Background The most popular hypothesis of the etiology of Achilles tendinopathy suggests that excessive pronation causes conflicting rotational motions, leading to a vascular constriction of the Achilles tendon, described as the “whipping phenomenon” (Clement et al., 1984). Although several studies identified pronation as a risk factor for the development of Achilles tendinopathy, the hypothesis that foot pronation causes vascular constriction of the tendon has not been investigated.

Objective Therefore, the aim of this study was to investigate whether the amount of pronation during running influences the Achilles tendon blood flow.

Design Cross-sectional study.

Setting Runners Service Lab, Zwijndrecht, Belgium.

Patients (or Participants) Twenty-five experienced runners (average running time 6.1±2.4 hours/week) aged 34.5±10.2 years participated in this study.

Interventions (or Assessment of Risk Factors) 2D-lower limb kinematics during barefoot and shod running in both frontal and sagittal plane were assessed. Blood flow of the Achilles tendon was measured before and after barefoot and shod running, by use of the oxygen-to-see device.

Main Outcome Measurements The increase of blood flow after running was registered as outcome variable. Mixed models analyses were executed with kinematic variables as fixed predictors.

Results The results of this study confirm that the more pronation excursion during running, the lower the increase in blood flow after running in both insertion and midportion (p=0.085 and 0.037, respectively). In addition, this study revealed that a higher average pronation velocity indicates a lower increase in blood flow after running at insertion (p=0.044). Next, this study also showed that the dorsiflexion excursion or the foot strike pattern was not correlated with blood flow after running.

Conclusions We can conclude that our findings indicate that foot pronation causes vascular constriction of the Achilles tendon, and therefore support this aspect of the whipping phenomenon.