Tendon rehabilitation: isolated eccentric loading invokes a greater reduction in Achilles tendon thickness than concentric loading

Eccentric calf muscle exercise has been advocated as the treatment of choice in Achilles tendinopathy. However, mechanisms underlying the efficacy of eccentric, as opposed to concentric, exercise remain unknown. This research investigated the acute change in the sagittal thickness of the Achilles tendon (AT) in response to bouts of isolated eccentric or concentric calf muscle exercise. Eleven healthy males, without AT pathology (age, 25.9 ± 4.9 years; mass, 74.2 ± 11.8 kg), completed an exercise protocol involving isolated eccentric loading of the AT (ankle dorsi-flexion), while the contra-lateral AT experienced isolated concentric loading (ankle plantar-flexion). Six sets of 15 repetitions were performed against body weight, with an additional 20% bodyweight added via a backpack. AT thickness 2 cm proximal to the calcaneal insertion was determined from sagittal sonograms taken immediately prior to and following exercise. Consistent with earlier research, calf muscle exercise resulted in an abrupt decrease in AT thickness. However, isolated eccentric loading induced a significantly greater decrease (−20.8 ± 5.5%) than concentric loading (−5.3 ± 4.7%, p = 0.013). It is hypothesised that eccentric muscle action may invoke a differential stress field within the AT that results in a localised increase in collagen strain and extrusion of water from the tendon. Shear stress arising from such fluid flow may, in turn, stimulate tenocytes to produce matrix proteins that promote tendon remodelling, as has been shown in other collagen-rich tissues. The findings of this research have broad implications for the treatment of tendinopathy and provide greater insight into the clinically perceived benefit of eccentric over concentric exercise in AT rehabilitation.