank). On average, arms were

in stance for approximately 70% of the crawling cycle, while shanks

were in stance for approximately 55–60% of the cycle.
3.3. Interlimb coordination in four-point crawling

Interlimb coordination patterns varied between the subjects,

with several distinct IPL groupings observed. Two subjects

exhibited IPLs between 41 and 43% (approximating trot-like

behavior). Three subjects had IPLs between 35 and 39%,

intermediate between trot-like and no-limb pairing patterns.

Three subjects demonstrated IPLs between 25 and 29%, indicative

of no-limb pairing. One subject exhibited an IPL of 19.6, a value

between no-limb pairing and pace-like patterns.

3.4. Electromyography

Fig. 3 provides a summary of average EMG activity for right and

left muscle groups for each locomotion method studied.

Fig. 3. Left thigh (top row) and right thigh (bottom row) average EMG activity observed in the various locomotion modalities.

The

locomotion mode employed significantly affected the activity of 7

of the 10 muscles studied (p < 0.05). Stoopwalking resulted in

comparative increases in activity of the LVL, LRF, RVL, RVM, RBF,

and RRF muscles according to post hoc tests. Two-point crawling

resulted in higher activity of hamstring muscles, specifically the

RST and LBF (p < 0.05). Presence or absence of kneepads did not

affect muscle activity (p > 0.05). The interaction between locomotion mode and kneepad condition was not significant for any

muscle (p > 0.05).

4. Discussion

As different positions are adopted for locomotion in confined

space, physical demands and loads are redistributed throughout

the body. This study specifically examined demands on the lower

extremities associated with crawling and stoopwalking activities.

Results illustrate differences in EMG activity of the thigh muscles,

kinematics of the lower limb, and gait parameters during natural

cadence in the three methods studied.

Stoopwalking is clearly the most rapid method of covering

ground in restricted vertical space. In fact, stoopwalking was

approximately twice as fast as four-point crawling and three times

as fast as two-point crawling. The increased gait velocity achieved

in stoopwalking is a function of increased stride length and

cadence compared to crawling methods. Stride length in crawling

activities is handicapped by the decreased lower-limb lever arm

afforded in crawling postures (hip-to-knee as opposed to hip-tofoot). Based on anthropometric data presented in Chaffin et al.

[16], the effective lever arm for the lower limb would be decreased

54% when hip-to-knee length is compared to hip-to-foot length.

However, the reduced lever arm is only one factor in determining

stride length – a fact made apparent when comparing the stride

length achieved in two-point versus four-point crawling. In fourpoint crawling, the increased support provided by the hands and

arms appeared to free the lower leg for a greater swing resulting in

increased stride length.

In comparing existing literature on upright walking to

stoopwalking in this study, several previously unidentified

differences were documented regarding lower limb kinematics

and EMG activity. In kinematic terms, it appears that the range of

included knee angles is reduced in stoopwalking when compared

to upright walking. The knee joint angles in normal gait reported

by Winter [15] range from about a 1808 included knee angle (full

extension)to approximately 1158. The current study found thatfull

extension of the knee never occurred in stoopwalking. The

maximum included knee angle during stoopwalking was only

1558. The lowest included knee angle observed in normal walking

by Winter [15] was approximately 1158; whereas, in the present

study the knee was flexed almost 908 during stoopwalking. It

appears that as vertical space is reduced, joints need to be

increasingly flexed to allow the body adequate clearance.

During the weight acceptance phase of normal walking, the

knee begins at full extension and there is a slight (208) knee

flexion–extension curve that occurs as weight is transferred from

heel to toe [15]. Such a curve was not observed in stoopwalking. In

stoopwalking,there is a prolonged flexion phase that subsumes the

brief knee flexion–extension phase seen in normal walking.

Comparing the data for thigh muscle activity in normal walking

[15] to the current study, several differences in EMG activity can be

observed. One overall difference is that the thigh muscles are

activated for a much longer portion of the cycle in stoopwalking

compared to upright walking. The expanded period of muscle

activity identified in this study may be one reason for the higher

metabolic load observed in stoopwalking compared to upright

walking [2–5]. Another notable difference in EMG activity between
walking modalities was observed in the RF. In upright walking, the

RF exhibits two periods of activation. The first occurs with

extension of the leg prior to heel contact and extension of the knee

during the mid-stance phase. However, the current study showed

this RF activity is absentin stoopwalking, perhaps due to the lack of

full knee extension. A second (minor) burst of RF EMG activity is

seen in normal walking just after toe-off and is associated with hip

flexion that allows the lower limb to be pulled forward and the

knee extension to decelerate the backward swinging ofthe leg [15].

