ECCENTRIC TRAINING AND THE SCIENCE BEHIND

Dear Editor-in-Chief: I read with great interest the recent paper by Dr. Mahieu et al. (6). They found that the dorsiflexion range of motion was significantly increased only in the eccentric training group after 6 wk of training. The mechanism behind the efficacy of eccentric training is currently not thoroughly understood. Given the current publications on eccentric training, I would suggest discriminating acute effects of eccentric training in contrast to midterm effects such as after at least 12 wk of eccentric training in addition to the fact whether healthy or tendinopathic individuals were tested. In a clinical perspective, it is not incontrovertible that short-term eccentric training of 4-6 wk necessarily leads to the same clinical results as at least 12 wk of eccentric training in Achilles tendinopathy. To date, however, there is no published randomized trial evaluating such differences and different intensities of the eccentric training. Therefore, I would appreciate if the authors could speculate whether a prolonged eccentric training program of 12 wk or more would be associated with the same biomechanical result or not, and if not, why not. As far as connective tissue turnover is concerned, 12 wk of eccentric training in Achilles tendinopathy leads to a significantly increased rate of collagen synthesis (5). Using color Doppler sonography, 12 wk of eccentric training in tendinopathy has been reported to reduce the degree of neovascularization (7). In line with this, quantitative assessment of Achilles tendon microcirculation with real-time spectrophotometry and laser Doppler flowmetry revealed a significantly increased capillary blood flow at the point of pain in both insertional and midportion tendinopathy at rest (4). Twelve weeks of eccentric training reduces the pathologically increased capillary blood flow without changes of tendon oxygen saturation (2). In addition, Achilles tendon venous outflow is facilitated after 12 wk of eccentric training. Twelve weeks eccentric training reduced the increased paratendinous capillary blood flow in Achilles tendinopathy by as much as 45% and decreased pain level on the basis of a visual analog scale (3). Local paratendon oxygenation was preserved, whereas paratendinous postcapillary venous filling pressures were reduced after 12 wk of eccentric training, which seems to be beneficial from the perspective of microcirculation. On the basis of these observations, one might speculate that the increased dorsiflexion found in the healthy volunteers might be associated with a decreased tendinous and paratendinous capillary blood flow after eccentric training. Local resolution of the capillary blood flow might be the result of the repetitive eccentric training. Alfredson reported that his patients were instructed to do their eccentric exercises with bent and straight leg with 3 × 15 repetitions each, twice daily, 7 d·wk−1, for 12 wk (1). This correlates to 1260 repetitions per leg per week and 15,120 for 12 wk. Given that ankle dorsiflexion limits the blood flow in the neovessels (3), it is tempting to speculate that the reasonably good results of eccentric training in midportion Achilles tendinopathy might be at least mediated or influenced by the intermittent disruption of the blood flow in the tiny neovessels. Taking this into account, the number of repetitions per day resulting in a temporal disruption of the blood flow might well be correlated to the effectiveness of any eccentric training intervention. In sum, several distinct mechanisms underlying the eccentric training in both healthy and injured Achilles tendons in an acute and midterm setting have been published to date. KARSTEN KNOBLOCH, MD, PhD Plastic, Hand and Reconstructive Surgery Hannover Medical School Hannover, Germany