Increase in Volitional Muscle Activation from Childhood to Adulthood: A Systematic Review and Meta-analysis

Abstract

Introduction Children’s maximal muscle strength is consistently lower than adults’, even when normalized to body size. Lower volitional muscle activation (VA) in children is often considered one of the main reasons for age-related differences in muscular performance. However, some recent studies have reported similar VA in children and adults, bringing into question whether there is indeed an age-related increase in VA. The purpose of this review was to determine the effect of age on VA during maximal isometric contractions. Methods Literature examining VA differences, using twitch interpolation in children (7–14 yr) and adults (16–28 yr), was systematically reviewed. Of the 1915 studies initially identified, 19 data sets were eligible for inclusion in the qualitative analysis and 14 in the quantitative meta-analysis (comprising 207 children and 193 adults). Results Significantly lower VA in children was reported in 9/19 (47%) studies. A random-effects meta-analysis found a strong effect of age on VA, supporting lower VA in children compared with adults (Hedges’ g = 1.55; confidence interval: 0.9–2.13). Moderator analysis included muscle group, sex, children’s age, stimulation number (singlet, multiple), type (electric, magnetic), and location (muscle, nerve), of which only muscle group was significant ( P < 0.001). A significant Egger’s regression test and asymmetrical funnel plot suggest that publication bias may be present. Conclusions Overall, these findings suggest that compared with adults, children activate their motor-unit pool less compared with adults. Moreover, that the degree of VA increase with age may be influenced by the muscle examined (upper vs lower extremity). However, more research is needed to elucidate the influence of this possible factor, as the current review contains limited data from upper body muscles. The developmental mechanism responsible for children’s lower VA requires further research.

Influence of Growth, Maturation, and Sex on Maximal Power, Force, and Velocity During Overground Sprinting

“…At the neural level, multiple studies have confirmed that maturity-related increases in muscle activation, as a result of greater recruitment and utilization of motor units (particularly higher threshold/type II motor units), also lead to increases in force production without any changes to muscle cross-sectional area (13,17,19,43,46). Furthermore, maturity-related improvements in intermuscular coordination reduce agonist-antagonist coactivation, especially during dynamic contractions, and increase effective force without any changes in muscle dimensions (15,16).…”

Building for the Future: A Systematic Review of the Effects of Eccentric Resistance Training on Measures of Physical Performance in Youth Athletes

“…The key physiological mechanisms underpinning these preferential responses in youth include lower post-exercise blood lactate levels and faster clearance rates [22][23][24][25][26], an improved blood acid-base regulation [27], lower phosphocreatine depletion and faster resynthesis [28,29], and faster heart rate recovery [23,26]. Additional factors such as intramuscular synchronisation [30], agonist-antagonist coactivation [31], degree of volitional activation [32] and a reduced capability to recruit or fully employ type-II motor units may explain the lesser fatigue and muscle damage symptoms experienced by children and adolescents in contrast with adults (for detailed reviews on these topics, please see Drury et al [33] and Woods et al [32]).…”

Perceived Discomfort and Voluntary Activation of Quadriceps Muscle Assessed with Interpolated Paired or Triple Electrical Stimuli

“…In particular, quadriceps weakness is frequently studied in anterior cruciate ligament injuries [ 5 , 6 , 7 ], because a clear distinction between neural and muscular factors contributing to this weakness is critical for optimizing rehabilitation interventions [ 8 , 9 ]. In recent years, VA of the quadriceps muscle has been commonly measured and quantified using the interpolation twitch technique (ITT), in which an electrical or magnetic pulse is applied transcutaneously to the muscle or femoral nerve during maximal voluntary knee extension [ 10 ]. Accordingly, ITT typically compares the amplitude of the double-evoked superimposed torque during the plateau of maximal voluntary isometric torque to the amplitude of the double-evoked torque at rest [ 8 ].…”

“…In the ITT, the proportion of the control twitch that remains superimposed during a maximal voluntary isometric contraction (MVIC) is considered an indicator of the proportion of muscle that is not activated, and therefore the ratio of the superimposed evoked torque to the evoked torque at rest is commonly referred to as the activation deficit. Voluntary activation is then calculated as follows: [1 − superimposed evoked torque/evoked torque at rest × 100] [ 10 ]. However, one of the limitations of the observed percentage of VA is the large variability between repeated measurements [ 11 ], which can be explained, at least in part, by the ITT method chosen (e.g., type and number of superimposed stimuli) [ 12 ].…”