Human tendon behaviour and adaptation, in vivo

Abstract

Tendon properties contribute to the complex interaction of the central nervous system, muscle-tendon unit and bony structures to produce joint movement. Until recently limited information on human tendon behaviour in vivo was available; however, novel methodological advancements have enabled new insights to be gained in this area. The present review summarizes the progress made with respect to human tendon and aponeurosis function in vivo, and how tendons adapt to ageing, loading and unloading conditions. During low tensile loading or with passive lengthening not only the muscle is elongated, but also the tendon undergoes significant length changes, which may have implications for reflex responses. During active loading, the length change of the tendon far exceeds that of the aponeurosis, indicating that the aponeurosis may more effectively transfer force onto the tendon, which lengthens and stores elastic energy subsequently released during unloading, in a spring-like manner. In fact, data recently obtained in vivo confirm that, during walking, the human Achilles tendon provides elastic strain energy that can decrease the energy cost of locomotion. Also, new experimental evidence shows that, contrary to earlier beliefs, the metabolic activity in human tendon is remarkably high and this affords the tendon the ability to adapt to changing demands. With ageing and disuse there is a reduction in tendon stiffness, which can be mitigated with resistance exercises. Such adaptations seem advantageous for maintaining movement rapidity, reducing tendon stress and risk of injury, and possibly, for enabling muscles to operate closer to the optimum region of the length-tension relationship.

Figure 1. The contribution of the rabbit soleus tendon to changes in muscle–tendon unit length during passive extension

Continuous line is tendon in series with proximal muscle fascicle, and dotted line is tendon in series with distal muscle fascicle. From Herbert & Crosbie (1997) with kind permission of Springer Science and Business Media.

Figure 2. The displacement for the free tendon and separate aponeurosis is expressed as strain (%)

Note the considerable difference between the strain of the aponurosis and free tendon. From Magnusson et al. 2003b and used with permission.

Figure 3. Anterior–posterior shear between medial gastrocnemius and soleus aponeuroses

From Bojsen-Møller et al. 2004 used with permission.

Figure 4. Typical data from one subject in one step cycle

a, changes in qastrocnemis medialis fascicular length (thick line), musculotendon length (dashed line) and tendon length (thin line). Positive and negative values indicate elongation and shortening, respectively. All values are given relative to heel-strike. b, EMG recordings from the GM muscle. c, ankle-joint (thick line) and knee-joint (thin line) angles. The 0 deg joint position corresponds to the anatomically neutral ankle- and full knee-extension positions. For the ankle joint, plantarflexion and dorsiflexion positions are indicated by positive and negative values, respectively. d, vertical component of ground reaction force. From Fukunaga et al. (2001) and used with permission.

Figure 5. Gastrocnemius tendon force–elongation properties in young, middle-aged and older individuals

From Onambele et al. 2006 and used with permission.

Figure 6. Patella tendon force–elongation relations before and after strength training for 14 weeks

The arrows indicate the loading and unloading directions. From Reeves et al. 2003b.

Figure 7. Gastrocnemius tendon stress–strain relation for the bed rest and bed rest + exercise groups

From Reeves et al. 2005a and used with permission.