Issues related to standardization, normalization, and reliability

Buchheit et al. [42] reported a large correlation be- tween eccentric knee-flexor strength, measured using the Nordbord during NHEs, and BM (r = 0.55, 90% confidence limits: 0.42;0.64). In their study, they found that heavier and older athletes outperformed lighter athletes. To address variations in BM, the authors recommended using a regression equation (eccentric strength in N = 4 × BM in kg + 26.1) to estimate ex- pected strength values based on individual BM (N/kg). This approach allowed for a more accurate compari- son of actual test scores using the Nordbord among players with differing body masses. Although it could sound logical to use mathematical regression analysis for further prediction, we are not aware if the formula has been validated. In addition, to their findings, Buch- heit et al [42] observed that professional soccer players displayed 20% greater relative strength than the ex- pected values for their BM compared to youth players.

In a study conducted by Wik et al. [43] on profes- sional soccer players in Qatar, they explored physical characteristics and positional differences. Goalkeep- ers were found to have the greatest BM compared to other positions (p < 0.05) and exhibited greater abso- lute strength compared to strikers and midfielders. However, when comparing eccentric hamstring strength measured using the Nordbord to the expected results based on BM, defenders reported significantly higher values (p < 0.05, effect size d = 0.80) than goalkeepers. This suggests that defenders demonstrate remarkable eccentric hamstring strength in relation to their BM.

Soós et al. [44] conducted a study on Hungarian youth male soccer players and reported that goalkeep- ers displayed significantly higher scores for dominant (p = 0.01) and non-dominant (p = 0.04) legs on the Nordbord compared to other outfield players, espe- cially midfielders. There was no difference between age groups for HES, which contradicts Buchheit et al. [42]. Notably, this study did not standardize the results based on individual BM, and the values provided are absolute scores.

Sherwood et al. [45] observed significant age-re- lated effects on HES among elite youth soccer players. They found that older age groups, specifically U18 and U23, exhibited significantly higher average HESs than the younger U16 group. However, no significant differ- ences were observed between the U18 and U23 players. These age-related differences in HES were attributed to the influence of structured strength and condition- ing programs commonly found in elite-level acade- mies, as well as natural growth and maturation. The authors concluded that the lack of difference between U18 and U23 groups was due to a shift in training focus from greater strength volume to more technical and tactical-oriented training.

In a review by Claudino et al. [46] covering four new technologies for evaluating ESKF, the Nordbord de- vice emerged as the most commonly used instrument for measuring HES. Its popularity stems from its cost- effectiveness compared to isokinetic dynamometers and its technical superiority compared to handheld dynamometers. They found that the bilateral assess- ment of the NHE using Nordbord demonstrated high to moderate test-retest reliability (ICC = 0.83–0.90, SEM = 6–9%), while lower reliability was observed during unilateral testing (ICC = 0.56–0.73 and SEM = 10–11%). Reliable results for between-limb imbalances were achieved when peak forces were averaged over six trials (ICC = 0.85; 95% CI 0.71–0.93; SEM = 5%, 95% CI 4–6%). Importantly, Claudino et al. [46] high- lighted the relatively poor correlation between forces measured by the Nordbord and peak isokinetic ham- string eccentric torque from the gold standard isoki- netic dynamometer (r = 0.35; r² = 12%). They empha- sized the need for standardized methods to assess ESKF on the Nordbord, including averaging results from six trials and normalizing for BM.

In contrast, Ogborn et al. [47] reported highly sig- nificant and moderate to high correlations between eccentric hamstring torque measured on the Nord- bord and isometric torque measured on an isokinetic dynamometer (r = 0.61–0.86). These correlations varied slightly with different body and foot positions. Addi- tionally, peak eccentric hamstring force measured on the Nordbord demonstrated high test-retest reliability, with ICC values of 0.993 (95% CI 0.983–0.997) for the left leg and 0.992 (95% CI 0.979–0.997) for the right leg.

Bishop et al. [48] monitored 14 professional foot- ballers weekly for eccentric strength over six weeks. They reported moderate reliability between test ses- sions, with ICC values ranging from 0.54 to 0.73. Ab- solute reliability remained under the 10% threshold, with only a 0.06% increase between sessions 1 and 2 before dropping below 10% for subsequent sessions. The authors concluded that two familiarization ses- sions were necessary to achieve reliable peak force measurements using the Nordbord for the NHE.

