Mechanisms of Strength Adaptation – Neurological adaptations
As well as the morphological adaptations to strength training we mentioned in part 2 of this series, there are also a number of neurological adaptations that occur following strength training which are arguably even more important when it comes to generating maximal force. Your nervous system is what causes your muscle fibre to contract in the first place, so without these adaptations changes in muscle physiology aren’t going to have much impact.
Your nervous system is responsible for recruiting motor units – bundles of muscle fibres grouped together. It is the recruitment and firing frequency of these motor units that is responsible for both maximal muscular contractions and the speed of repeated contractions. Motor units are grouped by fibre type, and during muscle contraction type 1 muscle fibres are recruited first, followed by type 2a and type 2b as the need for higher levels of force production increases. This is known as the size principle. The size principle holds true for muscle contractions at both low and high speeds (ballistic contractions) and during isometric holds. The amount of force different motor units can produce can vary by up to 50 times so the recruitment of motor units containing type 2a and type 2b muscle fibres is of great importance when we want to produce maximal force.
Following strength training, it is thought that more motor units can be recruited, but also that the threshold at which high force output motor units can be recruited is reduced. The combination of these two factors is highly beneficial when we want to maximise the force we are producing in each contraction.
Motor unit firing frequency, is the rate that neural impulses are transmitted to the muscle fibres. Firing frequency can impact on both the amount of force generated during a single contraction, as well as the rate of force development during repeated ballistic contractions. The combination of these two factors means that firing frequency plays a big role in both maximal force development and power. Strength training has been shown to lead to greater motor unit firing frequency.
Motor unit synchronisation occurs when two or more motor units are activated at the same time, which as you can imagine has been hypothesised to lead to increased force development – two is always better than one right? This is a neurological adaptation that leads to the coactivation of numerous motor units enhancing maximal force and rate of force development. Again, this is thought to be something that is enhanced through strength training.
Now, whilst all this is happening in the muscles that are contracting to produce force in the direction we want it too, lets not forget that muscles work in antagonistic pairs. With agonists contracting to produce force and antagonists relaxing. The co-activation of antagonists during muscle contractions is always going to counterproductive when we are trying to produce maximal force but is also somewhat necessary to prevent overextension of joints and provide stability. There is also some evidence that following training, that the co-activation of antagonist muscle fibres can be reduced allowing for greater force output.
As you can see, developing your strength is an interaction between both morphological and neurological factors and to maximise your improvements we need to develop both. Typically when you first start strength training you see rapid improvements, often referred to as beginners gains. These improvements tend to be more neurological than morphological as these adaptations occur much quicker. Once you reach this point, the adaptations required to continue gaining strength become more physiological, requiring longer periods of training and different training methods. In the next blog post we will begin to look at how we may elicit these adaptations using different training methods.