Effects of long-term stretching training on muscle-tendon morphology, mechanics and function

Abstract

The aim of the present thesis was to examine the effects of long-term stretching training on hamstrings and triceps surae muscle-tendon morphology, mechanics and function. Elite rhythmic gymnasts (study I) and professional ballet dancers (study II), who had undertaken years of systematic stretching, were compared to control subjects with no history of stretching. In a within-subjects randomized controlled trial, healthy, recreationally active adults underwent 24 weeks of stretching training of one leg, while the contralateral leg served as control (study III).

Altogether, the studies demonstrate that joint range of motion (ROM) is associated with a number of morphological, mechanical and neural factors, many of which were modified with 24 weeks of stretching. Specifically, passive resistance to stretch was lower in subjects with greater ROM (gymnasts and dancers), and in the group that stretched for 24 weeks, passive resistance of the plantar flexors was reduced. Maximally tolerated passive torque was increased after 24 weeks of stretching. The role of neural factors in determining ROM was underscored by lower electromyographic amplitudes (EMG) at standardized joint angles in ballet dancers, by reduced EMG at standardized angles after 24 weeks of stretching, and by bilateral changes in ROM, passive resistance, EMG and tissue elongation after 24 weeks of stretching. Ballet dancers had longer gastrocnemius medialis muscle fascicles and longer and more compliant Achilles tendons, but neither of these variables nor the amount of intramuscular collagen were altered by 24 weeks of stretching. Hence, this thesis does not provide direct evidence of morphological adaptations. However, the changes in passive resistance after 24 weeks of stretching cannot fully be explained by the observed changes in neural activation, and 24 weeks of stretching led to bilateral increases in tendon elongation, observed using two different methods. These findings may represent genuine structural adaptations, but further research is needed to confirm this.

The gymnasts presented specialized contractile properties, producing greater work, reaching peak knee flexion torque with the knee more extended and displaying a greater functional ROM, despite similar strength compared to controls. Such differences were not evident in dancers. However, 24 weeks of stretching shifted angle of peak torque to a more dorsiflexed ankle angle.

In conclusion, the present thesis shows that long term stretching training leads to neural adaptations and probably structural adaptations, which together translate into altered mechanical properties and have the potential to modify contractile function. The role of central mechanisms for increases in ROM was confirmed by bilateral responses to 24 weeks of stretching, which had previously not been demonstrated.

Paper I: Moltubakk, M. M., Eriksrud, O., Paulsen, G., Seynnes, O. R., & Bojsen-Moller, J. (2016). Hamstrings functional properties in athletes with high musculo-skeletal flexibility. Scandinavian Journal of Medicine and Science in Sports, 26, 659-665. doi: 10.1111/sms.12488.

Paper II: Moltubakk, M. M., Magulas, M. M., Villars, F. O., Seynnes, O. R., & Bojsen-Moller, J. (2018). Specialized properties of the triceps surae muscle-tendon unit in professional ballet dancers. Scandinavian Journal of Medicine and Science in Sports. 2018 May 3. doi: 10.1111/sms.13207 [Epub ahead of print]

Paper III: Moltubakk, M. M., Villars, F. O., Magulas, M. M., Magnusson S. P., Seynnes, O. R., & Bojsen-Moller, J. Altered triceps surae muscle-tendon properties after 6 months of unilateral stretching. Manuscript.

Paper IV: Moltubakk, M. M., Svensson, R. B., Magnusson S. P., Suetta, C., Raastad, T., Seynnes, O. R., & Bojsen-Moller, J. Effects of 6-month stretching training on collagen content of the human soleus muscle. Manuscript.