The effect of eccentric and concentric calf muscle training on Achilles tendon stiffness

Objective: To compare in vivo effects of eccentric and concentric calf muscle training on Achilles tendon stiffness, in subjects without tendinopathy. Methods: Thirty-eight recreational athletes completed 6 weeks eccentric (6 males, 13 females, 21.6 +/- 2.2 years) or concentric training (8 males, 11 females, 21.1 +/- 2.0 years). Achilles tendon stiffness, tendon modulus and single-leg jump height were measured before and after intervention. Exercise adherence was recorded using a diary. Results: All data are reported as mean +/-SD. Groups were matched for height and weight but the eccentric training group were more active at baseline (P<0.05). Tendon stiffness was higher in the eccentrically trained group at baseline compared to the concentrically trained group (20.9 +/- 7.3 N/mm v 13.38 +/- 4.66 N/mm; P = 0.001) and decreased significantly after eccentric training (to 17.2 (+/-5.9) N/mm (P = 0.035)). There was no stiffness change in the concentric group (P = 0.405). Stiffness modulus showed similar changes to stiffness. An inverse correlation was found between initial, and subsequent, reduction in stiffness (r = -0.66). Jump height did not change and no correlation between stiffness change and adherence was observed in either group (r = 0.01). Conclusions: Six weeks of eccentric training can alter Achilles tendon stiffness while a matched concentric programme shows no similar effects. Studies in patients with Achilles tendinopathy are warranted.