The study by Rhodes et al. [49] aimed to establish the reliability of quantifying HES in elite youth foot- ballers using both the Nordbord and an isokinetic dynamometer. Their findings revealed remarkable dif- ferences between the two devices, with the Nordbord displaying greater variance in coefficient of correla- tion measurements compared to the isokinetic dyna- mometer.

Specifically, out of a plethora of data measurements gathered, greater variance in metrics was observed for the right limb concerning peak force and peak torque. In contrast, the difference between the left and right limbs was smaller for average force and average torque. Noteworthy, the study does not specify whether the differences observed between right and left limbs were due to dominance/preference. Nevertheless, statisti- cally significant correlations were identified between the two devices for several strength measures, as seen in Table 2.

Importantly, statistically significant correlation coefficients were reported for all metrics on both the isokinetic dynamometer and Nordbord (p < 0.01). How- ever, there were no significant correlations between the two devices for any other strength measurements, indicating some discrepancies in the assessment of HES.

These findings highlight the importance of consid- ering the specific metrics and measures used when comparing Nordbord and isokinetic dynamometer results. While some metrics exhibited statistically sig- nificant correlations, others did not, highlighting the potential variations in outcomes when using these devices.

Fernandes et al. [50] investigated the reliability of the Nordbord in male youth footballers aged 10–16 years, considering maturation profiles (pre and post- PHV). They reported coefficients of variation (CVs) across age groups for various measures of HES using the Nordbord, including left (5.6–7.4%), right (5.9– 8.7%), bilateral (6.1–7.4%), and relative peak force (6.3–6.4%). These CVs were generally favorable; how- ever, none of the typical errors (TEs) were able to detect the smallest worthwhile change (SWC). Additionally, the peak force imbalance between the left and right limbs demonstrated poor agreement between trials (CV = 33.1–38.3%). The reliability of HES measure- ments for pre-PHV players was generally better than that of post-PHV players, as indicated by lower CVs for the left (5.7 vs. 6.9%), right (4.8 vs. 8.5%), bilateral (4.9 vs. 7.3%), relative peak force (5.0 vs. 7.2%), and imbalance (35.3 vs. 36.1%).

Read et al. [51] examined 203 professional foot- ballers in Qatar, assessing HES using the Nordbord and other strength-related variables for quadriceps and hip muscles. The study aimed to establish a thresh- old for limb asymmetry, but due to variations in test- ing procedures between the Nordbord and the isoki- netic dynamometer, the authors recommended that testing should be task-specific. They reported that athletes categorized as having extremely high asym- metry (a difference of > 9% on the Nordbord between limbs, with the principal component being quadri- ceps and hamstring isokinetic strength asymmetry) exhibited reductions in unilateral jump height. No- tably, there were no significant positional differences in HES on the Nordbord, which contradicted the find- ings of a previous study by Wik et al. [43].

These studies stress the importance of consider- ing factors such as maturation profiles and task-spe- cific testing when assessing HES using the Nordbord. The findings also highlight the relevance of limb asym- metry thresholds and the potential impact of these fac- tors on athletes’ performance in related activities like jump height and on-field positions.

Other studies emphasized the value of the NHE as a reliable and practical tool for assessing hamstring strength and monitoring changes in inter-limb sym- metry, particularly in the context of female elite foot- ball players. For instance, the study conducted by Cuthbert et al. [52] provides valuable insights into the reliability and suitability of various tests, including the NHE and kneeling 90° knee flexion (kneeling ISO), for assessing hamstring strength and inter-limb asym- metry in elite female football players. Here, we present a concise summary of their findings.

The authors reported good to excellent between- session reliability for the NHE on the Nordbord, with intraclass correlation coefficients (ICC) exceeding 0.901 and a lower-bound 95% CI above 0.790. When testing kneeling 90° knee flexion on the Nordbord, they found fair to excellent reliability for the left limb, with ICC values ranging from 0.798 to 0.909 and a lower-bound 95% CI from 0.530 to 0.788.

