It wasn’t until 1961 that scientists at the University of California became the first to describe what came to be known as the “bilateral strength deficit.” It is the fact that your right and left limbs, when used separately, are stronger than when they are used together. Researchers later observed the effect with all sorts of arm and leg movements, and not just grip strength. As a result, you can usually press less than twice as much weight with both legs as you can with either leg individually.
When it comes to training one limb at a time or both at once, bilateral strength deficits have long been invoked. Those who prefer the former claim you get better adaptations overall if you train one limb at a time. The supporters of the latter argue that the body is challenged more with greater loads, leading presumably to greater adaptations.
It’s a bit ambiguous who is right. Earlier this year, an international team led by Jason Moran of the University of Essex published a major meta-analysis in Sports Medicine on single-limb versus dual-limb resistance training for sprint speed. Sprinting is a one-leg sport, so there was no reason not to believe that training one leg could have superior results. Neither method was found to have a significant difference between its effects on sprint performance, but overall, both training methods improved sprint performance. Moran and his colleagues say there might be situations where one approach would be most appropriate: a back pain sufferer might prefer a lower load in single-limb training, for example. There are no clear advantages to either approach, however.
However, endurance athletes may have different needs. Researchers from the German Sport University in Cologne have submitted yet another study testing a slightly different protocol. The subjects alternated legs after every rep, rather than simply doing a set of each leg. The reasoning: the central motor drives must cross from the left hemisphere to the right, just as they would when cycling. This specific pattern of neural activation could better parallel the demands of cycling.
Three groups of cyclists and triathletes were compared in this Journal of Strength and Conditioning Research study. As a control group, no strength training was done; as a second group, two sets of leg press, leg extension, and leg curl were performed twice a week for ten weeks. For each set, they did between four and ten repetitions, raising the weight for no more than a second and lowering it for one second. Each group had a matched total load between them and performed the exercises with both legs at the same time, and alternated lifting one leg at a time.
Strength training works, that much was clear. The maximum leg strength increased in both training groups (by 28 percent when training both legs and by 27 percent when training one leg at a time), compared to just six percent in the control group. The single-leg group witnessed a significant 67 percent increase in endurance in the half-hour time-to-exhaustion ride, while the double-leg participants saw a 43 percent increase. The control group, however, experienced only a 37 percent increase in endurance performance. The training groups improved slightly, but the control group slipped by 6 percent, as they had agreed not to train at all during the study.
It is more difficult to determine whether alternating-leg training is superior to double-leg training. There is no question that the alternating-leg group showed significantly better time-to-exhaustion. In addition, there were several other outcomes that seemed to favor the same group, such as acceleration during a 15-second sprint. In addition to increasing strength equally well, the researchers argue that their alternating protocol “seems better at optimizing the transfer of strengthened cycling sprint capability.” More research is needed to confirm this conclusion, they concede.
As of now, I haven’t decided on a winner. A study I read back in 2015 when learning about bilateral strength deficits suggested the problem doesn’t really stem from how signals are transmitted between the brain and the muscles during single- or dual-limb contractions. It is attributed instead to the way the body is positioned and acted upon when moving. You can generate more torque with one leg compared to both legs when you’re pressing with one at a time. I conclude that a one-legged animal is not necessarily better than a two-legged animal, but it’s different. It will affect how you move, how your muscles stabilize you, and what neural signals you generate.
On that basis, I would say the most important thing is not to replicate the control group in Wahl’s study, which lost sprint speed by omitting strength training completely. The best approach, though, appears to be to select whichever protocol you prefer-or to use both.
Chris Yates provided additional research for this post. Check out my book Endure, which is a mixed-reality look at the mysterious limits of human performance. Join me on Twitter and Facebook, sign up for the email newsletter, and have a look at the Sweat Science blog.