Bilateral and Unilateral Loading

by Brian Jones

One of the more enduring debates in the strength and conditioning field is in the relative value of bilateral (BL) versus unilateral (UL) loading. Like many other debates coaches’ opinions can become quite polarized and as usual the truth is probably somewhere between the extremes. This purpose of this article is to analyze these methods, describe their benefits and limitations, and allow the readers to develop their own reasoned ideas on how to best implement them in their own training.


First let us define our terms. BL loading is defined as the simultaneous loading of both limbs (either both arms or both legs) during an exercise. Examples of this include the barbell bench press and the barbell back squat. During these movements, force and power are generated by the combined efforts of both limbs working against a single load. A common point of confusion is the case of dumbbells (or kettlebells). A standard flat dumbbell bench press or two kettlebell overhead press is a BL exercise. The term bilateral refers to the limbs rather than the implement used.


UL loading is defined as the loading of one limb (arm or leg) during an exercise movement. Defining UL loading is a bit trickier because the other limb may indeed be moving, even under load, but not in the same direction. Let us take the case of the UL flat dumbbell bench press. There are several ways to perform it. You could lie on the bench while holding only one dumbbell and perform the lift or you could hold two dumbbells and keep one side stationary in either the bottom or top locked-out position while the other side moved. Finally you could perform the exercise in a see-saw fashion with one arm pressing while the other arm lowered. Lower body exercises can also be somewhat difficult to categorize. Obviously a single leg pistol squat is UL, but what about the standard front lunge. In most discussions the lunge would also qualify as UL. Although both legs are contributing to the movement, the lead leg does most of the work.



Rather than presenting a list of benefits and limitations for each type this section has been arranged thematically. The intention is to highlight major differences and points of contention in the debates about BL and UL. Each discussion concludes with a few suggestions on how the concepts should impact training.

Core Work

In BL both limbs exert force at the same time and thus the absolute load per rep will be greater than that of a similar UL exercise. The greater load will force the core muscles to work harder to stabilize the spine. This is the case for both upper and lower body exercises. There is however, another factor to consider. Even though the absolute load may be smaller in UL work, loading only introduces a rotational component not present in BL movements. During the concentric phase of the lift, core rotation may be employed to assist the lift. In the isometric and eccentric phases the core activates to resist rotation.

Practical Applications

  • BL provides the greatest stimulus for core stability under heavy loads.
  • The core stress imposed by BL may be too great in the case of injury or fatigue and could become a limiting factor in the training of the limbs. UL training may be used to help deload the core while keeping the training intensity for the limbs high.
  • UL work can allow for a greater degree of trunk rotational training if that is a program goal.

Bilateral Deficit

The phenomenon of bilateral deficit is well-documented in the exercise science research literature. The BL deficit appears when the force that can be generated with both limbs simultaneously is lower than the sum of the forces that can be generated with each limb individually. Although present in most people to some degree the deficit is most pronounced in those who do little bilateral activity or training. The exact cause is debated but it is thought to be due to incomplete motor unit recruitment. Research also shows that BL training can overcome the deficit. Athletes who regularly trained bilaterally had a much lower deficit than those who did no BL training.

One question that comes to most lifters when they first hear about the BL deficit is “Why then, if I can press 200 lbs on a barbell bench press cannot I not press 100 + lb dumbbells for the same number of reps?”. The research studies on this topic used a specialized machine that required no stabilization. Dumbbells are inherently more difficult to keep in the groove and this limits some of the force that can be applied in the direction of movement.

Practical Applications

  • It can be assumed that all trainees will have some degree of BL deficit. The degree of the deficit will be directly related to the amount of BL training.
  • All sports training programs should include both BL and UL work. The precise amount will depend on the sport and the training goals.
  • For athletes who are not competitive lifters (weightlifting or powerlifting) dumbbells or kettlebells are recommended for the majority of upper body BL training due to the greater stabilization component.


The term ‘functional’ is so overused in discussions of physical training as to be almost meaningless. All that functionality means is that the training method or exercise is useful in training some specified activity or goal. So in truth all training is functional in some sense. Even the exercise machines that are so often attacked as worthless for athletes can stimulate hypertrophy in bodybuilders and bestow health benefits to non-athletes.

So how do BL and UL training stack up in terms of functionality? For beginning trainees it makes little difference. There are advantages to sticking with BL training when building up base levels of strength including core stability and overcoming the bilateral deficit. In addition imitating UL sport movements is not recommended by novices. Due to their lack of basic strength, the loads they can handle on such specialized exercises will be too low to stimulate any real gains.  After establishing basic strength, more UL becomes more functional.

Practical Applications

  • Beginners should focus on BL bodyweight and dumbbell exercises until a basic level of strength is achieved.
  • When developing UL exercises make sure the exercise difficulty does not preclude using a moderately heavy load. A good rule of thumb is to keep loads at or above 25% 1RM of biomechanically similar standard BL exercises.

Brian Jones, MS, CSCS

TAKU’s NOTE: Brian has a masters degree in exercise physiology and is currently pursuing an PhD at the University of  Kentucky. In his experience as a personal trainer and strength coach he has helped athletes and non-athletes of all levels achieve their goals. Brian teaches Brazilian jiujitsu and judo and specializes in getting fighters and grapplers in competition shape. He has written for numerous print and online periodicals, is a regular contributor to MILO, and has published two books (The Complete Sandbag Training Course and The Conditioning Handbook: Getting In Top Shape) available at


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