Proceedings of The Physiological Society

King's College London (2009) Proc Physiol Soc 14, SA15

Research Symposium

Myotendinous adaptations to ageing, disuse and training

M. V. Narici1

1. Institute for Biomedical Research into Human Movement, Manchester Metropolitan University, Manchester, United Kingdom.

Muscles and tendons show a remarkable plasticity in response to chronic loading, unloading and ageing. In a recent unloading study we showed a significant decrease (-8%) in human vastus lateralis (VL) muscle fibre fascicle length after 23 days of unilateral lower limb suspension (ULLS) (1). Concomitantly, patellar tendon (PT) stiffness decreased by 29% after 23 days ULLS (1). These changes are similar to those found in ageing since lower limb fibre fascicle length and tendon stiffness are respectively reduced by 10% and 36%, in older humans (2). These changes are expected to have an impact on the VL length-force relation (L-F), since a decrease in tendon stiffness would cause a left shift in the VL fascicles L-F relation, away from the force plateau region (3). However, since in both disuse and ageing, fascicle length was shorter, lesser sarcomere shortening would be expected upon contraction, thus shifting the L-F relation to the right, closer to the optimum region (3). Thus a compensation between the fascicular and tendon changes seems to occur. However, this effect may only partly mitigate the force loss, since it cannot compensate the decline in force due to the loss of motor units (MUs) with ageing (4) or to the decrease in single fibre specific tension occuring with ageing and disuse (5). Loading, instead, produces the opposite effects: after 14 weeks of resistance training in older men, VL fascicle length increased by 8-10% (3), while PT tendon stiffness increased by 65%. The rapid muscle remodeling produced by these experimental paradigms, seems regulated by changes in the mechano-sensitive costameric protein focal adhesion kinase (FAK), believed to be an up-stream regulator of protein synthesis. Overloading of avian muscle causes a massive increase in FAK content (+112% after 1 day and 611% after 8 days) and activity (+370% after 1.5 and 13 days) (6). Instead, unloading produces opposite effects, since FAK content and activity decrease by 20 and 30%, respectively, within 10 days of ULLS in humans (7). Thus the loss of muscle force observed with ageing and unloading likely reflects the combined effect of muscular and tendinous adaptations and, limited to old age, also of neuropathic processes responsible of the loss of MUs. In both ageing and unloading these myotendinous alterations may be largely reversed with loading, though muscle weakness due to MUs loss with age, cannot be prevented. The structural remodeling of skeletal muscle, detectable within few days of loading or unloading, seems mediated by changes in costameric proteins involved in mechanotransduction.

Where applicable, experiments conform with Society ethical requirements