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Table 2 Summary of studies

From: Eccentric Strength Assessment of Hamstring Muscles with New Technologies: a Systematic Review of Current Methods and Clinical Implications

Study Sport (profile) Experimental design Main findings
2006 Tous-Fajardo et al. [14] Soccer and Rugby
(n = 20; M)
Age: 26 ± 4 y
Age: 25 ± 3 y
Cross-sectional study 1) The flywheel leg-curl device offered eccentric overload in a range of motion near complete extension about the knee joint. The magnitude of this overload is related to the athlete’s training background; in other words, the athletes which had previous experience using this novel technology showed greater eccentric strength performances than novice athletes of the same caliber. It was observed that biceps femoris muscle plays a more critical braking role than semitendinosus.
Device: YoYo (Technology AB, Stockholm, Sweden) instrumented with strain gauge (MuscleLab Force Sensor) to measure the force.
Parameters: peak force (N); average force (N); peak power (W); average power (W); peak velocity (m/s); average velocity (m/s). The data was recorded from 6 coupled concentric-eccentric actions. Parameters’ definition was not reported (bilateral analysis).
2013 Opar et al. [15] Australian Football, Rugby, Soccer and Sprinting
(n = 50; M)
Age: not reported
Elite and sub-elite
Reliability and case-control study 2) The experimental device to assess eccentric knee flexor strength showed high to moderate test-retest reliability for measurements when the Nordic hamstring exercise was performed bilaterally (ICC = 0.83–0.90 and SEM = 6–9%), but less reliability during unilateral testing (ICC = 0.56–0.73 and SEM = 10–11%). Moreover, between elite athletes who had a unilateral history of hamstring strain injury within the previous 12 months, there was a significant eccentric knee flexor weakness in their injured limb compared to their uninjured limb and to the limbs of uninjured recreational athletes.
Device: a novel device using uniaxial load cells (MLP-1 K; Transducer Techniques, Inc., Temecula, CA; NordBord prototype).
Parameters: peak force (average); the mean of the peak forces from 6 trials of Nordic hamstring exercise in absolute terms (N). Peak force (highest); the best of 6 trials of Nordic hamstring exercise in absolute terms (N), for each limb (left and right). For previously injured players, only the average peak force based on 6 bilateral trials was determined. Between-limb imbalance ratio was measured as the left-right limb ratio. These between-limb ratios were them converted to the percentage difference using log-transformed raw data followed by back transformation (bilateral and unilateral analyses).
2015 Bourne et al. [29] Rugby
(n = 194; M)
Age: 23 ± 4 y
Elite, sub-elite, and U19
Prospective cohort studies study 3) Lower eccentric knee flexor strength was not associated with an increased risk of hamstring strain injury (HSI). But, higher levels of between-limb imbalance (≥ 15%) were associated with a significantly increased risk of a subsequent HSI, and this was amplified in athletes who had suffered the same injury in the previous 12 months.
Device: a custom-made uniaxial load cells (Delphi Force Measurement, Gold Coast, Australia; NordBord prototype).
Parameters: peak force (highest); the best of 3 repetitions of Nordic hamstring exercise for each limb (left and right), in absolute terms (N) and relative to body weight (N/kg).
The between-limb imbalance was calculated as left/right limb ratio, for the uninjured group and as uninjured/injured limb ratio, for the injured group. These between-limb ratios were them converted to the percentage difference using log-transformed raw data followed by back transformation. In some analysis, to have a single measure of eccentric hamstring strength for each athlete, an averaging the peak forces from each limb (2-limb-average strength) was made (unilateral and bilateral analysis).
2015 Opar et al. [28] Australian Football (n = 210; M)
age: 23 ± 4
Elite
Prospective cohort studies study 4) Lower limbs that sustained a HSI were weaker than uninjured limbs at the start and end of preseason. Eccentric hamstring strength below 256 N at the preseason start and below 279 N at the end of preseason increases the risk of HSI. The between-limb imbalance in eccentric hamstring strength was not different between the injured and uninjured groups. And finally, the interaction between athlete age, history of hamstring strain injury, and eccentric hamstring strength provided better information on athlete’s injury risk profile than only athlete’s age and history of HSI.
