Introduction Achilles tendinopathy is a common musculoskeletal condition driven in part by alterations in muscle performance.1 Whilst eccentric loading has demonstrated clinical efficacy, and superiority to concentric loading, the underlying mechanisms behind this benefit are largely unknown.2 One potential mechanism is a shift of angle to peak torque (AtPT) of the plantarflexors to a lengthened position as has been demonstrated elsewhere.3 This study compared the influence of eccentric vs. concentric loading on the AtPT and peak torque (PT) of the plantarflexors.
Methods Double-blind randomised control design. 30 healthy volunteers (23 ± 4 years) were randomly allocated to eccentric (ECC) or concentric (CON) groups. Baseline measures of AtPT and PT were obtained using isokinetic dynamometry. The exercise intervention reflected that described by Alfredson , with the ECC and CON groups performing eccentric or concentric contractions only respectively; load was progressively increased over a 10-week intervention period. Repeated measures of AtPT and PT were obtained and statistically analysed.
Results AtPT was significantly increased (p < 0.001) in the ECC group (pre, 25.6º ±. 6 to post, 28.7º ±. 7) but not in the CON group (p = 0.498). Furthermore, post-exercise comparison of the 2 groups revealed a significant difference in AtPT (p < 0.001) (Figure 1). There was no significant difference in PT for the ECC (p = 0.814) or CON groups (p = 0.216) (Figure 2).
Discussion These findings demonstrate that a clinically-derived eccentric loading protocol results in a shift in AtPT towards a longer length (i.e. further towards dorsiflexion), whilst not altering peak torque. This is in contrast to a concentric loading protocol that did not induce such changes. These findings offer a novel explanation regarding the mechanism of efficacy for eccentric loading, consistent with a mechanical pathoaetiological model of stretch-shortening cycle failure.
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