Mechanics of Pushing a Loaded Cart up an Incline

D. Gordon E. Robertson, Ph.D.

Dianne Ellwood, B.Sc.

Paul Johnson, B.Sc.

Dany Lafontaine, B.Sc.

School of Human Kinetics,

University of Ottawa, Ottawa, CANADA

Supported in part by Canada Post Corporation

Purpose

• to determine the functional role of to determine the functional role of the three leg moments of force the three leg moments of force while subjects push a heavily while subjects push a heavily loaded cart up a 10 deg inclineloaded cart up a 10 deg incline

• to test whether resultant forces at to test whether resultant forces at L5/S1 exceed 3400 N (L5/S1 exceed 3400 N (NIOSH action NIOSH action limitlimit) )

Methods

• 10 female subjects• cart loaded to 100 kg• five steps to 10 deg inclined ramp• force plates for last two footfalls before

ramp• cinefilm at 50 fps, forces at 200 Hz• inverse dynamics to obtain moments of

force at three leg joints for both legs• computed work and power produced by

moments of force

Stick-figures of subject pushing loaded cart up incline. Force vectors are shown under two of the feet.

Results

• average velocity before ramp was 1.75 m/s

• peak powers were similar in magnitude to those of walking but differed in pattern

Force signatures of last two steps before ramp. Direction of motion is left to right. Note, second force is concentrated under ball of foot.

Ankle powers

• 8/10 subjects produced two or more bursts of work by the plantar flexors during second last step before ramp

• 9/10 subjects produced two or more bursts of work by the plantar flexors during the last step before ramp

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Time (seconds)

-500

0

500

-500

0

500

-500

0

500

1000

Pow

er (

wat

ts)

Powers of the ankle, knee and hip moments

Trial: CP02CHI AnkleI KneeI HipC AnkleC KneeC Hip

IFS CTO CFS ITO IFS CTO

Hip powers

Knee powers

Ankle powers

Knee powers

• 5/10 subjects produced bursts of positive work by the knee extensors during second last step

• 9/10 subjects produced bursts of positive work by the knee extensors during last step

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Time (seconds)

-500

0

500

-500

0

500

-500

0

500

1000

Pow

er (

wat

ts)

Powers of the ankle, knee and hip moments

Trial: CP06AUI AnkleI KneeI HipC AnkleC KneeC Hip

IFS CTO CFS ITO IFS CTO

Hip powers

Knee powers

Ankle powers

Hip powers

• hip extensors performed positive work during early part of stance

• hip flexors performed negative work during latter part of stance

• 6/10 subjects performed as described for second last step

• 8/10 subjects performed as described for last step

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Time (seconds)

-500

0

500

-500

0

500

-500

0

500

1000

Pow

er (

wat

ts)

Powers of the ankle, knee and hip moments

Trial: CP01TRI AnkleI KneeI HipC AnkleC KneeC Hip

IFS CTO CFS ITO IFS CTO

Hip powers

Knee powers

Ankle powers

Conclusions

• patterns of power production differed from those of walking

• peak powers were similar in magnitude to those of walking

• compressive forces at L5/S1 did NOT exceed NIOSH action limit (3400 N)

• standard lifting model is not applicable to pushing