Variables and categories used to describe the ACL injury circumstances and biomechanics
| Variable | Categories |
|---|---|
| Weather condition | |
| Precipitation preceding injury | Yes, no, unsure |
| Football-specific variables | |
| Playing situation preceding injury | Offensive, defensive, set play, other, unsure |
| Field location at injury* | Defensive third, midfield zone 1, midfield zone 2, offensive third, unsure |
| Player action preceding injury | Heading, dribbling, receiving, screening, turning, kicking (passing, shooting or clearing), blocking, other (e.g. goalkeeping), unsure, no ball possession |
| If kicking, which leg | Right, left, unsure |
| Duel type preceding injury | Collision (unintentional), tackling (other player), tackled (by other player), heading, screening, pressing, running, blocking, other, unsure, no duel |
| If tackled, from what direction | Front, behind, side, unsure |
| If tackled, what type | One-footed, two-footed, unsure |
| If tackled, what movement | Sliding, standing, unsure |
| If pressing, what type | Tackling, intention to tackle, no intention to tackle, unsure |
| Player contact preceding injury | Yes, no, unsure |
| If contact, what type | Direct contact (to injured knee or injured leg), indirect contact (to uninjured leg, trunk, head/neck or arm), unsure |
| Player contact at injury | Yes, no, unsure |
| If contact, what type | Direct contact (to injured knee or injured leg), indirect contact (to uninjured leg, trunk, head/neck or arm), unsure |
| Biomechanical variables | |
| In balance at IC | Yes, no, unsure |
| If out of balance, what direction | Forward, backward, sideways, combined directions, unsure |
| Player movement at IC | Forward, backward, sideways, upward, downward, combined directions, unsure |
| Cutting angle at IC | Intended change of direction 0–30°, intended change of direction 30–90°, intended stopping or change of direction >90°, unsure |
| Leg loading at IF | One leg, two legs with equal load, two legs with main load on injured leg, two legs with main load on uninjured leg, unsure |
| Horizontal speed at IC | High, low, zero, unsure |
| Vertical speed at IC | High, low, zero, unsure |
| Trunk rotation at IF† | Toward injured leg, toward uninjured leg, neutral, unsure |
| Foot rotation at IC‡ | Internal 0–45°, internal >45°, external, neutral, unsure |
| Foot strike at IC | Heel, toe, flat, unsure |
*Midfield zones 1 and 2 denote the first and second halves of the middle third of the playing field.
†Trunk rotation denotes the position in relation to the foot position.
‡Foot rotation denotes the position in relation to the player movement direction.
ACL, anterior cruciate ligament; IC, initial contact; IF, index frame.