A simple theory is proposed to account for the quantity of mitochondria present in aerobic muscles. Attention is restricted to muscles adapted to operate aerobically at a well-defined ‘operating frequency’. For this special case, it is shown that the volume ratio of mitochondria to myofibrils should depend on the power density of mitochondria, and the operating frequency, but not on the mechanical properties of the myofibrils. If the underlying assumptions are valid, this would mean that the specific power output of such muscles could be determined by examination of electron micrographs.

We provisionally estimate that the inverse power density of mitochondria, in flight muscles running at a high temperature, is in the range 1.010−6 to 1.310−6m3W−1, that is, that a little over 1 ml of mitochondria is required to sustain 1 W of mechanical power output. On this basis, a muscle with equal volumes of mitochondria and myofibrils should be able to deliver a specific power of about 430 W kg−1, at an operating frequency around 40 Hz for nonfibrillar, or 230 Hz for fibrillar muscle. The limiting specific power should be twice this level in either case, i.e. about 860Wkg−1.

It is predicted that a survey of flight muscles should yield a straight-line relationship between wing-beat frequency and the volume ratio of mitochondria to myofibrils, in a set of muscles of the same general type. It is not known whether lack of exercise, either on a long-term or short-term basis is likely to affect this. As a preliminary to such a survey, we have examined the pectoralis muscles of domesticated quail, and a wild house sparrow. Both showed a high level of variability in the mitochondria: myofibril ratio, but this may be due, at least in part, to sampling artifacts caused by the shape of mitochondrial arrays. The quail showed distinct populations of aerobic and non-aerobic fibres, apparently identical to those described in wild birds with similar flight requirements by George & Berger (1966), but the quantity of mitochondria in the aerobic fibres was less than half that expected, by comparison with other species.

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