Force-frequency relationship and potentiation in mammalian skeletal muscle

Repetitive activation of a skeletal muscle results in potentiation of the t witch contractile response. Incompletely fused tetanic contractions similar to those evoked by voluntary activation may also be potentiated by prior a ctivity. We aimed to investigate the role of stimulation frequency on the e nhancement of unfused isometric contractions in rat medial gastrocnemius mu scles in situ. Muscles set at optimal length were stimulated via the sciati c nerve with 50-mu s duration supramaximal pulses. Trials consisted of 8 s of repetitive trains [5 pulses (quintuplets) 2 times per second or 2 pulses (doublets) 5 times per second] at 20, 40, 50, 60, 70, and 80 Hz. These sti mulation frequencies represent a range over which voluntary activation woul d be expected to occur. When the frequency of stimulation was 20, 50, or 70 Hz, the peak active force (highest tension during a contraction - rest ten sion) of doublet contractions increased from 2.2 +/- 0.2, 4.1 +/- 0.4, and 4.3 +/- 0.5 to 3.1 +/- 0.3, 5.6 +/- 0.4, and 6.1 +/- 0.7 N, respectively. C orresponding measurements for quintuplet contractions increased from 2.2 +/ - 0.2, 6.1 +/- 0.5, and 8.7 +/- 0.7 to 3.2 +/- 0.3, 7.3 +/- 0.6, and 9.0 +/ - 0.7 N, respectively. Initial peak active force values were 27 +/- 1 and 6 1.5 +/- 5% of the maximal (tetanic) force for doublet and quintuplet contra ctions, respectively, at 80 Hz. With doublets, peak active force increased at all stimulation frequencies. With quintuplets, peak active force increas ed significantly for frequencies up to 60 Hz. Twitch enhancement at the end of the 8 s of repetitive stimulation was the same regardless of the patter n of stimulation during the 8 s, and twitch peak active force returned to p restimulation values by 5 min. These experiments confirm that activity-depe ndent potentiation is evident during repeated, incompletely fused tetanic c ontractions over a broad range of frequencies. This observation suggests th at, during voluntary motor unit recruitment, derecruitment or decreased fir ing frequency would be necessary to achieve a fixed (submaximal) target for ce during repeated isometric contractions over this time period.