Effects of running on human Achilles tendon length-tension properties in the free and gastrocnemius components

Abstract

SUMMARYThe elastic properties of the human Achilles tendon are important for locomotion; however, in vitro tests suggest that repeated cyclic contractions lead to tendon fatigue -an increase in length in response to stress applied. In vivo experiments have not, however, demonstrated mechanical fatigue in the Achilles tendon, possibly due to the limitations of using two-dimensional ultrasound imaging to assess tendon strain. This study used freehand three-dimensional ultrasound (3DUS) to determine whether the free Achilles tendon (calcaneus to soleus) or the gastrocnemius tendon (calcaneus to gastrocnemius) demonstrated tendon fatigue after running exercise. Participants (N=9) underwent 3DUS scans of the Achilles tendon during isometric contractions at four ankle torque levels (passive, and 14, 42 and 70 N m) before and after a 5 km run at a self-selected pace (10-14 km h −1). Running had a significant main effect on the length of the free Achilles tendon (P<0.01) with a small increase in length across the torque range. However, the mean lengthening effect was small (<1%) and was not accompanied by a change in free tendon stiffness. There was no significant change in the length of the gastrocnemius tendon or the free tendon cross-sectional area. While the free tendon was shown to lengthen, the lack of change in stiffness suggests the tendon exhibited mechanical creep rather than fatigue. These effects were much smaller than those predicted from in vitro experiments, possibly due to the different loading profile encountered and the ability of the tendon to repair in vivo.

Three-dimensional morphology and strain of the Achilles free tendon immediately following eccentric heel drop exercise

“…Effect of exercise on Achilles free tendon transverse morphology at rest Consistent with previous in vivo studies, we found no change in regional CSA measured at rest immediately after exercise (Burgess et al, 2009;Farris et al, 2012;Lichtwark et al, 2013;Obst et al, 2015;Ooi et al, 2015). While our results did show a small but consistent reduction in CSA across all measurement regions (∼7%), post hoc comparisons failed to reveal any significant differences between time points at any of the tendon regions.…”

“…All 3DUS calibration, acquisition and image segmentation procedures were performed using the Stradwin software package (Stradwin 4.7 1 , Cambridge University, UK; http://mi.eng.cam.ac.uk/∼rwp/stradwin/). Previous work has established the accuracy and repeatability of freehand 3DUS for in vivo measurement of human muscle morphology (length and volume) at rest (Barber et al, 2009) and Achilles free tendon morphology (length, volume, CSA, AP diameter and ML diameter) under passive and active loading conditions (Lichtwark et al, 2013;Obst et al, 2014aObst et al, ,b, 2015.…”

“…The mechanical and morphological properties of the distal Achilles tendon (i.e. Achilles free tendon) appear particularly susceptible to change following acute exercise (Grigg et al, 2009(Grigg et al, , 2012Lichtwark et al, 2013;Obst et al, 2015). Increased strain of the Achilles free tendon measured at the same external load after exercise has been demonstrated following running (Lichtwark et al, 2013) and repeated eccentric heel drops (Obst et al, 2015), and is suggestive of early stage tendon fatigue due to mechanical creep (Fung et al, 2010).…”

“…Achilles free tendon) appear particularly susceptible to change following acute exercise (Grigg et al, 2009(Grigg et al, , 2012Lichtwark et al, 2013;Obst et al, 2015). Increased strain of the Achilles free tendon measured at the same external load after exercise has been demonstrated following running (Lichtwark et al, 2013) and repeated eccentric heel drops (Obst et al, 2015), and is suggestive of early stage tendon fatigue due to mechanical creep (Fung et al, 2010). Because increased tendon longitudinal strain during tensile loading is closely associated with changes in tendon transverse morphology and strain (Obst et al, 2014b;Pokhai et al, 2009;Reeves and Cooper, 2014;Vergari et al, 2011), a similar relationship might be expected for the changes in tendon morphology and strain that occur in response to an exercise bout.…”

Achilles tendinopathy alters stretch shortening cycle behaviour during a sub-maximal hopping task

“…The most likely explanation for this observation is that the pain experienced during the contact phase of the SSC can trigger inhibition of neuromuscular activity 21 resulting in delays or reductions in EMG activity 9 . Alternatively, it is possible that the earlier offsets observed are a manifestation of a learnt behaviour, adopted in response to chronic changes in muscular performance or alterations in sensory input secondary to changes in tendon compliance 22 Some methodological issues require consideration. The descriptive nature of this study limits causative interpretations; it is not possible to elucidate the temporal relationship between altered SSC behaviour, AT symptoms and pathology.…”