LEAD IN
Exercise choice is one of the most important considerations when designing a strength and conditioning program. Although bilateral (meaning both limbs) exercises such as the back squat and bench press have a significant historical foothold, unilateral (meaning 1 limb at a time) alternatives such as the single-leg extension and 1-arm dumbbell press have recently been labeled more effective by numerous researchers and coaches. This column provides a discussion of the scientific and practical merits of these claims.
POINT
The importance and effectiveness of bilateral exercises (BIL) for the development of strength are well established and accepted among most strength and conditioning professionals. Unilateral exercises (UNI) have not traditionally received the same accolades. Many coaches and researchers now recognize the existence of the bilateral force deficit (BLFD), a phenomenon in which force production per limb, when added together, is greater than the total force generated when both limbs contract simultaneously (1,4–7). This has forced many to rethink their philosophy on the importance of UNI. The following will provide scientific and practical support for this approach.
The existence of the BLFD, particularly in untrained individuals, is unequivocal (1,4–7). This suggests that force production during UNI movements can account for more than 50% of the total force produced during an equivalent BIL exercise. For example, an individual may be able to perform a single-leg press with 60 lbs, yet only able to perform a 2-leg press with 100 lbs. Performing a 2-leg press would thus limit the load per leg to 50 lbs (as opposed to 60 lbs during the single-leg press), which potentially limits strength gains. The BLFD is an important consideration as a recent publication reported that the magnitude of the BLFD during a jumping test was inversely related to sprint start performance, total impulse of force, and velocity on the starting blocks (2). In other words, sprinters with larger BLFD were slower off the starting blocks.
The specificity of UNI and BIL training on unilateral and bilateral performance has been both supported and questioned. BIL and UNI have indeed shown equal improvement in strength and power when performing bilateral and unilateral tasks (3,8,9,11). For example, one study reported that BIL is no more efficacious in improving lower body strength, 40 m speed, and change of direction in academy level rugby players than UNI training (11). However, UNI increases single-leg vertical jumping ability more than BIL (8). This is particularly important considering the frequency in which unilateral jumping is required in sport performance. This demonstration of specificity suggests that UNI training would be more effective at reducing the BLFD, particularly in individuals with more pronounced BLFD.
Regardless of the presence of BLFD, performing an exercise unilaterally (as opposed to bilaterally) changes the neuromuscular demand in several ways. First, it can allow similar utilization of the primary target muscle group (e.g., gluteals, hamstrings, etc.) while altering the use of secondary muscle groups. As an example, a split squat still requires use of the same major leg muscles as a back squat, but may increase the contributions needed from the adductors. Similarly, the nature of UNI likely increases the demand for balance, antirotation, antiflexion/extension, and other forms of stability. Another advantage of UNI is the potential to overload the target muscle group while reducing the total load needed. This can be particularly useful during times of injury or other limiting circumstances. For example, an athlete may use a UNI variation of the squat to achieve optimal loading of the quadriceps while reducing the load of the barbell on the cervical spine (if the spine, shoulder, or neck is injured or irritated). A final advantage of UNI is that they provide immediate and constant feedback to the coach and athlete regarding limb symmetry, a significant factor in the management of major injury (10).
In conclusion, although studies have been performed for strength, limited data exist which directly compare UNI with BIL in terms of developing power, hypertrophy, or general musculoskeletal conditioning. The information available indicates reducing the BLFD probably benefits sport performance and may reduce the likelihood of injury. However, more research is needed before direct relationships between UNI training, reduction of the BLFD, and injury frequency can be firmly established. Nonetheless, although BIL are important, coaches and practitioners are encouraged to include UNI at the appropriate stages of macro-, meso-, and micro-cycles, particularly if a large BLFD exists.
REFERENCES
1. Bobbert MF, de Graaf WW, Jonk JN, Casius LJ. Explanation of the bilateral deficit in human vertical squat jumping. J Appl Physiol (1985) 100: 493–499, 2006.
2. Bracic M, Supej M, Peharec S, Bracic P, Coh M. An investigation of the influence of the bilateral deficit on the countermovement jump performance in elite sprinters. Kinesiology 73–81, 2010.
3. Janzen CL, Chilibeck PD, Davison KS. The effect of unilateral and bilateral strength training on the bilateral deficit and lean tissue mass in post-menopausal women. Eur J Appl Physiol 97: 253–260, 2006.
