Effects of Reduced Training Volume of Nordic Hamstring Exercise on Eccentric Knee Flexor Strength, and Fascicle Length and Stiffness of Biceps Femoris Long Head

ABSTRACT

This study investigated whether and how ultralow‐volume Nordic hamstring exercise (NHE) training can maintain neuromuscular adaptations achieved through initial moderate‐volume training. Specifically, we examined changes in eccentric knee flexor strength, fascicle length, and stiffness of the biceps femoris long head (BFlh) across different training volumes. Forty‐five resistance‐trained males were randomly assigned to three groups: low‐volume, ultralow‐volume, and control (n = 15 per group). Training groups performed standardized moderate‐volume NHE training (48 repetitions/week) for 2 weeks, followed by low‐volume (eight repetitions/week) or ultralow‐volume (four repetitions/week) training for 8 weeks. Eccentric knee flexor strength, BFlh fascicle length, and BFlh stiffness assessed via shear wave elastography were measured at four time points: weeks 0 (baseline), 2, 6, and 10. In both training groups, eccentric knee flexor strength increased significantly during the initial 2 weeks and continued to increase through week 6 despite the reduction in training volume, with no further increases beyond week 6. BFlh fascicle length increased, and stiffness decreased significantly after 2 weeks of moderate‐volume training, and these adaptations were maintained throughout the subsequent 8 weeks. No significant differences were found between the two training groups for any parameter. The control group showed no significant changes. These findings indicate that neuromuscular adaptations induced by 2 weeks of moderate‐volume NHE training can be effectively maintained with as little as four repetitions per week of supramaximal NHE training, providing practical implications for in‐season training programming.

1. Introduction

Hamstring strain injury (HSI) is a common injury in many sports involving sprinting [1]. Among the hamstring components, the biceps femoris long head (BFlh) is the most frequently injured, accounting for approximately 80% of HSI cases in European soccer [2]. Despite extensive research on prevention strategies, the incidence of HSI has not decreased in field‐based team sports (such as soccer, rugby, and field hockey) over the past 30 years [3]. Moreover, in UEFA Champions League soccer players, the incidence has doubled (from 12% to 24%) over the past 21 years [4]. Therefore, the prevention of HSI remains a critical challenge in sports medicine.

Research has identified several modifiable risk factors for HSI [5, 6]. Eccentric knee flexor strength [7, 8] and BFlh fascicle length [8] have received particular attention. Studies with elite soccer players have demonstrated that players with eccentric knee flexor strength below 256 N during the Nordic hamstring exercise (NHE) during the pre‐season period are 2.7 times more likely to sustain HSI during the in‐season period than those with greater strength [8]. Similarly, players with the BFlh fascicles shorter than 10.56 cm during the pre‐season period are approximately 4.1 times more likely to sustain HSI than those with longer fascicles [8]. Recent evidence also suggests that increased passive muscle stiffness of the BFlh, assessed using shear wave elastography (SWE), is a potential risk factor for muscle strain injuries [9, 10]. These findings indicate that interventions targeting increased eccentric knee flexor strength and BFlh fascicle length, along with reduced BFlh stiffness, could lower HSI risk.

NHE training has emerged as an effective intervention for simultaneously increasing eccentric knee flexor strength and BFlh fascicle length. NHE training has been reported to reduce HSI incidence by more than 50% across various sports when compliance is high [11, 12]. Studies report that 4–10 weeks of NHE training increases eccentric knee flexor strength by approximately 27% [13] and BFlh fascicle length by 1.9–2.2 cm [13, 14] in recreationally active individuals. However, these studies implemented high‐volume training protocols of nearly 100 repetitions per week (2–3 days/week). Such high‐volume eccentric training protocols are challenging to implement in practical sports settings where athletes must prioritize sport‐specific training/practicing. Indeed, athletes' compliance with such high‐volume training protocols is low [15]. Additionally, while research indicates that passive BFlh stiffness acutely decreases following eccentric knee flexor contractions [16], the long‐term impact of NHE training is not yet well established. Therefore, developing low‐volume NHE training protocols that effectively modify all these risk factors while remaining feasible for in‐season implementation is essential.

