The effects of age- and training-related changes in tendon stiffness on muscular force production and neuro-motor control during childhood

Abstract

The research described in this thesis examined age- and strength training-related changes in Achilles tendon stiffness and plantarflexor force production in prepubertal children. The measurement of both Achilles tendon stiffness and muscular force production requires in vivo moment arm lengths to be known. Currently, this is possible only by using expensive and time-consuming medical imaging methodologies. Therefore, the predictability of the Achilles tendon moment arm from surface anthropometric measurements was assessed in the first experimental study (Chapter 3). The results demonstrated that a combination of foot length and the distance between the calcaneal tuberosity and 1st metatarsal head could explain 49% of the variability in Achilles tendon moment arm length in 5 – 12 year-old children. This was considered to be unacceptable for further use, thus an ultrasound-based method was decided upon for obtaining moment arm length in subsequent experimental studies. In the second and third experimental studies (Chapters 4 and 5), age-related changes in tendon mechanical and structural properties were documented and their relationship with changes in force production ability were examined in prepubertal children (5 – 12 years) and adult men and women. In Chapter 4, Achilles tendon stiffness was shown to increase with age through to adulthood, and that changes in tendon stiffness were strongly and independently associated with body mass (R2 = 0.58) and peak force production capacity (R2 = 0.51),which may provide the tendon with an increasing mechanical stimulus for growth and microadaptation. These increases in tendon stiffness were associated with a greater increase in tendon CSA (~105%) than that found for tendon length (~60%), in addition to an increase in Young’s modulus (~139%), suggesting that gross increases in tendon size as well as changes in its microstructure underpinned the increase in stiffness. In Chapter 5, the relationships between Achilles tendon stiffness and both electro-mechanical delay (EMD) and rate of force development (RFD) were determined during maximal isometric plantarflexion contractions. Moderate correlations were found between tendon stiffness and both EMD (r = -0.66) and RFD (r = 0.58). RFD was significantly better predicted when muscle activation (estimated as the rate of EMG rise) was included in a regression model. These data clearly show that increases in tendon stiffness with age through to adulthood are associated with decreases in EMD and increases in RFD, and that the rate of muscle activation has an additional influence on RFD during growth. Given that 1) Achilles tendon stiffness was lower in children than adults, 2) this lower stiffness was associated with a longer EMD and slower RFD, and 3) that strength training in adults had previously been shown to increase tendon stiffness and RFD, the adaptability of the developing Achilles tendon to a resistance training programme, and consequence of the potential changes on force production capacity were examined in the final experimental study (Chapter 6). Significant increases in Achilles tendon stiffness and Young’s modulus were found after 10 weeks of twice-weekly plantarflexor strength training in 8-9 year-old boys and girls, which demonstrates that the larger muscle force production provided a sufficient stimulus for tendon microadaptation. The training also resulted in a decrease in EMD, which was moderately correlated with the change in tendon stiffness (r = 55), but no change in RFD. Thus, the increasing tendon stiffness with training was associated with a decreasing EMD, but had no detectable effect on RFD. This would likely have a significant effect on the performance of tasks requiring rapid muscle force production. Together, the results of the present series of investigations demonstrate that the tendon loading experienced from both normal ageing and overloading (strength training) can increase tendon stiffness in children, and that these changes have a detectable effect on rapid force production.