Use and misuse of the tape-measure as a means of assessing muscle strength and power.

The tape-measure is frequently used by the medical profession to assess muscle function following injury. It is assumed that increasing limb girth is reflected by increasing muscle power and strength; but this assumption is empirical and may not in practice be justified. The relationship between thigh circumference and muscle strength and power, measured as the torque produced on an isokinetic dynamometer, was assessed in 64 subjects--24 non-injured sportsmen and 40 injured sportsmen. The correlation between the torque produced at the knee by the knee extensors and flexors and the thigh circumference at three levels was not significant in either group. Comparison of the injured with the non-injured limb also failed to show any correlation. By contrast, in eight subjects, repeated measurements over a six to eight month period showed a significant relationship between change in thigh circumference and change in quadriceps power. It is concluded that a single measurement of circumference at the same point in both thighs of a subject with unilateral injury should not be used as an assessment of muscle function. Serial measurements, however, at the 4 and 10 cm levels may be of value as an index of quadriceps power. Reasons for this are discussed.