The effect of Nordic Hamstring Exercise on the muscle strength and architecture of Hamstring：A Systematic Review and Meta-analyses

doi:10.21203/rs.3.rs-3216148/v1

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The effect of Nordic Hamstring Exercise on the muscle strength and architecture of Hamstring：A Systematic Review and Meta-analyses

https://doi.org/10.21203/rs.3.rs-3216148/v1

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Purposes: In this paper, we used Meta-analysis to study the effect of NHE on hamstrings and to find the scientific evaluation index of NHE effect.

Method: In this paper, we systematically searched all data since the establishment of the database through PubMed, Embase, Web of science, and Cochrane Library databases, with PubMed search formula (((Nordic) OR (Russian)) OR (NHE)) AND (((((((Hamstring) OR (Hamstring Muscle)) OR (Muscle, Hamstring)) OR (Muscles, Hamstring)) OR (Biceps Femoris)) OR (Semimembranosus)) OR (Semitendinosus)).The search field was full text and the search date of April 7, 2022. The quality evaluation, Meta-analysis, sensitivity analysis and publication bias analysis of the included literature were performed using Review manager 5.4 software.

Results: A total of 950 studies were retrieved for this study, and after strict inclusion and exclusion, a total of 11 studies were included, with 497 subjects, including 282 in the test group and 215 in the control group, and the overall quality of the literature was high. NHE can be used as an effective intervention method, which can significantly improve the eccentric hamstring strength peak(MD=46.16, 95% Cl: 28.70-63.63, P=0.00001<0.01) and the eccentric peak torque of the hamstrings (MD=10.87，95%CI：3.86-17.89，P=0.002<0.01) and the eccentric peak torque of the hamstrings muscles to body weight (MD=0.17, 95% CI: 0.01-0.33, P=0.04<0.05), increase BFLH fascicle length (MD=0.62, 95% CL: 0.11-1.12，P=0.02<0.05),improve H:Q functional ratio(MD=0.07,95%CI: 0.02-0.13, P=0.01<0.05),and decrease the hamstrings eccentric angle of peak torque（APT） (MD=2.53, 95% CI: 4.63-0.43, P=0.02<0.05).

Nordic Hamstring Exercise(NHE)

Hamstring

Systematic Review

Meta- analyses

Scientific

Hamstring injuries are one of the more prevalent non-contact injuries in sports and are widely distributed across sports such as soccer ^[1], track and field^[2], rugby^[3], and baseball ^[4], which not only affects athletic performance but also causes serious financial losses to clubs^[5]. Although clubs spend significant resources each year to prevent hamstring injuries, the results have been unsatisfactory and the incidence of hamstring injuries continues to increase^[6][7].Studies ^[8] have shownthat the majority of hamstring injuries are recurrent,thus suggesting that the effectiveness of rehabilitation is debatable and the importance of primary and secondary prevention for hamstring injuries cannot be overstated ^[9].Despite extensive research, the causes of hamstring injury remain unclear; however, studies have shown^[10] that hamstring injuries often occur during running, and the movements are characterized by hip flexion accompanied by knee extension and hamstring muscle weakness has a high correlation with injury; accordingly, it is urgent to find ways to effectively improve hamstring strength, which also has great practical significance for the prevention of hamstring injury.

