Each month, Dr. Jeffrey Sankoff looks at a recent study or body of research to talk to the researchers, explain the process behind it, and break down the findings.

This month: A paper that looks into the effects of differing intervals rest times in weight training for women—and, more generally, what the research says about the differences in weight training for men and women.

The off-season for triathletes, and especially the time around the new year, is often a period in which the emphasis shifts from high-volume endurance work to building strength that is both swim-bike-run specific and more generally full-body. Coaches and athletes alike have come to understand and appreciate how strength training should be an integral part of triathlon training. Improved durability and injury resistance, along with overall gains in performance, are felt to be directly attributable to strength work.

While in the past the gym has often felt like a boys’ club, more and more women have entered this space and now there is a heightened interest in ensuring that strength training is also designed to account for physiological differences so that women can fully take advantage of this type of training.

“Women have physiological differences with respect to muscle types and distribution. Additionally, women may have more efficient energy systems that allow them to be more fatigue resistant than males,” said Desmond Millender, a doctoral student in the department of health, exercise, and sports sciences at the University of New Mexico. Millender and his co-authors recently published a paper investigating the effects of differing rest intervals on women’s performance in weight training.

Millender was interested in assessing if women’s specific muscle physiology necessitated a different approach to weight training, since the majority of research in this area has been done on men. He told me, “It has been speculated that female lifters might require shorter rest intervals than male lifters because of physiological differences that include muscle fiber distribution, muscle fiber types, size of type 1 muscle fibers, and glycogen utilization during high-intensity exercise. It has also been reported that females produce less blood lactate, experience smaller force decrements, and recover force quicker than males during and after resistance training.”

The study that the New Mexico researchers performed was to compare the effects of one-minute vs. three-minute rest intervals in female weight lifters. They looked at total amount of work performed, ability to perform repetitive sets without fatigue, and measurements of serum lactic acid, a marker of cellular fatigue. The researchers evaluated both upper and lower body exercises in this way. The hypothesis going in was that women would do as well or better with shorter rest intervals, as previous studies have suggested this would be a likely outcome. 

The results turned out differently than what was expected. 

The main findings were that for both upper and lower body exercises, when a rest short interval was used, lifters were significantly limited in the total amount of weight they could lift and had significant fatigue with large decrements in lifting ability between the first and second sets. Interestingly, despite reported fatigue and demonstrated decrements in ability to lift, lactate levels did not change. 

The findings were surprising to Millender. “Several studies suggest women might require less time to recover between sets when compared to men. Contrarily, our study suggests that rest interval length is just as important for women as it is for men.” He also found the findings related to lactate levels interesting, but difficult to explain. “Total volume lifted was decreased in short vs. long rest interval conditions. This could be attributed to metabolic stress and inability to resynthesize metabolites for energy, but this was not supported by findings of elevated lactate levels. Unfortunately our study was not equipped to determine why this occurred.”

The women in the study also had no major differences between upper and lower body exercises, with respect to performance, no matter the rest interval duration. This suggests that muscle groups in the two areas fatigued at a similar rate and benefitted equally from longer rest intervals. “This finding was quite interesting,” Millender said. “It has been suggested that upper body musculature generally has a greater distribution of fast-twitch muscle fibers and consequently have a reduced ability to extract oxygen during upper-body tasks. For this reason, we anticipated superior fatigue resistance during leg press than with chest press, but this was not the case.”

I asked Millender what the takeaway from this should be for women who use weight training as part of their overall program. “This is an interesting question. Currently the American College of Sports Medicine recommends that shorter rest intervals should be used (less than 90 seconds) when low-intensity resistance training is performed. However, our results show that repetition performance significantly declined with three-minute rest intervals, implying that 90 seconds may not suffice for such training.”

Millender believes that it is important that women utilize longer rest intervals when lifting in order to recover appropriately and be able to maximize their ability to lift a higher total volume of weight than might otherwise be possible. “In fact, there is evidence that longer rest intervals lead to superior strength, and a recent study confirmed that decreased total volume lifted explained the inferior effects of shorter rest intervals on long-term adaptations.” he said.

Based on this research, it seems as though the predicted effects of women’s specific muscle physiology, to be able to perform well with shorter rest intervals, was not borne out by the observed results. Consequently, women should consider longer rest intervals between sets when weight training—as the physiological differences in performance across sports continued to be studied.

RELATED: The Gender Gap in Sports Research is Holding Female Athletes Back