Strength training: are two limbs better than one?

The research

The researchers performed an exhaustive search of the literature, looking for any studies comparing the impact of unilateral and bilateral training on sports performance outcomes. After initially starting with over 1500 studies, they narrowed it down to 28 studies that met the inclusion criteria. They then performed a ‘meta-analysis’ of all the data. This is a statistical method that allows researchers to ‘combine’ different studies, as well as determine the impact of differences of study designs on the results.

The researchers wanted to know whether unilateral or bilateral training was superior for improving jumping ability, maximal strength, sprinting ability, change of direction, as well as balance. To go even deeper, the researchers compared the effect of the two approaches on both unilateral and bilateral maximal strength, and unilateral and bilateral jumping ability. Since unilateral and bilateral jumping and strength are different types of movement, they wanted to see if the specific test influenced the results. As sprinting, change of direction, and balance abilities are necessarily unilateral in nature, no such comparison was performed in their case.

What they found

The results were fairly clear cut; when it comes to improving sprinting, change of direction, and balance, unilateral training proved to be superior to bilateral training all cases. So if these attributes are important for performance in your sport, focusing on unilateral training is going to be a wise choice. However, some of the results were more nuanced. When it comes to unilateral jumping ability and unilateral maximum force production, unilateral training proved to be significantly more effective. But when these same outcomes were measured during bilateral activities, there was no advantage for unilateral training as the improvements were not significantly different. So, if your preferred sport involves exerting a lot of force or jumping activities leading with one leg, it appears that unilateral training is the better bet. However, if you’re interested in bilateral performance outcomes – eg jumping using both legs - you have some options in terms of what to do.

Practical applications for athletes

One of the clear take-home massages is that specificity remains an important aspect of performance training. When athletes need to perform on one leg, at least some of their strength training should be performed on one leg (eg lunges, single leg press etc). So, if that’s you, consider doing a significant portion of your strength training unilaterally. But if you perform mostly bilaterally (eg squats), you’re likely to get good results, especially if you combine that strength work with your sport training. Another key takeaway is that bilateral training still works, just perhaps not as well as unilateral training, and as such, it’s still a viable option. Conversely, if you love unilateral training go for it as it will be effective regardless of what your sport demands.

There is one caveat to these findings however. The subjects in all of these studies were not necessarily performing much or any sport activity. As a result, they were likely performing a lot less ‘unilateral training’ on a day-to-day basis. It’s reasonable to assume that if they were, some of the differences between unilateral and bilateral training would have been smaller because the subjects would have had more exposure to unilateral sports activities, thereby being able to perform better in the single leg test.

Therefore, if you are performing a lot of sport activity, and you really enjoy bilateral training, you’re likely okay to do bilaterally focused training.  However, you can hedge your bets by doing including one or two sets of unilateral training in your bilateral training program. Another recommendation is that the less sport training you do, the more unilateral your strength training should be (remember, performing sport training helps the transfer of bilateral strength into unilateral activity).

In summary, the knowledge from the above study can help us make more educated decisions about how to implement strength training. Unilateral training clearly offers distinct benefits, and its inclusion makes sense. Bilateral training on the other hand is still effective, especially if it is topped off with unilateral sports activities. Therefore, including it as well won’t hurt your progress, and may even help it.

Reference

1. Front Physiol. 2023 Apr 13;14:1128250.  doi: 10.3389/fphys.2023.1128250. eCollection 2023.