Currently, there are numerous methods to improve hamstring strength, such as isometric plyometric training, stiff-leg deadlift (SLD)^[11], hip-dominant flywheel (HDF) ^[12], hamstring curl exercises (LCE) ^[13], neuromuscular electrical stimulation (NMES)^[13] , hip extension exercise (HEE) ^[14], and so on. Due to the expensive cost and maintenance of the isometric trainer and its inability to meet the frequent daily training of the club, Nordic Hamstring Exercise (NHE) is used as an emerging hamstring training method, as shown in Figure 1. The athlete starts in a kneeling position with the knee flexed at 90° and the trunk kept upright, the training partner fixes his or her ankle, and then, makes the trunk slowly leaning forward, during this phase the hamstrings contract centrifugally to overcome their own body weight, the athlete is asked to stop falling forward for as long as possible, touching the ground with arms and hands to cushion the fall, and immediately pushing back to the starting position with hands when the chest touches the ground to minimize the centripetal contraction of the hamstrings. NHE is the optimal choice for clubs because of its simplicity and operability compared to training methods such as SLD, HDF, LCE, NMES, and HEE, which are not limited by space and equipment assistance. Currently, most clubs are using NHE as part of their sports performance improvement and prevention strategies ^[15], e.g., FIFA uses NHE as one of the few strength exercises in their warm-up program^[16], which can reduce medical expenses ^[17].Al Attar^[18]conducted a meta-analysis of the effectiveness of NHE in reducing the hamstring injury rate, The results showed that NHE significantly reduced the hamstring injury risk ratio, and the injury rate decreased by 51% compared with the team that did not use NHE. However, the potential mechanism of how NHE can prevent hamstring injury is little known. Previous studies^[19] have systematically reviewed the effects of NHE on hamstring muscle activity, the research shows that compared with other training methods, NHE can significantly improve hamstring muscle activity, but does NHE only affect hamstring muscle strength and muscle activity? It is not known. Therefore, the purpose of this study was to understand the specific effects of NHE on hamstrings, to explore the potential mechanisms of NHE to prevent HSI, and then to find scientific evaluation indexes for the effects of NHE .

2.1 Information sources and retrieval tactics

The retrieval tactics were strictly formulated according to PICO principles.

Population: healthy adults without lower limb injuries;

Intervention: training with NHE;

Control: placebo or conventional training;

Outcome: changes in hamstring-related indicators.

The key words of the retrieval tactics are shown in Table 1:

Table 1：The Key Words of the Retrieval Tactics

NHE Training

Hamstring

Nordic

Russian

NHE

Hamstring Muscle

Muscle, Hamstring

Muscles, Hamstring

Biceps Femoris

Semimembranosus

Semitendinosus

In this paper, all data from the database establishment to date were systematically searched through PubMed, Embase, Web of science, and Cochrane Library databases with full-text search fields and a search date of April 7, 2022. The tactics of PubMed database retrieval are shown in Table 2.

Table 2 ：PubMed Database Search Formula

PubMed Database Search Formula

(((Nordic) OR (Russian)) OR (NHE)) AND (((((((Hamstring) OR (Hamstring Muscle)) OR (Muscle, Hamstring)) OR (Muscles, Hamstring)) OR (Biceps Femoris)) OR (Semimembranosus)) OR (Semitendinosus))

2.2 Inclusion and exclusion criteria of the literature

Inclusion criteria:

1. Randomized controlled trial (RCT);

2. The study population was professional athletes, healthy adults, student athletes or sports enthusiasts;

3. The intervention was NHE;

4. The language of the literature was English;

5. The outcome indicators were hamstring-related indicators.

Exclusion criteria：

1.The article was a systematic evaluation, meta-analysis, conference paper;

2. The study subjects had lower limb injuries;

3. The intervention method was not NHE ;

4. Outcome indicators were not hamstring-related indicators;

5. Full-text literature was not available;

6. Complete data were not provided.

Articles were screened by 2 investigators using an independent double-blind approach, and in case of disagreement, a third investigator made a determination by joint discussion to determine inclusion in the study.

2.3 Data extraction

Data were extracted from the final included literature by two investigators using an independent double-blind approach, including: first author of the literature, year of publication, country and region, sample size, age, intervention protocol, intervention period, and type of outcome indicators. In case of disagreement, a third investigator made a determination by joint discussion to determine inclusion in the study.

2.4 Literature quality assessment

As the study design type of the included literature in this article was a randomized controlled trial, the quality of the included literature was assessed independently by two investigators using Review manager 5.4 software according to the Cochrane Risk of Bias Assessment Tool, which included a total of seven areas of selection bias (including random sequence generation and allocation concealment), implementation bias (trial participant blinding), measurement bias (experimental analysis blinded), attrition bias (incomplete outcome data), reporting bias (selective outcome reporting), and other biases in a total of seven domains. Risk assessment of the included literature was performed according to three different risk levels: "low risk", "unknown risk of bias" and "high risk". In case of disagreement, a third investigator made a decision through joint discussion to determine whether to include the study.

2.5 Clinical Outcomes

Clinical outcomes related to hamstrings, including:

1. Indicators reflecting the concentric hamstring strength: the concentric peak torque of the hamstrings; the Concentric peak torque of the hamstrings muscles to body weight; the concentric hamstring strength peak, etc.

