Exploring acute physiological adaptations to circuit strength training: the role of work-to-rest interval variations

We investigated the acute physiological responses to circuit strength training performed with different work-to-rest interval durations (10:10, 20:20, and 30:30 s) under a constant 1:1 ratio. Thirty-four trained adults (15 males and 19 females) completed a 14-min circuit protocol consisting of two blocks of 6-min separated by a 2-min rest period. Each session included six alternating upper- and lower-body exercises performed at maximal intended velocity. Heart rate (HR) was continuously monitored during the protocol, while blood lactate concentration, countermovement jump (CMJ) height, and 10-m sprint performance were assessed pre-, mid-, and post-circuit session. Repeated-measures ANOVA was used for CMJ height, blood lactate concentration and 10-m sprint performance, and one-way ANOVA for HR data. The 30:30 protocol elicited the highest post-exercise lactate concentrations (p < 0.05) and the largest decrements in CMJ height. In contrast, HR was consistently higher in the 10:10 protocol across several time points (p < 0.01), indicating greater cardiovascular demand. Sprint performance declined from pre- to post-exercise across all protocols, indicating a time-dependent fatigue response. These findings demonstrate that manipulating absolute work-to-rest durations, even under a fixed 1:1 ratio, produces distinct physiological responses, while performance decrements may occur independently of protocol configuration. Longer intervals (30:30) maximize metabolic stress and neuromuscular fatigue, whereas shorter intervals (10:10) enhance cardiovascular load. The 20:20 protocol represents a balanced alternative between both stimuli.