Bilateral deficit: one or two legs better
Strength and conditioning has come a long way since its inception of the Boyd Epley days of Nebraska. It has long been a profession that has been influenced by powerlifting, weightlifting, bodybuilding, which need to be respected as their own separate sports. It’s likely why for the longest time that bilateral exercises were considered the king of exercises, and the evolution of the big three for most sports: back squat, deadlift, power clean. In the 90’s a physical therapy influence emerged that suggested adding single leg exercises to the box of tools in athlete development. Balance, proprioception became key words that drove exercise selection. It hasn’t been until recently that heavy single leg training has been proposed to be equal, or even more effective in developing athleticism then its two legged counterparts. Mike Boyle is considered the single leg coach, who champions unilateral loading to enhance athleticism. On reason is the bilateral deficit, or that in certain conditions, the sum of strength or power or a two individual legs (unilateral), are greater than a bilateral counterpart (1). As a field, it’s extremely important to see new ideas emerge, and ensure we’re using the right tools for the right job. Although research is still emerging, evidence for bilateral loading lead the way to develop athletic measures. Conceptually, unilateral training makes sense, and should be implemented. We need to first understand where this came from and what it means for us as coaches.
Vertical squat jumping and the bilateral deficit
The countermovement jump is a standard measure of athletic performance, and whether strength training has a carryover effect to athleticism. Several studies have looked at this and found that the jump height is 58.1% (2), 58.5 % (3), and a difference of 9cm (4) unilaterally compared to half of bilaterally. From this information, a coach could easily be persuaded that single leg only, is the way to go. However, just taking a few results at face value. The proposed mechanism that circulates the strength and conditioning world to explain this phenomenon is that there is a reduced neural drive when using two legs simultaneously (2). However, in countermovement jumping, there’s an alternative theory that suggests that in single leg jumping, muscles are already pre-activated to a higher level, therefore able to reach greater contraction then two legs (3). Another mechanism that may explain this difference is the use of the non-support leg as a swing, using the stretch shortening cycle to increase elastic contributions to the jump (4). More recently, Bobbert (6), found that this bilateral deficit in the countermovement jump is due to the increased depth of the countermovement. This allows the athlete to increase the total mechanical energy, to help displace higher. Understanding this research the bilateral deficit phenomenon in vertical jumping may be misleading, and taken in context with strength measures, and athletic development.
Displaying strength also appears to be impacted by the bilateral deficit. Both knee flexion and extension appear to have a bilateral deficit in adolescent, and adult and older females (7). This is supported by Luiz (8) who found force produced by both limbs (718 N), exceeded bilateral (663 N) with significance (p=0.001). Meanwhile, there are almost as many studies that show no bilateral deficit, however, most them use untrained, or novice trained athletes (2). An issue with these studies (and several like them), is that they’re looking at open chain, single joint exercises, which are not like conventional athlete training that involve squats, splits squats, etc. We need to respect that when looking to science, there is always going to be trade-off between reliability in the lab, and validity in the weight room. The leg press represents the best trade-off between the lab and the field. A deficit occurs here as well, which range from 6.1-20.9%, based on lifting at 1x or 2x relative body weight loads (9). Several reasons have been proposed why this strength deficit occurs, including neural activity, antagonist muscle activation, and stabilizer muscle function (1, 10). However most of the literature is mixed, and varies depending on population, type of exercise, and load.
Can we out-train it? What about bilateral facilitation?
Once an athlete has shown that they have a bilateral deficit, the type of training we as coaches select can potentially increase or decrease this deficit. Botton (11), found that by selecting only unilateral or bilateral training, found an increase in the bilateral deficit (-6.5%), or created a bilateral facilitation (5.9%). Howard and Enoka (12) found that high level weightlifters display a bilateral facilitation, where their unilateral strength is poor compared to their competitive, bilateral lifts. When we evaluate this information, it shows that there certainly is a specificity to the training of our exercises. This in turn, creates the potential for bias in our coaching styles. If we are more unilateral dominant, then our own results champion what we believe. The same can be said for embracing a purely bilateral approach.
Can unilateral exercises, alone, improve performance?
Initially, most research done on unilateral training has been determining the amount of bilateral deficit, or to see how much it can be changed. Less is known whether pure unilateral training is superior to bilateral training. Recently, evidence is emerging that is providing possible answers. Speirs and his colleagues looked at unilateral only versus bilateral strength training in well trained academy rugby players. Sprint times, change of direction, and leg strength all improved in both groups (13), with 10m sprint improvement being slightly greater in the unilateral group. Transfer amongst lifts was also found, which tells us as a coach, that lower body strength improves lower body strength regardless of the mode. The closest supporting evidence available is work by McCurdy (14), who found the unilateral strength training group performed better in single leg vertical jump (p<0.001), and power (p<0.01) vs. a bilateral group. Strength measures were found to be equally improved across both groups, and had a transfer effect to the other condition. Unfortunately, the groups were relatively untrained, which limits its practicality.
