Changes in Muscle Function and Activation with Growth and Maturation

Muscle performance capacity, such as maximal force, power and rate of force development, generally improves with growth and maturation. This improvement is explained not only by increased muscle mass, but also by neuromotor changes. On the other hand, growth and maturation are associated with increased muscle fatigability and slower recovery from intense exercise. These attributes are muscle-size-independent.

Some of the above developmental changes may be explained by a change in muscle composition (increase in type-II fibres), change in metabolic characteristics (greater glycolytic capacity), an increase in musculotendinous stiffness, and changes in neuromotor function (e.g., increased voluntary motor-unit activation, decreased agonist-antagonist co-activation). We propose that the deficit in motor unit activation mainly involves the higher-threshold (type-II) motor units. While experimental evidence for this proposal is scarce (mainly due to ethical and methodological reasons), there is much supportive evidence from studies of voluntary and involuntary muscle performance, along with studies of electromyographic (EMG) pattern during muscle contraction. Such studies have demonstrated lower rates of motor-unit activation during maximal isometric contractions, as well as a higher EMG breakpoint (EMG threshold) during progressive cycling exercise in children compared with adults. The latter reflects delayed and hence lesser utilization of the higher-threshold motor units in children. This presentation will provide some of the evidence for the hypothesized increase in higher-threshold motor unit utilization with maturation, and will suggest some directions for future research.