Recently, a few studies have examined the effects of low‐volume NHE training on eccentric knee flexor strength and BFlh fascicle length. Presland et al. compared 4 weeks of low‐volume (8 repetitions per week, 1 day/week) and high‐volume [64–100 repetitions per week (progressively increasing), 2 days/week] NHE training following 2 weeks of standardized moderate‐volume NHE training (48 repetitions per week, 2 days/week) [17]. These authors showed similar increases in eccentric knee flexor strength at 6 weeks and in BFlh fascicle length at both 2 and 6 weeks between low‐ and high‐volume NHE training. These findings suggest that moderate‐volume NHE training for 2 weeks can sufficiently increase BFlh fascicle length, with subsequent low‐volume training potentially maintaining these gains. Additionally, eccentric knee flexor strength can be significantly increased over 6 weeks by combining 2 weeks of moderate‐volume NHE training followed by 4 weeks of low‐volume training.

From a practical perspective in competitive sports, determining whether the improvements in eccentric knee flexor strength, BFlh fascicle length, and BFlh stiffness achieved during the pre‐season period can be maintained throughout the in‐season period with even lower training volume would be invaluable for athletes and teams. However, several critical questions remain: (1) whether ultralow‐volume NHE training (half the volume of previously studied low‐volume protocols) can maintain adaptations gained through moderate‐volume NHE training; (2) whether the effects of low‐volume NHE training following 2 weeks of moderate‐volume training persist beyond 4 weeks; and (3) how different NHE training volumes affect BFlh stiffness over time. Therefore, the present study aimed to examine the effects of 8 weeks of low‐ or ultralow‐volume NHE training following 2 weeks of moderate‐volume training on eccentric knee flexor strength, BFlh fascicle length, and BFlh stiffness.

2. Methods

2.1. Participants

Forty‐five young male adults (age 21.8 ± 4.4 years, height 170.7 ± 5.2 cm, body mass 76.3 ± 11.7 kg) participated in the study. The inclusion criteria were individuals who performed resistance training at least three times per week for at least the past 6 months. The exclusion criteria were (i) those who had undergone NHE training within the past 6 months, (ii) those with a history of HSI within the past 6 months, and (iii) those with any current orthopedic injuries or illnesses that could interfere with measurements. Of the 45 participants, 30 were university rugby players and the remaining 15 were resistance‐trained individuals. Prior to the experiment, each participant was informed of the experimental procedures and possible risks as well as the purpose of the study and provided written consent to participate. The present study was approved by the ethics committee of Juntendo University, Graduate School of Health and Sports Science (approval number: 2024‐44) and was conducted in accordance with the Declaration of Helsinki.

2.2. Study Design

Participants were allocated to one of three groups: ultralow‐volume (n = 15), low‐volume (n = 15), and control (n = 15). All participants underwent an NHE familiarization session at their first visit to determine the load for the first measurement and training session. In the experimental session, measurements of eccentric knee flexor strength, BFlh fascicle length, and BFlh stiffness (see below) were performed for all groups at four time points: before the intervention (week 0), after 2 weeks of intervention (week 2), after 6 weeks of intervention (week 6), and after 10 weeks of intervention (week 10). Measurements at weeks 2, 6, and 10 were taken 3–7 days after the last NHE training session to avoid possible acute effects of the exercise. Randomization was performed based on dynamic allocation to maintain balance among the groups in terms of height, body mass, and eccentric knee flexor strength relative to the body mass assessed at the familiarization session. During the 10‐week intervention period, all participants were instructed to maintain the same level of physical activity (i.e., their regular lower limb resistance training and participation in sports) as before the intervention. However, they were instructed to refrain from performing exercises that emphasize the eccentric phase of the hamstring (e.g., Romanian deadlifts, stiff‐leg deadlifts, prolonged continuous stretching) that they had not performed before the intervention.

