When tasked with designing a resistance training program for the first time, one of the biggest challenges that new health and fitness professionals encounter is understanding all of the different variables that can be integrated into a resistance training program. This task can oftentimes feel overwhelming, and for good reason, because it has been estimated that there are 10 to the 67th power (that’s a 1 followed by 67 zeroes!) of different workout combinations that can be manipulated when considering all of the of acute program design variables (1)! Whereas some of the more obvious elements that need to be addressed include variables such as frequency, intensity, rest periods, and volume, the decision of what exercises to choose can lead to a plethora of choices. Single-joint or multiple-joint exercises? Free weights or machines? Open-chain or closed-chain exercises? Without question, all of the aforementioned exercise choices have a time and place and can be easily justified or omitted, dependent on the specific needs analysis of a client.
One additional area of exercise selection that has received increased scrutiny as of late involves the choice of bilateral or unilateral exercises. For many years, bilateral, multiple-joint barbell exercises such as the back squat, bench press, and deadlift have been staples of resistance training programs, and for good reason, because exercises such as these are well established in their efficacy for improving muscle strength, size, and power (2). However, in many strength and conditioning circles, favoring unilateral exercises over bilateral exercise has become more prevalent (3–5), with the rationale that bilateral exercises contribute to a phenomenon known as the bilateral deficit. Thus, unilateral exercises are more functional and better adhere to the principle of specificity than bilateral exercise choices. Is the exclusion of bilateral exercises in favor of unilateral exercises a valid concern that is justified and well supported by research? The purpose of this article is to examine the pros and cons of bilateral and unilateral exercise selection, with the end goal of helping health and fitness professionals design programs based on evidence-based information that can best benefit their respective clientele.
BILATERAL AND UNILATERAL MOVEMENTS
Before we proceed, a few operational definitions are in order. A bilateral exercise movement is when both limbs are used in unison to contract the muscles, which creates force, and subsequently moves a given load (6). A unilateral exercise movement is when each limb works independently of the other to create the desired movement (6).
As with any attempt to classify exercise-based movement patterns, there will always be exercises that do not necessarily fit neatly into a classification scheme. For example, consider upper body exercises that use dumbbells. Whereas few would dispute that pressing and pulling dumbbell movements performed with one arm or in an alternating manner would be classified as unilateral exercises, what if the right and left limbs move simultaneously? For example, if an individual performs a dumbbell shoulder press and the right and left limbs concurrently press the dumbbells overhead, both limbs are clearly contracting in unison, but because of the dumbbell, each hand is independent of the other. Is this a bilateral or unilateral exercise? Furthermore, lower body exercises that are performed in a split-stance position, such as lunges and step-ups, also can be a challenge to classify. For example, when performing a lunge, although the lead leg has been shown to bear ~75% to 85% of the overall load (7), the trail leg is still needed to successfully execute the movement. Would this exercise be best classified as bilateral or unilateral? Whether the aforementioned exercises are classified as bilateral or unilateral is largely a matter of semantics because logical and defensible arguments clearly can be made on either side. A more centrist approach may be to simply encourage the health and fitness professional to adopt a consistent method of dumbbell and lower body exercise classification, acknowledging that shades of grey certainly do exist, and the classification of exercises is much more art than science. Table 1 provides examples of bilateral and unilateral exercises for the lower and upper body via a sample push-pull training split using multiple-joint movements.
THE BILATERAL DEFICIT
Muscular strength has been defined as the maximal amount of force that can be generated during a specific movement pattern at a specified velocity of contraction (8). However, when comparing the force production between bilateral and unilateral movements, a curious and much less understood phenomenon often is observed in which the force produced when the left and right limbs simultaneously contract is less than the sum of the forces produced from the left and right limbs separately (9). For example, let’s say an individual was performing a one-repetition maximum (1RM) effort leg press, resulting in the following maxes: 1) both legs at the same time, 1 RM = 500 lb; 2) just the right leg, 1RM = 280 lb; 3) just the left leg, 1RM = 270 lb. In this hypothetical scenario, the sum of the right and left legs 1RM (280 + 270 = 550) is greater than the 1RM of both legs (500 lb) working in unison. In other words, the total amount of force generated from a single bilateral contraction often is less than the total force generated by two separate unilateral contractions. Accordingly, this neuromuscular anomaly has been termed the bilateral deficit (BLD) (9). (See Figure for how to calculate and interpret).
The BLD is hypothesized to be caused by neural inhibition when two homologous contralateral limbs (e.g., left leg, right leg) are attempting to simultaneously contract (10). Interestingly, the BLD is not observed when nonhomologous muscle groups (e.g., left arm, right leg) simultaneously contract (11), suggesting that neural inhibition is limited to homologous bilateral contractions (9). In addition, a wide range of factors such as training, age, motor disorders, fatigue, fiber type and right-left dominance have each been theorized to contribute a role to the BLD (12). If the phenomenon of the BLD is largely agreed to be a result of an alteration or limitation of the neuromuscular system (9,11), what then are the potential impacts on exercise selection for health and fitness professionals?
