In vivo registration of Achilles tendon forces in man. I. Methodological development.

Mechanical behavior of muscle and its control is largely unknown under normal movement conditions. The present report deals with the methodological development to record directly in vivo forces from the human achilles tendon (AT) when the subjects perform normal movements ranging from slow walking to maximal sprinting and jumping. The development began with animal experiments, which dealt with the transducer design, surgical operation procedures, and duration of implantation. The first human experiment utilized an E form transducer implanted around the AT under local anesthesia. The transducer was kept in situ for 7 days, and on the 8th day recordings were made on simple plantar flexion movements and during slow walking. Further development led to final selection of a "buckle"-type transducer, and the measurements can be made immediately after operation and they usually last 2-3 h. The AT transducer can be calibrated by placing the subject in a prone position on to a calibration table. His operated foot was placed in a special shoe, the axis of which coincided with that of the ankle joint. A pulley system with known weights was used to dorsiflex the foot. Taking into consideration the geometrical arrangement of the AT transducer, axis of rotation, and the pulley system, the exact values of AT forces could be calculated. The actual measurements incorporate the use of EMG recordings of the leg extensor muscles and various external force measurements, such as long force platform or oscillating ergometers. The transducer is removed immediately after the measurements and the subjects recover.