Effect of altering neural, muscular and tendinous factors associated with aging on balance recovery using the ankle strategy: a simulation study.

Aging is associated with declines in neuromuscular function and reduced ability to recover balance from an imbalance episode. However, little is known about the relations amongst these factors. The purpose of this study was to determine the relative influence of age-related changes in neural, muscular and tendinous properties on the ability to recovery balance from a forward leaning position using the ankle strategy. A computer simulation was developed which consisted of an inverted pendulum with one rotational degree of freedom controlled by two muscles representing the ankle joint plantar flexor (PF) and dorsi flexor (DF) muscle groups. Model parameter values were adjusted so that the isometric torque-angle relation was in agreement with experimental ankle joint torque-angle curves from the literature. Muscle excitation was adjusted to match an experimentally determined maximum recoverable lean angle (MRLA) of 7.2 degrees (baseline condition). The effect of 20% alterations to maximum isometric force, optimum muscle fibre length, maximum shortening velocity, tendon stiffness, reaction time delay (RTD), activation time constant and the maximum excitation of the PF muscles, and maximum excitation of the DF muscles (co-activation) on MRLA was then assessed. The parameters that had the greatest influence on MRLA were maximum isometric force, the maximum excitation of the ankle joint PFs and RTD, which, respectively, resulted in 19.0%, 17.8% and 4.6% reductions in MRLA. Individual changes to other parameters influenced MRLA by less than 1.9%. When selected parameter values were adjusted in accordance with age-related changes reported in the literature, MRLA was reduced to 5.3 degrees , a value in relative agreement with experimental values reported in the literature (4.6+/-1.8 degrees ). In general, these results suggest that MRLA is most sensitive to PF muscle mass and the ability to maximally activate the PFs, and that the combined effect of multiple changes in neural, muscular and tendinous parameters reported to occur with aging can have a profound effect on the ability to recover balance from a forward fall using the ankle strategy.