The loss of muscle mass with age (Sarcopenia) has received growing attention over
the past decade. Despite efforts to provide a universal definition with clinically
meaningful cut-off points for diagnosis, there is no clear consensus on how to best
quantify and assess the impact of loss of muscle mass and function on functional
limitations. Whilst most previous studies have used dual energy x-ray absorptiometry
(DXA) to quantify this loss, chapter 2 of this thesis shows that DXA underestimates
the loss of muscle mass with age in comparison to the gold standard MRI.
Muscle mass per se is not enough to determine whether a person has an
exceptionally low muscle mass, as it can be readily seen that a healthy tall person
will have a larger muscle mass than a small person. Clinicians and researchers thus
need an index of muscle mass that takes differences in stature into account and also
gives an objective cut off point to define low muscle mass. In Chapter3, we show that
femur volume does not significantly differ between young and old. We used this
observation to introduce a new index: thigh muscle mass normalised to femur
volume, or the muscle to bone ratio. This index allows the examination of the true
extent of muscle atrophy within an individual.
In previous studies the appendicular lean mass (determined with DXA)
divided by height squared appeared to be a relatively poor predictor of functional
performance. In Chapter 4, the index introduced in Chapter 3, the muscle to bone
ratio, proved to be a somewhat better predictor of functional performance in the
overall cohort. This was, however, not true when examining the intra-group
relationships. A similar situation applied to the maximal muscle strength. In older
adults, the parameter which predicted functional performance best was muscle
power per body mass, measured during a counter-movement jump.
Chapter 5 shows that part of the larger loss power and force than muscle
mass is attributable to a left-ward shift of the torque-frequency relationship, indicative
of a slowing of the muscle, and reduction in maximal voluntary activation, as
assessed using the interpolated twitch technique in older adults.
Chapter 5 also shows that the fatigue resistance during a series of intermittent
contractions was similar in young and older adults. However, older adults could
sustain a 50% maximal voluntary contraction force longer than young people. Part of
this discrepancy maybe due to an age-related slowing of the muscle