What happens when trained or tired the opposite side

In 1894 a simple and interesting experiment took place: one participant practiced for two weeks a rubber ball partition using only her right palm. At the end of the training period, they discovered a surprising result – the left hand strengthened significantly (43%) even though it was not practiced at all.

More than a hundred years have passed since that famous case description, and under its influence many studies have been published on the “Contralateral strength training effect”: a phenomenon in which strengthening one limb leads to strengthening the opposite limb even in cases of complete inactivity. This is not a negligible improvement, one study found that strengthening one limb led to an average improvement of 7.5% in the other limb. No laughing.

If one limb is made tired

In recent years, researchers have begun to ask a question of a similar nature: If strengthening one limb leads to strengthening the opposite limb, will a tiring practice of one limb lead to fatigue in the opposite limb?  Unlike the phenomenon of power transfer between limbs, there are very few studies and answers to the question of fatigue. In fact, to date there are only about ten studies that have examined the phenomenon and the results found to be different in the departments.

For example, a 2007 study found that the maximum force of the non-dominant leg dropped by 13% after a tiring protocol of the dominant leg that consisted of exerting a maximum isometric force for 100 seconds. In addition, a 9% decrease was measured in the relative contribution of the central parts to fatigue (it is customary to divide peripheral and central fatigue). Interestingly the effect is found only in men.

In another study equilibrium while standing on one leg was significantly impaired after a tiring protocol of the opposite leg (40% difference compared to the first measurement). Equilibrium was measured by the number of oscillations while standing on a force plate and the tiring protocol consisted of ten sets of 50 repetitions of isometric knee raids with a resistance equal to 10% of maximum force.

In contrast, other studies have failed to find a similar effect in the upper torso or when using less intense tiring protocols. A notable disadvantage of the studies conducted to date is the exclusive use of isometric contraction in the tiring protocol, and here I come into the picture. The idea is to test whether tiring practice of one limb dynamically will affect fatigue in the other limb. In other words, are the neural properties of concentric and eccentric contractions different from isometric contractions in their effect on the opposing limb.


Why is it important?

Great question. After all, one way or another, the results will not have much meaning in the practical aspect, so why even study the phenomenon? The main reason is that the results may add another piece to a very complex and very complicated puzzle – what is the source of fatigue. As of today we know more or less what is associated with fatigue, but very little about what causes fatigue. Typically it is common to divide fatigue into “muscular” (peripheral) and “nervous” (central) components.

The first refers to processes that deafen the muscle itself and are usually characterized by metabolic byproducts that impair the muscle’s ability to contract (at the same time I will share with you that there is no connection between fatigue and lactic acid). The second, and the more interesting of the two (in my opinion), is related to everything that is not muscular: brain, spinal cord, peripheral nerves, and even the amount of neurotransmitters that the nerve secretes. When does fatigue occur on a neural background and when on a muscular background? By and large, this is an open, controversial, and unusually fascinating question.

There are several ways to partially map the source of fatigue, and in my work I will use a research tool called the Interpolated twitch technique. In short, it involves delivering a strong electric shock during maximum muscle contraction. The electric shock is given just as the force exerted by the muscle reaches the plane. If immediately after the electric shock there is an increase in power we can conclude that not all the motor units were recruited or that their firing rate was impaired. As the force as a result of the electric shock increases, it can be assumed that these are changes on a neural and less muscular background.

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