Physiologic methods have been developed to objectively quantify muscle strength, endurance, and fatigability. Isometric force and rectified/integrated electromyogram were simultaneously recorded during the three phases of a recording session: pre-fatigue, fatigue (1 min duration) and post-fatigue recovery (up to 10 min). Five parameters of muscle performance were computed: Maximum force (MF) exerted during isometric voluntary contraction (muscle strength); Force-time integral--area under force-time plot (endurance); Fatigue index (FI) (% reduction in MF); Neuromuscular efficiency (force/mV of EMG recruited), and Recovery time (RT). Normal values based on data from 20 normal subjects were determined for four muscles: index finger abductor, elbow flexors, knee extensors, and ankle dorsiflexors. Neuromuscular efficiency (NME) decreased significantly (20 to 70%) at the end of the fatigue phase; it generally increased to the pre-fatigue level in 2 to 10 min, during the recovery phase. The period needed to reach pre-fatigue level was referred to as RT. The elbow flexors had the highest mean FI (48%) and the longest RT (greater than 6 min); the ankle dorsiflexors had the lowest mean FI (34%) and the shortest RT (1.5 min). These methods have been used also to evaluate the effects of weight training in two patients with neuromuscular disorders.