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Q: I would like to ask you a question regarding rehabilitation and performance enhancement. In your opinion, what’s the best way to increase motor unit recruitment number within a muscle? Strength training? Power training? Maximal isometric contractions? Adding electrostimulation?

A: Anaerobic training promote several adaptations at levels such as neural, muscular, connective tissue, endocrine, cardiovascular and respiratory, where resistance training is included.

Regarding force production, neural factors have a large impact. We can divide it in two groups: intramuscular coordination and intermuscular coordination.

Intramuscular coordination relates to the number of motor units that are recruited and the rate at which a motor unit fires. These two characteristics are extremely important to achieve maximal force production.

Intermuscular coordination, as the name indicates, relates to the coordination between different muscles, and can be seen as the optimal recruitment of agonist, antagonist and synergist muscles.

Strength is the capacity of producing force at a specified velocity. Power is described as the time of doing work, or the product of force on an object and the object’s velocity in which the force is exerted (can be resumed as the product of force and velocity).

The vast majority of human muscles have Type I (low threshold, small size) and Type II (high threshold, large size) fibres in their arrangement. Also, a muscle can have several motor units, which means that a muscle can contract using only some fibres, rather than using all fibres. Muscle contraction follows the size principle. It means that fibres activate regarding their size and threshold level. A muscle contracts starting with Type I fibres activity, until the point it reaches Type II fibres threshold. The more intense an activity is, the greater the number of fibres activating will be. So, to stimulate type II fibres, one needs to go through intense activities, such as heavy resistance training. Motor units activity also follow another principle, called all or none principle. That means that a motor unit only has 0% or 100% output, like pulling the trigger of a gun: if pulled harder, the gun will fire the same way if pulled softer. So, a muscle produces more force mainly if a) fibres increase their size, b) more fibres are recruited, and c) the firing rate becomes higher.

In order to enhance an athlete’s performance, the first step is to perform a needs analysis. What kind of activity does the athlete have? Regarding practical programming, I will relate to an athlete from a strength/power sport, which is the majority of colective sports, such as soccer and basketball.

To achieve maximal strength, it takes time. It has been described that one needs >500ms to achieve maximal strength. The problem regarding sports performance is that in order to perform certain skills, such as jogging, jumping, kicking or throwing, it takes <250ms, and even less than 150ms during activities like sprinting. Maximal strength is the most fundamental capacity, and it affects power production. While charging an opponent, maximal strength is extremely important. However, during other activities like jumping or sprinting, power is the most important output, ie., the ability to produce force rapidly (P=FxV).

So, in order to enhance performance, one have to increase the number of recruited motor units. As stated above, that can be accomplished with maximal strength training (>85% RM), following the size principle. One can use isometric maximal contractions, however, after needs analysis, isotonic contractions have more transfer to activities such as jumping and sprinting. But one still need to express high force rapidly by increasing power output and rate of force development, since most sports actions take <250ms.

So, training with a variety of loads and speed is necessary, in order to cover all force-velocity spectrum. Maximal strength (high load/low speed), strength-speed (high/moderate load/high speed) and speed-strength (low load/high speed) training are necessary to elicit the most complete neural adaptations and to achieve a bigger transfer to the final activity.

Regarding electrical stimulation, “there is no doubt that it is possible to improve muscle strength by means of ES training programmes. However , it should be remembered that ES training-induced strength gains for healthy muscles are not greater than those that can be achieved with traditional voluntary training. In a recent systematic review of ES studies, Bax, Staes and Verhagen (2005) concluded that for unimpaired quadriceps the effectiveness of ES training is generally lower than with voluntary modalities, while for impaired (completely or partially immobilized) quadriceps, ES training could be more effective than voluntary training. This has important implications for the use of ES in the context of post-injury and/or postoperative rehabilitation. Training studies performed in the last 20 years have also demonstrated that it is possible to obtain significant improvements in muscle strength – particularly for the lower-extremity muscles – in amateur and competitive athletes of all levels.” (Mafilletti & Cardinale, 2011).

Luís Mesquita