2.3. NHE Training Intervention

All NHE training was conducted under the supervision of a National Strength and Conditioning Association (NSCA)‐certified strength and conditioning specialist. Participants were instructed to kneel on a platform at a height of 20 cm, lean forward slowly from the kneeling position with arms crossed in front of the chest, while keeping the hips and torso extended, and then resist maximally until the arms touched the ground (partners applied pressure to fix the ankles to ensure that their feet did not leave the platform during the motion). During each training session, the supervisor assessed participants' ability to control the eccentric phase of the movement through visual observation. If participants demonstrated full control throughout the entire range of motion, particularly during the last 10°–20° before ground contact (characterized by controlled eccentric descent without rapid or uncontrolled movement), they were asked to hold additional weight plates in front of their chest to make the NHE supramaximal, with weights being increased in 2.5 kg increments (weight range: 2.5–15 kg). To encourage maximum effort during all testing and training sessions, verbal encouragement was provided, with a 30‐s rest between repetitions and a 2‐min rest between sets. The training groups completed the same standardized sessions for 2 weeks, after which they were divided into ultralow‐volume and low‐volume groups for an additional 8 weeks of training (Table 1). During the standardized sessions, a minimum of 48 h was allowed between the sessions. The control group was prohibited from performing the NHE during the intervention period while maintaining their regular lower limb resistance training and sports participation, following the same activity guidelines as the training groups (i.e., refraining from exercises that emphasize the eccentric phase of the hamstring that they had not performed before the intervention).

TABLE 1.

Training volume of Nordic hamstring exercise in ultralow‐volume and low‐volume groups.

Week	Ultralow‐volume				Low‐volume
	Frequency	Sets	Reps	Total reps	Frequency	Sets	Reps	Total reps
1	2	4	6	48	2	4	6	48	Standardized training period
2	2	4	6	48	2	4	6	48
3	1	1	4	4	1	2	4	8	Varied volume training period
4	1	1	4	4	1	2	4	8
5	1	1	4	4	1	2	4	8
6	1	1	4	4	1	2	4	8
7	1	1	4	4	1	2	4	8
8	1	1	4	4	1	2	4	8
9	1	1	4	4	1	2	4	8
10	1	1	4	4	1	2	4	8
Sum				128				160

2.4. Eccentric Knee Flexor Strength

Eccentric knee flexor strength was assessed using a custom‐made apparatus [18] at weeks 0, 2, 6, and 10 (Figure 1). Participants knelt on a padded board, and each ankle was secured with a non‐elastic brace. The brace was fastened 4 cm proximal to the malleolus, and the load cell was positioned perpendicular to the tibia. Prior to testing, participants performed warm‐up repetitions at 50%, 75%, and 90% of their maximal perceived effort, one repetition each. After a 2‐min rest, participants performed three maximal efforts of the NHE with their body mass. If participants were deemed to have achieved full control throughout the entire range of motion, particularly during the last 10°–20° of the NHE, an additional 2‐min rest was provided. Subsequently, a supramaximal load (determined by adding weight plates in 2.5 kg increments) was applied, and the participants performed three repetitions under this condition. The peak force of each leg during each repetition was recorded in newtons. For each condition (body‐mass and loaded), the peak forces from both legs were averaged for each trial, and then the mean of the three trials within each condition was calculated. The higher value between the body mass and loaded conditions was selected as the participant's representative eccentric knee flexor strength for subsequent analysis.

FIGURE 1.

Postural changes during Nordic hamstring exercise.

2.5. BFlh Fascicle Length

Following established protocols from previous studies [13, 14], the fascicle length of the BFlh was assessed with participants in a prone position (hips and knees in neutral position) (Figure 2), using an ultrasound scanner (Aixplorer, Supersonic Imagine, France) coupled with a linear array probe (SL15‐4, Supersonic Imagine, France). The probe had a field of view width of 50 mm, and the imaging depth was set to 50 mm. The probe was placed at 50% of the thigh length (defined between the greater trochanter and the popliteal crease). The probe orientation was manually aligned with the BFlh fascicle direction so that several fascicles and the proximal intramuscular tendon (i.e., intermediate aponeurosis) could be identified within the B‐mode image in a certain plane. At each time point, the measurements were performed three times (i.e., three B‐mode images were acquired). Care was taken to avoid pressing the probe onto the skin surface [19]. All ultrasound measurements were performed by a single examiner with more than 10 years of experience in ultrasound measurements. The acquired B‐mode images were exported to a personal computer for analysis. When the entire fascicle was not visible within the field of view of the B‐mode image, the BFlh fascicle length was estimated using the following equation [20] and reported in absolute values (cm):

$$\mathrm{FL}=\sin \left(\mathrm{AA}+{90}^{°}\right)\times \mathrm{MT}÷\sin \left({180}^{°}-\left(\mathrm{AA}+{180}^{°}-\mathrm{FA}\right)\right)$$

where FL, fascicle length; MT, muscle thickness; AA, anatomical angle (angle between the superficial and intermediate aponeuroses); FA, fascicle angle.

