title: The manipulation of strain, when stress is controlled, modulates in vivo tendon mechanical properties but not systemic TGF-β1 levels.
abstract: Modulators of loading-induced in vivo adaptations in muscle-tendon complex (MTC) mechanical properties remain unclear. Similarly contentious, is whether changes in MTC characteristics are associated with growth factor levels. Four groups were subjected to varying magnitudes of stress/strain: Group 1 trained with the MTC at a shortened position (MTCS; n = 10); Group 2 at a lengthened position (MTCL; n = 11; stress levels matched to MTCS); Group 3 over a wide range of motion (MTCX; n = 11); and Group 4 (n = 10) was the control population (no training). Patella tendon Stiffness (P < 0.001), Young's modulus, and quadriceps torque (P < 0.05) increments (only seen in the training groups), showed MTCL and MTCX groups responses to be superior to those of MTCS (P < 0.05). In addition, MTCL and MTCX better maintained adaptations compared to MTCS (P < 0.05) following detraining, with a pattern of slower loss of improvements at the early phase of detraining in all training groups. There were no significant changes (P > 0.05) in antagonist cocontraction, patella tendon dimensions or circulating transforming growth factor beta (TGF-β1) levels following training or detraining in any of the groups. We conclude that chronically loading the MTC in a relatively lengthened position (which involves greater strains) enhances its mechanical properties, more so than loading in a shortened position. This is true even after normalizing for internal stress. The underlying endocrine mechanisms do not appear to be mediated via TGF-β1, at least not at the systemic level. Our findings have implications with regard to the effectiveness of eccentric loading on improved tendon structural and mechanical properties.
authors.affiliation: Department of Exercise and Sport Science, Institute for Performance Research, Centre for Lifespan and Wellbeing, Manchester Metropolitan University Crewe Green Road, Crewe, CW1 5DU, United Kingdom ; Sports Institute Northern Ireland, University of Ulster Shore Rd, Newtownabbey, BT37 0QB, United Kingdom.
authors.forename: Gerard E
authors.initials: GE
authors.lastname: McMahon
authors.forename: Christopher I
authors.initials: CI
authors.lastname: Morse
authors.forename: Adrian
authors.initials: A
authors.lastname: Burden
authors.forename: Keith
authors.initials: K
authors.lastname: Winwood
authors.forename: Gladys L
authors.initials: GL
authors.lastname: Onambélé-Pearson
keywords: Muscle–tendon complex; soft tissue biomechanics; stiffness; stress/strain; stretch