Intense training leads to skeletal-muscle microtrauma. This microtrauma initiates remodeling of the muscle cells, making them larger and stronger than before. Factors most closely associated with microtrauma are weight loads, volume, and the condition of the muscle at the time training occurs. Besides load, volume, and conditioning, rest between sets is also a fundamental variable of training, although not often associated with muscle damage. Brazilian researchers working in collaboration with Eastern Illinois University wanted to see if altering the rest between sets would have an impact on muscle damage. In the process, they uncovered something that might explain why some consider themselves “hard-gainers.”

Researchers measured creatine kinase (CK) levels following bouts of resistance exercise, utilizing long (3 min.) or short (1 min.) rest intervals between sets. CK is an indicator of muscle-fiber damage: Intense training results in “leaky” muscle cells, and CK seeps out of damaged cells, allowing a quantifiable measure of it in the bloodstream to estimate the extent of muscle damage after training. Fifty male subjects did two workouts separated by seven days and had CK levels measured after each session. The workout consisted of four sets of curls (seated curl machine) using 85% of their one-rep max (1RM).

The study results indicated that rest between sets does indeed have an effect 
on muscle damage. Resting one minute between sets induces more muscle damage than three minutes’ rest, assuming all other things remain equal (i.e., volume and weight loads). But here’s the interesting part. Some individuals consistently experienced higher CK levels and more muscle soreness following four sets of curls regardless of rest period length. The researchers categorized them as high, medium, and low responders. The high and medium responders were most affected by rest intervals, but the low responders had such low CK levels that it didn’t matter how long the rest period was. Keep in mind all subjects used 85% of their 1RM for four sets. So it wasn’t a matter of different weight loads or volume. It was simply a matter of some individuals responding more robustly to resistance exercise.

The take-home message is to try shortening your rest periods. It’s OK if you have to lighten things up for a bit. You can always ramp up the loads over time. Try it and see if you can’t get some “good” soreness going for a few days to kick-start some new gains.