In the late 90s, Alfredson published a widely referenced paper showing that heavy-load eccentric calf exercises reduced pain and improved function in even the worst cases of Achilles tendinopathy. More than 20 years later, it is still unclear exactly why heavy-load eccentric strength training stimulates tendon repair. There is also controversy over the best exercise prescription necessary to stimulate remodeling as some authors recommend performing 4 sets of 80 repetitions with light weights, while others recommend maximum resistance with very few repetitions. In this paper, the authors had 16 subjects perform different isometric exercise routines in which they held the isometric contraction for either a short duration (2 seconds) or a long duration (8 seconds).
The exercise intensity of the exercises also varied, as subjects held the isometric contractions at either a low intensity (35% full effort) or a high intensity (75% full effort). Ultrasonography was used to evaluate Achilles tendon volume at rest and during activity pre-and post-intervention. At the end of the 3 week study, the authors noted that only the individuals performing the long-duration, high-intensity isometric contractions had significant improvements in Achilles tendon volume and stiffness. In fact, the long-duration hold exercises resulted in a 13.5% reduction in Achilles tendon volume, which compares to the 5.4% reduction associated with the long-duration contractions performed at 35% full effort. The authors suggest that the long-duration contractions increased hydrostatic pressure within the tendon, which in turn produces a transient fluid flow out of the tendon, explaining the reduced tendon volume associated with the long-duration hold exercises. The authors theorized that the flow of fluid from the tendon core generates shear stress on the membranes of the tenocytes (the cells responsible for tendon repair), which in turn stimulates the cellular repair process. In their conclusion, the authors state to effectively stimulate repair in the Achilles tendon, the applied load must be heavy and sustained for a long duration to maximize tendon remodeling.