Our body has a remarkable abilty to adapt. There are hundreds of processes and systems which work in unison to keep us functioning. When we apply stimuli or stress to our body, it responds in such a way that allows it to effectively continue to function under that stress. This response is what we use to become faster, stronger or fitter. The downside of this adaptation is that there is usually a tradeoff between the systems. It is extremely difficult to train all capabilities at once. This is the main challenge for any coach or trainer. They must construct an appropriate training program which achieves an improvement in certain capabilities while not negatively affecting the others.
One common scenario is related to body composition. Often an athlete will need to increase body mass while simultaneously reducing body fat. These goals directly conflict with each other. To increase body mass we need a calorie surplus but to reduce body fat we need a calorie deficit. It is contradictory. Many athletes attempt this believing that if they increase muscle mass there will be an increase in energy expenditure associated with greater muscle mass. While in theory this is possible it is a very difficult task to achieve in a real world scenario. A more effective strategy would be to alternate between periods of surplus and deficit, carefully monitoring both variables to ensure gradual progress in both. This would result in small body mass fluctuations but over time it would achieve the goal.
Another example is the athlete who wishes to improve both aerobic and strength capabilities simultaneously. While it is completely achievable, progress will be relatively slow. This is simply because while one promotes the development of type 2 fibres, the other is promoting development of type 1 fibres. This is not the most efficient approach to the task. Depending on time frame it may be necessary, but it is not as effective as partitioning the goals and focusing directly on one capability.
There are many training program designs and methodologies which look to solve the challenge of training multiple abilities at once. The problem is that combining certain training goals can be extremely counterproductive. The strategy for an athlete should be to always look for maximum gains with minimal effort and interference with other capabilities. This is not to advocate a lazy athlete. Instead it advocates a smart athlete who looks to effectively promote some qualities without negatively impacting others.
In terms of programming for an athlete, it is important to keep things as simple as possible. Athletes should have few but specific targets to work towards. Often high level athletes have so many targets to hit that they get lost. A wheelspin effect is created where their efforts counteract each other leading to very little progress. As simple as it sounds athletes should have a clear goal and stick to the process which achieves it. When they achieve this goal, they should identify their next weakness and follow the process to improve it and so on. Keeping goals clear and simple is the most effective way to make solid and consistent progress.