In vivo human tendinous tissue stretch upon maximum muscle force generation

In the present study, we examined the hypothesis that stretch of tendinous tissue in the human tibialis anterior (TA) muscle–tendon unit upon isometric dorsiflexion maximum voluntary contraction (MVC) varies along the entire tendinous component length. Ultrasound-based measurements of the excursions of the TA tendon origin and proximal end of the TA central aponeurosis were taken in the transition from rest to MVC in six men. Subtracting the TA tendon origin excursion from the excursion of the aponeurosis proximal end, the aponeurosis excursion was estimated. Estimation of the aponeurosis proximal region excursion was obtained subtracting the excursion of the insertion point of a central region fascicle on the aponeurosis from the whole aponeurosis excursion. Subtracting tendon excursion from the excursion of the central fascicle insertion point, the aponeurosis distal region excursion was estimated. Strain values were calculated dividing the excursions obtained by the original resting lengths. All excursions and lengths were measured in the mid-longitudinal axis of the TA muscle–tendon unit at the neutral anatomical ankle position. Tendon excursion and strain were 0.5±0.08 cm (mean±SE) and 3.1±0.2%, respectively. Aponeurosis excursion and strain were 1.1±0.15 cm and 6.5±0.6%, respectively. Aponeurosis distal region excursion and strain were 0.3±0.05 cm and 3.5±0.3%, respectively. Aponeurosis proximal region excursion and strain were 0.8±0.12 cm and 9.2±1%, respectively. Aponeurosis excursion and strain were larger by 110–120% (P<0.05) compared with tendon. Aponeurosis proximal region excursion and strain were larger by 165–170% (P<0.05) compared with aponeurosis distal region. These findings are in line with results from in vitro animal material testing and have important implications for theoretical models of muscle function.