The function and mechanical behaviour of human skeletal muscle are in many ways unknown during natural locomotion. To gain more insight into these questions a method was developed to record directly in vivo forces from the human Achilles tendon (AT). The paper focuses on the details of the various techniques including the design, surgical implantation and calibration of the transducers. The implantation is performed under local anaesthesia and the measurements can last up to three hours, after which the transducer is removed. Exemplar results are presented from the measurements during walking, running and jumping. The loading of AT reached in some cases values as high as 9 KN, corresponding to 12.5 times the body weight or, when expressed per cross-sectional area of the tendon, the value was 11 100 N cm⁻². During the early contact phase of running the rate of AT force development increased linearly with the increase of running speed. Indirect measurements of the length changes of the muscle-tendon complex was used to plot the force-length and force-velocity relationships in the various activity situations. The observed results demonstrated that in normal locomotion involving the stretch-shortening cycle (SSC) muscle actions, the mechanical response of the triceps surae muscle is very different from the classical curves obtained in isolated muscle preparations. In agreement with the animal experiments using a similar in vivo technique, the natural locomotion with primarily SSC actions may produce muscle outputs which can be very different from the various conditions of the isolated preparations, where activation levels are held constant and storage and utilization of strain energy is limited. It is suggested that despite some limitations (due to e.g. difficulties in obtaining volunteers for AT force measurements, possible inaccuracies in transducer calibration and in muscle length estimates) the in vivo force measurement technique has an important role in studying the mechanical behaviour of muscle and its control under normal movement conditions.