Should I do a single- or a double-limb exercise?

What's the benefit of completing single-arm or single-leg movements over doing double limb movements? Well, it depends on your current goals. Here are some thoughts on when choosing one- or two-limb exercises matters for reaching common exercise goals, including improved muscle growth, muscle strength, and pain or injury recovery.

Your goal is muscle growth

Both single-limb and double-limb exercises are equally effective at increasing muscle mass, as long as sufficient weekly volume (sets x reps x weight) is maintained or increased over months to years. However, most effective muscle group programs will also plan to include both single limb and double limb movements to increase exercise variety (Single vs. Multi-Joint Resistance Exercises: Effects on Muscle Strength and Hypertrophy). 

Why is exercise variety helpful for muscle growth? Because of the “regional hypertrophy” effect of different exercises. All muscles are composed of muscle fibers of varying lengths within the muscle group. Therefore, no single exercise can place large amounts of tension on all the muscle fibres of a single muscle group (The role of exercise selection in regional Muscle Hypertrophy). More than one exercise is often needed to do that. So, each exercise you choose will have regional hypertrophic effects. Placing progression tension on all muscle fibres of a muscle group is one of the most essential principles for stimulating long-term muscle growth (Resistance Training Variables for Optimization of Muscle Hypertrophy). 

If muscle growth is your primary goal, consider varying your program throughout the year to stimulate all regions of a single muscle group during your lifting sessions. Apart from adding variety, single limb exercises:  

1. Use fewer muscles, so they tend to induce less fatigue per set of the same intensity as double limb movements. So, more sets with less fatigue will help you increase your monthly volume with less fatigue, which is a win/win

2. might be necessary in a context where you do not have the weight to challenge a double limb movement sufficiently. For example, when traveling, or limited in transport options to get to a gym, to maintain your strength gains, you might only be able to complete bodyweight or resistance band exercises. If squatting is too easy with bodyweight, a lunge or single-leg squat may be a more beneficial option. If push-ups are too easy, consider a single-arm push-up on the floor or against a bench, chair, or bed edge for a more beneficial exercise. However, single-limb movements take more time than double-limb movements because they require training both limbs. There are trade-offs to any training decision. 

Your goal is maximal strength

Strength is always task-specific—practice strength-based training with exercises that target the areas you want to strengthen (The Misunderstanding Of Muscle Strength - Get Back To Sport). You will always be stronger in absolute amounts, completing a double-limb version of an exercise versus a single-limb version. However, you might find that you will be stronger with single limb movements in relative amounts to double limb movements due to several neurological factors (Muscle mass and motor unit recruitment | Patreon + Bilateral deficit in maximal force production). The most obvious example is that although you will be able to squat more than you can lunge, your lunge could become more than half as strong as your squat, rather than less than half of your squat strength, as some would expect. So, if your squat is 60 kgs, you will find that you can lunge more than 30 kgs. So picking single limb exercise for some movements might also be helpful for strength development. Picking double limb strength exercises, while also helping with increasing overall strength development, will reduce the relative difference of force you can produce between one or combined limbs, if you believe symmetry matters for you as discussed next.

This graph displays how you can produce more force when you combine your total strength when testing each limb at a time compared to when you test your strength only on a double limb task (Bilateral deficit in maximal force production)

You might also prioritise single-limb movement if you have a strength symmetry goal, particularly if one limb is significantly weaker than the other. We would not encourage avoiding training the stronger side, as it's better to have one flat tyre than two. Still, this is an example where you could prioritize a single limb strength exercise and complete the weaker side first to become more symmetrical (What influence does resistance exercise order have on muscular strength gains and muscle hypertrophy?). However, strength symmetry is not as important as getting stronger on both sides. Asymmetry seems to be normal across most populations and not associated with increased injury risk in many settings, which is different from what is commonly believed  (The Relationship Between Asymmetry and Athletic Performance: A Critical Review + Why Sports Should Embrace Bilateral Asymmetry). In the long term, focus on getting both limbs strong rather than only symmetrical. 

You are strength training with pain in only one limb or joint. 

Double-limb exercises can be helpful in distracting you from single-sided joint pain. For example, double-arm lateral raises or shoulder presses might be more tolerable than single-arm lateral raises or shoulder presses for a sore shoulder. As already said, don’t be too concerned about short term or small asymmetries between limbs. Your confidence in your body should increase over time anyways. Using the other limb at the same time can be kind of “distracting” in the short term to help you continue your strength training habits while the effects of progressive movement, patience and optimism do their thing on changing the complex “neuro-immune” system adaptations contributing to persistent pain (Physical Activity as a Central Pillar of Lifestyle Modification in the Management of Chronic Musculoskeletal Pain + Influence of routine exercise on the peripheral immune system to prevent and alleviate pain). In some cases, persistent pain can lead to significant reductions in strength or range of motion in a single limb. In this context, adding single-limb exercises will be helpful in reducing the effects of limb disuse due to persistent pain, much like you would approach a strength asymmetry, side to side. If one joint is in a cast, brace, or has reduced tolerance to movement, intensely training the other limb can cause a “cross education effect,” reducing how much muscle and strength is lost while you complete your bracing recommendation and return to increased joint movement again (Planned vs Reactive Deloads/Exercise Breaks + Exercise prescription and strategies to promote the cross-education of strength). In some cases, the usual dosage of weekly single leg exercise might be non-tolerable, such as for some types of leg, hip and lower back pain. In these cases it is sensible to temporarily (less than a month) reduce single leg loading, but make a plan to gradually progress single leg loading as these areas become less sensitive (Changing the Narrative in Diagnosis and Management of Pain in the Sacroiliac Joint Area + The SI Joint + The Grumpy Achilles Tendon + Glute/Hip Tendon Pain). Any advice to avoid single leg loading for these conditions is often outdated and leads to persistent concerns (Pain “Mediators"). Double leg movements can be insufficient to regain a normal human levels of asymmetry, as explained in the strength section above. Context where someone has had persistent pain or an injury in only one limb is a unique context, like a large knee injury (Age-, Sex-, and Graft-Specific Reference Values From 783 Adolescent Patients at 5 to 7 Months After ACL Reconstruction) or muscle strain (The Effect of Single Leg Romanian Deadlift on the Risk of Hamstring Strain Injuries) where single limb options become important to prioritise until you have reached a normal level of asymmetry again (A Complete Guide to Single Leg Training - YouTube). Your body is very good at hiding from these areas after injury or previous periods of pain if you only use double leg movement. This is the reason double limb movements are more "safe" early during sore periods, because they allow you to "offload" the affected limb. Seek a second opinion if you have been led to believe single limb leg/limb loading is “bad” for your concerns as you might be speaking to a "zombie" health care provider who's well-intended beliefs infect many to harmfully fear single leg movements. 

Generally, a good strength training program should incorporate both single- and double-limb movements to enhance overall body resilience and confidence. However, there are specific contexts where either approach could be more beneficial to prioritise for a certain period. That’s it. Hopefully making a decision between a single or a double limb movement is more informed now. Happy lifting!