FIGURE 2.

Example of fascicle length measurement of the biceps femoris long head using B‐mode ultrasonography.

2.6. BFlh Stiffness

Passive muscle stiffness of the BFlh was assessed following previously established methods [9, 10, 21]. Briefly, participants were seated on a bench, with the hip flexed at 70° (0° = lying position), the knee fully extended, and the ankle at approximately 90°, following the procedure used in previous studies [9, 10, 21] (Figure 3). Measurements were conducted using an ultrasound SWE scanner (Aixplorer Ver.12, Supersonic Imagine, France) in ‘SWE’ mode (preset: ‘MSK’; persistence: off; smoothing: 5), coupled with a linear array probe (SL10‐2, Supersonic Imagine, France) with a field of view width of 36 mm. The probe was placed at 50% of the thigh length (defined between the greater trochanter and the popliteal crease). The probe orientation was manually aligned with the direction of BFlh fascicles so that several fascicles could be identified within the B‐mode image in a certain plane. At each time point, the measurements were performed three times (i.e., three SWE images were acquired). Care was taken to avoid pressing the probe onto the skin surface. All ultrasound measurements were performed by a single examiner with more than 10 years of experience in ultrasound measurements, as in the assessment of fascicle length.

FIGURE 3.

Example of muscle stiffness measurement of the biceps femoris long head using ultrasound shear wave elastography.

The acquired SWE images were exported in DICOM format and analyzed using a custom‐written MATLAB algorithm. The mean shear wave speed (SWS) of the region of interest, which was manually maximized while excluding other tissues (e.g., aponeurosis and subcutaneous fat), was calculated. Only pixels with a quality index of ≥ 0.7 (maximum: 1.0) were included in the analysis [22]. No pixels in the region of interest reached the upper limit of SWS for the SWE system (16.3 m/s). Following previous studies [23] we report the squared value of the SWS as a proxy for stiffness. The average value of the squared‐SWS from three measurements was used for subsequent analyses.

2.7. Statistical Analysis

All statistical analyses were conducted using statistical software (SPSS Statistics Ver. 26, IBM Japan, Japan). Normality and homogeneity of variance of the data were confirmed using the Shapiro–Wilk test and Levene's test, respectively. Since there were no significant differences between the left and right legs for eccentric knee flexor strength, BFlh fascicle length, and BFlh squared‐SWS, the average of both limbs was used in further analyses. Analyses of eccentric knee flexor strength, BFlh fascicle length, and BFlh squared‐SWS were performed using a two‐way analysis of variance (ANOVA) for mixed design [within‐subject factor: time (weeks 0, 2, 6, 10), between‐subject factor: group (ultralow‐volume, low‐volume, control)]. When significant main effects or interactions were found, post hoc Bonferroni tests were performed. Additionally, to compare the intervention effects between groups from week 2 onward, analysis of covariance (ANCOVA) was performed, adjusting for baseline values (week 0) as covariates. The significance level was set at 0.05 for all analyses. Where possible, Cohen's d effect sizes were also reported, with the magnitude of the effects classified as small (0.20 ≤ d < 0.50), medium (0.50 ≤ d < 0.80), and large (d ≥ 0.80) [24].

A priori sample size calculation was performed using G*Power, version 3.1.9.6. The effect size was derived from a study that reported a ~20% increase in BFlh fascicle length (d = 2.1) following 10 weeks of NHE training compared to the control group [13]. For the present study, the effect size was conservatively set at approximately half of the reported value, resulting in an effect size of 1.0. With a power of 0.80 and an alpha level of 0.05, the minimum sample size was calculated to be 10 participants per group. Considering potential dropouts during the intervention period (due to injuries or other reasons), 15 participants per group were included in the present study.

3. Results

No significant differences were observed in age, height, body mass, or strength relative to body mass at baseline (week 0) between the three groups (p ≥ 0.38) (Table 2). Two participants in the ultralow‐volume group dropped out of the study due to injuries sustained during sports activities unrelated to the experimental intervention and were excluded from data analysis. The remaining participants completed all training sessions with 100% compliance.

TABLE 2.

Baseline characteristics of participants.

