Amyotrophic Lateral Sclerosis (ALS) is a progressive neurological disorder characterized by amongst others asymmetric muscle weakness, respiratory insufficiency and spasticity. The disease is usually fatal within 2-3 years and until now there is no cure. ALS patients are usually supported by a multidisciplinary team. One of the members of this team is the physical therapist. The aim of physical therapy might be to enhance or to preserve cardiovascular fitness and muscle strength. Some authors suggest, however, that a moderate to high intensive exercise programme might lead to overuse weakness (an undesired fast progression of muscle weakness). The primary objective of this study is therefore to investigate whether regular moderate to high intensity exercise program in ALS can maintain or optimize cardiorespiratory fitness and muscle strength. A secondary objective is to investigate whether such a programme leads to overuse weakness and if there is a positive influence on patient's disability, fatigue and quality of life.