Purpose: To identify modifiable biomechanical and neuromuscular anterior cruciate ligament (ACL) injury risk factors for first‐time ACL injury in adolescent female elite football and team handball players.Methods: Adolescent female elite football and handball players with no previous ACL injury participated in the present study. At baseline, players were tested during side‐cutting manoeuvres performed in a 3‐dimensional motion analysis laboratory with concomitant electromyography (EMG) measurements. Maximal isometric lower limb muscle strength was assessed by handheld dynamometry. Players were prospectively followed for 2 years after baseline testing, and all magnetic resonance imaging (MRI) verified ACL injuries were registered. The effect of 16 risk factor candidates on the relative risk (RR) of ACL injury was estimated using Poisson regression analysis.Results: Ninety players (age 16.9 ± 1.2 years) were included in the analyses. Nine first‐time ACL injuries (injury incidence 10.0% (95% confidence interval (CI) 5.4–18.6%)) were registered during the 2‐year follow‐up period. Four risk factor candidates were significantly associated with the risk of ACL injury: (1) hip flexion angle at initial contact (IC) [RR 0.56, 95% confidence interval (CI) 0.34–0.92], (2) internal knee rotation angle at IC [RR 1.13, 95% CI 1.08–1.19], (3) semitendinosus EMG activity 50 ms prior to IC [RR: 0.62, 95% CI 0.43–0.89], and (4) external hip rotator strength [RR: 0.77, 95% CI 0.66–0.89].Conclusion: Four distinct ACL injury risk factors related to the side‐cutting manoeuvre were identified in a population of adolescent female elite football and team handball players with no previous ACL injury. As ACL injury typically occur during side‐cutting, intervention programmes to modify these risk factors pose a promising strategy for ACL injury prevention in adolescent female elite football and team handball.Level of evidence: II.

The aim of this study was to evaluate muscle activity during hamstring rehabilitation exercises performed in training machine compared with elastic resistance. Six women and 13 men aged 28-67 yrs participated in a crossover study. Electromyographic (EMG) activity was recorded in the biceps femoris and the semitendinosus during the concentric and the eccentric phase of hamstring curls performed with TheraBand elastic tubing and Technogym training machines and normalized to maximal voluntary isometric contraction-EMG (normalized EMG). Knee joint angle was measured using electronic inclinometers. Training machines and elastic resistance showed similar high levels of muscle activity (biceps femoris and semitendinosus peak normalized EMG >80%). EMG during the concentric phase was higher than during the eccentric phase regardless of exercise and muscle. However, compared with machine exercise, slightly lower (P < 0.05) normalized EMG values were observed using elastic resistance at 30- to 50-degree knee joint angle for the semitendinosus and the biceps femoris during the concentric and the eccentric phase, respectively. Perceived loading (Borg CR10) was significantly higher (P < 0.001) during hamstring curl performed with elastic resistance (7.58 ± 0.08) compared with hamstring curl performed in a machine (5.92 ± 0.03). Hamstring rehabilitation exercise performed with elastic resistance induces similar peak hamstring muscle activity but slightly lower EMG values at more extended knee angles and with higher perceived loading as hamstring curls using training machines.

Muscle strength assessment is fundamental to track the progress of performance and prescribe correct exercise intensity. In field settings, simple tests are preferred. This study develops equations to estimate maximal muscle strength in upper- and lower-extremity muscles based on submaximal elastic resistance tests. Healthy adults (n = 26) performed a maximal test (1 RM) to validate the ability of the subsequent submaximal tests to determine maximal muscle strength, with elastic bands. Using a within-group repeated measures design, three submaximal tests of 40%, 60%, and 80% during (1) shoulder abduction, (2) shoulder external rotation, (3) hip adduction, and (4) prone knee flexion were performed. The association between number of repetitions and relative intensity was modeled with both 1st and 2nd order polynomials to determine the best predictive validity. For both upper-extremity tests, a strong linear association between repetitions and relative intensity was found (R² = 0.97–1.00). By contrast, for the lower-extremity tests, the associations were fitted better with a 2nd order polynomial (R² = 1.00). The results from the present study provide formulas for predicting maximal muscles strength based on submaximal resistance in four different muscles groups and show a muscle-group-specific association between repetitions and intensity.

The aim of the present single‐case study was to investigate the effect of 6 weeks’ kettlebell training on the neuromuscular risk profile for ACL injury in a high‐risk athlete returning to sport after ACL reconstruction. A female elite soccer player (age 21 years) with no previous history of ACL injury went through neuromuscular screening as measured by EMG preactivity of vastus lateralis and semitendinosus during a standardized sidecutting maneuver. Subsequently, the player experienced a noncontact ACL injury. The player was screened again following postreconstruction rehabilitation, then underwent 6‐week kettlebell training, and was subsequently screened again at 6‐week follow‐up. Prior to and after postreconstruction rehabilitation the player demonstrated a neuromuscular profile during sidecutting known to increase the risk for noncontact ACL injury, that is, reduced EMG preactivity for semitendinosus and elevated EMG preactivity for vastus lateralis. Subsequently, the 6‐week kettlebell training increased semitendinosus muscle preactivity during sidecutting by 38 percentage points to a level equivalent to a neuromuscular low‐risk profile. An ACL rehabilitated female athlete with a high‐risk neuromuscular profile changed to low‐risk in response to 6 weeks of kettlebell training. Thus, short‐term kettlebell exercise with documented high levels of medial hamstring activation was found to transfer into high medial hamstring preactivation during a sidecutting maneuver.