Device: a custom-made uniaxial load cells (Delphi Force Measurement, Gold Coast, Australia; NordBord prototype).
Parameters: peak force (average); the mean of the peak forces from 3 trails at Nordic hamstring exercise for each limb (left and right) resulting in a left and right limb measure, reported in absolute terms (N) and relative to body weight (N/kg). For the athletes that remained injury free, the strength measurement of right and left limb was averaged. The between-limb imbalance was calculated as left/right ratio for uninjured group and as an uninjured/injured limb ratio in the injured group; these between limb ratios were them converted to the percentage difference using log-transformed raw data followed by back transformation (unilateral and bilateral analysis).
2015 Opar et al. [30] Australian Football
(n = 99; M)
Age: 23 ± 3 y
Elite
Prospective cohort study 5) Athletes with a unilateral history of HSI's displayed smaller increases in eccentric hamstring strength compared with the control group athletes, who had no history of injury, during a preseason training. Interestingly, the smaller increase in eccentric strength across the preseason was not restricted to the previously injured limb, as the contralateral limb displayed small increases too, which might suggest that the effects of a prior HSI may be centrally mediated.
Device: a custom-made uniaxial load cells (Delphi Force Measurement, Gold Coast, Australia; NordBord prototype).
Parameters: peak force (average); the mean of the peak forces from 3 contractions during Nordic hamstring exercise for each limb (left and right) in absolute terms (N) and relative to early preseason strength measure expressed as the late preseason/early preseason ratio. In some analysis, to have a single measure of eccentric hamstring strength for each athlete, an averaging the peak forces from each limb (2-limb-average strength) was made (bilateral and unilateral analysis).
2016 Buchheit et al. [16] Soccer and Australian Football
(n = 122; M)
Age: 22 ± 7 y
Elite, 4th Division, U21, U19, and U17
Cross-sectional study 6) Eccentric knee flexor strength, as assessed with the NordBord device is largely body mass dependent, but simply dividing eccentric strength by units of BM (i.e., N/Kg) may not be an optimal strategy. To control body mass (BM) effect, practitioners may compare actual test performances with the expected strength for a given BM, using the following predictive equation: eccentric strength (N) = 4 × BM (kg) + 26.1. Value deviating from body mass expected values by last 40 N (12%) may be considered greater or lower.
Device: a custom-made uniaxial load cells (Delphi Force Measurement, Gold Coast, Australia; NordBord prototype).
Parameters: peak force (highest); the best of 3 repetitions of Nordic hamstring exercise for right and left limbs expressed in absolute terms (N). In the between-limb imbalance, the average strength of left and right legs was used for analysis (bilateral analysis).
2016 Timmins et al. [31] Soccer
(n = 152; M)
Age: 25 ± 5y
Elite
Prospective cohort study 7) Athletes with biceps femoris long head fascicle length (BFlh) shorter than 11 cm were 4.1 times more likely to suffer a HSI, and low levels of eccentric knee flexor strength (below 337 N) increases in 4.4 times the risk of HSI. For every increase in eccentric knee flexor strength, the injury risk was reduced by 9%. No measure of MVIC strength or between-limb imbalances (eccentric strength and BFlh fascicle) led to a statistically increase in HSI risk.
Device: a custom-made uniaxial load cells (Delphi Force Measurement, Gold Coast, Australia; NordBord prototype).
Parameters: peak force (average); the mean of the peak forces from 3 contractions during Nordic hamstring exercise for each limb (left and right), reported in absolute terms (N) and relative to body weight (N/kg). Peak torque (average); the product of peak force (average) by the shank length (m) for each limb (left and right), reported in absolute terms (Nm) and relative to body weight (Nm/kg). The shank length (m) was determined as the distance from the lateral tibial condyle to the mid-point of the brace that was placed around the ankle during Nordic hamstring exercise. Between-limb imbalance: calculated as left/right limb ratio, for the uninjured group and as uninjured/injured limb ratio, for the injured group. These between limbs imbalance ratios were them converted to the percentage difference using log transformed was data followed by back transformation. In some analysis, to have a single measure of eccentric knee flexor for each athlete, an averaging the peak forces from each limb (2-limb-average strength) was made (unilateral and bilateral analyses).