In terms of inter-limb asymmetry, they observed that the magnitude of asymmetry for the kneeling ISO was –2.76% to +13.94%, while for the NHE, it ranged from 3.94% to 7.41% between sessions. The direction of asymmetry between sessions demonstrated kappa coefficient values that indicated slight to moderate agreement (k = 0.03–0.47) for isometric assessments and substantial agreement (k = 0.62) for the NHE.

Based on their findings, Cuthbert et al. [52] con- cluded that the NHE on the Nordbord was the most reliable test both within and between sessions. It also demonstrated substantial agreement in identifying inter-limb asymmetry between sessions when used as a bilateral test. This suggests that the NHE on the Nordbord is the most appropriate method for assess- ing hamstring strength and inter-limb asymmetry over time in elite female football players.

The results of the above-reviewed studies present a conflicting landscape when it comes to comparing the reliability of the Nordbord with the gold standard, the isokinetic dynamometer. Ogborn et al. [47] con- ducted a comprehensive analysis of various methods for testing knee flexor muscle strength and reported a high degree of correlation between Nordbord meas- urements and isometric knee flexion torque as meas- ured via the Biodex Dynamometer (r = 0.61–0.86). They also found high test-retest reliability for Nordbord measurements, with values of 0.993 (95% CI 0.983– 0.997, n = 19), and established a Minimal Detectable Change with 95% CI of 26.88 N and 28.76 N for the left and right limbs, respectively. Similarly to Rhodes et al. [49], the author does not specify the dominant limb of the participants. However, there are only a couple of Newtons within these 95% CI to detect a significant im- plication. These findings are in line with several other authors [38, 39, 53] who have also found Nordbord to be a reliable alternative for measuring HES.

However, the literature also includes studies sug- gesting contradictory results. Authors have indicated that Nordbord may have poor correlation with the isoki- netic dynamometer [46, 54], greater variance in the coefficients of correlation measurements [49], and even a tendency to overestimate HES [55].

This divergence in findings raises uncertainty about the extent to which Nordbord can offer a reliable and cost-effective alternative to the expensive gold standard of the isokinetic dynamometer. As suggested by Clau- dino et al. [46], there is a pressing need for standard- ized methods for assessing the ESKF on the Nordbord. Additionally, a regression equation, such as the one proposed by Buchheit et al. [42], is required to be vali- dated and tailored specifically for football players. Furthermore, the literature reveals a gap in our under- standing of the effects of age, position, gender, and the level of participation for footballers when it comes to Nordbord measurements. These factors need to be con- sidered to establish normalized data that can assist practitioners in various footballing contexts and levels in assessing the HES of their players.

In short, while Nordbord shows promise as a prac- tical and cost-effective tool for assessing HES, the con- flicting findings in the literature and the identified gaps in research suggest the need for further investigation, standardization, and refinement to fully establish its reliability and utility in the context of football. This will be essential to ensure its effectiveness as a valu- able alternative to the isokinetic dynamometer.

The effects of training volume and protocol specificity

Some studies suggest that increasing training vol- ume may not necessarily lead to greater improvements in HES. The findings of this review also reveal the im- portance of the specificity of training protocols and the need for monitoring and assessing different aspects of strength and performance in footballers. For exam- ple, Lacome et al. [56] conducted a study comparing the effects of low and high volumes of NHE and stiff-legged deadlifts in elite U19 French footballers. After a 6-week training phase, both the low-volume and high-volume groups exhibited moderate increases in HES, along with small gains in biceps femoris and semimembra- nosus fascicle length. Notably, there were no significant differences or further improvements between the two groups during the subsequent 6-week phase, suggest- ing a potential ceiling effect. As a result, the authors recommended the use of low-volume, high-intensity eccentric exercises as an effective part of injury preven- tion protocols, as they were just as effective as high- volume protocols.

Amundsen et al. [54] conducted a randomized trial involving female soccer players from a Norwegian Di- vision two football team. This study aimed to determine the effects of high-volume and low-volume Nordics ex- ercise protocols on HES, jump height, and sprinting performance. After 8 weeks of training, both the high- volume and low-volume groups showed significant improvements in maximal eccentric force on the Nord- bord, regardless of the added weight. However, there was no significant difference between the groups for maximal eccentric force. The study also found poor cor- relations between Nordbord assessments and isoki- netic dynamometer measurements, as well as limited changes in sprinting and jump performance. These results indicate that the training volume did not sig- nificantly impact HES in this context.

