Developmental differences in dynamic muscle-tendon behaviour: implications for movement efficiency.

Children perform cyclic motor tasks less efficiently than adults; however, the mechanisms underlying such differences are not fully understood. One mechanism that may contribute to these age-related differences is a differential contribution of muscles and tendons to a given muscle-tendon unit (MTU) excursion. The aims of this study were to (i) compare muscle and tendon excursion between children and adults performing vertical hopping, and (ii) determine whether children and adults choose a hopping frequency that maximizes movement efficiency, based on the utilization of energy-saving mechanisms. Twelve children (8.8±0.3 years) and 12 adults (26.0±2.1 years) performed 20 s of two-legged hopping at a self-selected frequency and at 1.33, 2.00, 2.67 and 3.33 Hz. Gastrocnemius medialis MTU excursion was estimated from kinematic data and muscle and tendon excursions were derived using a combination of 3D-motion capture and ultrasonography. Optimum hopping frequency was determined as the frequency that maximized surrogate measures of elastic energy storage potential of the tendon and minimized muscle excursion. Adults presented a significantly greater potential for elastic energy storage in combination with lower muscle excursion than children at their self-selected frequency, suggesting that children do not utilize these energy-saving mechanisms as effectively as adults. However, tendon elastic energy storage was maximized and muscle excursion minimized at the preferred frequency in both children and adults, indicating that children may select their preferred hopping frequency based on the same criteria as adults. These findings increase our understanding of the mechanisms contributing to the higher energy cost of movement performance in children, and have implications for the interpretation of age-related differences in complex task performance.