4. Kuruganti U, Murphy T. Bilateral deficit expressions and myoelectric signal activity during submaximal and maximal isometric knee extensions in young, athletic males. Eur J Appl Physiol 102: 721–726, 2008.
5. Kuruganti U, Murphy T, Pardy T. Bilateral deficit phenomenon and the role of antagonist muscle activity during maximal isometric knee extensions in young, athletic men. Eur J Appl Physiol 111: 1533–1539, 2011.
6. Kuruganti U, Parker P, Rickards J, Tingley M, Sexsmith J. Bilateral isokinetic training reduces the bilateral leg strength deficit for both old and young adults. Eur J Appl Physiol 94: 175–179, 2005.
7. Kuruganti U, Seaman K. The bilateral leg strength deficit is present in old, young and adolescent females during isokinetic knee extension and flexion. Eur J Appl Physiol 97: 322–326, 2006.
8. McCurdy KW, Langford GA, Doscher MW, Wiley LP, Mallard KG. The effects of short-term unilateral and bilateral lower-body resistance training on measures of strength and power. J Strength Cond Res 19: 9–15, 2005.
9. Rube N, Secher NH. Effect of training on central factors in fatigue following two- and one-leg static exercise in man. Acta Physiol Scand 141: 87–95, 1991.
10. Schmitt L, Paterno M, Ford K, Myer G, Hewett T. Strength asymmetry and landing mechanics at return to sport after anterior cruciate. Med Sci Sports Exerc 47: 1426–1434, 2015.
11. Secher NH. Isometric rowing strength of experienced and inexperienced oarsmen. Med Sci Sports 7: 280–283, 1975.
REFERENCES
1. Bobbert MF, de Graaf WW, Jonk JN, Casius LJ. Explanation of the bilateral deficit in human vertical squat jumping. J Appl Physiol (1985) 100: 493–499, 2006.
2. Bracic M, Supej M, Peharec S, Bracic P, Coh M. An investigation of the influence of the bilateral deficit on the countermovement jump performance in elite sprinters. Kinesiology 42: 73–81, 2010.
3. Hakkinen K, Kraemer WJ, Kallinen M, Linnamo V, Pastinen UM, Newton RU. Bilateral and unilateral neuromuscular function and muscle cross-sectional area in middle-aged and elderly men and women. J Gerontol A Biol Sci Med Sci 51: B21–B29, 1996.
4. Hakkinen K, Kraemer WJ, Newton RU. Muscle activation and force production during bilateral and unilateral concentric and isometric contractions of the knee extensors in men and women at different ages. Electromyogr Clin Neurophysiol 37: 131–142, 1997.
5. Howard JD, Enoka RM. Maximum bilateral contractions are modified by neurally mediated interlimb effects. J Appl Physiol (1985) 70: 306–316, 1991.
6. Janzen CL, Chilibeck PD, Davison KS. The effect of unilateral and bilateral strength training on the bilateral deficit and lean tissue mass in post-menopausal women. Eur J Appl Physiol 97: 253–260, 2006.
7. Kuruganti U, Murphy T. Bilateral deficit expressions and myoelectric signal activity during submaximal and maximal isometric knee extensions in young, athletic males. Eur J Appl Physiol 102: 721–726, 2008.
8. Kuruganti U, Murphy T, Pardy T. Bilateral deficit phenomenon and the role of antagonist muscle activity during maximal isometric knee extensions in young, athletic men. Eur J Appl Physiol 111: 1533–1539, 2011.
9. Kuruganti U, Parker P, Rickards J, Tingley M, Sexsmith J. Bilateral isokinetic training reduces the bilateral leg strength deficit for both old and young adults. Eur J Appl Physiol 94: 175–179, 2005.
10. Kuruganti U, Seaman K. The bilateral leg strength deficit is present in old, young and adolescent females during isokinetic knee extension and flexion. Eur J Appl Physiol 97: 322–326, 2006.
11. Makaruk H, Winchester JB, Sadowski J, Czaplicki A, Sacewicz T. Effects of unilateral and bilateral plyometric training on power and jumping ability in women. J Strength Cond Res 25: 3311–3318, 2011.
12. McCurdy KW, Langford GA, Doscher MW, Wiley LP, Mallard KG. The effects of short-term unilateral and bilateral lower-body resistance training on measures of strength and power. J Strength Cond Res 19: 9–15, 2005.
