Contraction, relaxation, and energetics of normal human diaphragm strips (n = 10) were investigated over the whole load continuum in both twitch and tetanus modes. For a given level of isotonic total force and over a large part of the contraction phase, instantaneous velocity was shown to be a unique function of instantaneous length, regardless of time and initial length. In afterloaded tetanic contractions and over a wide range of loads, the peak lengthening velocity (VL) linearly decreased when maximum extent of muscle shortening (delta L) decreased. Stimulation mode modulated the VL versus delta L relationship, the slope of this linear regression being greater in tetanus than in twitch. Conversely, over a wide range of load, an increase in load linearly accelerated the peak rate of force decay, regardless of the stimulation mode. The energetics of human diaphragm muscle was evaluated in terms of both mechanical activity and economy of force generation. Maximum mechanical work (Wmax) differed significantly according to the stimulation mode, and the relative force at which Wmax occurred was higher in tetanus than in twitch (p < 0.05). The G curvature of the P-V hyperbola and maximum mechanical efficiency were significantly higher in tetanus than in twitch. This strongly suggests that the economy of force generation is higher in tetanus contractions than in twitch.