Minimization of the knee shear joint load in leg-extension equipment

Abstract

We developed an analytical biomechanical model for leg-extension equipment and the associated knee-extension/flexion exercises. The shear component, ϕ t , of the tibiofemoral joint load was calculated taking into account all the fundamental elements of the equipment mechanics (resistance pad placement, cam/pulley system geometry, selected weight stack, etc.) and the instantaneous values of the relevant kinematical parameters (knee-flexion angle ( θ f), angular velocity, and angular acceleration). The optimal distance ( a R ) OPT between the knee-flexion/extension axis and the resistance pad placement point was derived by minimizing ϕ t . ( a R ) OPT is nearly independent of joint angular velocity and, for appreciably high resistance torques, becomes nearly independent of resistance level and cam/pulley geometry: for θ f > 40°, ϕ t is minimized by placing the resistance pad distally along the lower leg; for θ f ≤ 40°, ϕ t can be completely eliminated by continuously moving the resistance pad proximally during knee-extension phase and distally during knee-flexion phase (0.17 m ≤ ( a R ) OPT ≤ 0.4 m). In the presence of knee angular accelerations and hamstrings co-contractions, not predictable in advance, the value of ( a R ) OPT obtained neglecting these effects still represents a good compromise for joint protection. This work establishes the rational basis for the design and clinical use of a leg-extension equipment that minimizes ϕ t .