In addition to the specificity of exercise protocols used in training programs that can influence the out- comes, other findings suggest that NHE interventions may have a positive impact on certain aspects of sprint- ing and change of direction (COD) performance, par- ticularly in amateur football players. However, the re- lationship between HES and sprinting ability is complex and may not be a significant predictor of sprint per- formance.

In a 10-week intervention with amateur football players, Ishoi and their team [20] found that partici- pants who underwent NHE showed improvements in their sprinting performance. Although the group dif- ferences in total sprint times were not statistically sig- nificant, they favored the intervention group. Signifi- cantly, the intervention group exhibitedimprovements in the fastest sprint time, with a very likely medium effect size, and also in the last 10 m sprint time. The improvements in total sprint times, fastest 10 m sprint time, and last 10 m sprint time ranged from 1.8% to 3.2%. These findings suggest that NHE interventions may positively impact sprinting performance among amateur football players.

However, in a study involving youth soccer players, Jones et al. [18], examined the relationship between body weight-scaled HES measurements using the Nordbord and COD performance. They found that body weight-scaled HES was significantly related to COD performance, specifically on a 505-agility test. How- ever, when predicting sprinting ability at various dis- tances (5, 10, and 20 m), HES did not emerge as a sig- nificant predictor. Additionally, the study found that imbalances in HES had minimal effects on both sprint- ing and COD performance. These results indicate that HES may play a more prominent role in COD perfor- mance than in sprinting ability.

Adding to the above, Harøy et al. [55], compared two training programs: one following the standard FIFA 11+ program with NHE and the other replacing NHE with the Copenhagen Adduction (CA) exercise. They found no significant differences in HES between the two programs. However, the group following the stand- ard FIFA + program showed a significant within-group increase in HES. Neither group exhibited significant improvements in sprint times at various distances (5, 10, 15, and 20 m). The study also highlighted the potential overestimation of HES measured on the Nordbord com- pared to the isokinetic dynamometer, in line with previous research.

Similar to the different findings reported in sprint- ing and agility, a few studies provided insights into the effects of various NHE interventions on hamstring strength among football players. While some studies reported no significant changes [22], others demon- strated improvements in hamstring strength [20, 54], highlighting, yet again, the need for a better understand- ing of the specific protocols, participant characteristics, and intervention durations influencing the outcomes.

In a study involving 20 professional football players over 24 weeks with biweekly interventions, Quiceno et al. [57] examined the effects of a hamstring-specific training protocol. The study reported no significant changes in hamstring strength pre- and post-interven- tion. However, it’s essential to note that the participants’ scores were not standardized according to their body weight. Further details about the intervention are nec- essary to make meaningful comparisons with other studies.

However, Quiceno et al. [58] observed the effects of an 8-week intervention performed at a biweekly fre- quency in female Colombian football players. The in- tervention plan included NHEs with incremental vol- ume and other exercises, such as Kbox squats with progressive overload. The study reported a significant difference between pre- and post-intervention, indi- cating an improvement in hamstring strength.

Similarly, Drury et al. [59] investigated the effects of a 6-week NHE program among youth male soccer players. The participants were divided into pre-PHV and mid/post-PHV groups and further categorized into control and experimental groups (1–2 sessions of NHE per week). Within-group analyses showed an increase in relative peak force in both experimental groups, with a greater effect size observed in the pre-PHV group com- pared to the mid/post-PHV group. Between-group analyses revealed moderate increases in hamstring strength in both maturity groups, with the pre-PHV group showing a larger effect size.

The studies presented in this section highlight the intricate relationship between ESKF, sprint perfor- mance, and the role of NHEs in enhancing hamstring strength. Notably, ESKF is associated with improved sprint performance, emphasizing the significance of hamstring strength in athletic endeavors [60]. Several studies, including Ishøi et al. [20], Siddle et al. [21], and Bautista et al. [22], have proposed NHE as a crucial element for enhancing both hamstring strength and sprint performance.

