Introduction In the aftermath of Achilles tendon rupture (ATR) a period of immobilization is always needed. For ATR, the immobilization period is normally 6–8 weeks which can cause significant leg muscle deconditioning and muscle weakness. Descriptive studies of limb deconditioning in the early stages after ATR injury are lacking and could yield new insights on how to best limit limb deconditioning after injury.

Material and Methods 15 patients with unilateral non-operated ATR were included from a randomised controlled trial. At 6 weeks after injury, all patients underwent computer tomography investigation from mid-thigh to plantar foot. Muscle CSA and attenuation were measured and associated with both patient-reported- and functional outcome one year after injury.

Results The soleus muscle of the injured limb contained at mean 19.5% more intramuscular fat than the uninjured limb at 6 weeks after injury(p<0.05). The lateral and medial gastrocnemius contained at mean 8.3% and 14.8% more fat than the uninjured limb respectively (p>0.05). Mean CSA of the uninjured lateral-, medial gastrocnemius and soleus were 11.4%, 8.4% and 6.7% larger than the injured limb (p>0.05). No association between CSA or fatty infiltration and patient outcome were observed. There was no association between patient weightbearing during immobilization and muscle deconditioning.

Conclusion Muscle deconditioning occur early after ATR. Significant fatty infiltration in the muscle had occurred, with no significant difference in muscle CSA. However, no significant association to patient outcome were observed at one year, which might be due to the low number of patients in this study.