The ability to exert high levels of strength in a short period of time, known as explosive strength or power, is one of the pillars of performance in many sports, from those involving actions such as jumps, throws, sprints or changes of direction to those in which it seems to be less important, such as resistance training. Specifically, it has been demonstrated that the rate of strength development, which is the capacity to generate strength from the first milliseconds of the action (usually assessed during the first 100-200 milliseconds), is a better indicator of resistance in these types of actions than the maximum strength reached, although the latter has typically been the most popular one.¹

In spite of the fact that muscle mass plays an important role in the development of muscle strength, being, in fact, one of the determinants that affects the rate of force development,¹ it has been shown that the latter is largely influenced by neural factors. In fact, the improvement of neural factors is one of the adaptations that benefits the optimisation of the rate of strength development the most and in performance in general, being evident from the first weeks of training–in contrast, muscle mass gains can take weeks. For instance, a research study has shown that 5 weeks of training could lead to improvements in muscle strength without producing changes in muscle thickness, muscle rigidity or muscle structure, which can indicate that the first improvements in strength are mainly due to neural adaptations.²

Knowing the role performed by neural factors in strength, several studies have demonstrated that the ability to produce a rapid neuromuscular activation at the beginning of the contraction, measured by an electromyography, is key to attain high levels of strength development ratio, contrary to maximum strength which does not require a rapid neuromuscular activation.^3,4 Therefore, training programmes intended to improve actions such as jumping must aim to increase that neuromuscular activation from the first milliseconds of the actions. In this sense, it has been observed that resistance training with high loads (finishing with 4-6 maximum repetitions) increases not only maximum strength but also neuromuscular activation during the first phases of contraction (0-200 milliseconds).⁵ In addition, Del Vechio and partners have recently monitored motor neurons before and after 4 weeks of resistance training, making contractions with >75% of the maximum strength at high-speed, and observed that there was a motor neurons recruitment before, speeding up its firing rate.⁶

Moreover, with resistance training, not only can motor neurons changes be observed but also cerebral cortex changes. Therefore, it has been demonstrated that training only with one side of the body, the untrained part of the body can also gain strength. For example, a study showed that after 9 resistance training sessions with one leg, the strength of that leg increased by 41%; but interestingly, the strength of the untrained side also increased by 35%, and so did the cortical activation.⁷ Thus, these neural adaptations can build up, for instance, the strength of an injured player by training with the body, parts that are not injured.

Conclusions

To sum up, although the focus for improving strength is often placed on structural factors such as muscle volume, neural factors perform an even more important role, especially in actions that involve achieving high levels of strength in a short period of time (jumping, sprinting, etc). Explosive resistance training with high loads has been demonstrated to be effective to enhance those neural factors, not requiring a large volume of training–unlike what happens when trying to achieve muscle hypertrophy–to obtain these benefits.

Pedro L. Valenzuela

References

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