2. Indicators reflecting the eccentric hamstring strength: the eccentric peak torque of the hamstrings; the eccentric peak torque of the hamstrings muscles to body weight; the eccentric hamstring strength peak, etc.

3. Hamstring isometric peak torque.

4. The hamstrings eccentric angle of peak torque（APT）: reflecting the optimal initial length of the hamstrings.

5.Hamstring fascicle length, muscle thickness, pennation angle, stiffness, flexibility, etc.

6. H/Q ratio: including 1. H:Q conventional ratio: reflecting the balance of knee flexion and extension muscle strength[20]; 2. H:Q functional ratio: reflecting the dynamic stability of the knee joint[21].

2.6 Data Analysis

This study used Review manager 5.4 software for data analysis. Effect sizes of the ending indicators were expressed as mean ± standard deviation. Heterogeneity was tested by the I² statistic, and I² ≥ 50% and P ≤ 0.05 indicated a large heterogeneity in the included literature, which was analyzed using a random-effects model. The reasons for heterogeneity were also explored using sensitivity analysis and publication bias. I²<50% and P>0.05 indicated less heterogeneity in the included literature and could be analyzed using a fixed-effects model. When effect sizes were combined, the combined effect values of the included literature included mean difference (MD) and 95% confidential interval (CI), and forest plots were also performed. P<0.05 indicated significant differences in the results. Sensitivity analysis was used to explore the sources of heterogeneity in randomized controlled trials in the literature. Funnel plots were produced to determine the presence of publication bias.

3.1Literature retrieval results

Based on the retrieval tactics, a total of 950 studies were searched in this study, including PubMed (n=261), Embase (n=324), Web of science (n=267), and Cochrane Library (n=98). After removing duplicates, 519 studies were initially included, and after excluding literature such as reviews, systematic evaluations, and Meta-analyses, 468 studies were obtained after initial screening, and 59 studies were included after excluding literature with inconsistent study content or interventions by reading titles and abstracts, after which a total of 11 studies that met the criteria were finally included by reading the full text, and systematic review and Meta-analyses were performed The PRISMA flow chart provides details of the reasons for study selection and exclusion of articles, as detailed in Figure 2.

The reasons for exclusion of 52 studies contained:

1. the indicator was EMG of hamstring muscle activity (n=13), a systematic review of the effect of NHE on hamstring muscle activity has been conducted in a previous study ^[¹⁹^].

2. the index is the incidence of HSI (n=5), the previous study^[¹⁸^] has conducted a Meta-analysis on the incidence of hamstring injury in soccer players by NHE.

3. the index is human balance function (n=4)

4. the indicator is the change of red blood cells (n=1)

5. the index was kinematic analysis (n=2)

6. incomplete data(n=19)

7. full text not available(n=4).

3.2 Basic characteristics of included studies

The study involved 497 participants, ranging in age from 19 to 30 years old. The basic information, sample size of subjects, intervention programs, clinical indicators, etc. from the included studies were extracted. In the 11 included studies[^{22][23][24][25][26][27][28][29][30][31][32]}, the intervention methods in the experimental group were all NHE, and the control group was conventional training or training placebo. All studies confirmed that the subjects were in a healthy state and had no sports injuries, and the intervention period ranged from 4-27 weeks. See table 3 for details.

