Potentiation of isometric and isotonic contractions during high-frequency stimulation

Activity dependent potentiation is an enhanced contractile response resulting from previous contractile activity. It has been proposed that even a maximal effort contraction may be enhanced by prior activity if there is an increase in the peak rate of force development. This should increase the peak active force during a very brief maximal effort contraction. The purpose of these experiments was to evaluate potentiation during brief sequential contractions with high-frequency stimulation. For this experiment, the rat medial gastrocnemius muscle was isolated in situ. Sequential stimulation (two contractions per second for 4 s) with 200, 300, or 400 Hz doublets, triplets, and quadruplets was applied. A small degree of force potentiation was observed in isometric contractions at the reference length (RL), but the activity dependent potentiation of isometric contractions was greater at short muscle length. For example, peak rate of force development for 200 Hz doublets increased significantly from the first to the eighth contraction (from 0.30 ± 0.02 to 0.34 ± 0.02 N·s⁻¹ at RL and from 0.18 ± 0.02 to 0.28 ± 0.01 N·s⁻¹ at RL-3 mm). During isotonic contractions, there were significant increases in peak shortening from the first to the eighth contraction. With 200 Hz doublet stimulation, shortening increased from 0.85 ± 0.14 to 1.14 ± 0.17 mm, and this corresponded with an increase in peak velocity (from 116 ± 18 to 136 ± 19 mm·s⁻¹). Remarkably, even 400 Hz quadruplets showed a significant increase in shortening during repeated contractions (2 s⁻¹). These observations indicate the possibility that activity dependent potentiation can result in significant improvement in both isometric and dynamic contractions, even when activated at very high frequency.