THE DAMPED OSCILLATIONS OF PASSIVE LIMBS AND THEIR ROLE IN HUMAN LOCOMOTION MECHANICS

The mechanics of locomotion classically take into account the work done by muscle force to raise and accelerate the body center of mass and to accelerate limbs with respect to it at each step. This last component, named Internal Work (W_INT), considers only the cost to overcome segment inertia, inherently assuming frictionless joints. Thus, the unavoidable damping opposing segmental oscillation due to anatomical structures within or around the pivoting centers has never been measured so far. The frictional coefficient (b, N.m.s.rad-1) of such a biological rotational damper has been here assessed by sampling the time course of passive oscillation (with respect to the vertical axis) of upper and lower limbs and by analyzing its motion. This experiment (straight pendulum) was performed to assess joint energy dissipation during the swing phase of locomotion. A custom mathematical model, leading to a 2nd Order Non-Linear Ordinary Differential Equation, allowed to infer b values for upper (bUU = 0.39 ± 0.08) and lower (bUL = 2.24 ± 0.56 N.m.s.rad-1) limbs in 16 healthy males. Phase planes ensured that no muscle activity was involved. In the same population, the passive swing of a lower limb, behaving as an inverted pendulum after a push (body upside-down), was also sampled while loading the leg as to replicate the compressive stress to which the hip joint is exposed during stance phase. Loads ranged from 0 N (mass of leg only) to 118 N. Damper values (b) for the inverted swing of a loaded lower limb increased with the load and ranged from 4.89 ± 1.29 to 8.92 ± 1.74 N.m.s.rad-1. The influence on locomotion mechanics has been here evaluated. In walking, for instance, each step includes 3 'passively' swinging, unloaded segments (2 upper limbs and the swinging lower limb with joints under tensile stress) and 1 'actively' oscillating, almost fully loaded segment (stance lower limb, joint under compressive stress). The actual experimental results have been combined to provide an estimate of the internal mechanical work due to tissue and joint damping. In walking that is comparable (and should be added) to the estimate obtained by means of a kinematics-based model (Minetti, 1998) and experimental data from the literature of the traditional ‘kinematic’ W_INT. In the discussion, the potential overestimation and underestimation of those two types of internal work are presented, together with the implications of the presented additional work (and its metabolic equivalent) to the energy balance and efficiency of human locomotion.