Summary:
The fact is athletes can and do improve in explosive disciplines such as sprint, high and long jump, throw or martial arts. We have been following results of the very best athletes who were already subjected to specialized training programs in their own disciplines prior to their very first appearance at international competitions. Therefore their initial results already set a high bar. These athletes improved their results on average between 5 and 20%, depending on the discipline and on the individual.
For efficiency in sports such as football, basketball, volleyball, handball, tennis and so on, multiple explosive features must be present at the same time (sprinting, reaction speed and time, throwing, changing directions, hitting). If an athlete improves each of them by 'only' 10%, the overall improvement becomes very impressive.
Many athletes start off weak and have a very low relative power. That could be due to different factors, for example because of a lower proportion of muscle mass, excess fat or poor activation (or all of them combined). In this case it is sensible to expect significantly higher increment of explosiveness and speed.
Body composition is an important area of improvement. The ratio muscle mass/body mass is the key to the effectiveness of acceleration and sudden deceleration of athlete's own body (jumps, sprints, change of direction). This means the improvement of explosiveness of an athlete with a low muscle mass and relatively high body mass would be much greater and much more noticeable than the improvement of a fit athlete, especially if we assume they are in fact healthy individuals who happened to gain high proportion of excess fat due to unhealthy lifestyle.
Just how much an individual who does not show a great athletic potential can improve is very difficult to say. We must remember that most of the times we can only notice how the improvement of explosiveness and speed manifests, and not what potential the athlete actually possesses.
We can only speculate on the exact amount of improvements. We could probably expect to see improvements of 10 to 20 % for individuals that did not have specialized training programs devoted to improve specific explosive feature.
Let us think about a specific theoretical example. Imagine two individuals; one whose pre-dispositional long jump measures 5 m and one whose pre-dispositional long jump is 7 m long. We can claim that genetically more 'talented' long jumper can improve his jumping abilities by 10%, which would mean his long jump improvement would measure 70 cm. If we take the same 10% improvement for genetically less 'talented' long jumper, he would improve just for 50 cm. The same is also true for high jump, javelin and weightlifting disciplines.
However, would the difference between the best and the worst individual also increase accordingly in reality? No. Based on our experiences we can vouch the 'worse' individuals would improve much faster and more visibly than 'better' individuals. The difference in the results of these two individuals would not increase. On the contrary – it would decrease. That means that genetically less 'talented' athlete can improve more percentagewise than genetically 'talented' athlete. We already pointed out a number of reasons exactly why that happens, but we will talk more about this another time.