This RF activity is observed during stoopwalking, as similar actions

occur in both walking modes after toe-off.

The knee joint range of motion was decreased in both fourpoint and two-point crawling when compared to stoopwalking.

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

approximately 508 (Fig. 2). Fig. 2 also shows that approximately

60% of the cycle time was spent in extending the knee (mostly

during stance) while the flexion phase was quicker. Fig. 3

illustrates that EMG magnitude for four-point crawling was lower

than for stoopwalking. This may be the result of less body weight

being supported by the legs in four-point crawling because the

arms support the upper body.

Two-point crawling exhibited a restricted range of knee motion

(included angles were approximately 50–708), and differences in

muscle activation, compared to the other locomotion modes.

Specifically, the hamstrings showed greater activity in two-point

crawling, and even exceeded the activity of the vasti muscles, the

only posture in which this was observed. It should be noted that

the stride length in two-point crawling was only half that observed

in four-point crawling, and subjects found two-point crawling to

be very uncomfortable. The greater body weight borne by the

knees using this technique would appear to exacerbate knee

discomfort. This study is the first (to the authors knowledge) to

quantify gait measures and thigh EMG activity in two-point

crawling.

The velocity of four-point crawling observed in the current

study (0.50 m/s) is generally similar to velocities observed in

previous studies [7,17,18]; however, these velocities are somewhat slower than those observed in studies in which crawling was

examined in a building escape context [6,19]. One study reported a

maximum crawling speed of 0.64 m/s [17]; in another study,

subjects averaged 0.38, 0.59, and 0.82 m/s for slow, medium, and

fast crawling gaits [18]; and a third study examined a wide-range

of crawling velocities from 0.22 to 1.34 m/s [7]. The crawling

speeds for the escape context studies reported a normal crawling

speed of 0.71 m/s and maximum crawling speed of 1.47 m/s [6,19].

It should be noted that such velocities may not be possible in mines

given the rough/wet ground conditions usually present. The

stoopwalking velocity observed in the current study (1.01 m/s) can

be compared to another study where the natural cadence upright

walking velocity was 1.33 m/s, representing a 26% decrease for

stoopwalking [15]. Although the cadence of 112.8 strides/min for

stoopwalking is similar to the 105.3 strides/min observed in the

previous study for natural walking [15], the stride length observed

for stoopwalking in this study was 1.04 m, considerably less than

the 1.51 m for natural walking [15]. Thus, the reduced stride length

in stoopwalking is not fully compensated for by the modest

increase in cadence, resulting in a reduction in speed.

Several limitations must be noted with the current study.

Firstly, it must be acknowledged that the sample size was rather

limited. Furthermore, the circumstances of the present study did

not allow testing of actual low-seam underground miners. It would

be expected that workers who must function everyday in the

mining environment would adapt and develop greater efficiency in

their movement patterns in restricted postures. Therefore, the

results of the current study should be understood to represent the

gait responses of relative novices to this environment. Even so, it

must be recognized that many of the gait influences demonstrated

in this paper are intrinsic to the environment, and while

individuals may become more efficient at gait, many of the

influences demonstrated in this study would be present no matter

the individual skill level. In addition, the current subjects over

represent the proportion of females typically found in mining.

Finally, results of this study provide gait data over a fairly short

distance, so the data would not reflect influences such as fatigue on

gait patterns. Nonetheless, results ofthis study would seem to have

important implications for activities such as escape or taking

refuge in underground mines. The distance covered in escape

situations is important for mine evacuation/escape situations, and

the locomotion technique employed will clearly impact the time

needed to cover a given distance in such circumstances.

5. Conclusions

The following conclusions are drawn from the current study:

1. The highest gait velocity in confined space is achieved with

stoopwalking, followed by four-point and two-point crawling.

2. Wearing kneepads did not influence thigh muscle EMG or knee

kinematics during stoopwalking or crawling; however, wearing

kneepads in two-point crawling decreased cadence and

increased stride length.

3. Compared to upright walking, EMG activity of the thigh muscles

during stance in stoopwalking is more prolonged.

4. A variety of interlimb coordination patterns were observed in

four-point crawling, including trot-like, pace-like, and no-limb

pairing contact patterns.

5. Two-point crawling was unique among the techniques in that

the hamstrings exhibited greater activity than the vasti muscles.

Acknowledgements

The authors wish to acknowledge Albert Cook and Mary Ellen

Nelson for their assistance in the conduct of this research.

Conflict of interest. The authors are unaware of any conflicts of

interest that would inappropriately influence their work.

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