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Here's Why Training One Limb at a Time Builds More Muscle

Single limb, or unilateral training, is an often overlooked part of resistance training but there are a few really good reasons why you should start adding them to your lifting regimen. 

By
Brock Armstrong,
March 21, 2018

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Woman doing single arm bicep curl

This may come as a surprise to you barbell kings and queens, but there is scientific evidence that training one limb at a time can actually give you more bang for your buck in a few different ways. This style of lifting is often referred to by us gym nerds as Unilateral Work, which simply means that you train your limbs separately, rather than engaging both sides at the same time. 

Unilateral lifting has three main advantages over bilateral:

  1. It recruits more muscle fibers and fatigues more motor unit pools with each rep. This simply means that it makes each lift more effective.
  2. It allows you to concentrate and focus more on one specific muscle. This can help you maintain good form which leads to an increase in performance and a decrease in injuries. 
  3. It helps to repair strength imbalances between your right and left sides. This is through a phenomenon called Contralateral Effect, and we'll get into that in a moment. 

Even though bodybuilders have been doing single limb lifts for a long time now, science has recently revealed the reason behind these benefits. Even findings in the field of neurophysiology indicate that single limb work is more beneficial than bilateral work when looking at recruitment of higher threshold motor units. 

Recruitment of Muscle Fibers

A motor unit is made up of a motor neuron and the muscle fibers that are activated by that motor neuron's axonal terminals (where the synapse contacts with nerve cells) and groups of motor units can work together to coordinate the contractions of a single muscle. Exercise science has known for years that some types of muscle fibers (specifically type IIb fibers) have the most potential for hypertrophy (muscle growth) but that they are also quite hard to recruit. This is the sweet spot where unilateral lifting excels. 

The Contralateral Effect

In 2017, a meta-analysis suggested that unilateral strength training actually produces adaptations in the opposite limb. So if you were doing a bicep curl with your left arm, you would actually see some benefits in your right arm. This is called the Contralateral Effect of unilateral strength training. 

There are two theories of why this happens. The first hypothesis is that unilateral resistance training may activate neural circuits that will enhance the motor pathways that connect to the opposite untrained limb. This may lead to an increased capacity to drive the untrained muscles and thus result in increased strength. The second hypothesis is that unilateral resistance training induces adaptations in motor areas that are primarily involved in the control of movements of the trained limb. The opposite untrained limb may access these modified neural circuits in ways that are similar to actually exercising that limb.

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