Short communicationSlack length of gastrocnemius medialis and Achilles tendon occurs at different ankle angles
Introduction
Muscle–tendon slack length is defined as the length beyond which muscle and tendon begin to develop passive elastic force and determines the compliance of individual tendons. Although this is a crucial parameter used in muscle models (Ackland et al., 2012, Garner and Pandy, 2003, Hoy et al., 1990, Zajac, 1989) this is one of the most difficult measures to estimate in vivo (Garner and Pandy, 2003).
Passive torque–angle curves of a joint are classically used to assess mechanical properties of the passive musculo-articular complex. Some studies determined the slack length as the muscle–tendon length at which the passive joint torque first exceeds zero (Barber et al., 2012, Muraoka et al., 2004, Muraoka et al., 2005). However, passive torque is influenced by all structures crossing the joint [e.g., all the agonist/antagonist muscles, skin, and tendons; (Riemann et al., 2001)], therefore this measure is unlikely to correspond to the true slack length of an individual musculo-tendon complex. Other studies have used ultrasonography to isolate the slack length of one muscle–tendon unit (Herbert et al., 2011, Hoang et al., 2007). As muscle fascicles, aponeurosis and free tendon are classically considered to be in-series (Zajac, 1989), the slack length of these structures has been considered to occur at the same muscle–tendon length or joint angle (Hoang et al., 2005, Hoang et al., 2007). However, this assumption has not been confirmed experimentally via direct measurements of muscle and tendon stress.
Elastographic techniques can be used to determine the local mechanical properties (e.g., shear elastic modulus) of soft tissues by measuring the propagation velocity of shear waves. We have recently shown that an ultrasound shear wave elastographic technique called “supersonic shear imaging” (SSI) is able to accurately quantify shear elastic modulus of a targeted muscle during passive stretching (Koo et al., 2013, Maisetti et al., 2012). Taking advantage of this technique, muscle slack length was determined on gastrocnemius medialis (Maisetti et al., 2012) and on each head of biceps brachii (Lacourpaille et al., 2013).
During pilot experiments that aimed to test the reliability of shear elastic modulus measurement on Achilles tendon, preliminary observations suggested that the slack length of Achilles tendon and gastrocnemius medialis occurs at different ankle angles. Therefore, the aim of this study was to extend this preliminary observation. For that purpose, the onset of the rise in passive tension (i.e., slack length) was determined during passive stretching, independently in Achilles tendon and gastrocnemius medialis.
Section snippets
Participants
Nine healthy males volunteered to participate in this study (age: 22.6±1.8 years). Participants were informed of the purpose of the study and methods used before providing written consent. The local ethics committee approved the experiment and all procedures adhered to the Declaration of Helsinki.
Ergometer
An isokinetic dynamometer (Biodex 3 medical, Shirley, NY, USA) was used to measure ankle angle, joint angular velocity, and torque during passive ankle dorsiflexions. The participant's right foot was
Results
A significant main effect of “structure” (P<0.0001) was found for the resting shear elastic modulus value measured at 50° of plantarflexion (5.0±1.2 vs. 35.3±6.7 kPa for gastrocnemius medialis and Achilles tendon, respectively). There was neither no significant main effect of “knee angle” (P=0.20), nor significant “structure×knee angle” interaction (P=0.14).
Typical examples of ultrasound images and of the shear elastic modulus–angle relationship are depicted in Fig. 1. As tendon is stiffer than
Discussion
There is experimental evidence that the shear elastic modulus measured by SSI is linearly related to muscle stress during both isometric contraction (Bouillard et al., 2011, Bouillard et al., 2012) and passive stretching (Maisetti et al., 2012). Taking advantage of this technique, Maisetti et al., (2012) showed that the slack length of gastrocnemius medialis can be reliably determined and reported values of ≈−20° of plantarflexion (knee fully extended), which are very close to the data reported
Conflict of interest
None
Acknowledgments
This study was supported by grants from the European Regional Development Fund (ERDF, no. 37400), the Association Française contre les Myopathie (AFM, no. 14597) and the Region des Pays de la Loire.
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