Group	Age (years)	Height (cm)	Body mass (kg)	Eccentric knee flexor strength/body mass (N/kg)
Ultralow‐volume	22.8 ± 5.3	171.3 ± 5.4	77.6 ± 14.1	5.5 ± 0.6
Low‐volume	22.0 ± 4.3	170.7 ± 4.6	78.0 ± 11.4	5.3 ± 0.4
Control	21.1 ± 4.2	170.2 ± 6.1	72.5 ± 10.3	5.4 ± 0.4

3.1. Eccentric Knee Flexor Strength

Two‐way ANOVA showed a significant main effect of time (p < 0.001) and time × group interaction (p < 0.001) (Figure 4). Post hoc Bonferroni tests demonstrated significant differences between the control and low‐volume groups at weeks 2, 6, and 10 (see Table S1 for detailed p‐values and Cohen's d effect sizes) and between the control and ultralow‐volume groups at weeks 2, 6, and 10. In contrast, no significant differences were observed between the low‐volume and ultralow‐volume groups at weeks 2, 6, and 10. ANCOVA with baseline values as covariates (i.e., after adjusting for the nonsignificant differences at week 0) showed a significant main effect of group (p < 0.001) and significant time × group interaction (p = 0.007), without a significant main effect of time. Post hoc Bonferroni tests revealed significant differences between the control and low‐volume groups at weeks 2, 6, and 10 (all time points: p ≤ 0.006) and between the control and ultralow‐volume groups at weeks 2, 6, and 10 (all time points: p ≤ 0.005). In contrast, no significant differences were observed between the low‐volume and ultralow‐volume groups at weeks 2, 6, and 10 (all time points: p > 0.999). According to post hoc Bonferroni tests, in the ultralow‐volume group, eccentric knee flexor strength increased significantly from week 0 to week 2 (p < 0.001, d = 0.56) and from week 2 to week 6 (p = 0.031, d = 0.27), with no further changes observed from week 6 to week 10 (p > 0.999, d = 0.03). In the low‐volume group, eccentric knee flexor strength increased significantly from week 0 to week 2 (p < 0.001, d = 0.67) and from week 2 to week 6 (p < 0.001, d = 0.57), with no further changes observed from week 6 to week 10 (p > 0.999, d = 0.02). In the control group, no significant changes were observed during any period: from week 0 to week 2 (p > 0.999, d = 0.07), from week 2 to week 6 (p > 0.999, d = 0.07), or from week 6 to week 10 (p > 0.999, d = 0.08).

FIGURE 4.

Time‐course changes in eccentric knee flexor strength in response to different Nordic hamstring exercise protocols. Values are expressed as delta (Δ) from baseline to enhance visual clarity, while statistical comparisons were performed on absolute measurements. See Figure S1 for absolute values. *p < 0.05 vs. baseline (Week 0); †p < 0.05 versus Week 2; #p < 0.05 versus control group.

3.2. BFlh Fascicle Length

Two‐way ANOVA showed a significant main effect of time (p < 0.001) and time × group interaction (p < 0.001) for BFlh fascicle length (Figure 5). Post hoc Bonferroni tests demonstrated no significant differences between the control and low‐volume groups at weeks 2, 6, and 10 (see Table S1 for detailed p‐values and Cohen's d effect sizes) and between the control and ultralow‐volume groups at weeks 2, 6, and 10 or between the low‐volume and ultralow‐volume groups at weeks 2, 6, and 10. ANCOVA with baseline values as covariates showed a significant main effect of group (p < 0.001) without significant time × group interaction or main effect of time. Post hoc Bonferroni tests revealed significant differences between the control and low‐volume groups at weeks 2, 6, and 10 (all time points: p < 0.001) and between the control and ultralow‐volume groups at weeks 2, 6, and 10 (all time points: p < 0.001). In contrast, no significant differences were observed between the low‐volume and ultralow‐volume groups at weeks 2, 6, and 10 (all time points: p ≥ 0.246). According to post hoc Bonferroni tests in the ultralow‐volume group, BFlh fascicle length increased significantly from week 0 to week 2 (p < 0.001, d = 1.08), with no further changes observed from week 2 to week 6 (p = 0.999, d = 0.03) and from week 6 to week 10 (p > 0.999, d = 0.03). In the low‐volume group, BFlh fascicle length increased significantly from week 0 to week 2 (p < 0.001, d = 1.28), with no further changes from week 2 to week 6 (p = 0.418, d = 0.12) or from week 6 to week 10 (p > 0.999, d = 0.01). In the control group, no significant changes were observed during any period: from week 0 to week 2 (p > 0.999, d = 0.09), from week 2 to week 6 (p > 0.999, d = 0.04), or from week 6 to week 10 (p > 0.999, d = 0.00).

