Consequences of endurance exercise

1. Muscle damage and soreness

Mild muscle damage actually stimulates the rebuilding process which results in new and stronger muscle protein. More severe damage can result in muscle stiffness and soreness, limiting recovery and inhibiting performance. There are three primary causes of muscle damage.

a) Contractile stress

- Muscle contraction (especially the eccentric phase) places great stress on the muscles -> small tearing of muscle fibre.

- An acute inflammatory response is triggered by an injury -> swelling at injured site -> further muscle damage. This response peaks after 24 hours which explains why soreness is often felt some time after the workout is completed.

b) Hormonal shifts

- Catabolic hormone, cortisol is released when blood glucose is low or during high

intensity exercise.

- Primary function of cortisol is to generate fuel for working muscles by activating gluconeogenesis, lipolysis and proteolysis. Proteolysis can cause muscle damage.

c) Free radical reactions

- Free radicals are generated during exercise which can damage muscle protein and membranes and may even affect proper functioning of the immune system.

- These radicals must be neutralized by antioxidants such as vitamins C and E.

2. Immune system suppression

Athletes who train intensely -> more likely to catch colds and infections. Moderately intense exercises stimulate the immune system but strenuous exercises coupled with work-life stress -> suppress immune function.

Several reasons exist for the immunosuppressive effects of strenuous exercise. These include an increase in blood cortisol and other stress hormones, and a reduction in blood glutamine and glucose. Cortisol is the main contributor which increases during strenuous exercise, low blood glucose and periods of mental stress. It lowers the concentration and activities of many important immune cells that fight infection. The immune system suppression can last up to 73 hours after exercise and significantly increase vulnerability to infection.

References:

Antonio, J., Kalman, D., Stout, J. R., Greenwood, M., Willoughby, D. S., & Haff, G. G. (Eds.). (2008). Nutrition before, during, and after exercise. In A. Ivy, J.L. (Eds.), Essentials of sports nutrition and supplements (pp. 624-625). Chapter Humana Press.