Table 2.
Baseline characteristics of participants
| Early lengthening (N=44*) | Delayed lengthening (N=44*) | |
| Age (years) | 26 (±4) | 25 (±5) |
| Gender (male) | 44 (100%) | 44 (100%) |
| Sports | ||
| Football | 30 (68.2%) | 27 (61.4%) |
| Futsal | 3 (6.8%) | 2 (4.5%) |
| Handball | 4 (9.1%) | 5 (11.4%) |
| Basketball | 4 (9.1%) | 4 (9.1%) |
| Athletics | 1 (2.3%) | 4 (9.1%) |
| Volleyball | 1 (2.3%) | 0 |
| Field hockey | 1 (2.3%) | 1 (2.3%) |
| Rugby | 0 | 1 (2.3%) |
| Level of sports | ||
| Professional | 40 (90.9%) | 42 (95.5%) |
| Competitive | 4 (9.1%) | 2 (4.5%) |
| Previous hamstring injury, n (%) | 21 (47.7%) | 21 (47.7%) |
| Previous ipsilateral hamstring injury, n (%) | 13 (29.5%) | 17 (38.6%) |
| Previous ipsilateral hamstring autograft for ACL reconstruction, n (%) | 2 (4.5%) | 0 |
| Injury during | ||
| Match, n (%) | 32 (72.7%) | 32 (72.7%) |
| Training, n (%) | 12 (27.3%) | 12 (27.3%) |
| Time of injury during match or training, n (%) | ||
| Beginning (first quarter) | 5 (12.5%) | 15 (36.6%) |
| Middle (second/third quarter) | 16 (40%) | 12 (29.3%) |
| End (fourth quarter) | 19 (47.5%) | 14 (34.1%) |
| Mechanism of injury, n (%) | ||
| Sprinting | 26 (59.1%) | 26 (59.1%) |
| Stretching | 5 (11.4%) | 3 (6.8%) |
| Kicking/shooting | 5 (11.4%) | 5 (11.4%) |
| High kick | 0 (0%) | 1 (2.3%) |
| Sliding/tackling | 3 (6.8%) | 0 (0%) |
| Other | 5 (11.4%) | 9 (20.5%) |
| Dominant leg injured, n (%) | 30 (69.8%) | 27 (61.4%) |
| Training per week (in hours) | 10.1 (±3.5) | 10.6 (±3.6) |
| Time between injury and start of blinded assessment or rehabilitation (in days) | 2 (IQR 2–4) | 3.5 (IQR 2–4) |
| Max pain at time of injury (NRS 0–10) | 7 (IQR 6–8) | 7.8 (IQR 5.8–9) |
| Participant predicted RTS (in days) | 14 (IQR 10–21) | 14 (IQR 10–18) |
| Participant expectation of performance after recovery (as compared with before injury, in %) | 100 (IQR 100–101) | 100 (IQR 100–120) |
| Palpation pain at blinded assessment (yes/no) | 40/4 | 41/3 |
| Length of palpation pain (in cm) | 7 (IQR 5.3–12) | 8.5 (IQR 6–10.5) |
| Width of palpation pain (in cm) | 4.5 (IQR 3.5–6.8) | 5.5 (IQR 4–7.5) |
| Distance from ischium to maximal painful area on palpation (in cm) | 15.6 (±7.4) | 13.9 (±7.8) |
| Range of motion | ||
| PKET relative deficit (in % of uninjured leg) | 59.8 (IQR 38.8–86.9) | 66.1 (IQR 39.8–91.1) |
| SLR relative deficit (in % of uninjured leg) | 72.7 (IQR 60.1–93.9) | 81.9 (IQR 62.1–91.7) |
| MHFAKE relative deficit (in % of uninjured leg) | 58 (IQR 40.9–83.9) | 59.2 (IQR 19–88) |
| Strength | ||
| Able to do ‘inner’ test (yes/no) | 42/2 | 43/0 |
| ‘Inner’ position relative deficit (compared with contralateral leg, in %) | 73 (IQR 47–81.5) | 72.1 (IQR 49.5–82) |
| Able to do ‘mid’ test (yes/no) | 41/3 | 44/0 |
| ‘Mid’ position relative deficit (compared with contralateral leg, in %) | 58.8 (±30.7) | 50 (±26.3) |
| Able to do ‘’outer’ test (yes/no) | 41/3 | 42/2 |
| ‘Outer’ position relative deficit (compared with contralateral leg, in %) | 38.9 (IQR 29.7–53.7) | 36.6 (IQR 24.3–56.5) |
| MRI | ||
| Injury location, n (%) | ||
| Biceps Femoris long head | 36 (82%) | 37 (84%) |
| Biceps femoris short head | 0 | 0 |
| Semimembranosus | 5 (11%) | 6 (14%) |
| Semitendinosus | 3 (7%) | 1 (2%) |
| Grade I/II (modified Peetrons), n (%) | ||
| Grade I | 20 (45%) | 18 (41%) |
| Grade II | 24 (55%) | 26 (59%) |
| Oedema craniocaudal length (in cm) | 13.5 (IQR 8.9–19.4) | 15.8 (IQR 11.1–22.6) |
| Cross-sectional area of injury (in % of maximum CSA of the involved muscle) | 61.5 (IQR 36.6–86.1) | 58.5 (IQR 44.3–85) |
| Distance from ischial tuberosity to (in cm): | ||
| Start of oedema | 15.2 (IQR 8.5–22.3) | 11.8 (IQR 5.5–20) |
| Maximal SI of oedema | 23.4 (IQR 15.4–28.4) | 22.4 (IQR 16.3–25.8) |
| Intramuscular tendon involvement, n (%) | 26 (59%) | 28 (64%) |
| 0% (no involvement) | 18 (40.9%) | 16 (36.4%) |
| <50% CSA | 9 (20.5%) | 10 (22.7%) |
| 50%–99% CSA | 9 (20.5%) | 12 (27.3%) |
| 100% CSA | 8 (18.2%) | 6 (13.6%) |
| Compliance to rehabilitation (in %) | 77 (IQR 60–96) | 76 (IQR 63–90) |
*For cases that do not add up to 44, data were missing.
ACL, anterior cruciate ligament; CSA, cross-sectional area; Inner, prone knee flexion 90°; MHFAKE, Maximum Hip Flexion Active Knee Extension test; Mid, prone knee flexion 30°; NRS, Numeric Rating Scale; Outer, supine knee/hip flexion 90°/90; PKET, Passive Knee Extension Test; SI, signal intensity; SLR, Straight Leg Raise test.