Thigh muscle strength in below-knee amputees.

The aim of this investigation has been to study the magnitude of the reduction of thigh muscle strength of below-knee amputees and to correlate the strength to the degree of atrophy and to the clinical function. Thirty-two below-knee amputees participated in examinations of the isometric and isokinetic knee extension and flexion strength using an isokinetic dynamometer (Cybex II). The atrophy was evaluated by measuring the mean muscle fibre area on muscle biopsies of the vastus lateralis and the cross-sectional area of the quadriceps and hamstring muscles measured with computed tomography. The isometric and isokinetic knee extension and flexion strength in the amputated leg with and without prosthesis was significantly lower than the strength in the non-amputated leg (p less than 0.01). Knee extension strength was significantly correlated to the mean muscle fibre area of the vastus lateralis. There was no correlation between strength and the cross-sectional area of the quadriceps muscles. There was, however, a significant correlation between the side differences (amputated leg versus non-amputated leg) in isometric knee extension and flexion strength and the side differences in cross-sectional areas. The reduction in isometric and isokinetic knee extension and flexion strength in the amputated leg both with and without prosthesis compared with the non-amputated leg was larger than the reduction in cross-sectional areas of the quadriceps and hamstring muscles and in muscle fibre areas of the vastus lateralis. This indicates that besides muscle atrophy changes in motor unit recruitment pattern may be of importance for the reduction in muscle strength. Isometric and isokinetic knee extension and flexion strength values in the amputated leg with prosthesis were significantly correlated to step length, maximal walking speed and circumference of the thigh. These results indicate that below-knee amputees with a better preserved thigh muscle strength have a good walking capacity.