Neuromuscular coordination of squat lifting, I: Effect of load magnitude.

BACKGROUND AND PURPOSE In this study, we examined changes in kinematic and electromyographic (EMG) measurements of the coordination (ie, the relative timing of joint movements and muscle activity) of a squat-lifting task in response to lifting increasing loads. SUBJECTS Fifteen male industrial workers served as a sample of convenience. METHODS Subjects lifted a weighted crate containing 15% to 75% of their maximum lifting capacity using a symmetrical squat-lift technique. Movement kinematics were obtained with videography. The relative phase between joint motions was derived. The EMG activity of the vastus lateralis muscle (VL) and the erector spinae muscle (ES) was recorded, and the relative timing of their onsets and peaks was estimated. RESULTS The relative phase of movement between joints such as the knee and lumbar spine changed in a quasi-linear fashion with increasing load during lifting but not during lowering. The relative time of onset of ES EMG activity and its peak activity changed in a manner consistent with the interjoint relative phase results. The timing of VL events were not affected by increasing the load. CONCLUSION AND DISCUSSION Relatively continuous changes in interlimb coordination occur when increasing the load lifted from an initial squatting posture. Changes in EMG relative timing partially corroborate the kinematic evidence for changes in coordination with load scaling. The results indicate the need for further study to determine whether the observed changes in coordination are beneficial or detrimental to the musculoskeletal system. Clinicians should evaluate performance of this task under a range of task conditions.