The quadriceps femoris muscle in 20-70-year-old subjects: relationship between knee extension torque, electrophysiological parameters, and muscle fiber characteristics.

Comparisons have been made between torque (isometric and isokinetic), electrophysiological (SFEMG, Macro EMG), and muscle fiber characteristics in the vastus lateralis muscle of both legs in healthy subjects aged between 20 and 70 years. Torque was greater in males and decreased with age in both sexes. Multifactorial analysis showed a positive correlation between torque, body surface area, and mean fiber area. These variables explained only about 30-40% of the torque changes. The electrophysiological parameters (Marco EMG amplitudes and fiber density) revealed evidence of reinnervation, indicating preceeding denervation and therefore loss of motor units. It was concluded that this fall out of motor units also contributes to the reduction in torque, when compensatory reinnervation begins to fall. Other factors, such as reduction in muscle fiber contractility, metabolic factors, and central factors, may also play a role in age-related reduction in torque.