Causes of fatigue during aerobic exercise

The three main causes of fatigue during prolonged aerobic exercise are thermal stress caused by dehydration, muscle glycogen depletion and limited blood glucose availability caused by a decline in liver glucose output.

1. Dehydration

- Primary means of heat dissipation during exercise is by sweat evaporation which accounts for about 80% of total heat loss.

- Consequence of body water loss (dehydration) -> reduced blood volume -> stress on cardiovascular system -> negative impact on physical performance.

- A loss of body fluid as little as 1% can have significant impact. 4% -> heat cramps and exhaustion. 6% -> heat stroke (life threatening)

- Second consequence of dehydration -> Loss or imbalance in electrolytes (affect normal metabolic functioning of tissues and organs).

2. Depletion of carbohydrate stores

- During aerobic exercise of moderate intensity (60-75% VO2 max), 40% of energy requirements must come from carbohydrate. Higher intensity -> higher % of carbohydrate to sustain muscle contraction.

- Insufficient blood glucose level (<3.5mM) -> fatigue.

3. Depletion of muscle glycogen

- During aerobic exercise of high intensity (75%-85% VO2 max), >70% of energy requirements must come from carbohydrates. Of which, ~85% of the carbohydrates required are supplied by muscle glycogen.

- Even at normal blood glucose level, fatigue sets in as muscle glucose uptake is too slow to support carbohydrate needs of muscles. Adequate muscle glycogen stores are essential for maximal performance for both short and long duration aerobic exercises.

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. 622-624). Chapter Humana Press.