Effects of leg function constraints on a lifting task examined through inverse kinematics analyses, direct dynamics modeling, and electromyographic biofeedback techniques.

Abstract

Numerous epidemiological surveys indicate that a large number of older workers, particularly women, have difficulty performing lifting tasks of 25 pounds (11kg) or more. The thesis explored in this dissertation is that functional limitations in forward balance, leg joint ranges of motion, and/or leg muscle strengths, which can occur in middle-aged and older women, significantly affect the kinematics, kinetics, and performance capabilities of lifting moderate loads. To investigate this thesis, three different methods of investigation were utilized. The first study empirically correlated these functional abilities (balance, range of motion, and strengths) with initial lifting postures chosen by twelve women when lifting a 15kg box from floor level. The second study developed a forward dynamics model which assumed the body to be a three-link structure, with muscles modeled as applied torque actuators of varied capabilities. The third study utilized a novel application of EMG biofeedback to train eight healthy women to lift a 15kg box from floor level, while having the gastrocnemii and rectus femorii muscles selectively inhibited. These studies found: (1) that leg strength, in particular ankle plantarflexion strength, predicted initial lifting postural angles (with R$\sp2$ = 0.39-0.43) better than forward balance and leg joint range of motion attributes; (2) that model predicted leg strength declines of 25% to 65% could be accommodated by alterations in the initial squat-style lifting postures to a more stoop-like posture; and (3) that inhibiting leg muscle strengths to below 45% of maximum EMG levels altered lifting kinematics to a stoop lifting posture similar to that demonstrated by subjects with weak leg strengths. It is concluded from these results that diminished leg strengths systematically alter lifting capabilities and movements to reduce the leg strength requirements while lifting from floor level. It is believed that these results underscore the need to maintain leg strengths to permit lifting moderate loads. Training programs which emphasize specific lifting techniques will need to carefully assess client leg strengths for such programs to be effective.