Journal scan

Why older people experience muscle loss

Study shows reinnervation of muscle fibres does not expand motor unit size in those with sarcopenia

Study shows reinnervation of muscle fibres does not expand motor unit size in those with sarcopenia

Sarcopenia, measured by grip strength and walking speed, affects 10%–20% of over 65s. Picture: iStock

As people grow older leg muscles become smaller and weaker, leading to frailty and disability. 

Of those aged over 65, 10%–20% have sarcopenia: a loss of muscle mass and strength measured by grip strength and speed of walking. These people are at risk of social isolation, bone fracture, disability and hospital admission. Sarcopenia is associated with a reduction in the number of motor cells innervating limb muscles. 

By the age of 75, individuals typically have 30%–50% fewer nerves controlling their legs. This leaves parts of their muscles disconnected from the nervous system, making them functionally useless so they waste away. Healthy muscles have a form of protection in that surviving nerves can send out new branches to rescue some of the detached muscle fibres. It is not fully understood why connections between muscles and nerves remain healthy in some people and not in others.

This study compared motor unit size and number between 48 young men, 13 non-sarcopenic and 53 pre-sarcopenic older men, and 29 with sarcopenia. The results show that extensive motor unit remodelling occurs relatively early during ageing. In non-sarcopenic and pre-sarcopenic older men, reinnervation of muscle fibres seems to expand the motor unit size but this does not happen in those with sarcopenia. 

This suggests that it is the failure to expand the motor unit size that distinguishes sarcopenic from pre-sarcopenic muscles. The race is now on to see if regular exercise in middle and older age can slow the process of muscles becoming disconnected from the nervous system or improve nerve branching to rescue detached muscle fibres. 

Piasecki M, Ireland A, Piasecki J et al (2018) Failure to expand the motor unit size to compensate for declining motor unit numbers distinguishes sarcopenic from non-sarcopenic older men. The Journal of Physiology. doi: 10.1113/JP275520

Compiled by Ruth Sander, independent consultant in care of the older person

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