Postural perturbation and muscular response following sudden release during symmetric squat and stoop lifting

Squat and stoop lifting have been examined in some detail, but limited data exist regarding the sudden release of load during such lifting. Ten participants performed squat and stoop lifting trials with loads of 20, 40, 60 and 80N, and sudden release was randomly included in one of the lifting cycles. Postural perturbation was recorded via centre of pressure displacement using a force platform and the electromyographic response of trunk and lower limb muscles was recorded. Results indicated that irrespective of lifting posture, an 'ankle' response strategy to sudden release was elicited, where the anterior muscles of the lower limb contracted first, followed by the anterior trunk muscles, relaxation of the posterior trunk muscles and, finally, relaxation of the posterior lower limb muscles. The latency of muscles responding by contraction tended to decrease slightly with increasing load for both postures, while the latency of muscles responding by relaxation increased, resulting in increased trunk muscle co-contraction durations. The postural disturbance appeared to be greater for squat lifting than stoop lifting at the higher loads of 60 and 80N, as the centre of pressure moves significantly closer to the posterior limit of static stability (the line joining the heels).In terms of stability and muscular response, squat lifting may not be the most appropriate strategy if a sudden release of loads greater than approximately 50N is likely.