Figure 2.

In vivo recording of TA muscle and motor unit forces and glycogen depletion of a single muscle unit. The sciatic nerve was stimulated to evoke isometric twitch and tetanic forces of the TA muscle (A, D) and dissected ventral root filaments in L4 and L5 stimulated to evoke a maximum of four all-or-none increments in twitch force (B) and electromyographic signals (data not shown). The motor unit forces were obtained by subtraction (C). The number of reinnervated motor units was calculated as the ratio of the muscle and average motor unit (MU) twitch forces (E). A single motor unit was isolated by being evoked by 2× threshold stimulation of a teased ventral root filament. The muscle unit twitch and maximal tetanic isometric forces were recorded; fatigue resistance was calculated from the ratio of the unfused tetanic forces at 40 Hz at 0 and 2 min of a fatigue test in which the nerve filament was stimulated with 13 pulses at 40 Hz at a repetition rate of 1 Hz for 2 min (F). Thereafter, the isolated single motor unit was stimulated at 100 Hz repeated at a rate of 1 Hz until the force stabilized, then at 2 Hz, and then at 5 Hz until the force was reduced to 5% of the initial force (G). The motor unit force was then allowed to recover at a stimulation rate of 0.1 Hz. When the force had completely recovered, a second bout of fatiguing stimulation was initiated. This stimulation regime was repeated until the unit tetanic force failed to recover beyond ∼50% of the initial force levels.