Muscle fibre size and composition as well as cross sectional area of the Quadriceos muscle were measured in needle biopsies from the Vastus Lateralis of 84 young male soccer players. Measurements were made before and after treatment for an acute lesion in the Medial Collateral Ligament of the knee. A highly significant decrease was found in the relative number of type I fibres, from 42.3% to 37.2%. The total area of the Quadriceps cross section occupied by type I fibres decreased significantly from 34.7 cm2 to 21.8 cm2 in the immobilized Quadriceps muscle. The calculated total number of muscle fibres in the immobilized muscle decreased from 1.5 x 10(4) to 1.1 x 10(4). Only insignificant changes were found in the cross sectional area of the two main fibre types during immobilization. Several explanations could be given for these changes in fibercomposition and absolute fibernumber. The most reasonable explanation might be changes in the length and orientation of the fibres, so the results do not necessarily reflect a real loss of type I fibres. Isometric exercise or indirect electrical stimulation with 10 Hz or 50 Hz were found ineffective in attenuating these immoblization-induced muscle fibre changes. It is concluded that immobilization in Plaster of Paris for 4 to 6 weeks has profound effects especially upon the oxidative type I fibres of the human Quadriceps muscle, and this hypotrophy is not prevented by Quadriceps settings or electrical stimulation.

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