Background Calf muscle strain injuries (CMSI) are prevalent in sport, but investigations into the variable recovery periods and re-injury are lacking.

Objective To determine whether clinical and MRI findings are predictive of recovery and re-injury following CMSI.

Design Retrospective audit of injury data (2014–2017).

Setting Data from the Soft Tissue injury Registry of the Australian Football League (STRAFL).

Patients (or Participants) 149 Australian Football League players.

Interventions (or Assessment of Risk Factors) Blinded radiologists assessed MRI characteristics including: the anatomical location of injury, waviness, and severity of aponeurotic disruption (AD): none (0%), mild (<50%), severe (50%-99%), complete (100%). Clinical factors included: age, mechanism, and injury history.

Main Outcome Measurements The recovery period consisted of four milestones: pain free walking, running at >90% of maximum speed, return to full training, return to play (RTP). Risk factors for re-injury were evaluated for early re-injury (<2 months), and overall (<2 seasons). Multi-variable regression was used to evaluate clinical and MRI predictors of recovery and re-injury.

Results 149 CMSI (114 index, 35 re-injuries) were included: 126 soleus, 17 gastrocnemius, and 6 other. The anatomical location of injury did not impact recovery (all, p>0.05). CMSI with severe AD took on average 11.9 days longer to RTP than injuries with no AD (p=0.003), and the presence of AD (p=0.03) and a running-related mechanism of injury (p=0.01) were the strongest predictors of a longer RTP for soleus injuries. Older age (p=0.001) and a history of a previous ankle injury (p=0.03) were risk factors for early re-injury, while older age (p=0.01) and a history of previous CMSI (p=0.002) were risk factors for re-injury overall. Baseline MRI findings were not associated with re-injury (all, p>0.05).

Conclusions Presence of aponeurotic disruption and a running-related mechanism of injury resulted in longer return to play times for soleus injuries. Only clinical data, not MRI characteristics, were associated with risk of re-injury.