Mixed
Adding 30-second no-load isometric holds (iso-holds) during the inter-set rest period of resistance training enhances mid-thigh (quadriceps) muscle hypertrophy compared to passive rest, but may impair lower-body maximal strength gains.
If you want to maximize quadriceps growth without spending extra time in the gym, try holding a strong isometric contraction (like a seated knee extension) for 30 seconds immediately after each set of squats or leg presses. Do this for the first 30 seconds of your 2-minute rest, then relax for the remaining 90 seconds. Note: This might slightly reduce your ability to increase the weight on the bar for lower body strength, so prioritize this strategy if hypertrophy is your main goal.
Results indicated a favorable effect of ISO on MT in the mid-thigh. Alternatively, there was a possible detrimental effect for ISO on leg press strength. No other notable differences were seen between conditions.
Why this rating
Randomized controlled trial with resistance-trained subjects, though the strength finding was driven by a single influential outlier.
Source
To Flex or Rest: Does Adding No-Load Isometric Actions to the Inter-Set Rest Period in Resistance Training Enhance Muscular Adaptations? A Randomized-Controlled Trial
Brad J. Schöenfeld et al. · Frontiers in Physiology · 2020
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