An Investigation of the Effects of Fatigue and Stance Width on Ground Reaction Forces and Trunk Kinematics

Abstract

Manual material handling (MMH) is often required in challenging outdoor environments (e.g. agriculture and construction) that may require fatiguing, repetitive lifting while standing on slippery ground surfaces. Slips and falls can result in a variety of injuries as a result of both the impact of the fall and extreme muscular forces used to attempt to counteract a fall. Few studies have explored the effects of a fatiguing lifting task on slip potential. The goal of the study was to assess the effects of fatigue and stance width on ground reaction forces and trunk motion during an asymmetric, repetitive lifting task.

Twelve individuals participated in this study. They were required to lift a load equal to 40% of their maximum lifting capacity at an angle of 45 degrees in the transverse plane. Participants lifted the load (eccentric followed by concentric) for 10 minutes at a rate of 12 lifts per minute. Stance was changed once per minute from shoulder width to twice shoulder width or vice versa. Ground reaction forces were recorded using a force plate for each foot, and trunk motion data were recorded by using a Lumbar Motion Monitor (LMM). Fatigue was verified by both electromyographic (EMG) median frequency shift of the erector spinae muscle group and a verbal questionnaire. MANOVA and ANOVA statistical analysis techniques were used to analyze these data, and the modified Levene's test was used to identify changes in variability of the dependent measures as a result of the main effects.

The results showed a statistically significant (p < 0.05) increase in lateral ground reaction forces (from 43 N to 51 N) as Time into the lifting task increased in the narrow stance. Stance was also found to affect lateral shear forces (resulting in an increase from about 40 N to about 80 N as stance width doubled) and anteroposterior shear forces (decreased from 40 N to 30 N as stance width doubled). Further, Time into the lifting task resulted in parabolic shifts in peak sagittal flexion (ranging from 75 degrees at the beginning and end to 70 degrees in the middle of the trial) and peak sagittal acceleration (ranging from around 600 deg/s2 at the beginning and end of trials to around 540 deg/s2 in the middle) in both stances. Coronal peak flexion followed a similar parabolic pattern with respect to Time in the wide stance (10 degrees at the beginning and end and 8.5 degrees in the middle), but showed a decreasing trend in the narrow stance (moving from 9 degrees in the first Time block to 8 degrees by the final Time period). Coronal acceleration was found to increase slightly as a function of Time in the narrow stance (moving from 61 deg/s2 to 68 deg/s2).

The results of the study indicate clear, but small magnitude , increases in shear forces due to Time into task, but only in the narrow stance. Even larger increases in shear forces are shown as a result of the change from a narrow to a wider stance. In order for the recorded shear forces to cause a slip, the coefficient of friction would have to be very low. Employers in industries such as agriculture and construction, who occasionally work on wet, icy, or muddy ground, should take caution to help limit employees' fatigue and teach proper lifting techniques with a shoulder-width stance.

Ultimately, this research can be used to further the information base on the effects of fatigue on external ground reaction forces. These results are largely consistent with similar studies, and it is hoped that, with those studies, more accurate and safe lifting techniques and guides may be developed.