FIGURE 5.

Time‐course changes in fascicle length of the biceps femoris long head (BFlh) in response to different Nordic hamstring exercise protocols. Values are expressed as delta (Δ) from baseline to enhance visual clarity, while statistical comparisons were performed on absolute measurements. See Figure S2 for absolute values. *p < 0.05 versus baseline (Week 0).

3.3. BFlh Shear Wave Speed (Proxy for Stiffness)

Two‐way ANOVA showed a significant main effect of time (p < 0.001) and time × group interaction (p < 0.001) (Figure 6). Post hoc Bonferroni tests demonstrated that significant differences were observed between the control and low‐volume groups at week 2 and week 10, while no significant difference was found at week 6 (see Table S1 for detailed p‐values and Cohen's d effect sizes). In contrast, no significant differences were observed between the control and ultralow‐volume groups at weeks 2, 6, and 10. No significant differences were observed between the low‐volume and ultralow‐volume groups at weeks 2, 6, and 10. ANCOVA with baseline values as covariates showed a significant main effect of group (p < 0.001) without significant time × group interaction or main effect of time. Post hoc Bonferroni tests revealed significant differences between the control and low‐volume groups at weeks 2, 6, and 10 (all time points: p < 0.001) and between the control and ultralow‐volume groups at weeks 2, 6, and 10 (all time points: p < 0.001). In contrast, no significant differences were observed between the low‐volume and ultralow‐volume groups at weeks 2, 6, and 10 (all time points: p ≥ 0.341). According to post hoc Bonferroni tests in the ultralow‐volume group, BFlh squared‐SWS decreased significantly from week 0 to week 2 (p < 0.001, d = 0.84), with no further changes observed from week 2 to week 6 (p > 0.999, d = 0.02) and from week 6 to week 10 (p = 0.321, d = 0.08). In the low‐volume group, BFlh squared‐SWS decreased significantly from week 0 to week 2 (p < 0.001, d = 0.87), with no further changes from week 2 to week 6 (p > 0.999, d = 0.01) and from week 6 to week 10 (p > 0.999, d = 0.01). In the control group, no significant changes were observed during any period: from week 0 to week 2 (p > 0.999, d = 0.01), from week 2 to week 6 (p > 0.999, d = 0.05), or from week 6 to week 10 (p = 0.392, d = 0.05).

FIGURE 6.

Time‐course changes in the squared value of shear wave speed (SWS) of the biceps femoris long head (BFlh) in response to different Nordic hamstring exercise protocols. Values are expressed as delta (Δ) from baseline to enhance visual clarity, while statistical comparisons were performed on absolute measurements. See Figure S3 for absolute values. *p < 0.05 versus baseline (Week 0); #p < 0.05 versus control group.

4. Discussion

The present study is the first investigation of how 8 weeks of low‐ and ultralow‐volume NHE training, following 2 weeks of standardized moderate‐volume training, affects eccentric knee flexor strength, BFlh fascicle length, and BFlh stiffness. The novel findings were: (a) eccentric knee flexor strength increased during the initial 2 weeks and continued to increase through week 6 despite the reduction in training volume, with no differences between the two groups; (b) BFlh fascicle length increased significantly after just 2 weeks of moderate‐volume training and remained elevated throughout eight subsequent weeks of either low‐ or ultralow‐volume training, with no differences between these groups; (c) BFlh stiffness (evaluated by squared‐SWS) decreased significantly after 2 weeks of moderate‐volume training and remained reduced throughout the subsequent 8 weeks in both training groups, with no significant differences between them.

