The metabolic, respiratory, and cardiovascular characteristics of fatigue during lifting tasks and bicycle ergometry were investigated. Experiments were designed to determine the relationship of the surface electromyograph to vascular fatigue, the relationship of metabolic, ventilatory, and circulatory responses to lifting and cycling tasks, and the correlation of physiological fatigue in lifting tasks to maximal oxygen uptake. Electrokymographic (EMG) recordings were obtained from electrodes placed on the forearm, hands or, spinal erector muscles of volunteers during brief isometric contractions or contractions held to fatigue. Minute ventilation, heart rate, and oxygen consumption were measured during lifting tasks or bicycle ergometry. The EMG amplitude was related to the tension exerted, the degree of fatigue developed during exertion, and the previous degree of fatigue developed in the muscle. The average oxygen uptake, the oxygen uptake at the maximal working capacity, and the heart rates showed much higher values during positive work at maximal capacity than during negative work at the same load. More than 30 percent of the maximum oxygen uptake fatigue occurred even in men well trained in their work. The authors suggest that the given maximum oxygen uptake above which fatigue occurs should be set in terms of the particular job and should not relate to another kind of work.