DATASET FOR STUDY: Morphological and Functional Adaptations of the Triceps Surae Complex to Eccentric Quasi-Isometric versus Eccentric Resistance Exercise: A Randomised Controlled Trial

PURPOSE: Eccentric quasi-isometric (EQI) contractions consist of maintaining a joint position until isometric failure, followed by resisting the subsequent eccentric (ECC) phase. Despite growing interest in EQI exercise, chronic effects have not been explored. The aim of this study was therefore to determine the chronic morphological and functional adaptations induced by EQI exercise compared to traditional ECC resistance exercise. METHODS: Participants (n = 48) were randomly allocated (using sex-stratified randomisation) into two experimental (EQI and ECC; n = 16 each) and one control group (n = 16). The participants in the experimental groups performed a six-week EQI or ECC exercise protocol with equal load and time under tension within the exercise session. Both interventions were executed using a Smith machine. The condition was assessed before the start and after the end of the exercise protocol. RESULTS: Significant group × time interactions showed that Achilles tendon thickness, gastrocnemius fascicle length, muscle thickness, and lateral gastrocnemius pennation angle increased from baseline to post-intervention in both EQI and ECC groups, with no meaningful change in controls (all interaction p ≤ 0.001; η² = 0.53–0.79) and with no difference between the two protocols (p = 0.227–0.965). Both protocols also improved ankle dorsiflexion range of motion in open and closed kinetic chains and increased concentric, ECC and isometric maximal voluntary contraction torque compared to control group (p ≤ 0.019; η² = 0.33–0.77), with no difference between the protocols (p = 0.561–0.976). Only ECC exercise improved isokinetic dynamometer tested EQI time under tension and EQI torque impulse (p = 0.012–0.020; η² = 0.33–0.37). Tendon stiffness, medial gastrocnemius pennation angle, and isokinetic dynamometer tested EQI angular velocity did not change (p ≥ 0.065). CONCLUSION: The implementation of a six-week EQI or ECC protocol has a comparable effect on improving the morphological characteristics of the ankle plantar flexor muscle-tendon complex, muscle performance, and increasing dorsal flexion of the ankle. Based on these findings, potential for optimising training and exercise-based therapeutic interventions are discussed. Trial registration: ISRCTN (International Standard Randomised Controlled Trial Number): ISRCTN35199955.