Load-elongation characteristics of in vivo human tendon and aponeurosis

In the present study, we measured the in vivo load-elongation characteristics of the human tibialis anterior tendon and its central aponeurosis. Measurements were taken in five men using dynamometry, muscle electrical stimulation and ultrasonography. Percutaneous tetanic stimulation of the muscle at successive voltages corresponding to 20, 40, 60, 80 and 100 % of maximum isometric dorsiflexion moment was applied. During electrical stimulation, we recorded the displacements of the tibialis anterior tendon origin and its aponeurosis proximal end using B-mode ultrasonography. Aponeurosis displacement was calculated by subtracting tendon displacement from the displacement of the aponeurosis proximal end. Tendon and aponeurosis displacements increased curvilinearly from 1.3 to 4 mm and from 3.7 to 12 mm, respectively, as a function of dorsiflexion load. Scaling of the displacements recorded to the resting lengths (measured over the skin) yielded strain values that increased curvilinearly with load, from 0.8 to 2.5% in the tendon and from 2.1 to 7% in the aponeurosis. Tendon strain was smaller by between 61 and 64% compared with aponeurosis strain at any given contraction level. These findings are in line with reports from in vitro isolated material testing and have important implications for muscle modelling.