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.

Acute effects of muscle stretching on physical performance, range of motion, and injury incidence in healthy active individuals: a systematic review

“…However, despite correlations being observed between tendon stiffness (or changes with exercise training) and electromechanical delay (or its change with training) (Waugh et al 2013(Waugh et al , 2014, this effect has not been shown explicitly. Reductions in tendon stiffness are also thought to affect the rate of force development (Bojsen-Møller et al 2005;Waugh et al 2013).…”

“…Changes in tendon stiffness also speculatively influence electromechanical delay (Cresswell et al 1995;Waugh et al 2013Waugh et al , 2014 and thus reduce the rate of force production. However, despite correlations being observed between tendon stiffness (or changes with exercise training) and electromechanical delay (or its change with training) (Waugh et al 2013(Waugh et al , 2014, this effect has not been shown explicitly.…”

“…Reductions in tendon stiffness are also thought to affect the rate of force development (Bojsen-Møller et al 2005;Waugh et al 2013). However, small changes in tendon stiffness (e.g., a 30% increase after strength training in children) do not appear to influence the rate of force development (Waugh et al 2014) so it is unlikely that the changes in tendon stiffness elicited by acute muscle stretching meaningfully influence it.…”

Avaliação dos ligamentos longitudinais da coluna de ratos Wistar em modelo experimental da terapia Suit

“…Waugh et al 18 evaluated the calcaneal tendon of children subjected to resisted plantar flexion exercises, noting that there were changes in mechanical properties of tendons, with 29% increase of rigidity. Again it should be noted that in this study we used animals with healthy central nervous system, and therefore with normal muscle adaptive capacities.…”

Effects of Two Plyometric Protocols at Different Surfaces on Mechanical Properties of Achilles Tendon in Children

“…Recently, it has been shown that Achilles tendon stiffness significantly increased (29%) after a 10-week resistance training in pre-pubertal children (3,4)) whereas throughout the same study no change in tendon CSA has been mentioned (4). Therefore, the immature human Achilles tendons respond to short-term resistance training similarly to mature tendons (5).…”