2017 van Dyk et al. [32] Soccer
(n = 413; M)
Age: 26 ± 5 y
Elite
Prospective cohort study 8) Eccentric knee flexor strength assessed with the device did not differ between injured and healthy players and do not provide a clinical value in predicting risk of HSI. Muscle strength is part from a multifactorial complex model that may lead to injury.
Device: a custom-made uniaxial load cells (Delphi Force Measurement, Gold Coast, Australia; NordBord prototype).
Parameters: peak force (highest); the best of 3 repetitions of Nordic hamstring exercise for each limb (left and right), reported in absolute terms (N) and relative to body weight (N/kg). Peak force (average); the mean of the peak forces from 3 contractions during Nordic hamstring exercise for each limb (left and right), reported in absolute terms (N) and relative to body weight (N/kg). Between-limb imbalance was measured as the left-right limb peak force ratio (bilateral and unilateral analyses).
2018 Chalker et al. [33] Cricket
(n = 44; M)
Age: 18 ± 2 y
U23 senior sub-elite and school level
Interventional cross-over study 9) The augmented feedback significantly increased mean eccentric knee flexor force, with the majority of this increase occurring in the weaker compared to stronger limb. A single session with the use of real-time visual feedback during the performance of the Nordic hamstring exercise did not improve between limb knee flexor strength asymmetries, but resulted in a non-significant, albeit small to moderate effect size decrease in between limb force asymmetries.
Device: a custom-made uniaxial load cells (Delphi Force Measurement, Gold Coast, Australia; NordBord prototype).
Parameters: peak force (average); the mean of the peak forces from 3 contractions during Nordic hamstring exercise for each limb (left and right), reported in absolute terms (N). Between-limb imbalance; calculated as a left/right limb ratio of the peak force (average), using log transformed raw data followed by back transformation. In some analysis, to have a single measure of eccentric knee flexor for each athlete, an averaging the peak forces from each limb (2-limb-average strength) was made (unilateral and bilateral analysis).
2018 Isik et al. [34] Soccer
(n = 88; M)
Age: 16 ± 2 y
U19-U14
Cross-sectional study 10) The athlete’s body weight affects strength imbalance, and young soccer players with reported lower extremity injuries in the previous 2 years had similar eccentric hamstring strength imbalance in comparison to non-injured soccer players.
Device: NordBord Hamstring Testing device (Qutbluebox, Queensland, AUS).
Parameter: between-limb imbalance; calculated as stronger leg peak force (N) − weaker leg peak force (N), with each leg force data being obtained from the best of 3 repetitions of Nordic hamstring exercise for each limb (left and right), reported in absolute terms (N) (unilateral analysis).
2018 van Dyk et al. [23] Soccer
(n = 288; M)
Age: 25 ± 5 y
Elite
Prospective cohort study 11) There was a substantial variability for isokinetic measures between seasons, as demonstrated by the large measurement error for all the contraction modes. A poor correlation was found between peak isokinetic hamstring eccentric torque and peak eccentric knee strength measured during Nordic hamstring exercise. Also, no correlation between bilateral imbalances measured in isokinetic strength testing and Nordic hamstring exercise testing was found.
Device: a custom-made uniaxial load cells (Delphi Force Measurement, Gold Coast, Australia; NordBord prototype).
Parameter: peak force (highest); the highest value obtained from 3 repetitions for each limb (left and right) in absolute terms (N), resulting in a left and right limb measure. Peak force (average); the mean of the peak forces from 3 contractions during Nordic hamstring exercise for each limb (left and right), reported in absolute terms (N). The peak force (highest) from Nordic hamstring exercise was correlated to the peak torque of isokinetic eccentric contraction at 60°/s (r = 0.35; r2 = 12%). The between-limb imbalance was calculated as a percentage following the formula left/right limb ratio of peak force.
