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 (~20 per decade) were studied. Maximal strength of the quadriceps group was determined isometrically, on a specially designed bench, at 3 hip angles (45°, 90°, and 180° of extension), which represent three rectus femoris muscle lengths. Maximal strength was observed at a hip angle of 180° 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 (~15% per decade). The increase in strength as the muscle was lengthened from 45° to 180° hip angle was about 80% (of the force at 45°) in 20-year-olds; strength decreased progressively with age to a value of ~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.