Because in mitochondrial disease the cells cannot make ATP in sufficient quantity, anything that requires a lot of ATP such as exercising a muscle will mean that the body will not be able to keep up with demand. This has two main effects; firstly the muscle will become tired sooner than normal and secondly the muscle makes other compounds such as lactic acid in an attempt to keep up with energy demand. This can lead to pain and cramp in the muscles and patients often feel “like they have run a marathon” even after only moderate exercise. The temptation is to avoid all exercise so that you never get to this stage, but this is not recommended for two reasons. Firstly, it is important for general fitness to exercise and as well as exercising the muscles it is important to keep the heart and circulation healthy. Secondly, there is good evidence that if you become very unfit this will adversely affect your muscles. In many patients’ muscles there is a mixture of good and bad mitochondria and the hope is that exercise can increase the good mitochondria, boosting the level of ATP back to normal and so avoiding symptoms. At this stage this remains just a theory and there are large trials looking in to this idea. At the present time our advice is to exercise regularly at a level that feels comfortable, but without pushing yourself to the point that your muscles become painful.