Bilateral exercises use both limbs together such as during a back squat. Unilateral exercises use each limb one at a time such as during a split squat. Although not truly unilateral, I also count exercises where each arm works independently but at the same time, such as the dumbbell bench press, as unilateral. In the bilateral vs. unilateral debate, bilateral wins out for strength training.
Bilateral exercises allow the most stability, and stability is essential for strength training. Any exercise that requires too much balance and coordination can allow these factors to limit you ahead of the goal to create tension in the largest muscles. Tension is the squeeze you feel when muscles contract and acts as the main stimulus for more size and strength.
Similar to the compound vs. isolation exercises debate, the ability to use heavier loads is not a good argument. Leverage can change how challenging a load feels and more or less range of motion can do so as well. The bilateral advantage comes from other reasons.
Reasons
- Your body uses both limbs for the most heavy activities.
Pushing, pulling, and squatting to the maximum effect will always use both limbs. Unilateral actions usually allow for traversing, repositioning, or light tasks. You would always use as much muscle as possible when requiring the most strength.
Some argue that unilateral exercises are more functional. Keep in mind the stabilizers exist simply to hold positions and allow the large prime movers to get the work done. Their strength tends to mirror the larger muscles. Their growth is limited since they have great leverage and less space to grow versus the big muscles.
Allowing for the most strength to be expressed in an exercise will create more tension.
- Two limbs at once work more efficiently.
Unilateral exercises require more time since you have to work both sides of the body. This requires you to muster your effort for more sets. In the case of including both bilateral and unilateral exercises, this just adds redundancy.
- The bilateral deficit is questionable.
Some research shows a bilateral deficit.
This occurs when the combined strength of two limbs is less than the force produced by each limb added up. Other studies have bilateral strength as greater than each limb together. This occurs especially for athletes used to bilateral exercises.
Some of the research relied upon leg extension machines, horrible devices that ruin your knees and work the quads at weak lengths. Their insights for training seem doubtful. Exercise science on the more applied end of the spectrum always seems suspect.
Unilateral exercises often deceive you because they use more muscle relatively. For the dumbbell row, the core contributes through a slight twist. The entire core adds to the effort possible on each arm but with two arms you may have even less help.
This would mean you simply use more muscle. The target muscles receive similar amounts of work or perhaps less for unilateral. It seems the bilateral deficit goes away with training if it even occurs at all.
- Bilateral exercises rely upon less skill.
Too much skill for an exercise reduces the strength stimulus because it competes with creating tension.
Some argue that unilateral exercises are more sports-specific. Skills are either completely specific or not at all. Squatting improves your potential for sprinting but can never replace sprinting to sprint faster. Unilateral exercises are no more specific and by performing them you just get better at them alone.
If you expect to perform well unilaterally without practice, you would train poorly until you acquire the balance and coordination for it. So of course by this standard bilateral exercises will make you less skilled at unilateral exercises. This comes as much from a lack of learning as opposed to stabilizer issues.
- The stabilizers get hit enough during intervals and bracing.
Many experts argue you need it to develop the deep stabilizing muscles that hold positions. This argument in favor of unilateral exercise fades though if you include an athletic option for intervals. Interval training such as stairclimbing challenges the stabilizers.
This allows you to concentrate on strength training as opposed to balance and coordination. You also brace yourself on bilateral exercises. This may provide enough work already.
Focus on the big muscles and you will find the small stabilizers take care of themselves.
- Weaknesses tend to go away in time.
Some argue asymmetries can develop with bilateral exercises only. One limb will always be dominant though. My experience has been that the weaker limb catches up enough that most trainees will have balanced enough development over time regardless of what path they go.
- The greater safety of unilateral exercises is questionable.
Unilateral gives you more freedom of movement but this also allows more room for error, especially with heavy weights. No one can bench press with dumbbells anywhere near to their maximum possible with a barbell due to this fact. The barbell still gives enough freedom to choose a bar path that suits you. Unilateral exercises distribute the load asymmetrically to bring risks as well.
Unilateral exercises do tend to reduce the total load on certain joints. A split squat reduces the load on the back versus a barbell squat. Is this a good thing though? The squat compresses the spine, but the spine can handle this reasonably well versus other forces such as shear that come through isolation exercises. This compression can build bone density.
If you have a joint issue you need to determine for yourself if bilateral exercises will harm you. This would be the main exception in favor of unilateral options. You would judge this on a case-by-case basis.
Settling the Bilateral vs. Unilateral Battle
Someone may appear more reasonable by saying to use both. They just assume a middle ground works best. What if one extreme has negative value? Sensible people would not suggest you can strike a compromise on hardcore drug use.
Unilateral exercises give you more freedom of movement and lessen the load on certain joints. The stabilizers work more as well but this can harm your strength development. A good interval option addresses them enough along with the bracing on bilateral exercises. If you can stay stable though, such as on a dumbbell row, then this sort of unilateral option works fine.
I suggest bilateral exercises mainly due to the better stability essential to strength training. Using free weight, compound exercises ranks much more importantly than any choice here though. Machine and isolation exercises are poor no matter how you do them.