2019 Franchi et al. [35] Alpine Skiing (n = 170; 100 M, 70 F)
Age: 14 ± 1 y
Age: 22 ± 3 y
Elite and U15
Cross-sectional study 12) There is a description of maximal eccentric hamstrings strength in alpine skiers from the youth to the elite level. This study highlights the importance of biological maturation regarding maximal eccentric hamstrings strength values in youth athletes, more specifically those who are close to their growth spurts. This study presents novel data that may offer novel insights for anterior cruciate ligament injury prevention in Alpine Skiing.
Device: NordBord Hamstring Testing device (Vald Performance, Newstead, Australia).
Parameter: all participants performed one set of 3 repetitions with 5–10 s of rest between repetitions. The peak force was considered the maximum value between the 3 repetitions, for the left and the right limb. The limbs’ asymmetry during testing was calculated as the difference between stronger and weaker leg expressed as a percentage. The parameters analyzed were both right and left limbs as well as limbs asymmetry.
2019 Hegyi et al. [27] Soccer and Rugby
(n = 13; M)
Age: 23 ± 3 y
Regional
Cross-sectional study 13) During Nordic hamstring exercise with hip flexed in 90°, there were higher eccentric knee flexor torque and lower hamstring EMG (biceps femoral long head and semitendinosus) levels compared to Nordic hamstring exercise with hip in neutral position in most phases of movement. There was similar peak eccentric knee flexor torque and EMG levels in Nordic hamstring exercise performed unilateral and bilateral modes, independently of hip position. During Nordic hamstring exercise with hip in neutral position, there was higher semitendinosus activity during the movement’s early phase and lower during movement’s final phase (toward full knee extension) than biceps femoral long head activity. During Nordic hamstring, exercise with hip flexed in 90°, there was higher semitendinosus activity than biceps femoral long head activity in the second half of bilateral movement and in the final phase of unilateral movement.
Device: a device with load cells (ELAF, 1250 N; TE Connectivity, Schaffhausen, Switzerland) and a potentiometer (P4500; Novotechnik, Ostfildern, Germany).
Parameter: peak torque (average); mean of instantaneous force in the dominant leg (N) multiplied by the lever arm (distance between femur’s lateral epicondyle and the center of the load cells) in meters, recorded in two repetitions.
2020 Giakoumis et al. [26] Track and Field
(n = 44; 23 M, 21 F)
Age: 19–33 y
Elite
Cross-sectional study 14) Male athletes produced greater absolute force levels compared to female athletes, but relative force (normalized to body weight) was similar between sexes. Long sprinters (400 m) presented stronger right leg than left leg, and short sprinters (100 m/200 m/110 m) showed similar strength between legs. There were no differences in eccentric hamstring strength between previously injured and uninjured athletes.
Device: NordBord Hamstring Testing device (Qutbluebox, Queensland, AUS).
Parameter: peak force (highest); the highest value obtained from 3 repetitions for each limb (left and right) in absolute terms (N) and relative to bodyweight (N/kg), resulting in a left and right limb measure. Peak force (average); the mean of the peak forces from 3 contractions during Nordic hamstring exercise for each limb (left and right), reported in absolute terms (N) and relative to bodyweight (N/kg). Peak torque (highest) (Nm); calculated by multiplying the length of the shank by the peak force (highest), reported in absolute terms (Nm) and relative to body weight (Nm/kg). The shank length (m) was determined as the distance from the lateral tibial condyle to the mid-point of the brace that was placed around the ankle during Nordic hamstring exercise. Between-limb imbalance: calculated by dividing torque or force of right limb by the measures of left limb. 0% indicates no imbalance, imbalance > 0% indicates more force/torque on the right side (unilateral and bilateral analysis)
2020 Markovic et al. [22] Soccer
(n = 155; M)
Age: 11–17 y to 19–30 y
Elite and U18
Cross-sectional study 15) The absolute and relative Nordic hamstring strength increases with players’ age, but this increase was not linear and there was an abrupt increase in Nordic hamstring strength in U16 category. In general, the body size is largely responsible for the observed age-related increase in absolute Nordic hamstring strength. Bilateral Nordic hamstring strength asymmetry varied non-significant (8–16%) between age groups, with the highest asymmetries being observed in U12 and U13 age groups (> 15%).
