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 less than 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 less than 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 fibers.