A new method for measuring passive length–tension properties of human gastrocnemius muscle in vivo

The study of muscle growth and muscle length adaptations requires measurement of passive length–tension properties of individual muscles, but until now such measurements have only been made in animal muscles. We describe a new method for measuring passive length–tension properties of human gastrocnemius muscles in vivo. Passive ankle torque and ankle angle data were obtained as the ankle was rotated through its full range with the knee in a range of positions. To extract gastrocnemius passive length–tension curves from passive torque–angle data it was assumed that passive ankle torque was the sum of torque due to structures which crossed only the ankle joint (this torque was a 6-parameter function of ankle joint angle) and a torque due to the gastrocnemius muscle (a 3-parameter function of knee and ankle angle). Parameter values were estimated with non-linear regression and used to reconstruct passive length–tension curves of the gastrocnemius. The reliability of the method was examined in 11 subjects by comparing three sets of measurements: two on the same day and the other at least a week later. Length–tension curves were reproducible: the average root mean square error was for pairs of measurements taken within a day and for pairs of measurements taken at least a week apart (about 3% and 6% of maximal passive tension, respectively). Length–tension curves were sensitive to mis-specification of moment arms, but changes in length–tension curves were not. The new method enables reliable measurement of passive length–tension properties of human gastrocnemius in vivo, and is likely to be useful for investigation of changes in length–tension curves over time.