Maximal isometric torque of knee extension as a function of muscle length in subjects of advancing age.

Many neuromuscular diseases are associated with muscle weakness. Assessment of this weakness by manual muscle testing or with hand-held equipment has been criticized. Furthermore, muscle length influences peak force development. One hundred fifty-three female and 116 male asymptomatic subjects between the ages of 20 and 80 (approximately 20 per decade) were studied. Maximal strength of the quadriceps group was determined isometrically, on a specially designed bench, at 3 hip angles (45 degrees, 90 degrees, and 180 degrees of extension), which represent three rectus femoris muscle lengths. Maximal strength was observed at a hip angle of 180 degrees and did not decrease significantly from 20 to 50 years of age, although the values for the male and female subjects over 50 years were significantly decreased (approximately 15% per decade). The increase in strength as the muscle was lengthened from 45 degrees to 180 degrees hip angle was about 80% (of the force at 45 degrees) in 20-year-olds; strength decreased progressively with age to a value of approximately 50% in the 70-year-olds. This greater reduction in maximal strength at longer quadriceps muscle lengths has functional significance in rising from a chair, climbing stairs, and eventually, walking and standing.