Table 3: Basic information of included research literature

Study	Location	Population	Subjects and gender(M/F)		Subject's age		Duration (weeks)	Intervention (reps × sets × frequency)		Outcomes
Study	Location	Population	test group	control group	test group	control group	Duration (weeks)	test group	control group	Outcomes
Iga et al. ^[22]	Britain	professional soccer players	10M	8M	23.4±3.3	22.3±3.9	4	1-3×2-3×5-12	Φ	C,M
Delextrat et al.^[23]	Britain	university-level hockey players	9F	8F	19.7±1.4	19.6±1.4	6	3×2-3×6-10	Φ	C,H,M
Suarez-Arrones et al.^[24]	Spain	professional football players	group 1：16M	17M	18.8±0.8		17	1-2×2-3×5-10	Φ	E,F
Suarez-Arrones et al.^[24]	Spain	professional football players	group 2：17M	17M	18.8±0.8		15	1-2×2-3×5-10	Φ	E,F
Ishøi et al. ^[25]	Denmark	amateur football players	18M	17M	---		10	1-3×2-3×5-12	Φ	C,E
Ribeiro-Alvares et al.^[26]	Brazil	healthy young adults	3M/7F	3M/7F	23.7 ± 3.3	26.0 ± 2.7	4	2×3×6-10	Φ	A,C,G,H,I,J,K,N
Seymore et al. ^[27]	America	healthy young adults	4M/6F	2M/8F	18.3 ± 0.5	19.9 ± 1.2	6	1-3×2-3×5-12	Φ	B,D,J,K,L,M,O
Delahunt et al. ^[28]	Republic of Ireland	healthy young adults	15M	14M	22±1.38		6	1-3×2-3×5-12	Φ	C,D,M
Vianna et al. ^[29]	Brazil	professional soccer players	17F	16F	24±5	24±4	8	2×2-4×6-10	Φ	E,K
Salci et al. ^[30]	Turkey	recreational athletes	13F	12F	20.5±4.9	21.0±1.6	10	1-3×2-3×5-12	Φ	B,D
Siddle et al. ^[31]	United Kingdom	amateur Soccer and Rugby players	7M	7M	20.13±1.55	20.86±1.57	6	1-2×2-3×5-10	Φ	C
Hasebe et al. ^[32]	Japan	high school soccer club level players	156M	103M	16.7 ± 0.5	16.3 ± 0.6	27	1-2×2-3×6-10	Φ	G

“---”: Not stated; reps: repetitions; “Φ”: normal training or placebo; Outcomes: “A”: The concentric hamstring peak torque (conH PT); “B”: The normalized concentric hamstring peak torque (Normalized conH PT); “C”: The eccentric hamstring peak torque (eccH PT); “D”: The normalized eccentric hamstring peak torque (Normalized eccH PT);“E”: The eccentric hamstring strength peak; “F”: The normalized eccentric hamstring strength peak; “G”: The conventional H:Q ratio; “H”: The functional H:Q ratio ;“I”: BFLH thickness; “J”: BFLH Pennation angle; “K”: BFLH Fascicle length; “L”: BFLH volume; “M”: The hamstrings eccentric angle of peak torque（APT）;“N”: The hamstring flexibility ;“O”: The hamstring stiffness.

3.3 Results of the quality evaluation of the included studies

The quality evaluation of the included 11 articles was performed using Review manager 5.3 software, and the results are shown in Figures 3 and 4. The evaluation entries included selection bias, performance bias, detection bias, attrition bias, reporting bias, and other bias. bias, reporting bias, and other bias, with selection bias subdivided into random sequence generation and allocation concealment. All included literature had a total score of ≥4, and the overall quality was high.

3.4 Meta-analysis results

In this study, the clinical indicators of each experiment were summarized and found to be 15 indicators, as shown in Table 4. It was found that there was only one study reported on six indicators, including the concentric peak torque of the hamstrings^[²⁶^]; the eccentric hamstring strength peak to body weight[^33]; BFLH muscle thickness^[²⁶^];BFLH volume^[²⁷^]; flexibility of hamstrings^[²⁶^]and hamstrings muscle stiffness^[²⁷^], and three other indicators were reported in two studies,such as the concentric peak torque of the hamstrings muscles to body weight^[²⁷^,²⁸^],H:Q conventional ratio^[²⁶^,³²^],BFLH Pennation angle^[²⁶^,²⁷^].But the heterogeneity was large, and nine indicators such as the overview could not be Meta-analyzed.

Therefore, in this paper, Meta-analysis was performed on six clinical indicators including the eccentric hamstring strength peak, the eccentric peak torque of the hamstrings,the eccentric peak torque of the hamstrings muscles to body weight, the hamstrings eccentric angle of peak torque(APT),fascicle length of biceps femoris, and H:Q functional ratio in the included studies. Since some of the studies tested the peak moments of different groups or hamstrings in low [≤60 (°)/s], medium [120 (°)/s], and high [≥180 (°)/s] angular velocity conditions, the data from different trials of the same study were expressed as T1, T2···Tn.

