Comparing six weeks of full-body eccentric quasi-isometric and dynamic resistance training on muscular size, physical performance, and serum hormones in untrained men

Eccentric quasi-isometrics (EQI) have gained popularity as a potential means of promoting muscular adaptations when traditional resistance training is contraindicated. However, evidence regarding their chronic effects remains limited. Thirty untrained men (18–26 years) were randomized to full-body EQI (n = 10), dynamic resistance training (DYN; n = 10), or a non-training control group (CON; n = 10). Muscle size, strength, power, endurance, and serum hormone concentrations were assessed before and after the six-week (three sessions/week) intervention. Within-group changes were assessed using paired-samples Wilcoxon signed-rank tests, while between-group differences were analyzed using ANCOVA with baseline value covariates. Changes in all dependent variables were similar between EQI ( d = 0.23–3.03, Wilcoxon p ≤ 0.069) and DYN ( d = 0.22–4.33, Wilcoxon p ≤ 0.105), with both training groups improving more than CON ( d = -0.79–0.14, Wilcoxon p ≥ 0.103). However, no significant between-training-group differences were detected ( Bonferroni p = 0.400–1.00), though EQI tended to result in small-to-moderately greater beneficial changes in upper-body strength and endurance, and growth hormone and follistatin ( d = 0.35–0.70). In contrast, muscle size, lower-body strength and endurance, jump power, and cortisol concentrations improved by a greater magnitude following DYN ( d = 0.24–0.70). Both training groups generally improved more than CON. These findings suggest that six weeks of EQI training elicits beneficial morphological, neuromuscular, and endocrine adaptations comparable to traditional dynamic resistance training in untrained young men.