Modeling age-related neuromuscular changes in humans

Abstract

With aging, motoneurons and muscle tissue undergo significant changes, which influence function in terms of strength, mobility, and overall independence. Mathematical modeling provides a practical method of studying the relationships among recruitment, rate-coding, and force output in motor units, and may be used to predict functional neuromuscular changes related to aging. For this study, the Heckman–Binder model was used to examine changes in human quadriceps motor units. Relationships among current input, firing frequency, and force output were defined for both a younger and an older individual. Included in the model were age-related effects associated with reduced muscle contractile speed; reduced muscle-fibre number, size, and specific tension; reduced gain of the frequency–current relationship; decreased size of motoneurons; and altered motor unit remodeling. Adjustment of this model to reflect age-related changes resulted in a leftward shift of the force–frequency function, lower firing frequency for any given current injected into the motoneuron, and a reduction in maximal force output. The model suggests that older individuals are capable of reaching force levels up to approximately 50% of those attained by younger individuals, with relatively similar or even slightly lower levels of current input. This could mean that the sense of effort and the contribution of factors other than degree of effort from afferent inputs to the pool, including conscious supraspinal centres, might be different in the older adult.