Table 4：Statistical details of the clinical indicators for each experiment

Statistics of clinical indicators of each experiment(n=15)

the concentric peak torque of the hamstrings
the concentric peak torque of the hamstrings muscles to body weight
the eccentric peak torque of the hamstrings
the eccentric peak torque of the hamstrings muscles to body weight
the hamstrings eccentric angle of peak torque（APT）
the eccentric hamstring strength peak
the eccentric hamstring strength peak to body weight
H:Q conventional ratio
H:Q functional ratio
BFLH thickness
BFLH pennation angle
BFLH fascicle length
BFLH volume
flexibility of hamstrings
hamstrings muscle stiffness

3.4.1 Effect of NHE on the eccentric hamstring strength peak

In the included studies, a total of five experiments from three articles ^[²⁵^，²⁹^，³³^]investigated the effect of NHE on the eccentric hamstring strength peak, with a combined sample size of 76 and 76 for the experimental and control groups, respectively. Heterogeneity tests were performed, and the results are shown in Figure 5, showing no heterogeneity between the combined studies (I2=1%, P=0.04), so a fixed-effect model was used to calculate the combined the results of the effect size showed that NHE significantly enhanced the eccentric hamstring strength peak compared with the control group (MD=46.16, 95% CI: 58.70-63.63, z=5.18, P<0.001).

In the included studies, a total of 11 experiments from 5 articles^[²³^，²⁶^，²⁸^，³⁰^{, 34]} investigated the effect of NHE on the eccentric peak torque of the hamstrings with a combined sample size of 110 and 95 for the experimental and control groups, respectively. Heterogeneity test was performed, and the results are shown in Figure 6, showing no heterogeneity between the combined studies (I2=0%, p=0.93), so a fixed-effect model was used to calculate the combined effect size, and the results showed that NHE significantly enhanced the eccentric peak torque of the hamstrings compared with the control group (MD=10.87, 95% CI: 3.86-17.89, z=3.04, P<0.01).

3.4.3 Effect of NHE on the eccentric peak torque of the hamstrings muscles to body weight

In the included studies, a total of three experiments from three articles investigated the effect of NHE on the eccentric peak torque of the hamstrings muscles to body weight, with a combined sample size of 38 and 36 for the experimental and control groups, respectively. Heterogeneity test was performed, and the results are shown in Figure 7, which shows that there was no heterogeneity between the combined studies (I2=0%, P=0.82), so a fixed-effect model was used to calculate the combined effect size, and the results showed that NHE significantly enhanced the eccentric peak torque of the hamstrings muscles to body weight compared with the control group(MD=0.17, 95% CI: 0.01-0.33, z=2.02, P<0.05).

3.4.4 Effect of NHE on the hamstrings eccentric angle of peak torque（APT）

In the included studies, a total of 10 experiments from four articles^[²²^,²³^,²⁷^,²⁸^]investigated the effect of NHE on the hamstrings eccentric angle of peak torque（APT）, with a combined sample size of 103 and 88 for the experimental and control groups, respectively. Heterogeneity tests were performed and the results, as shown in Figure 8,showed no heterogeneity between the combined studies (I2=0%, P=0.76), so a fixed-effects model was used to calculate the combined effect size, and the results showed that NHE significantly reduced the hamstrings eccentric angle of peak torque（APT） compared with the control group(MD=2.53, 95% CI: 4.63-0.43, z=2.36, P<0.05).

3.4.5 Effect of NHE on BFLH fascicle length

In the included studies, a total of four experiments from three articles^[²⁶^,²⁷^,²⁹^]investigated the effect of NHE on BFLH fascicle length, with a combined sample size of 54 and 52 for the experimental and control groups, respectively. Heterogeneity tests were performed, and the results were shown in Figure 9, showing no heterogeneity between the combined studies (I2=0%, P=0.57), so a fixed-effects model was used to calculate the combined effect size. The results showed that NHE significantly increased the BFLH fascicle length compared with the control group (MD=0.62, 95% CI: 0.11-1.12, z=2.40, P<0.05).

