The control mechanisms underlying undisturbed movements were analysed in two series of experiments: (1) normal physiological responses were investigated in neurologically intact subjects; (2) an artificial motor control system for paraplegic patients using functional neuromuscular stimulation (FNS) of the paralysed leg muscles was developed and tested. In both series of experiments standing-up from a chair and sitting-down were studied. A three-link model of the human body was used for recording and processing biomechanical data. In 5 normal subjects ground reaction forces and the surface electromyogram of different leg muscles were also recorded. Basic physiological aspects of FNS such as muscle force regulation and fatigue could be documented. For the standing-up and sitting-down experiments in 2 paraplegic patients the gluteal and quadriceps muscles were stimulated. The best results were achieved with a combination of open-loop and closed-loop stimulation with position and velocity feedback. The importance of feedforward and feedback control during undisturbed movements is discussed for natural and artificial motor control systems. It is concluded that the control of knee joint angle during standing-up and sitting-down represents an unstable system which cannot be controlled open-loop only. Different aspects of sensory feedback including the regulated variables, gain and stability of the system are discussed on the basis of the experimental data and the literature.