How should we use this to our advantage?
Overall much of the research shows us there may or may not be a bilateral deficit with respect to strength exercises, especially at an untrained level. If we understand that an athlete will improve regardless, we should choose based on the athlete’s initial ability. At St. Mary’s, our director, Dan Cleather, proposes that the bilateral squat is a balance drill (15). Thinking of this concept practically, a novice athlete could have trouble balancing, and therefore limit neuromuscular improvements. If we put them in a split stance, it would lengthen their base of support, and reduce some of the balance required. Their centre of mass would stay within their supports, and they could load the exercise quicker to realise strength gains. While the athlete gains strength in unilateral training, the coach could teach the athlete bilateral versions and begin to load it. Additionally, with the recent findings that despite lower loads, similar improvements in athletic measurements are found, means that single leg versions could be a great choice for athletes who can’t perform bilateral versions due to injury or movement concerns. When dealing with athletes, our goal should be to reduce chance of injury in sport. Split squats, and variations, tend to have higher EMG of muscles that support the hip, and knee, compared to bilateral squats (16). I think with where the evidence currently sits a coach should use this information and make the best decision on an athlete by athlete basis. References 1. Nijem, R., & Galpin, A. (2014). Unilateral versus bilateral exercises and the role of the bilateral deficit. Strength and Conditioning Journal, 36(5), 113–118. 2. Challis, J. (1998). An investigation of the influence of bi-lateral deficit on human jumping. Human Movement Science, 17(3), 307–325. 3. Van Soest, A. J., Roebroeck, M. E., Robbert, M. F., Huijing, P. A., & Van Ingen Schenau, G. J. (1985). A comparison of one-legged an two-legged countermovement jumps. Medicine and Science in Sports & Exercise, 17(6), 635–639. 4. Vint, P., & Hinrichs, R. (1996). Differences between one-foot and two-foot vertical jump performances. Journal of Applied Biomechanics, 12(3), 338–358. 5. Bobbert, M. F., & Casius, L. J. (2005). Is the effect of a countermovement jump on jump height due to active state development. Medicine and Science in Sports & Exercise, 28(1402–1412), 1996. 6. Bobbert, M., de Graaf, W., Jonk, J., & Casius, R. (2006). Explanation of the bilateral deficit in human vertical jump squatting. Journal of Applied Physiology, 100, 493–499. 7. Kuruganti, U., & Seaman, K. (2006). The bilateral leg strength deficit is present in old, young and adolescent females during isokinetic knee extension and flexion. European Journal of Applied Physiology, 97, 322–326. 8. Teixeira, A., Narciso, J., Salomao, I., & Dias, M. (2013). Bilateral deficit in maximal isometric knee extension in trained men. Journal of Exercise Physiology, 16(1), 28–34. 9. Hay, D., de Souza, V., & Fukashiro, S. (2006). Human bilateral deficit during a dynamic multi-joint leg press movement. Human Movement Science, 25(2), 181–191. 10. Kuruganti, U., Murphy, T., & Pardy, T. (2011). Bilateral deficit phenomenon and the role of antagonist muscle activity during maximal isometric knee extension in young, athletic men. European Journal of Applied Physiology, 111, 1533–1539. 11. Botton, C., Radaelli, R., Wilhelm, E., Rech, A., & Brown, L. (2015). Neuromuscular adaptations to unilateral vs. bilateral strength training in women. Journal of Strength and Conditioning Research, 30(7), 1924–1932. 12. Howard, J., & Enoka, R. (1985). Maximum bilateral contractions are modified by neurally mediated interlimb effects. Journal of Applied Physiology, 70, 306–316. 13. Speirs, D., Bennett, M., Finn, C., & Tuner, A. (2016). Unilateral vs. bilateral squat training for strength, sprints, and agility in academy rugby players. Journal of Strength and Conditioning Research, 30(2), 386–392. 14. McCurdy, K., Langford, G., Doscher, M., Wiley, L., & Mallard, K. (2005). The effects of short-term unilateral and bilateral lower-body resistance training on measures of strength and power. Journal of Strength and Conditioning Research, 19(1), 9–15. 15. Cleather, D. (2012). Squatting is a balance skill: An alternative technical model. In Professional Strength and Conditioning (pp. 17–21). Lancashire: UKSCA. 16. DeForest, B., Cantrell, G., & Schilling, B. (2014). Muscle activity in single vs double leg squats. International Journal of Sport Science & Coaching, 7(4), 302–310.