Passive mechanical properties of human gastrocnemius muscle–tendon units, muscle fascicles and tendonsin vivo

Abstract

SUMMARY This study provides the first in vivo measures of the passive length–tension properties of relaxed human muscle fascicles and their tendons. A new method was used to derive passive length–tension properties of human gastrocnemius muscle–tendon units from measures of ankle stiffness obtained at a range of knee angles. Passive length–tension curves of the muscle–tendon unit were then combined with ultrasonographic measures of muscle fascicle length and pennation to determine passive length–tension curves of the muscle fascicles and tendons. Mean slack lengths of the fascicles, tendons and whole muscle–tendon units were 3.3±0.5 cm, 39.5±1.6 cm and 42.3±1.5 cm, respectively (means ± s.d., N=6). On average, the muscle–tendon units were slack (i.e. their passive tension was zero) over the shortest 2.3±1.2 cm of their range. With combined changes of knee and ankle angles, the maximal increase in length of the gastrocnemius muscle–tendon unit above slack length was 6.7±1.9 cm, of which 52.4±11.7% was due to elongation of the tendon. Muscle fascicles and tendons underwent strains of 86.4±26.8% and 9.2±4.1%, respectively, across the physiological range of lengths. We conclude that the relaxed human gastrocnemius muscle–tendon unit falls slack over about one-quarter of its in vivo length and that muscle fascicle strains are much greater than tendon strains. Nonetheless, because the tendons are much longer than the muscle fascicles, tendons contribute more than half of the total compliance of the muscle–tendon unit.

On the ascent: the soleus operating length is conserved to the ascending limb of the force-length curve across gait mechanics in humans

“…Our estimates of the passive F-L curves of the SOL fascicles were consistently less steep than the theoretical model (Fig.4) and compared to those from other mammalian species (Azizi and Roberts, 2010), despite the peak passive muscle forces and stiffness being commensurate with those reported for the human gastrocnemius muscle (Hoang et al, 2007;Gao et al, 2009) (taking into account muscle cross-sectional area). Our results show that the length at which passive SOL forces begin to develop correspond closely to the predicted L 0 , corroborating both standard models (Zajac, 1989) and previous experimental data (Azizi and Roberts, 2010;Winters et al, 2011), and provides further confidence in our estimates of subject-specific F-L data.…”

Effects of plyometric training on passive stiffness of gastrocnemii muscles and Achilles tendon

“…Firstly, the passive muscle stiffness was not measured directly. Hoang et al (2007b) showed that ultrasonography could be used to assess muscle and tendon force-length relationships, and this method has been very useful in determining the plyometric training effects on passive muscle stiffness. Secondly, the tendon stiffness was assessed during an isometric contraction, while MTC stiffness was measured during passive stretching.…”

Anatomical information is needed in ultrasound imaging of muscle to avoid potentially substantial errors in measurement of muscle geometry

“…Two commonly applied strategies to standardize probe orientation are: (1) a perpendicular orientation of the probe to the skin, 25,32,33,35 and (2) ultrasound visualization of an image with "clearly visible fascicles." 4,13,14,16,19,22,23,25,27,[30][31][32]34,38 However, no systematic experimental evidence seems to be available to validate these strategies. The reason for this is the noninvasiveness of ultrasound, which is generally a major advantage of the method.…”