The detection of force onset in fast, maximal muscle contractions would be expected to be slightly delayed by lesser activation of the muscle in general, and of fast-twitch motor units, in particular. Indeed, longer EMD (force-onset latency relative to EMG onset) has been consistently shown in children compared with adults (4,25,34,37), seemingly supporting the notion of children’s lower muscle activation. However, EMD has been shown to be largely and inversely related to musculo-tendinous stiffness (23) which is considerably lower in children (46,60,62). Consequently, while the direction of the available data conforms to the hypothesis, it is impossible to tease out and argue the purported effect of differential motor-unit involvement.
Mean Power Frequency (MPF)
The EMG signal is characterized by different signal magnitudes and frequencies. The power spectrum density describes the relative distribution of EMG frequencies and resembles a Gaussian curve, skewed toward the lower frequencies. The MPF is the weighted mean of that distribution. It may be affected by motoneuron firing rates as well as by the nature and frequency of the resulting motor-unit action potentials (56).
In adults, MPF has been shown to be affected by fiber-type distribution (58). Similarly, it has been argued that the MPF is related to the relative utilization of type-II motor units, with higher utilization rate resulting in a right shift (to higher frequencies). As type-II motor units fatigue faster than type-I units, a decrease (left shift) in MPF is typically observed in fatiguing exercise (56). In adults, this left-shift has been shown to be greater in persons with higher type-II fiber composition (61). Interestingly, an increase in MPF was demonstrated in adults following 3 explosive-training sessions (41), suggesting that subjects were able to recruit more higher-threshold motor units after such training. Halin et al. (49) showed a greater MPF-decrease during a 25-s elbow-flexion fatigue test, in 10-yr-old power-trained male gymnasts (
12%). It has been suggested that this shift toward the lower-frequencies, is due to selective accumulation of metabolites in the more fatigable type-II motor units (2,41,48).
Halin et al. (48) found higher MPF during maximal contraction in men compared with boys. The authors suggested that the difference was due to a higher composition or greater utilization of type-II motor units. Moreover, during a 28-s fatigue test, the men demonstrated a much greater MPF decrease (
16%), suggesting a greater drop-off of type-II motor units. More recently, Armatas et al. (2) examined MPF during maximal knee extension in boys and men. Although no differences could be shown during maximal contraction, MPF decreased substantially more in the men than in the boys during an isometric fatiguing task. The authors attributed the difference to a possibly greater lactic-acid accumulation in the men (not measured), a phenomenon expected more of type-II than of type-I motor units (see below).
Contractile and Power-Related Differences
Differential motor-unit activation ought to have implications that extend beyond maximal force. Because type-II motor units are faster contracting than their type-I counterparts, these differences should be observable in various parameters of whole-muscle performance, as described below. While conforming to the differential motor-unit activation hypothesis, this class of differences does not exclude differential muscle composition as an alternative or additional explanation.
Rate of Force Development (RFD)
Lower RFD should be expected if children use their fast-twitch motor units to a lesser extent than adults. Indeed, lower RFD during maximal isometric contractions has been repeatedly shown in children (4,25,34,37,45). A similar how much does maturesinglesonly cost? outcome could be expected if motor-unit contractility was inherently deficient in children. However, based on observations of similar twitch contractile characteristics (contraction time, half-relaxation time) in children and adults (45,72), the latter possibility seems unlikely.