On the Load-Displacement Behavior of the Tensed Adult Upper Extremity under Impulsive End-Loads: Theoretical and Experimental Studies of Age and Gender Effects

Abstract

The upper extremities are often used to protect the head and thorax by bracing for impact, particularly in falls to the ground. The impulsive loads they impose on the hand and wrist can be substantial, exceeding one body-weight. If the upper extremity then “gives way” or flexion buckles at the elbow then a head injury is likely, particularly in the elderly; but if the elbows are fully extended to prevent buckling, then the risk for wrist fracture increases. A current knowledge gap includes the biomechanical factors that determine the threshold load required to flexion-buckle the elbow of an end-loaded and pretensed human upper extremity. In this thesis we use computer simulations and in vivo experiments to explore how age, gender, initial elbow angle, arm muscle strength and pre-contraction level and lumped contractile properties about a joint affect upper extremity deflection under impulsive end-loading. The experimental results show that gender and age affect the rotational stiffness and damping coefficients of muscles acting about the elbow and shoulder when estimated by dynamic optimization. The pre-contraction levels of arm and shoulder muscles significantly affected these coefficients. Computer simulations predict that advancing age, female gender and insufficient arm and shoulder muscle pre-contraction level adversely affect upper extremity buckling loads. Kinetic, kinematic and myoelectric studies suggest the speed of propagation of the impulsive load along the upper extremity is such that arm and shoulder muscles must be pretensed prior to impact: no neuromuscular reflex is rapid enough to increase arm muscle tensile stiffness to prevent flexion buckling. Pre-contraction level and gender significantly affected the rate of propagation of an impulse along the upper extremity. The findings provide a framework for better understanding how biomechanical factors determine whether or not an arm will buckle when end-loaded during a fall arrest. We conclude that in order to help safely arrest falls older women and men need to avoid using hyperextended arms when possible, use an adequate pre-contraction level in the arm muscles to prevent buckling, and maintain as much arm protraction strength as possible, perhaps most conveniently by regular push-up exercises.