Objective: 

To describe the relationship between magnetic resonance imaging (MRI) findings and time to return to sport (RTS) from muscle injuries in youth athletes.

Design: 

Prospective collection of injury surveillance data over 6 seasons (2014-2015 to 2019-2020) and reanalysis of MRIs by a radiologist blinded to RTS time after the data collection period.

Setting: 

National sports academy and sports medicine hospital.

Participants: 

Male youth (11-19 years) athletes participating in the football (soccer) or athletics (track and field) programs with a muscle injury.

Independent Variables: 

Magnetic resonance imaging findings (eg, grade and location) in time-loss muscle injuries.

Main Outcome Variables: 

Injury incidence, distributions, and RTS time.

Results: 

In total, 353 time-loss muscle injuries were recorded for 1089 athlete-seasons, of which 85 satisfied our inclusion criteria (MRI within 14 days). Return to sport time was significantly longer for reinjuries compared with index injuries. Associations between MRI findings and RTS time were specific to the muscle group, with significant differences in RTS time observed between categories of structure, location, and grade. The largest number of injuries was to the hamstrings (n = 46), where injuries involving the tendon were more severe than those involving the myotendinous junction and muscle, proximal injuries were more severe than mid-portion and distal, and longer RTS time was observed for higher injury grade (P < 0.05).

Conclusions: 

Muscle injuries in youth are diverse and cannot be treated as a single injury type. Specific injury durations can be expected depending on injury characteristics such as reinjury, location, structure, and grade.