85–87%, respectively), their values were considerably lower than those reported earlier by Belanger and McComas (13; 94% vs. 99%, respectively), who used electrical stimulation of the same muscles. Thus, while the true percentage of volitional motor-unit activation in both children and adults remains unclear, the interpolated twitch technique nevertheless depicts a clearly lower overall activation level in children.

However, according to the size principle (51), it is the slower, low-threshold (type I) motor units which are recruited first. To increase force output, faster, higher-threshold (type II) motor units are recruited by increasing motoneuron firing frequency. It might be argued, therefore, that lower overall motor-unit activation, as described above, reflects lesser activation of high-threshold motor-units, since those are the ones typically activated last.

It may also be argued that children’s lower volitional/nonvolitional force ratio is a result of lesser motor-unit synchronization during volitional contraction. While this possibility cannot be dismissed, it has never been examined and is only speculative at present. Moreover, substantial synchronization differences are incongruent with other observed child-adult differences (see “Lactate response to maximal short-term exercise”, below).

Finally, it is important to underscore the fact that regardless of the exact reason, children’s lower volitional muscle activation cannot be attributed to differential muscle-fiber composition.

EMG-Derived Evidence

Much of the evidence supporting child-adult differences in muscle activation has been derived using surface EMG. Surface EMG can readily detect electrical activity within the muscle. However, the nature and amplitude of the detected signal are highly affected by muscle size and the filtering effect of the intervening skin and subcutaneous tissues (68). These factors differ between individuals, and particularly between children and adults, thus precluding direct quantitative comparisons. Some of these issues can be skirted by the use of rate- or timing-related parameters, as described below. However, while all of this EMG-related evidence is congruent with the differential motor-unit activation hypothesis, it cannot distinguish it from differences in muscle composition.

Rate of EMG Rise

The initial slope of the rectified surface-EMG trace has been thought of as reflecting the initial rate of muscle activation (25,34,37,42,44). Greater involvement of type-II motor units is expected to manifest itself by greater EMG activity immediately following neural stimulation. An index describing this is the Q₃₀, defined as the integral (area under the curve) of the rectified EMG activity during the first 30ms. In adults, performing 400 explosive elbow-flexion repetitions over four training sessions (i.e., time period sufficient for learning, but not substantial structural or metabolic adaptations), Gabriel & Boucher (42) showed Q₃₀ to increase with training (learning) and to be related to the speed of movement. Presumably, faster movements were attained as the subjects learned to activate their type-II motor units faster, or in a more synchronous manner. Although there are some unanswered reservations as to the validity of the Q₃₀ index (25), its theoretical and commonly observed relationship with velocity supports its use as an indicator of the activation level of faster, higher-threshold motor-units.

Similarly, we have shown both Q₃₀ and the rate of force development (RFD) to be significantly higher in young male gymnasts (typically trained for explosive muscular performance), than in young swimmers (mostly endurance-trained), or nonathletes (67). It may be argued that such differences reflect gymnasts’ preselection rather than true training effect. However, the observed differences were limited to the highly trained knee extensor muscles, but could not be shown in the marginally-trained knee flexors.

Along these lines, we have recently demonstrated consistently lower Q₃₀ and RFD values, in boys compared with men (25,34). Collectively, these results suggest that children have lower initial rate of muscle activation, as reflected by the lower Q₃₀ values. This could be due to differential motor-unit recruitment, but also to lesser motor-unit synchronization, or differential rate-coding of the higher-threshold (type II) motor units (41). Whatever the reason, children’s lower Q₃₀ appears strongly related to their lower RFD, which in turn explains their compromised explosive power compared with adults (see below).