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
 Football30 (68.2%)27 (61.4%)
 Futsal3 (6.8%)2 (4.5%)
 Handball4 (9.1%)5 (11.4%)
 Basketball4 (9.1%)4 (9.1%)
 Athletics1 (2.3%)4 (9.1%)
 Volleyball1 (2.3%)0
 Field hockey1 (2.3%)1 (2.3%)
 Rugby01 (2.3%)
Level of sports
 Professional40 (90.9%)42 (95.5%)
 Competitive4 (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 (%)
 Sprinting26 (59.1%)26 (59.1%)
 Stretching5 (11.4%)3 (6.8%)
 Kicking/shooting5 (11.4%)5 (11.4%)
 High kick0 (0%)1 (2.3%)
 Sliding/tackling3 (6.8%)0 (0%)
 Other5 (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)40/441/3
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)42/243/0
 ‘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)41/344/0
 ‘Mid’ position relative deficit (compared with contralateral leg, in %)58.8 (±30.7)50 (±26.3)
 Able to do ‘’outer’ test (yes/no)41/342/2
 ‘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 head36 (82%)37 (84%)
 Biceps femoris short head00
 Semimembranosus5 (11%)6 (14%)
 Semitendinosus3 (7%)1 (2%)
Grade I/II (modified Peetrons), n (%)
 Grade I20 (45%)18 (41%)
 Grade II24 (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 oedema15.2 (IQR 8.5–22.3)11.8 (IQR 5.5–20)
 Maximal SI of oedema23.4 (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% CSA9 (20.5%)10 (22.7%)
 50%–99% CSA9 (20.5%)12 (27.3%)
 100% CSA8 (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.