Simulation of elbow and forearm motion in vitro using a load controlled testing apparatus

The purpose of this study was to compare passive to active testing on the k inematics of the elbow and forearm using a load-controlled testing apparatu s that simulates muscle loading. Ten fresh-frozen upper extremities were te sted. Active control was achieved by employing computer-controlled pneumati c actuators attached to the tendons of the brachialis, biceps, triceps, bra chioradialis and pronator teres. Motion of the radius and ulna relative to the humerus was measured with an electromagnetic tracking system. Active el bow flexion produced more repeatable motion of the radius and ulna than whe n tested passively (p < 0.05). The decrease in variability, as determined f rom the standard deviation of five successive trials in each specimen, was 76.5 and 58.0% for the varus-valgus and internal-external motions respectiv ely (of the ulna relative to the humerus), The variability in flexion durin g simulated active forearm supination was 30.6% less than during passive te sting. Thus under passive control, in the absence of stability provided by muscular loading across the joint, these uncontrolled motions produce incre ased variability amongst trials. The smooth and repeatable motions resultin g from active control, that probably model more closely the physiologic sta te, appear to be beneficial in the evaluation of unconstrained kinematics o f the intact elbow and forearm. (C) 2000 Elsevier Science Ltd. All rights r eserved.