Achilles tendon ruptures are disabling injuries that lead to long-term deficits in strength, physical activity, and function. Despite the high prevalence of this injury, the optimal treatments for the management of Achilles tendon ruptures are poorly defined. The authors of this study used ultrasonography to evaluate movement patterns between tendon fibers in people with healthy Achilles tendons, and in people treated non-surgically 14 months after an acute Achilles tendon rupture. Interestingly, the authors note that when people with healthy Achilles tendons isometrically contract their calf muscles, the individual fibers that comprise the Achilles tendon move in a non-uniform pattern, with the deep anterior fibers displacing more than the superficial fibers. 

The authors state the ability of the tendon fascicles to slide relative to each other is considered a sign of a healthy tendon and is usually overlooked in the management of Achilles tendon ruptures. In contrast, people treated surgically or non-surgically following Achilles tendon rupture display more uniform within-tendon displacements one year after Achilles tendon rupture. 

The authors note the tendon fascicles from the medial gastrocnemius muscle are particularly prone to impaired gliding, and they reference research showing that the deeper flexors, in particular flexor hallucis longus, plays an important role in generating force after an Achilles tendon rupture. Prior studies have shown compensatory hypertrophy of flexor hallucis longus following Achilles tendon ruptures, as this muscle becomes more active in an attempt to offload the Achilles tendon. This was an important paper mostly because it emphasizes how little we know about tendon recovery. Perhaps early mobilization enhances motor recruitment patterns, allowing for non-uniform displacement of Achilles tendon fascicles. 

Another consideration is that prolonged isometric contractions could enhance neural drive, increasing recruitment of a larger number of muscle fibers, which in turn would vary gliding patterns of individual fascicles. The researcher Keith Barr has shown that performing 4, 30-second isometric gastroc/soleus contractions increases interfascicular gliding between tendon fibrils and strengthens tendons. The intensity of the isometric contractions can be gradually increased over time and one of the main takeaways from this paper is that you cannot overlook flexor hallucis longus when treating/preventing Achilles injuries. This important muscle can appreciably offload the Achilles tendon and clinicians should prescribe specific exercises to strengthen the flexor hallucis longus when managing Achilles tendon ruptures. Ideally the exercises will be performed with the toes in a dorsiflexed position, as strengthening the digital flexors while they are stretched produces 4 times the strength gains of conventional exercises.