Bilateral deficit – the reduction in force that accompanies maximal two-limb efforts relative to maximal single-limb performances (13).
As previously mentioned, the existence of the BLD often is the rationale provided by many health and fitness professionals as to why they may feel unilateral exercise movements are superior to bilateral exercises (3–5). However, previous research that has examined the difference in force production between bilateral and unilateral movements paints a much more nuanced picture, suggesting that the phenomenon of the BLD is actually highly variable among individuals (9). Whereas the BLD most often is observed in acute studies that have used untrained subjects performing novel movements, longer term bilateral training tends to mute the inhibitory effects of the BLD (3,9,11,13,14). Furthermore, in trained athletes, such as rowers and weightlifters who routinely perform bilateral movements, maximal bilateral force production can actually be greater than the sum of unilateral forces, a phenomenon termed bilateral facilitation (9,13), suggesting task familiarization and specificity of exercise can play a large role in reducing the effects of the BLD. Therefore, understanding the individual differences concerning bilateral strength production is perhaps best viewed along a continuum, ranging from deficit to facilitation, with some individuals showing no effect, all outcomes attributable to the wide variability in human subjects (13).
Given the ambiguity and inconsistency surrounding the BLD, what conclusions, if any, can the health and fitness professional derive from the research? Based on the results from review articles on the BLD (3,9,11), here are a few of the major takeaways and practical implications:
- Lower body movements generally exhibit a greater BLD than upper body movement. The increased postural stability requirements needed to produce force in the lower body may contribute to a larger BLD; thus, addressing core activation in training may be of value (15).
- The magnitude of the BLD tends to increase with the velocity of contraction during explosive or ballistic movements. Thus, for individuals training for activities that require one-legged jumping, addressing high-velocity, low-load (<30% 1RM) power training may be warranted (16).
- Bilateral training tends to reduce the BLD, whereas unilateral training tends to increase the BLD. Thus, bilateral training will affect performance on a bilateral task, and unilateral training will affect performance on a unilateral task (3). Therefore, the principle of specificity dictates that the optimal program design needs to reflect the specific adaptations that are desired (2,17).
- Lack of familiarity with a task often results in a BLD, suggesting the BLD may be larger in untrained versus trained individuals. It is well understood that the rapid improvements in strength during the early stages of training in untrained/novice individuals are predominantly due to neural adaptations, thus training interventions in as little as 4 weeks may be effective in reducing the BLD (2).
- Minimal studies to date have investigated the relationship between the BLD and athletic performance or injury, thus any definitive answers are simply unknown at this time.
For individuals not well versed in interpreting research articles, the aforementioned summaries may lead to feelings of confusion or dissatisfaction at the lack of definitive conclusions. The tendency to reach a conclusion without fully weighing all possible options equally, only accepting and interpreting information that confirms preconceptions, is a type of cognitive bias known as confirmation bias (18). Preconceived viewpoints can cloud our ability to look at a situation objectively, especially when we simply rely on anecdotal or past experiences (19). Does the BLD exist? Absolutely. However, individual differences and task specificity create enormous variability among the literature that has investigated the BLD; therefore, any claims as to the superiority of either unilateral or bilateral training methods are simply not supported at this time by research. There may certainly be situations where bilateral exercises may be a better option to unilateral movements and vice versa, but using the BLD as the scientific rationale that ultimately impacts exercise selection does not seem to be warranted at this time (3,9,11).
TRY THIS AT HOME
In a classic study from 1961, Henry and Smith (20) found that bilateral maximal handgrip strength was significantly less than the sum of the maximal right and left handgrip strength combined. For health and fitness professionals with access to a handgrip dynamometer, here’s how you can easily replicate this study! First, to assess your bilateral grip strength, grip the dynamometer with both hands, maximally contract, and record your measurement. After a couple minutes’ rest, grip the dynamometer with your dominant hand, maximally contract, and record your measurement. After a few minutes’ rest, repeat the same process with your nondominant hand. Using the formula shown in the Figure, calculate your bilateral index. Any evidence of a bilateral deficit or facilitation? Are there any potential limitations or confounding factors you might wish to consider before reaching any conclusions? To assess lower body strength, repeat the aforementioned steps on a leg extension or leg press machine.
THE CROSS-EDUCATION EFFECT
Another interesting neurophysiological phenomenon that may be of interest to the health and fitness professional is when strength increases are observed in the contralateral (opposite side) limb after performing unilateral exercises with the ipsilateral (same side) limb, a phenomenon most commonly referred to as the cross-education effect (21). For example, performing bicep curls with the left arm can actually increase the strength of the right arm! The cross-education effect has been observed via voluntary muscle contractions, electrical stimulation of muscle, and even via mental practice of muscle contractions (22). Furthermore, the cross-education effect can occur in both upper and lower body muscles, in both dynamic and isometric contractions, and in all ages and sexes (22). A recent meta-analysis quantified the typical contralateral limb strength gains to be approximately 11.9% after any form of unilateral training, with even greater improvements with specific training interventions such as dynamic-isotonic training (15.9%) and eccentric training (17.7%) (23). Much like the BLD, the exact mechanisms of the cross-education effect are not well understood, but are hypothesized to be as a result of neural adaptations, complex changes in the contralateral motor pathways, or motor learning (22).