Additionally, Opar et al. [61] discovered that pre- vious HIs had a negative impact on ESKF improve- ments during the pre-season. Players with a history of injuries experienced smaller gains in ESKF, which might be a risk factor for potential future injuries. Fur- thermore, sprint performance tends to decline during the off-season, independent of previous HIs, age, or the duration of the break [5]. This suggests that eccentric strengthening of the hamstring during the pre-season is vital for improving sprint performance and reducing the risk of HIs, underscoring the importance of tools like the Nordbord for assessment and monitoring.

From a physiological perspective, longer fascicle lengths of the muscles are associated with better sprint performance [62]. While Mendiguchia et al. [63] ar- gued that NHEs did not significantly impact biceps femoris long head fascicle length (BFlh FL) or sprint performance, and that sprint training was more effec- tive. Behan et al. [64] and Lacome et al. [56] reported positive outcomes from NHE interventions, including increased HES and BFlh FL. Behan suggested that con- sistent low-volume NHEs may not yield significant improvements, while Lacome did not make this dis- tinction but reported no increase in the second stage of their study. The differences in results may be attrib- uted to variations in intervention protocols, particu- larly the volume of NHE performed.

Lastly, it’s crucial to note that both HES and BFlh FL can be enhanced through acceleration or maximum speed training, serving as a method for HI prevention [65]. Given the critical role of ESKF in sprint perfor- mance, stakeholders can utilize tools like the Nordbord to monitor changes in hamstring strength through- out pre-season and during the season, helping to op- timize performance and reduce the risk of injuries.

Issues related to the effect of fatigue on ESKF

This section summarizes findings that provide val- uable insights into the effects of fatigue and physicalactivity on ESKF, emphasizing the importance of con- sidering variables such as absolute and relative strength, inter-limb differences, and physical demands in sports and training programs. The following studies shed some light on the complex relationship between ham-string strength, sprint performance, and stability, pro- viding more understanding of the multifaceted nature of athletic performance and injury prevention in soccer players.

In a study conducted by Brogden et al. [66], the ef- fects of fatigue on ESKF were investigated in a group of male academy soccer players using the Nordbord device. The results showed significant differences in absolute ESKF before and after the Yo-Yo Intermittent Recovery Level 1 (YYIR Level 1) test. Both dominant and non-dominant limbs displayed increased ESKF values post-exercise compared to pre-exercise. Addi- tionally, bilateral differences were observed, with the dominant limb ESKF being significantly higher than the non-dominant limb ESKF post-exercise. The study also revealed differences in ESKF relative to BM, both pre- and post-exercise, between the dominant and non- dominant limbs. Importantly, the authors noted that the Nordbord device was sensitive enough to detect these differences in absolute, relative strength, and in- ter-limb dominance for ESKF. Moreover, no signifi- cant correlations were found between ESKF meas- urements and the distance covered during the YYIR Level 1 test, suggesting that ESKF changes were not influenced by aerobic fitness.

In another study by Madison et al. [67], the effects of different small-sided games (SSGs) on hamstring isometric torque of the preferred kicking side were examined. Two SSG formats, one with a smaller play- ing area (3 v 3 in 300 m²) and the other with a larger area (4 v 4 in 1000 m²), were compared. The results indicated that the larger SSGs led to significantly greater physical demands, including increased total distance covered, distance covered in different speed zones, max speed, accelerations, decelerations, meta- bolic power, and heart rate. Importantly, the study re- vealed that SSGs with a larger relative area resulted in greater reductions in peak and mean isometric forces of the hamstring at a 90-degree knee angle. Post hoc tests confirmed significant differences in the peak and mean force reductions between the small and large SSG conditions. Additionally, the number of accelerations in both SSGs was positively correlated with the change in force at a 90-degree knee angle, highlighting the relationship between acceleration and hamstring force reduction during these activities.

Shah et al. [68] conducted a study to investigate the effects of weekly sprint distance and maximal velocity exposure on HES in professional soccer players. They observed an inverse relationship between total weekly sprint distances and the percentage change in HES, with a very small correlation. Similarly, they found an inverse relationship between total weekly efforts at maximal sprint speed above 90% and the percentage change in HES, albeit with a very small correlation. The mean percentage change in HES was influenced by weekly efforts at maximal sprint speeds above 90%, showing significant differences between specific sprint effort ranges. They also reported variations in HES among different player positions, with attackers having the highest HES, followed by defenders and midfield- ers. However, they did not provide a direct comparison of HES between positions, making it challenging to interpret the significance of these findings.

