Introduction Achilles tendinopathy is believed to be driven in part by eccentric fatigue.1 Likewise, interventions that improve resilience to eccentric fatigue demonstrate efficacy.2 An experimental model of eccentric fatigue has the potential to inform our understanding of the underlying operating mechanisms. The purpose of this study was therefore to explore the influence of a longitudinal experimental model of eccentric fatigue on stretch shortening cycle behaviour during a sub maximal hopping task in healthy volunteers. In doing so, our understanding of the role that fatigue, pain and structural changes play in the development and rehabilitation of Achilles tendinopathy may be progressed.
Methods Eleven healthy volunteers (23.2 ± 6.7 years) performed a sub-maximal hopping task on a custom-built sledge system. A 3D motion capture system and surface EMG was utilised to measure temporal kinematic measures (lower limb stiffness, ankle angle 80 ms pre-contact, ankle angle at contact, peak ankle angle, ankle stretch amplitude) and muscle timing measures (soleus and tibialis anterior onset, peak and offset), respectively over 4 testing occasions; baseline, acute fatigue (5 min), pain (2 days) and recovered (7 days).
Results Compared to baseline, volunteers hopped with greater stiffness in all experimental conditions (p < 0.001). Stiffness increased in fatigue and again in pain, before partially returning in the recovered state (Figure 1). Compared with baseline, when fatigued, participants ankle kinematics were unchanged; however in pain, participants hopped in greater dorsiflexion (p < 0.001) but had reduced stretch amplitude (Figures 1 and 2). Once recovered, ankle kinematics returned towards baseline levels, albeit shifted towards a more plantarflexed position (p < 0.001). Soleus activity became earlier in each experimental condition, whilst tibialis anterior activity also became earlier in each experimental condition, except in the recovered state where it became relatively delayed.
Discussion These findings indicate that experimentally induced fatigue alters stretch-shortening cycle behaviour in healthy volunteers. Likewise, the structural changes that occur following such an intervention also lead to alterations in behaviour. Specifically, lower limb stiffness increases, ankle kinematics shift towards a more plantarflexed position, and muscle activity becomes earlier. These findings have clinical applications supporting theoretical models of the pathoaetiology of Achilles tendinopathy and the mechanisms underpinning eccentric loading interventions.
References 1 Shepherd, et al. Int J Exp Path. 94:260–270
2 Magnuson, et al. Clin J Sports Med. 19:54–64
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