Table 1

Operational definitions

InjuryInjury resulting from playing football and leading to a player being unable to fully participate in future training or match play (ie, time-loss injury)
Thigh muscle injuryA traumatic distraction or overuse injury to the anterior or posterior thigh muscle groups leading to a player being unable to fully participate in training or match play
RehabilitationA player was considered injured until team medical staff allowed full participation in training and availability for match selection
Lay-offNumber of days until the player resumed full team training
Re-injuryInjury of the same type and at the same site as an index injury occurring no more than 2 months after a player's return to full participation from the index injury
Functional muscle disordersPainful muscle disorder without evidence of muscle fibre damage.
Fatigue-induced muscle disorderCircumscript longitudinal increase of muscle tone due to over-exertion, change of playing surface, or change in training patterns. Pain increased with activity. Increased muscle tone is noted in the affected muscle area with mild, dull pain. ‘Dry muscle’ changes without oedema formation are typical on imaging modalities.
Delayed Onset Muscle Soreness (DOMS)More generalised muscle pain following unaccustomed, eccentric deceleration movements with possible associated sarcomeric ruptures of Z-discs. Presents as dull, aching pain in the affected muscle groups usually peaking within 24–72 h after initiating activity. Pain is present at rest and less severe or absent with concentric muscle activity. No or minimal signal changes are observed in the involved muscle on imaging.
Neuromuscular muscle disorder—spine relatedCircumscript longitudinal increase of muscle tone due to functional or structural spinal disorder (including sacroiliac joint). Subjective tightness and pain with intense activity, stretching and palpation. Sometimes associated with altered skin sensation. Increased muscle tone over the entire muscle length on palpation. Discrete ‘seam-like’ fluid accumulation between muscle and fascia throughout the entire length of the involved muscle on imaging.
Neuromuscular muscle disorder—muscle relatedCircumscript spindle-shaped area of increased muscle firmness. Possibly resulting from dysfunctional neuromuscular control. Increasing pulling and cramp-like sensation within muscle. Aggravated with activity and improved by rest and gentle stretch. Spindle-like, longitudinal induration within the affected muscle belly on palpation. Spindle-like muscle oedema without haematoma or focal muscle defect on imaging.
Structural muscle disordersAny acute indirect muscle disorder with macroscopic evidence of muscle fibre damage.
Partial muscle injury—minorStructural muscle injury involving only Intra-Fascicle Tear. Acute, sharp pain often at muscle–tendon junction. Focal pain on palpation usually without palpable defect. No visible haematoma. Aggravation of pain by stretching and palpation. Intra-fascicle haematoma and focal muscle defect on imaging with intact surrounding fascia.
Partial muscle injury—moderateStructural muscle injury involving Inter-Fascicle or Muscle Bundle Tear. Acute severe, stabbing pain often at muscle–tendon junction, often associated with fall from reflectory unloading. Palpable, defined defect in affected muscle, painful to touch and gentle stretch. Quickly developing, visible haematoma. Defect of muscle, fascia and haematoma visible on imaging.
Subtotal/complete muscle injury/tendinous avulsionStructural muscle injury involving the subtotal (>90%) or complete muscle diameter or complete tendinous avulsion. Acute severe pain (‘someone kicked/hit me’) and reflectory unloading. Severe pain with passive motion and palpation. Immediate functional deficit with development of extensive haematoma. Large palpable defect often at the muscle–tendon junction or retraction of avulsed muscle. Obvious muscle defect or tendinous avulsion and with haematoma formation is observed on imaging.