The Older Muscle: Ageing or Disuse? - Prof. Harridge

Plenary Session "From the Cradle to the Grave - Sport and Physical Activity for a Sustainable Body" at ECSS Malmö 2015

The Older Muscle: Ageing or Disuse?
Harridge, S.
King`s College London

The prime role of muscle is to act as a biological machine - to produce force and generate power. This is to fulfil a number of different
functions from maintaining posture to allowing the performance of all the physical tasks needed for everyday living. But, muscle also
has a plethora of other roles which include protein being an important dynamic store, being the largest sink for the storage of glucose
and acting as a source of protective padding. As we get older there is an observable reduction in many of these functions. This is
primarily related to a loss of tissue mass, which when passing a given threshold is referred to as “sarcopenia”. Sarcopenia is
associated with an increased risk of falls and sustaining fractures and is an increased risk factor for morbidity and mortality. However,
the extent to which sarcopenia and its attendant side effects can be attributed to an inherent biological ageing process are far from
clear.
At any age muscle mass and function are acutely sensitive to activity pattern and usage – being highly responsive to both mechanical
and metabolic signals. For example, limb immobilisation, bed rest or exposure to microgravity (space flight) all result in muscle loss
and weakness- irrespective of age. The effects of many years of a sedentary lifestyle thus seriously confound our understanding of
the properties of a muscle we might expect for a given chronological age. For a typical sedentary older person, a smaller muscle
seems to relate to both a reduced number of muscle fibres (and motor units) and a selective atrophy of the fast-contracting type II
muscle fibres. Muscle “quality” also declines in older sedentary individuals with evidence of fat deposits and connective tissue
accumulation. This makes “contractile” tissue mass harder to determine and contributes to the decrease in force potential per unit
area (specific force) of a whole muscle in vivo. Furthermore, when rates of muscle protein synthesis are measured using tracer
techniques there is evidence of a reduced sensitivity to both exercise and amino acid feeding.
What is the cause of the decline in mass and function? To what extent are these phenomena attributable to the biological ageing
process and what can be done to ameliorate these deleterious effects? These are some of the key questions that need to be
addressed in the light of the dramatic changes in population demographics.
Recent experiments on highly active older people have thrown some light on answering these questions. These studies have shown
that despite ageing, muscle mass, function and quality can all be well maintained. These data suggest that we need to rethink our
perceptions on the interactions between ageing, exercise and physiological function.