For healthy individuals, cross-education may seem like a silly or trivial side effect because why would anyone only train one side of the body? However, the true value and application of the cross-education effect may be in the world of rehabilitation in situations where one limb may be immobilized because of an injury (24). For example, a football player who breaks his right hand, which is subsequently placed in a cast, can implement unilateral dumbbell exercises with his left hand to stem the typical strength loss and muscle atrophy that accompanies the disuse due to immobilization (24). Previous research has found promising applications of the cross-education effect in individuals with distal fractures or anterior cruciate ligament reconstruction (23). However, it is important to note that the majority of cross-education research has used individuals with a healthy immobilized limb; therefore, additional randomized control trials are needed before establishing evidence-based recommendations for clinical practice (23).
Another promising implementation of the cross-education effect may be within a clinical setting for individuals who have suffered a stroke. Approximately 80% to 85% of individuals recovering from a stroke will have a one-sided muscle weakness, known as hemiparesis, resulting in reduced functional ability (25). Consequently, occupational and physical therapists have explored cross-education interventions via resistance training to restore strength and function from asymmetrical deficits in stroke patients (24). Early investigations of the cross-education effect in poststroke rehabilitation have shown initial promise; however, further trials are needed before definitive conclusions can be made (25). Whereas the nonclinical health and fitness professional may not typically work with the aforementioned populations, the general fitness population is no stranger to aches and pains, so when limitations do inevitably arise with clientele, applications of the cross-education effect may be implemented to maintain training capacity.
Cross-education effect – when training one side of the body increases the strength on the other side of the body (26).
Now that we have established some of the neuromuscular physiology that can affect bilateral and unilateral movements, let us now turn our focus to applying this knowledge toward designing resistance training programs. According to the 2009 ACSM position stand on resistance training, there is a rich body of evidence that supports the inclusion of bilateral and unilateral (both single- and multiple-joint) exercises when designing resistance training programs for novice, intermediate, and advanced individuals (2). Therefore, the logical next step for the health and fitness professional is to conduct a needs analysis that is individualized to his or her respective client’s goals and needs, while employing ACSM’s evidence-based FITT-VP (F = frequency, I = intensity, T = time, T = type, V = volume, P = progression) principles of exercise prescription (27). Accordingly, here are some considerations to think about when deciding when to prescribe bilateral or unilateral exercises:
- For an individual with limited time, choosing bilateral movements may be a better choice because of the reduced time commitment — e.g., 3 sets of 10 repetitions of goblet squat = 30 total repetitions versus 3 sets of 10 repetitions of split squats = 60 total repetitions (30 reps right leg, 30 reps left leg).
- Unilateral and bilateral exercises are equally effective at improving upper and lower body measures of strength and power; therefore, the inclusion of both are warranted throughout the course of a training cycle (3).
- For individuals seeking increased activation of the core musculature, unilateral movements can create a greater core challenge, especially in the frontal and transverse planes because alternating or off-set loads require the core musculature to counter the destabilizing forces acting on it (28) — e.g., increased demands on the core from an alternating dumbbell bench press or a one-arm dumbbell bench press.
- Similarly, for individuals seeking increased activation of the muscles that stabilize the pelvis and knee, unilateral exercises such as split squats, lunges, step-ups, and single-leg deadlifts all require greater frontal plane control and subsequent neuromuscular activity than bilateral exercises (7). Activation of the muscles of the pelvis and knee can be increased even further by using asymmetrical or offset loads in ipsilateral or contralateral progressions — e.g., single-leg deadlift on right leg with a dumbbell in the right hand (ipsilateral) or single-leg deadlift on the right leg with a dumbbell in the left hand (contralateral).
- However, for individuals with a history of low back pain, loaded unilateral exercises, especially those that challenge the frontal and transverse planes, may trigger discomfort because exercises that increase muscle activation also dramatically increase the compressive loads on the spine (29). Therefore, health and fitness professionals working with individuals with a history of low back pain should progress very conservatively from sagittal to frontal to transverse plane challenges to stay within their client’s respective tolerance and capacity to training.
- For individuals planning to train each muscle group or movement pattern at least twice a week, one day could be dedicated to just bilateral movements, one day could be dedicated to just unilateral movements, or both bilateral and unilateral movements could be spread out across the training split (see Table 2 for examples).
BRIDGING THE GAP
The health and fitness professional has a myriad of options to consider when tailoring a resistance training program for a client. Through a basic understanding of the neuromuscular physiology that can affect both bilateral and unilateral exercises, the health and fitness professional is encouraged to use an evidence-based approach to program design and exercise prescription. Ultimately, the decision of what type of resistance training exercise movements to implement — bilateral, unilateral, or a combination of both — will be dependent on 1) the initial needs analysis and assessments performed, 2) the goals of the client, and 3) which exercises will lead to the greatest exercise adherence and self-efficacy as per ACSM recommendations (30).
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