Introduction Achilles tendon rupture (ATR) leads to long-term structural and functional impairments (1,2). Currently, the predictors of good recovery after ATR are poorly understood. Thus, we applied multivariable linear regression analysis to identify factors that explain good recovery.

Materials and Methods A total of 35 unilateral ATR patients (6 females) were recruited. Structural, mechanical, and neuromuscular parameters were measured 1-year after rupture. Multivariable linear regression was used to predict differences between limbs (Δ) in: 1) tendon length at rest, 2) non-uniformity of tendon displacement, and 3) flexor hallucis longus (FHL) surface electromyography (EMG) activation% during isometric submaximal contraction. Relevant covariates were included in the models based on previous knowledge (1,3–5). We also investigated the relative contribution of FHL to total triceps surae EMG activity during submaximal contraction between limbs.

Results Medial Gastrocnemius (MG)-tendon Δstiffness was significantly associated with both ΔMG (p=0.007) and Δlateral gastrocnemius (p=0.030) subtendon lengths. FHL EMG% difference between limbs was associated with MG (p=0.003) and soleus (p=0.040) Δsubtendon lengths. The relative contribution of MG to plantarflexion was lower in the injured limb with a mean difference of 0.061 (95%CI [0.02–1.0]; p=0.007). This was accompanied by an increased FHL contribution in the injured limb of -0.061 (95%CI [-1.06- -0.016]; p=0.011).

Conclusions The increased contribution of FHL appears to counteract deficits caused by the elongated tendon and smaller contribution of MG in the injured limb. Excessive lengthening of the tendon post-rupture could result in lower stiffness, reducing maximal isometric force production capacity, and worsening the ramifications after ATR.