3.4.6 Effect of NHE on H:Q functional ratio

In the included studies, a total of 3 experiments from 2 articles^[²³^,²⁶^]investigated the effect of NHE on H:Q functional ratio with a combined sample size of 28 and 26 for the experimental and control groups, respectively. Heterogeneity tests were performed and the results were shown in Figure 10, showing no heterogeneity between the combined studies (I2=0%, P=0.43), so a fixed effects model was used to calculate the combined effect size. The results showed that NHE training significantly increased the H:Q functional ratio compared with the control group (MD=0.07, 95% CI: 0.02-0.13, z=2.53, P<0.05).

3.4.7 Sensitivity analysis

Sensitivity analysis was performed on the studies included in each research, and none of the studies caused significant interference with the results of this Meta-analysis, implying that this research has a good stability.

3.4.8 Publication Bias Analysis

The software was used for publication bias analysis, and a funnel plot was drawn to check whether there was publication bias in this study. The symmetry of the funnel plot meant that there was no publication bias. See Figure 11-16 for details.

4.1 Discussion

As an emerging training method, NHE has a great influence on hamstrings, such as on muscle activity during exercise, hamstring injury incidence, muscle strength^[33], muscle morphological bodies^[26,27], and muscle erythrocytes, etc. The present study is the first Meta-analysis to specifically investigate the effects of NHE on hamstrings, and a total of 11 articles were eventually included, which showed that the effects on hamstrings were mainly reflected in enhanced eccentric muscle strength and increased muscle length.

The impact of NHE on hamstring strength is mainly reflected in enhancing the eccentric hamstring strength peak, increasing the eccentric peak torque of the hamstrings, increasing the eccentric peak torque of the hamstrings muscles to body weight, and improving H:Q functional ratio, which is the ratio of the eccentric peak torque of the hamstrings to the concentric peak torque of the quadriceps femoris^[^].As hamstring strain injury occurs mostly during running, it was found that the injury occurs mainly in the early stance^[^] and late swing phase^[^].For one thing, in the early stance, the ground reaction force passes in front of the knee and hip joints, producing a extension torque at the knee and a flexion torque at the hip, and in order to resist this torque, the hamstrings contract to produce a extension torque at the knee and a flexion torque at the hip, so the hamstrings are under great load in the early stance, which is a high-risk period for injury. For another thing, during the late swing phase, hamstrings need to perform eccentric contraction to prevent rapid extension of the lower leg, which in turn prepares the foot to effectively complete the skimming stirrup when it lands next; therefore, insufficient the eccentric hamstring strength decreases the contraction capacity, which may lead to acute HSI. Moreover, studies have shown^[^]that athletes with lower the eccentric hamstring strength have a 2.7-fold increased incidence of injury compared to normal athletes. Numerous studies^[11,12,13] have shown that NHE is more effective in improving hamstring strength compared to other training methods. Therefore, NHE should be incorporated into injury prevention programs.

Previous studies^[22,28] showed that the APT did not change after NHE, but the present study found that NHE significantly reduced the APT, and when this angle is smaller, it means that the optimal initial length of the hamstring is longer. Studies has shown^[^]that the excessive magnitude of the muscle strains is a direct mechanism leading to HSI, and muscle strain is the ratio of the muscle deformation to the optimal initial length of the muscle, and the optimal initial length is the length of the muscle when it generates the contraction peak force. Injuries mainly occur when muscles are stretched. It is known that the occurrence of injuries is directly related to muscle strain, but not to muscle contraction force and velocity. When the initial length of the muscle is longer, the strain when stretched is smaller. Luckily, NHE can effectively optimize the optimal initial length of hamstring, so it can prevent the occurrence of injury. In the process of fast running, the muscle activity of muscle fibers in the late stage of swing will be greater than that of the semitendon and semimembranous muscles, and the risk of injury may be increased, and this study also found that NNE training can increase the length of the biceps head of the femoris, consistent with previous research results^[^,^,^,^], but the relationship between the length increase of the long head of the biceps femoris and the optimal initial length of the hamstring muscles is not clear, and further research is needed.

