Unless the goal of exercise is to help clear depression, usually exercise of all types needs to be reasonably hard in order to improve or maintain performance. The problem is that if we work too hard for too long, we increasingly get an unproductive stress response that is actually counterproductive to the exercise goals. I am going to start with the most extreme example. In virtually every ironman race; this is typically a 2.4 mile swim, 112 mile bike ride and then a 26.2 mile run, one competitor suffers from part of the bowel dying. When we exercise hard, as in every stress response, the blood is diverted from the digestive system and to the working muscles. Anybody who has eaten too close to exercising knows the joy of vomiting up the food, so they rapidly learn to leave a little time for the food to pass out of the stomach. However, blood is diverted from the whole digestive system, and that includes the small intestine and the colons or bowels. So if the exercise is long and hard then the bowel gets no blood for the hours involved and added to this is the stress of competition. As a result, just as cutting off the blood supply to a foot or finger results in amputation, so it is necessary to amputate part of the bowel.
In any endurance event, like a straightforwards marathon, it is common for the competitors to experience stomach discomfort, bloating, nausea, vomiting and diarrhoea and the main reason for this is the stress response along with the jiggling and jostling of the guts. If you want to read more about this, then click on the linked article called The Tao of Poo.
So all exercise events that are reasonably strenuous work by stressing the body. In his paper on the stress and cortisol, ((Sapolsky RM, Romero CM, Munck AU. How do glucocorticoids influence stress responses? Integrating, permissive, suppressive, stimulatory and preparative actions. Endocrine Reviews; Feb 1, 2000; vol.21 no.1 55-89)) Sapolsky gives an excellent analogy for the stress response, likening it to an army. Upon the start of stress, in this case exercise, the first response is to shoot the enemy and this calls on the first stress hormones to be released which includes adrenalin and the human growth hormone. There are defences already in place – and this represents a good training programme, so the body responds well to the exercise stress and does not get quickly overwhelmed by it. As the attack/exercise progresses, active combatants are called up from the reserves, which is an analogy for cortisol and other long term stress hormones, collectively known as glucocorticoids. There are suppressive actions by the body to prevent friendly fire or an autoimmune response. There are also preparative actions for attack/exercise and these could be to institute rationing; if we regularly undertake long lasting (events of about 40 mins and longer) exercise bouts, the body becomes more sparing in its use of energy. Another preparative action could be to enhance responsiveness to the next invasion/exercise bout. The stress hormones called glucocorticoids play a role in all parts of the action except for the initial first response.
There are 2 take home points from all this: “Excess glucocorticoids, while beneficial or harmless for a few days, can be fatal if prolonged”. And
For the best response we need to keep the primary response from overshooting and, in the aftermath, reduce the actions of those primary defenses to bring about recovery.
So all this means train hard, but train smart and allow good recovery time plus excellent diet that includes sufficient animal protein and high quality fat and a back up strengthening programme for the cardio athlete . Rest and recuperation, good diet and attention to weaknesses is just as vital as hard training – or the army is depleted, runs out of bullets, is overrun and ultimately turns on itself.
So how long to train for depends upon the goal. If it is for general fitness, then cardio training is better shorter than longer. The longer it is, the more efficient the body becomes at burning fuel and the more recovery time is needed. And the same goes for weight loss. As my page on How To Exercise To Lose Weight went into, long slow training does nothing to help weightloss, it merely increases the appetite.
If the goal of exercise is the long endurance events, then take time about building up the exercise times to allow the army to build good defences, a good back up army, good energy rationing and good recovery ability.
If the goal of the exercise session is to build or strengthen muscle, then the exercise bout must be no longer than 1 hour. Shorter is better. In the first 20 minutes, the initial shooting of the enemy, testosterone and human growth hormone rise, both critical in building muscle mass and strength. After that it starts falling and cortisol rises steadily. While cortisol is mandatory for fuel mobilisation, it is antagonistic to muscle growth. ((Exercise training and the hypothalamo-pituitary-adrenal axis. Martin Duclos and Antoine Tabarin)) So exercise for too long here, and we end up with less strength and smaller muscles. Of course, this exercise time does not include stretching and mobilisation time.
So train for an appropriate length of time according to the goal of the session, put in the back up forces of rest, good food and structural stuff and exercise will help lead a long and active life. Abuse any of these and exercise will ultimately cause pain, discomfort and illness.