Rise of the tendon research
Venue: Scandinavian journal of medicine science in sports

Muscles are our motors for movements and physical activity, but they cannot do the job by themselves. They are bound and surrounded by a vast connective tissue network including endo epi and perimysium surrounding the muscle fibers, and aponeurosis surrounding and protruding into the muscle, and finally the external tendon attaching the muscle to bones. Exercise provides stimulus to muscle adaptation, and also to tendon adaptation. Alternatively, if our research focuses on tendon adaptation, e.g. implementing strength training to increase tendon stiffness, we need to acknowledge that the adaptation goes hand in hand with the muscle adaptation. It is perhaps not surprising then that we see an association between muscle force and tendon stiffness regardless of age (Stenroth et al. 2015) or training background (Arampatzis et al. 2007) The mechanical behavior of connective tissue like tendon is traditionally thought to be akin to a simple spring, which elongates in response to tension. Textbooks typically show a monotonous length tension curve for tendon. However, tendon is not an ideal spring. It loses about 10% of its strain energy and this property, called hysteresis, is well established based on early animal studies but may be difficult to capture with adequate accuracy during in vivo human studies (Finni et al. 2013) Tendon also displays another history dependent behavior where stretching to given stress causes gradual elongation of the tendon, termed tendon creep. During cyclic loading, like walking or running, the phenomenon of tendon elongation is partly due to creep (due to the net positive load on the tendon) but may involve "conditioning" and "fatigue" Conditioning effect has been shown to stabilize after about five maximal contractions (Maganaris, 2003) and with prolonged tasks such as running and hopping, a healthy human Achilles tendon does not appear to show fatigue although the results vary considerably depending on methodologies used. In a recent issue of SJMSS, Obst et al. (2016) report measured changes in the free Achilles tendon and gastrocnemius tendon in response to eccentric heel drop exercise of 3 9 15 repetitions. The 10 min exercise caused lengthening and "creep" to occur in the free Achilles tendon but not in the gastrocnemius tendon. A similar result that the free Achilles tendon experiences a small elongation <1% without elongation of the gastrocnemius tendon was reported after a 5 km run (Lichtwark et al. 2013) Both of these papers used three dimensional ultrasound scanning to assess the tendon lengths. Functionally, SOL and GA muscles are synergistic muscles but they span a different number of joints. This may require flexibility from the mechanical system to provide enough connectivity and also allow some independency to optimize the tendomuscular function. The intriguing question raised by these studies is, however, why would the free Achilles tendon show a greater elongation than the longer tendon of the gastrocnemius muscle? Detailed micromechanical experiments have demonstrated that tendon fascicles have capacity to slide relative to each other (Haraldsson et al. 2008; Thorpe et al. 2013) and in human walking non uniform strain within the free AT has been reported (Franz et al. 2015) The finding by Obst et al. that the tendon portion attached to the distal soleus is more affected than gastrocnemius is consistent with recent findings by Franz and Thelen (2015) showing greater tendon displacement in the deep (anterior) part than in the superficial (posterior) part in the level of free AT during walking. Anatomically, the very distal end of the soleus is attached to the deep region of the tendon either anteriorly or medially (see fig. 8 in Finni et al. 2003) and therefore it is possible that the deep anterior motion of the tendon may represent movement of the distal soleus. Besides reporting free AT and gastrocnemius tendon lengths, Obst et al. also report a variable they have termed gastrocnemius aponeurosis. The "MG and LG aponeurosis" as Obst et al. defined in the Methods and fig. 1, represents the flat, sheet like tendinous structure that serves as insertion to fibers from the soleus. Therefore, this structure may better be defined as the SOL aponeurosis, as it lies anterior to the gastrocnemius tendon and binds the soleus. However, it is not clear which regions of the fascicles of this structure attach to which muscle and therefore defining this parameter becomes difficult. The terminology is important because the underlying anatomy is associated with function and needs to be fully understood. The Achilles tendon triceps surae unit has a very complex structure (e.g. Edama et al. 2015) As the tendon fibers running distally toward the calcaneus rotate laterally, a linear measure between a distal and proximal point may not represent the same portion of the tendon.