MUSCLE WEAKNESS IN OLD AGE: HOW MUCH THIS ORIGINATES FROM THE MUSCLE ITSELF?

Muscle wasting and weakness are typical features of old age. However, when one compares the losses of mus-cle strength and power to those of muscle size and vol-ume, the decline in muscle function largely exceeds that of muscle mass and size. In vivo, differences of 20-30% in force normalised to cross-sectional area have been re-ported between young and older individuals (3). The dif-ference is even larger when specific power is considered, for muscle peak power/volume is 45 % lower in older males (6). Clearly, simple dimensional properties, only partly ac-count for the loss of muscle function. Several studies have shown that a deficit in agonists activation capacity as well as an increase in antagonist co-activation contribute to this phenomenon, but even after accounting for these as well as for changes in muscle architecture, the specific force of older individual is still significantly lower than that of younger adults (by about 30 % for the plantarflexors, 4). Recent evidence suggests that age-related changes in ten-don properties may also be involved (5). Older tendons are more compliant than younger tendons which may cause muscle fibres to operate in a non-optimal region of their length-tension relation. Single fibre studies show that older muscle fibres are intrinsically weaker (2) and significantly contribute to the observed loss in whole muscle specific force. On the other hand, increased attention is being given to the role of the extracellular matrix (ECM) in force trans-mission and in regulating muscle mass. Within the ECM, intramuscular connective tissue has been shown to con-tribute significantly to myofascial force transmission (1) and it cannot be excluded that age-related alterations in intra-muscular connective tissue also affect muscle specific ten-sion. The above factors undoubtedly also account for the decline in specific power in old age but in addition, intrinsic slowing of the myosin molecules likely plays a role