When you do physical work you use up energy but you become fitter in the process enabling you to do more in the future. Mental work relies on a fixed number of neurone The brains capacity to concentrate diminishes with time and any progress you make is stored through various forms of memory. This might be analytical , subconscious our muscle memory depending on the activity concerned.
However mental activity can create improvement by laying down new pathways for the neurone to traverse. The brain will improve due to fresh combination of these pathways which are unique in nature. Most cells will replenish but much research suggests that the neurone numbers are limited. The number is so great that when multiplying the neurones by the number of connections through the fresh pathways the brain becomes super efficient. However this requires a great deal of energy to build these pathways and will respond best with plenty of sleep and rest.
A good metaphor for this is using a railway system as an example. Physical activity is the train running along the track. Your neurones are the number of trains in the network and the pathways are the different routes these trains can take to reach a particular destination. The brain uses up masses of energy just thinking. As stated this requires plenty of rest, brain rich foods including omega rich fish oils etc. lots of physical exercise and good hydration. Intelligence is not the main factor that leads to success it is rather how you optimise the brain you have.
Peter Bull ( Author of the Get In Touch Series)