This commentary complements data reported in Clinical Biomechanics [1] reporting reduced maximal handgrip strength in numerous patients with myalgic encephalomyelitis / chronic fatigue syndrome (ME/CFS) in proportion to their lowered maximal physical performances. The causes of muscle weakness in these patients are open to discussion. Literature data reveal a reduction of central command to skeletal muscles in some ME/CFS patients, related to encephalomyelitis. Altered muscle membrane excitability, that is “peripheral fatigue”, is also described in relation with an imbalance of the oxidant / anti-oxidant status. On the other hand, subgroups of chronically fatigued patients with clinical criteria of ME/CFS do not suffer from any muscle weakness. Thus, clinical data do not sufficiently clarify homogeneous ME/CFS pathology.