Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, C108
Human in vivo tendon adaptations to resistance training & detraining at different muscle lengths
G. McMahon1, C. I. Morse1, A. M. Burden1, K. Winwood1, G. L. Onambélé1
1. Institute for Performance Research, Manchester Metropolitan University, Cheshire, United Kingdom.
Figure 1. (A) Changes in K pre- and post-training in SL, LL & Con groups. * Significant relative increase compared to baseline (P<0.01), # Significant compared to other training group (P<0.05). (B) Changes in Tendon Force-Elongation pre- and post-training, # significant compared to other training group (P<0.05).
The material properties of the muscle-tendon complex (MTC) alter in relation to mechanical stress (e.g. resistance training, RT (1, 2)). However, performing exercise at different muscle lengths will alter the mechanical stress delivered to the MTC via changes to the moment arm of the in-series elastic component (3). The in vivo response of the MTC to dynamic resistance training and detraining at two different muscle lengths are yet unknown. Two training groups - LL (n=8) and SL (n=8) undertook 8 weeks of dynamic RT and 4 weeks detraining at either a short muscle length (SL) or longer muscle length (LL). Tendon dimensions (CSA and length), stiffness (K), Young’s Modulus (E), antagonist co-contraction (TAnt) and maximal voluntary (isometric) muscle torque (MVC) over a range of angles (30-90o knee flexion) were measured at weeks 0, 8, 10 and 12 using ultrasonography, EMG and dynamometry. A control group (n=10) was also monitored during this period. The local Ethics committee approved this study and participants gave written informed consent. Results are mean ± S.E.M., and compared via ANOVA (alpha ≤ 0.05). There was a significant effect of training (P<0.01) in SL and LL groups in MVC, K (figure 1) and E compared to baseline and control data. There was no training effect (P>0.05) on patellar tendon dimensions or TAnt in training groups. SL increased MVCs at knee joint angles between 50-75o, whereas LL increased MVCs over a wider range (30-90o). There was a main group effect (P<0.05) in K (45±6% vs. 32±2%) and E (42±5% vs. 30±2%) at week 8, with LL exhibiting greater adaptations than SL (P<0.05). The group effect in K and E remained at week 10. Detraining resulted in significant (P<0.05) deteriorations in tendon mechanical properties measured in both groups by week 12 compared to week 8, although they remained significantly elevated relative (P<0.01) to week 0. Our results show that adaptations to the MTC are superior immediately following a period of chronic resistance training at a longer muscle length in comparison to a shorter muscle length. The relative workloads between groups were similar, with the differential mechanical stress to the unit (3) likely to form the basis for the differential magnitude of responses between LL and SL, as observed during isometric training (4). In order to determine the underlying mechanisms of these specific adaptations, an investigation into endocrine signalling is warranted, as it has been suggested that IGF-I and TGF-β1 (i.e. markers also of the potential hypertrophic response to muscle-loading induced stress) may be mediators of collagen expression in response to mechanical loading of tendon (5).
Where applicable, experiments conform with Society ethical requirements