Effects of resistance training on tendon mechanical properties and rapid force production in prepubertal children

Abstract

Children develop lower levels of muscle force, and at slower rates, than adults. Although strength training in children is expected to reduce this differential, a synchronous adaptation in the tendon must be achieved to ensure forces continue to be transmitted to the skeleton with efficiency while minimizing the risk of strain-related tendon injury. We hypothesized that resistance training (RT) would alter tendon mechanical properties in children concomitantly with changes in force production characteristics. Twenty prepubertal children (age 8.9 ± 0.3 yr) were equally divided into control (nontraining) and experimental (training) groups. The training group completed a 10-wk RT intervention consisting of 2-3 sets of 8-15 plantar flexion contractions performed twice weekly on a recumbent calf-raise machine. Achilles tendon properties (cross-sectional area, elongation, stress, strain, stiffness, and Young's modulus), electromechanical delay (EMD; time between the onset of muscle activity and force), rate of force development (RFD; slope of the force-time curve), and rate of electromyographic (EMG) increase (REI; slope of the EMG time curve) were measured before and after RT. Tendon stiffness and Young's modulus increased significantly after RT in the experimental group only (∼29% and ∼25%, respectively); all other tendon properties were not significantly altered, although there were mean decreases in both peak tendon strain and strain at a given force level (14% and 24%, respectively; not significant) which may have implications for tendon injury risk and muscle fiber mechanics. A decrease of ∼13% in EMD was found after RT for the experimental group, which paralleled the increase in tendon stiffness (r = -0.59); however, RFD and REI were unchanged. The present data show that the Achilles tendon adapts to RT in prepubertal children and is paralleled by a change in EMD, although the magnitude of this change did not appear to be sufficient to influence RFD. These findings are of importance within the context of the efficiency and execution of movement.

Keywords: Achilles; elastic modulus; electromechanical delay; strength.

Fig. 1.

Resistance machine configuration. Children performed plantar flexion contractions by pushing the footplate with the balls of their feet.

Fig. 2.

Mean (black) and individual (gray) changes in tendon mechanical and material properties [calculated using maximum voluntary contraction after resistance training (RT) (MVC_post)] for control (CON; n = 9) and experimental (EXP; n = 9) participants before (PRE) and after (POST) the training intervention. *P < 0.05.

Fig. 3.

A: representative trace of dynamometer moment and gastrocnemius muscle (GM) electromyographic (EMG) signal used to obtain electromechanical delay (EMD) measurement. B: relationship between changes in tendon stiffness and changes in EMD. Filled circles and open diamonds represent the experimental and control groups, respectively. Trend lines for the experimental (solid line) and control groups (dashed line) are shown. The relationship between the changes in tendon stiffness and EMD for the experimental group, examined using a regression analysis, was nonsignificant (P = 0.077).

Fig. 4.

Rate of EMG increase (REI) measured before (open circles) and after (filled circles) training, calculated as % peak amplitude EMG per second. A: REI measured to 25, 50, 75, and 100 ms after EMG onset. B: time (ms) to reach relative (to peak) EMG levels.

Fig. 5.

Rate of force development (RFD) measured before (open circles) and after (filled circles) training, calculated as %MVC per second. A: RFD measured to 50, 100, 200, and 400 ms after EMG onset. B: RFD measured to each 10% interval of peak MVC up to 90% MVC.