The relative isometric strength of type I and type II muscle fibres in the human quadriceps

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Summary. In isometric contractions, the type II fibres of animal muscle may have a greater specific strength than the type I fibres. This paper reports two retrospective analyses of the influence of fibre‐type composition on the ratio of the voluntary isometric strength of the quadriceps to its cross‐sectional area at mid‐thigh.In 15 normal quadriceps, the ratio of the muscle's strength to its cross‐sectional area was weakly correlated with the percentage contribution of type II fibres to a biopsy from the lateral mass of the muscle (r = 0·55, P<0·05). Linear regression suggested that type II fibres might have about three times the specific strength of type I fibres.Measurements of the atrophied quadriceps of 11 patients with unilateral knee injury/immobilization were standardized by comparison with the contralateral muscles. Strength/cross‐sectional area was again correlated with the percentage area as type II fibres (r = 0·62, P<0·05). The data suggested that type II fibres might have about twice the specific strength of type I fibres.Despite the wide confidence limits of each of the analyses, they agree that, in isometric contractions, the type II fibres of the human quadriceps seem stronger, for their cross‐sectional area, than the type I fibres.

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