INTRODUCTION: Unfused tetanic contractions of fast motor units exhibit a transitional decline in force following the initial extra-efficient force development, known as sag. Sag is sensitive to changing energy demands and the fuels metabolized to meet these demands. AIM(S): Since different training modes have varying effects on cellular energy systems in muscles, we aimed to determine how endurance, strength, and vibration training would affect sag in fast motor units of rat medial gastrocnemius. METHOD(S): Separate control groups were used for each training mode, with activity limited to normal cage movements. Endurance training (ET): 2, 4, or 8 weeks of treadmill training with weekly progressions in duration and speed. Strength training (ST): 5 weeks of voluntary progressive weight‑lifting. Vibration training (VT): 3 or 6 months of whole‑body vibration training at 50 Hz (4 × 30 s with 60 s rest periods). ET, ST, and VT groups were trained 5 days per week. Following training, functionally isolated fast motor units (divided into fast fatigable (FF) and fast fatigue-resistant (FR)) were investigated, and profiles of their unfused tetanic contractions at 40 Hz with sag were analyzed. RESULTS: The 40 Hz contractions of trained groups were less fused than those of untrained groups; this effect was attributable to shorter twitch time parameters in trained animals. Accordingly, numerous differences appeared in the sag profiles. However, when limiting the comparisons to motor units with comparable levels of fusion (fusion index 0.2 – 0.8), few differences were observed. With this constraint 1) ET had no effect on sag profiles, 2) the force decreased after the initial peak was delayed in FF of ST rats, and 3) VT resulted in a shorter duration of sag in FF and a smaller force decrease after the initial peak in FR. CONCLUSIONS: We conclude that while different training modes have differing effects on sag, these effects are primarily due to altered twitch time parameters.
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