13. Rube N, Secher NH. Effect of training on central factors in fatigue following two- and one-leg static exercise in man. Acta Physiol Scand 141: 87–95, 1991.
14. Secher NH. Isometric rowing strength of experienced and inexperienced oarsmen. Med Sci Sports 7: 280–283, 1975.
15. Speirs DE, Bennett M, Finn CV, Turner AP. Unilateral vs bilateral squat training for strength, sprints and agility in academy Rugby players. J Strength Cond Res 11 July 2015. [Epub Ahead of Print].
16. Taniguchi Y. Lateral specificity in resistance training: The effect of bilateral and unilateral training. Eur J Appl Physiol Occup Physiol 75: 144–150, 1997.
17. Taniguchi Y. Relationship between the modifications of bilateral deficit in upper and lower limbs by resistance training in humans. Eur J Appl Physiol Occup Physiol 78: 226–230, 1998.
COUNTERPOINT
Practitioners often reference the BLFD as rationale for prioritizing UNI in strength and conditioning programs. In fact, some have gone as far as cutting out many (if not all) BIL in favor of their UNI variations (e.g., rear-foot elevated split squat in place of a back squat). Though this approach may appear logical, the current evidence base does not support it. The subsequent paragraphs will question the relevance of the BLFD and the assertion that UNI are superior to BIL for strength development.
The BLFD has indeed been documented in several populations (1,7–10), though it does not always exist (3–5). In fact, other studies have actually reported a bilateral facilitation, where the force produced bilaterally is greater than the unilateral forces combined (3,5,14). Thus, the general ambiguity of the scientific literature makes drawing strict conclusions difficult. The BLFD appears to be more prominent in untrained or less trained populations, suggesting the phenomenon is influenced by training and sport history. For example, national caliber rowers have a less pronounced BLFD than their club-level counterparts, while international rowers have no BLFD whatsoever (14). Likewise, Howard and Enoka (5) found a BLFD in sedentary individuals, but not in weightlifters with BIL experience and competitive cyclists. In fact, the weightlifters and international rowers of the aforementioned studies demonstrated a bilateral facilitation (5,14). Thus, these data suggest the BLFD, while existent, is only a concern for untrained individuals.
The BLFD also appears highly responsive to different training modes such that UNI tends to increase it, while BIL decreases it (6,16,17). Though the data are admittedly limited, the available research suggests decreasing the BLFD via BIL may be beneficial for physical performance. However, caution must be taken when making exercise selection decisions based on this; given the relationship between the BLFD and physical performance is limited to a single correlational study (2). Exercise selection should therefore be based on how they independently influence actual performance, regardless of how what happens to the BLFD. The concept of specificity seemingly suggests UNI are better suited for activities which require unilateral performance (e.g., running, kicking a ball, throwing, etc.) while BIL are superior for activities which require bilateral performance (e.g., 2-foot jumping, weightlifting, deadlifting, etc.). Interestingly, the available data do not completely support the conclusion that such rigid specificity is required (6,11–13,15).
Early work on the topic found BIL improved performance during a bilateral muscular endurance task, but not unilateral endurance. Likewise, UNI improved unilateral endurance, but not bilateral (13). However, BIL and UNI equally improve bilateral and unilateral strength and power (6,12,13). A recent study of rugby players showed equal improvements in measures of unilateral and bilateral leg strength, sprint speed, and agility with UNI compared to BIL (15). Another particularly well designed study showed it may take longer for the divergent adaptations of BIL and UNI to differentiate themselves (11). The study found UNI improved performance more rapidly than BIL. However, the UNI group plateaued after 6 weeks and regressed during a 4-week detraining period. In contrast, although BIL improved performance more slowly than UNI, BIL did not suffer a plateau during the 6 training weeks or a regression (during detraining). These data tentatively indicate UNI provides more rapid, yet shorter lasting effect, while BIL offers a slower, more sustainable improvement. This is particularly important when considering times of the year that strength and conditioning training is limited (e.g., during season).
Given these differences, both UNI and BIL appear valuable and warrant inclusion in a periodized program, irrespective of the BLFD. Due to the current lack of understanding of the relationship between the BLFD and physical performance (i.e., sprint speed, sport performance, etc.), exercise selection should be made based on the needs analysis of the athlete and sport and the context of the training period (e.g., season, previous volume, fatigue, injury history, etc.).
