Neuromuscular coordination of squat lifting, II: Individual differences

This article reports individual differences in the coordination (ie, the relative timing of joint movements and muscle activity) of squat lifting identified by extended analysis of data reported in the authors' companion article in this issue.

Two post hoc groups of 6 subjects each were identified from the original sample of 15 subjects based on qualitative differences in knee-lumbar spine relative motion plots during load acceleration.

Subjects lifted a crate containing 15% to 75% of their maximum lifting capacity using a symmetrical squat-lift technique. Movement kinematic data were obtained with videography, and the electromyographic (EMG) activity of the vastus lateralis and erector spinae muscles was recorded with surface EMG. Measurements of coordination derived both kinematically and via EMG and the kinematic data were examined for group differences.

Subjects in group 2 limited lumbar spine motion during load acceleration for all loads lifted, whereas those in group 1 limited lumbar spine motion more when lifting the heaviest loads. These differences were obvious both qualitatively, via knee-lumbar spine relative motion plots, and quantitatively, via measures of the relative timing of joint motions early in the lift. The effect of load on the coordination of these joints was the same for both post hoc groups after initial load acceleration. Significant differences in other kinematic measurements were also found between these groups.

Despite specific instructions about how to lift the load, individual subjects coordinated their joints differently during the initial, accelerative phase of squat lifting. Individual differences in coordination in response to load increases could be categorized into two patterns, although the data of 2 subjects were difficult to categorize and thus not included in these analyses. Whether the two dominant patterns have consequences for stress to the joints during lifting remains to be determined.