LOSS OF MUSCLE MASS AND THE CONTRIBUTION IT MAY MAKE TO THE AGE-RELATED LOSS OF MOBILITY
David A Jones, Mathew Piasecki, Hans Degens, Jamie McPhee – School of Health Care Science, Manchester Metropolitan University, Manchester, United Kingdom
Loss of mobility has a major impact on the quality of life for older people, not only does it reduce the person’s ability to look after themselves but slow and hesitant movements also make a person more liable to trips and falls that can have life-threatening consequences. Muscle wasting and loss of strength are well documented features of ageing and there are arguments for various activities that may improve strength, or at least slow the decline. It is, however, important to understand the reasons for the loss before putting limited resources into rehabilitation programmes. Muscle mass declines by about 30 % by the 8th decade and muscle fibre atrophy, predominantly of the type 2 fibres, contributes about half of this loss, the other half appears to be to loss of fibres, the number in the vastus lateralis decreasing from 1.22 x 106 to 1.03 x 106 . This loss of fibres is the result of motor unit loss due to motor neuron death. Recent estimates suggest that around 50 % of motor units are lost by the 8th decade although the impact on muscle fibre number is lessened by axons of surviving motor neurons reinnervating denervated fibres, resulting in a smaller number of larger motor units. This helps to preserve strength but may have implications for fine motor control. The loss of motor units is not prevented by life-long physical activity as master athletes show the same pattern of motor unit loss. At the moment we know of no way of preventing, reversing or even slowing this process. There is then the question of whether the 30 % loss of strength is the cause of the decrease in mobility and while, for instance, there is a strong relationship between muscle strength and walking speed if the whole range from young to old is considered, within an age cohort (either young or old) there is little or no relationship. Elderly men have very similar strength to young women, yet young women walk faster and are more agile and mobile than old men. Other factors that affect walking speed are balance, which is severely affected by age, eyesight, and probably cognitive function and the ability to dual task. People of all ages tend to pace themselves at around 70 % of their maximum heart rate and, since the maximum decreases with age, older people inevitably choose to walk more slowly than the young. There is, unfortunately, no easy answer to maintaining mobility in old age and we need to explore not only physiological interventions but also social and psychological barriers to mobility that would benefit not only older people but the whole of society.