Although this study did not perform a meta-analysis of hamstring volume, thickness, stiffness, and pennation angle, it does not mean that NHE has no effect on these aspects. On the contrary, some studies^[^,^]showed that NHE can increase muscle volume, while previous studies^[^] suggested that muscle size, volume and muscle thickness are closely related to muscle strength. Against this context, the relationship needs to be further explored and there needs to a further study whether there is a correlation with hamstring injury. The hamstrings muscle stiffness is affected by many factors such as the collagen content of the muscle, the connection between collagen, and the composition of the tissue fluid^[27]. The greater the stiffness, the stronger the resistance to deformation. In terms of hamstrings, the greater the stiffness, the lower the strain on the muscle and the lower the chance of sports injury. It has been shown^[^]that exercise can lead to increased muscle stiffness, but the current study^[27]found that NHE had no significant effect on hamstring stiffness. The larger the muscle pennation angle, the more sarcomere side by side, and the fewer sarcomere connected in series. In this way, it is more conducive to generate strength, but the maximum speed is slower. On the contrary, the muscle with smaller pennation angle has more consecutive sarcomere, conducive to generating greater velocity. Seymore^[27] found that NHE can increase the muscle pennation angle, but Ribeiro-Alvares^[26]came to the opposite conclusion. Therefore, the impact of NHE on the above four aspects of hamstring remains to be studied and requires a lot of experimental evidence.

4.2 Conclusion

NHE training plans an important role on the muscle strength and architecture of hamstrings through the following aspects. First of all, it can significantly increase the maximum centrifugal contraction force of the hamstrings, so that the muscles can produce more force during exercise. Secondly, NNE training can also improve the peak moment of eccentric contraction of the hamstrings, which means that the hamstrings can withstand greater loads during exercise. Thirdly, the normalized peak moment of centrifugal contraction will also be improved, which means that the strength performance of the hamstring muscles will be more stable and reliable. Fourthly, the knee angle corresponding to the peak moment of hamstring centrifugal contraction is reduced, which helps reduce pressure and injury to the knee joint. In addition, NNE training is able to increase the muscle length of the biceps longhead, which further improves athletic performance. Finally, by improving the functional H/Q ratio, NNE training can also help people better control muscle strength and balance, thereby improving exercise capacity and preventing sports injuries. In summary, the above indicators can be used to evaluate the effectiveness of NHE training.

4.3 Future research direction

Since the present study has confirmed that NHE mainly affects the eccentric strength and optimal initial length of hamstring, future studies should go deeper on the effects of NHE on muscle morphology and structure, such as the volume, muscle thickness, muscle stiffness and pennation angle, among many other factors. Besides, based on Unver’s studies,NHE did not alter erythrocyte deformability, hematocrit and oxidative stress, but increased erythrocyte aggregation index and decreased erythrocyte aggregation half-life. Therefore, the influence of NHE training on hamstring molecular level needs to be further studied. Moreover, NHE is mostly used in football, but whether it is also applicable to other sports remains to be studied.

4.4 Weaknesses of this study

Sadly, this study did not differentiate between the gender and occupation of the study population. But it is assuredly meaningful to spot the mechanisms of gender differences in NHE, as there are significant differences in height, weight, and hamstring strength and length in women compared to men^[^].Since researchers' occupations included soccer players, field hockey players and sports enthusiasts, the effects of NHE on different genders and occupations need to be further refined. In addition, most of the included articles belong to small sample size studies, and there are certain differences in the intensity and cycle of NHE, which may have a certain impact on the results.

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Availability of data and materials

The date that support the findings of this study are available from the corresponding author upon reasonable request.

Competing interests

There is no conflict of interest in this article.

Funding

This work was supported by Fujian Province Natural Science Fund of Grant No.2019J01755 , also it is supported by the cultivation grant(No.C520132) from Jimei University.

Authors' contributions

This paper summarizes and analyzes the research on the training effect of NHE in recent years and analyzes and discusses it.

Z.W. and Y.X.wrote the main manuscript text

J.J. As a discussant, discuss whether the article is eligible for inclusion.

D.C. and Y.Z.prepared figures 1-10.

M.S. As a correspondent, I revised the article many times.

Acknowledgements

Not applicable.

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No competing interests reported.

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The effect of Nordic Hamstring Exercise on the muscle strength and architecture of Hamstring：A Systematic Review and Meta-analyses

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1. Introduction

2. Research methods

3. Outcomes

4. Discussion and analysis

4.1 Discussion

4.2 Conclusion

4.3 Future research direction

4.4 Weaknesses of this study

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