There was a large negative correlation between eccentric knee flexor strength and sprint performance, with 27% spring performance variance being explained by relative Nordic hamstring strength (Nm/kg).
Device: a custom-made uniaxial load cells (FL34-100 kg; Forsentek Co., Shenzhen, China; NordBord prototype).
Parameter: peak force (highest): the highest value obtained from 3 repetitions for each limb (left and right) in absolute terms (N) and relative to bodyweight (N/kg), resulting in a left and right limb measure. Peak torque (highest): the product of peak force by the shank length (m), left and right, in absolute terms (Nm) and relative to body weight (Nm/kg). The shank length (m) was determined as the distance from the lateral tibial condyle to the mid-point of the brace that was placed around the ankle during Nordic hamstring exercise. The bilateral strength asymmetry of knee eccentric flexor strength was expressed in % following the formula: 1 − (dominant leg strength/non-dominant leg strength) × 100. In some analysis, to have a single measure of eccentric knee flexor for each athlete, an averaging the peak forces or peak torque from each limb (2-limb-average strength) was made (unilateral and bilateral analysis).
2020 Ribeiro-Alvares et al. [25] Soccer
(n = 210; M)
Age: 24 ± 5 y
Elite
Cross-sectional study 16) Previously injured players presented strength deficit on injured limb in relation to their contralateral limb and to the uninjured player’s limb. Previously injured and uninjured players displayed similar between-limb asymmetry values, but half of the previously injured players and 37% of uninjured players presented between limb strength asymmetry > 10%.
Device: device was based on the prototype validated by Opar et al. (2013) with two independent commercially available load cells (Elastic, E-sporte Soluções Esportivas, Brasilia, Brazil).
Parameter: peak force (highest); the highest value obtained from 3 repetitions for each limb (left and right) in absolute terms (N), resulting in a left and right limb measure. Between-limb imbalance: calculated as the player’s stronger limb minus the weaker limb with the stronger limb being used as the reference value (i.e., 100%). For the injured group, limbs were analyzed separately; for the control group, the eccentric knee flexor peak force was average between limbs (bilateral and unilateral analyses).
2020 Vicens-Bordas et al. [24] Soccer
(n = 284; M)
Age: 23 ± 4y
Regional
Cross-sectional study 17) Athlete’s age was negative associated with preseason eccentric hamstring strength with a mean reduction on knee flexor strength of 0.9% per year increased. Players with previous hamstring injury duration of more than 3 weeks had 9% lower preseason knee flexor eccentric strength compared to players with no previous hamstring injury.
Device: a custom-made uniaxial load cells (Delphi Force Measurement, Gold Coast, Australia; NordBord prototype).
Parameter: peak force (average); the mean of the peak forces from 3 contractions during Nordic hamstring exercise for each limb (left and right), reported in relative terms (N/kg). Between-limb imbalance: analyzed using the formula: (strongest limb − weakest limb)/(sum of both limbs) (bilateral and unilateral analyses).
  1. Cross-sectional: consist in assessing a population at a single point in time, cannot demonstrate temporality, therefore can present prevalence and associated factors [36]
  2. Interventional cross-over study: interventional study where participants underwent for all treatment’s arms
  3. Prospective cohort study: it is suggested to be the gold standard of observational research and can identify the potential risk factors for an injury or disease, due to the temporality [36]
  4. Reliability and case-control study: in a reliability study design, researchers are interested in measuring the consistency of some measure across time, the consistency of people’s responses on a measure, and the consistency of different observers and their judgments of some measures. Case-control study designs refers to retrospective observational study in which participants are identified and selected based on their case status (i.e., injured or not injured); this type of study allows the establishment of a statistical association between the exposure for some variables and outcomes [36]
  5. n number of subjects, F female, M male, y years old, N Newton, BM body mass, Kg kilogram, HSI hamstring strain injury, ICC intraclass correlation coefficient, SEM standard error of measurement