Sore muscles

The weekend before last, several friends and I travelled to Cinque Terre, where we hiked through the trails connecting each of the five coastal towns. By the time we got back to Siena however, my legs were quite stiff and sore, just like the soreness one feels after a long run or a day at the gym. At any rate, within a day or two, my legs were back to normal. Just as I did at one time, some of you may be wondering why your muscles get sore after a tough workout.

Your cells obtain the energy needed to do work (i.e. hiking over a mountain) by breaking down glucose to produce ATP (adenosine triphosphate). This is typically done through an aerobic process in which glucose is broken down to pyruvate, which is then broken down further to carbon dioxide and water through the electron transport chain. This process is aerobic because oxygen is the final electron acceptor at the end of the electron transport chain, where it is converted to water. However, in times when large amounts of energy are required, such as weight lifting or sprinting, the amount of ATP needed is typically far greater than the amount of oxygen the lungs can provide.

When this happens, the body switches to an anaerobic process, in which oxygen is not used. Although this process does not produce nearly as much energy, it does produce enough to allow body functions to continue. As your breathing and heart rate increase, oxygen levels in muscle tissue eventually peak. When this happens, the electron transport chain function also peaks. However, because glucose is being broken down at a much faster rate, pyruvate eventually begins to build up inside the cell. This excess pyruvate is then diverted into an alternative pathway, through which it is converted to lactic acid. It is the presence of this lactic acid that leads to muscle soreness because it cannot be readily eliminated by the cell, unlike carbon dioxide and water.

When the body’s energy requirements subsequently drop, the anaerobic processes eventually stop. However, the muscle cells still require a minimal amount of energy to for normal daily function, and so all the pyruvate produced is once again pushed into the electron transport chain. In addition, all the lactic acid that has built up in the cells begins to be converted back to pyruvate, which then can also be sent to the electron transport chain. Consequently, within a day or two, all the lactic acid that has built up in the cells will have been converted back into pyruvate and all the muscle soreness will be gone.