On the other hand, Brünn et al. [69] found no sig- nificant correlation between HES and a higher sprint- ing speed on either the right or left side. Instead, they identified correlations between HES and weight and age, which align with findings from other studies in this area.

In contrast, Rhodes et al. [70] reported significant correlations between various functional hamstring strength metrics measured with the Nordbord and pos- teromedial and posterolateral stability, indicating the relationship between HES and aspects of stability. These findings suggest that hamstring strength is associated with stability in specific directions, which has implica- tions for injury prevention and athletic performance. However, no significant correlations were observed between HES metrics and anterior stability.

The studies discussed in this section highlight the impact of fatigue on ESKF and knee stability in soccer players. It is evident that a football match or intense soccer-specific activities can lead to a reduction in ESKF and knee stability, which may persist for several days. This reduction in strength and stability could potentially increase the risk of HIs and other lower limb issues in players.

Draganidis et al. [71] emphasized that the total ec- centric actions during a match, combined with the player’s physical condition, play a crucial role in deter- mining the loss of strength, which can last for up to 60 hours post-match. Matthews et al. [72] showed that a conditioning program focusing on muscular strength or endurance could mitigate the loss of eccentric ham- string strength during and after soccer activities. Kakavas and Kekelekis [73] and Salter et al. [74] pre- sented mixed results, with Kakavas reporting a sig- nificant decrease in ESKF after matches and Salter finding no substantial changes following sim- ulated soccer play.

The discrepancies in the findings might be attrib- uted to variations in the test protocols, such as the use of a friendly match, a simulated soccer match, or fa- tigue-inducing exercises. The choice of test may influ- ence the level and duration of strength reduction ob- served. The importance of utilizing tests that accurately replicate the eccentric strength demands of a real foot- ball match is crucial for obtaining reliable data.

Greig [75] and Dewig et al. [76] provided insights into the relationship between fatigue and knee stability. Greig found that fatigue led to a significant reduction in ESKF and an increase in knee varus at touchdown. Dewig associated hamstring strength with a decrease in various moments during single-leg landings, indi- cating the importance of strong hamstrings in main- taining knee stability under fatigue.

All in all, the use of tools like the Nordbord can as- sist performance specialists in evaluating a player’s fatigue level and monitoring recovery following matches or intense training sessions. This information can be invaluable for coaches and medical staff in making informed decisions about player readiness and injury prevention strategies. However, more research and data are needed to better understand the effects of fatigue on HES during different post-match and post-training time points, such as match day +1, +2, and +3, to pro- vide a comprehensive picture of a player’s recovery dynamics.

The Nordbord as a tool for injury prevention, return to play, and performance monitoring in football

The literature search suggests that previous lower extremity (LE) injuries or non-contact lower limb in- juries (NCLLI) may not have a substantial impact on hamstring strength or imbalance [60, 61]. Factors such as body weight and baseline measurements appear to play a more significant role in strength differences among players [77]. However, imbalances in ESKF may be associated with an increased risk of non-contact ACL injuries [78]. The Nordics exercise has been dem- onstrated as a feasible and well-accepted tool for as- sessing hamstring strength in soccer players [39, 50].

The effect of previous LE injuries on hamstring strength imbalance (HSI) in soccer players has been a topic of investigation in the literature. Isik et al. [77] conducted a study with young soccer players aged 14 to 19 and found no significant effect of previous LE in- juries on HSI. This suggests that prior LE injuries did not have a discernible impact on the imbalance of ham- string strength. It is noteworthy that body weight had a significant effect on HSI, which aligns with previous research, indicating the influence of body weight on this measure.

Similarly, Jones et al. [60] explored the impact of NCLLI on hip, groin, and knee flexor strength devel- opment in male academy footballers. They reported that players who sustained an NCLLI had statistically stronger absolute strength for adduction, abduction, and ESKF when compared to their uninjured coun- terparts. However, when the data was controlled for baseline measures and scaled for body weight, these differences were not significant. This suggests that the differences in strength were likely influenced by body weight and baseline measurements rather than the injury itself. The study also highlighted the potential for strength fluctuations during the season, with stronger players at the beginning of the season losing strength while players with less strength initially gained strength over time.

