Eccentric Quasi-Isometric Exercise Produces Greater Impulse with Less Pain than Isokinetic Heavy–Slow Resistance Exercise in Ankle Plantar Flexors: Quasi-Randomized Controlled Trial

Recently, there has been growing interest in optimizing exercise protocols in sports training and rehabilitation, with particular attention to eccentric quasi-isometric (EQI) contractions, which involve maintaining joint position until isometric failure and then resisting the subsequent eccentric phase. Evidence directly comparing EQI with other contraction modes remains scarce. This quasi-randomized controlled trial examined the short-term effects of EQI versus isokinetic heavy–slow resistance (IHSR) exercises on ankle plantar flexors, focusing on pain, range of motion (RoM), and strength performance. Thirty-two physically active participants were allocated to EQI (n = 16) or IHSR (n = 16) groups and assessed at baseline, immediately post-exercise, and 24 and 48 h later. Both groups performed three exercise sets with 3 min breaks. The protocols were designed to approximate matched loading, based on preliminary testing. Nevertheless, the EQI group achieved a significantly greater total impulse (p = 0.028), a shorter time under tension (p = 0.001), and lower effort scores (p < 0.001). Group × time analysis revealed less decline in maximal voluntary isometric contraction torque (p = 0.002; η2 = 0.16), as well as lower general (p < 0.001; η2 = 0.32) and activity-related pain (p < 0.001; η2 = 0.32) in the EQI group, with no significant differences in dorsiflexion RoM (p = 0.893). In conclusion, EQI produced a higher torque impulse while inducing less fatigue and post-exercise pain than IHSR, suggesting it may be a more efficient loading strategy for the ankle plantar flexors. The results contribute to the understanding of contraction-specific efficiency, and may inform the design of future training and rehabilitation protocols targeting the ankle plantar flexors.