Collings et al. [78] investigated the relationship be- tween various factors, including isometric hip abduc- tor and adductor strength, ESKF, countermovement jump (CMJ) kinetics, and previous ACL injury history, in the context of non-contact ACL injury prediction. The study found no significant relationship between directional leg asymmetry in strength, ESKF, or CMJ kinetics and the likelihood of sustaining an ACL in- jury. However, non-directional between-leg asymme- try for ESKF had a significant odds ratio, suggesting that imbalances in ESKF may be associated with an increased risk of non-contact ACL injuries.

Haller et al. [79] focused on the feasibility and ac- ceptance of a series of tests and questionnaires to eval- uate the training load in soccer players. The NHE was included as a part of the strength and conditioning ses- sion before games. The study found that the best per- formance in Nordics most often occurred on the first attempt. Additionally, the testing procedures, includ- ing the NHE, were generally well-accepted by the play- ers, with only a small amount of missing data attrib- uted to injury or discomfort during the test.

The utilization of ESKF as a performance meas- urement in football has gained significant attention, particularly in the context of injury prevention, reha- bilitation, recovery, and sprint performance. Research suggests that strength deficits, inter-limb imbalances, and previous HIs are predictive risk factors for future injuries in football [8, 80]. It has been proposed that players with ESKF below a certain threshold, such as 2.4 Nm/kg [80], may be at an increased risk of acute HIs, with age potentially exacerbating this risk [7, 81]. The Nordbord, as a tool for assessing ESKF, can be instrumental in monitoring and mitigating this risk.

Numerous studies have suggested NHEs as an ef- fective injury prevention measure to reduce the risk of HIs [6, 82, 83]. However, the success of such pro- grams is closely tied to player compliance [82]. While many studies have mentioned imbalances in hamstring strength between the previously injured and non-in- jured limbs, some research, like the study by Giako- umis et al. [84], has found no association between previous injuries and hamstring strength, indicating the complexity of this relationship. Additionally, there are no significant differences in relative force per kilo- gram between males and females among elite track and field athletes [84]. The Nordbord offers a quick and precise method for measuring hamstring strength and identifying imbalances, which can inform and en- hance injury prevention programs.

Regarding rehabilitation from HIs, a consensus among stakeholders often involves reaching “pre-in- jury levels” and achieving “similar strength” as core criteria for the return to play (RTP) decision-making process [85]. Recent research has proposed HI reha- bilitation protocols that focus on eccentric hamstring strengthening [9–15]. The Nordbord plays a critical role in this context, offering an accurate and objective means of assessing hamstring strength and monitor- ing progress on a daily basis. By using the Nordbord, therapists can ensure that the criteria for RTP are met, contributing to more effective and individualized re- habilitation programs.

In summary, the Nordbord has the potential to significantly impact injury prevention, rehabilitation, and performance monitoring in football. It provides a quantifiable and objective measurement of EKFS, which can be invaluable for stakeholders in making informed decisions related to a player’s health and performance. However, further research is needed to establish normative levels of ESKF for football players at different levels and to better understand its asso- ciations with previous HIs and future injury risk.

The use of Nordbord to assess player recovery, mitigate injury risk, and enhance performance

Recovery and injury prevention are crucial aspects of maintaining player health and performance in foot- ball. Several studies have explored interventions and programs aimed at improving hamstring strength re- covery and reducing the risk of injuries, with the use of Nordbord playing a significant role in these inves- tigations.

In football, interventions such as cold-water im- mersion (CWI) and NHE programs have been identi- fied as effective means to enhance recovery of EKFS and reduce the risk of HIs [6, 82, 86]. The Nordbord is a valuable tool for measuring hamstring strength and monitoring a player’s response to these interven- tions [6], ultimately contributing to player wellbeing and performance in the sport.

Alexander et al. [86] conducted a study comparing the effects of CWI and passive rest (PR) on EKFS, iso- metric adductor strength, skin temperature, and sub- jective questionnaires. Their findings revealed signifi- cant effects for time points and groups in HES, peak torque, and peak force. The study showed that CWI had a stronger individual response compared to PR for HES, resulting in higher strength output for the CWI group, especially 24 hours post-intervention. This indi- cates the potential benefits of CWI in enhancing re- covery and HES, which can be critical for injury pre- vention.

Buckthorpe et al. [87] conducted a review of HI pre- vention strategies and identified various risk factors contributing to HIs in football. The review highlighted that NHE programs have been effective in reducing hamstring strain injuries by 65–70%. Moreover, it em- phasized that both low and high-volume NHE-based training can lead to similar adaptations in fascicle length and HES. This finding aligns with the results from Lacome et al. [56], which indicated that lower- volume, high-intensity eccentric exercises can be just as effective in preventing HIs as high-volume protocols. These insights suggest that tailored NHE programs can play a crucial role in reducing the risk of HIs and promoting player health.

As noticed, this section provides valuable insights into the use of the Nordbord device in professional foot- ball for assessing recovery, evaluating injury risk, and optimizing player performance. It also highlights the increasing physical demands of the sport and the im- portance of sprint ability in modern football.

In professional football, various recovery techniques are employed to optimize eccentric strength measures and knee stability during the competition phase. These techniques included massage and CWI, as studied by Rhodes [88] and Alexander et al. [86, 89]. The Nord- bord can play a pivotal role in assessing player recov- ery after a match. It helps ensure the effectiveness of recovery methods, informs coaches about player read- iness for the next match, evaluates the risk of HIs due to overuse, and assists in decision-making, especially for players who may need to participate but have physi- cal limitations.

We should also emphasize on the changing nature of professional football, with increased speed and physi- cal demands [90–92]. The importance of sprint ability in decisive moments, as supported by studies [93, 94], underscores the significance of optimal hamstring strength and injury prevention strategies. The increase in HIs during training over the past decade, as noted by Ekstrand [2], suggests that while injury prevention programs have been effective, the physical demands of matches have also risen. This emphasizes the need for ongoing research to adapt injury prevention meth- ods and optimize player performance.

In summary, the use of the Nordbord in professional football is a valuable tool for assessing player recovery, mitigating the risk of injury, and enhancing perfor- mance. It is crucial in an environment where the physi- cal demands of the sport continue to evolve, and the effectiveness of existing injury prevention programs must be continually assessed and improved.

Any issue related to conflict of interest?

The potential conflict of interest related to the in- volvement of one of the creators of the Nordbord in many of the published studies is a valid concern. How- ever, it is also important to consider that scientific re- search is typically subject to rigorous peer review, and the findings should be evaluated based on their scien- tific merit and methodology rather than solely on po- tential conflicts of interest.

It is reassuring that other researchers, who are not directly affiliated with the Nordbord creator, have con- firmed the findings and contributed to studies using the Nordbord. This multi-author and collaborative approach helps ensure that the research is subject to diverse perspectives and scrutiny.

In assessing the validity of the findings, it is essen- tial to focus on the scientific methods, reproducibility, and the overall body of evidence rather than solely on the potential conflict of interest. This approach helps maintain the integrity and objectivity of scientific re- search in the field of hamstring strength assessment and injury prevention.

A summary of the pros and cons of the Nordbord device See table 3.

Practical recommendations

To optimize the utility of the Nordbord in football, several recommendations have emerged from our re- view. Firstly, standardized testing protocols should be established. In addition, a consensus is needed on the normalization of the data for body weight (N/Kg) to maximize the comparability across different levels of the sport to ensure consistency and comparability of the results. Furthermore, research efforts should focus on establishing normative values of ESKF tailored to various player positions, demographics, and skill levels, as well as the bilateral relationship between the dom- inant and non-dominant foot. Additionally, investi- gations into the threshold values of HES indicative of hamstring injury risk and the recovery method’s ef- fectiveness are crucial. Comparative studies should evaluate the measurements of Nordbord in relation to other devices like the gold standard isokinetic dy- namometer to eliminate the uncertainty of validity and reliability. Longitudinal studies tracking ESKF changes over time, and their implications for injury risk and performance outcomes are needed. Collabo- ration among researchers and practitioners can foster a consensus on the best practices for Nordbord utili- zation and result interpretation in football.