Eccentric knee flexor strength plays a crucial role in HSI prevention, although absolute muscle strength thresholds must be interpreted with caution as they are influenced by individual body mass. Previous research with Australian football players demonstrated that players with eccentric knee flexor strength below 256 N during the pre‐season period faced approximately a three times higher risk of sustaining HSI compared to stronger counterparts [7]. This highlights the potential value of interventions targeting eccentric knee flexor strength improvement. Previous studies employing the same training protocols as used in the low‐volume group of the present study (i.e., 2 weeks of moderate‐volume followed by 4 weeks of low‐volume NHE training) reported eccentric strength increases of 17%–33% after 6 weeks in recreationally active individuals without prior experience in eccentric hamstring exercises [17, 25]. The present study observed a 14% ± 9% strength increase after 6 weeks in the low‐volume group, despite our participants already engaging in regular lower‐limb resistance training including eccentric hamstring exercises. Importantly, we demonstrated that gains in eccentric knee flexor strength occur during both the initial 2 weeks of moderate‐volume training phase and the subsequent four‐week low‐volume phase, with no further increases beyond week 6. Furthermore, all participants in the present study exceeded the HSI risk threshold of 256 N regardless of NHE training volume, suggesting that even ultralow‐volume training protocols may effectively reduce HSI risk. However, it should be noted that eccentric knee flexor strength evaluates the overall function of the hamstrings, and it remains unclear whether functional increases occur in the BFlh, which is the most commonly injured muscle among the hamstring components [26, 27, 28, 29].

As a muscle‐specific risk factor in the BFlh rather than the entire hamstring, BFlh fascicle length has received considerable attention [5, 6, 8]. Elite soccer players with BFlh fascicle lengths shorter than 10.56 cm at the start of the pre‐season period exhibited an approximately four times higher HSI risk compared to those with longer fascicles [8], although such absolute thresholds should be interpreted with caution as fascicle length is influenced by individual thigh length. Indeed, we found that baseline BFlh fascicle length significantly correlated with thigh length (r = 0.546, p < 0.001), suggesting that relative rather than absolute values might provide more accurate risk assessment. Despite this limitation of absolute thresholds, interventions targeting increased BFlh fascicle length may help reduce HSI risk. Previous studies employing similar training protocols to ours observed that NHE training primarily increased BFlh fascicle length during the initial 2 weeks of moderate‐volume training (15%–24% [17, 25]), with no further increases during subsequent low‐volume training. The present study showed a similar pattern of changes, with BFlh fascicle length increasing during the initial 2 weeks and remaining elevated thereafter in both training groups. These findings suggest that even the ultralow‐volume NHE training protocol used in the present study may be effective in reducing HSI risk. However, the magnitude of increase in the present study (4% ± 2% and 5% ± 2% for ultralow‐volume and low‐volume groups, respectively) was smaller than previously reported (15%–24%) [17, 25], possibly due to our participants' prior exposure to the hamstring training. Importantly, we found that baseline BFlh fascicle length significantly correlated with thigh length (r = 0.546, p < 0.001), and participants whose fascicle length remained below the risk threshold after training (three and one participants in the low‐volume and ultralow‐volume groups, respectively) had significantly shorter thighs than those who exceeded the threshold. This reinforces the need for individualized thresholds based on anthropometric characteristics rather than absolute values.

Furthermore, in addition to fascicle length, passive muscle stiffness assessed using ultrasound SWE has recently emerged as another muscle‐specific risk factor for muscle strain injuries. Passive muscle stiffness is strongly influenced by the extracellular matrix, including the perimysium and endomysium [30, 31]. While increased BFlh fascicle length induced by NHE training has been hypothesized to be due to sarcomerogenesis (addition of serially arranged sarcomeres) [32, 33], recent evidence has challenged this hypothesis. Pincheira et al. suggested that increased BFlh fascicle length following NHE training resulted from increased sarcomere length rather than sarcomerogenesis [34]. Although more recent findings from the same group indicate that sarcomeres may be added in series following 9 weeks of NHE training [35], this interpretation has been challenged, highlighting ongoing debate about adaptation mechanisms [36].

Regardless of the specific mechanism for fascicle length changes, eccentric training appears to influence muscle mechanical properties. Previous studies have reported that eccentric knee flexor contractions acutely reduce BFlh stiffness, while concentric contractions do not elicit such changes [16]; suggesting that eccentric loading modifies extracellular matrix properties. Specifically, the wavy collagen fibrils (crimps) within extracellular matrix can become flattened and reduced in number during eccentric loading, and these changes may persist after physiological elongation [37, 38]. Interestingly, a recent study by Pieters et al. reported no changes in BFlh stiffness following 10 weeks of NHE training, contradicting our findings. However, this discrepancy might be explained by methodological differences; specifically, muscle length during measurements. It has been established that the effects of various interventions on passive muscle stiffness, such as stretching exercises and nutritional supplementation, are detectable only when muscles are in lengthened positions rather than shortened positions [39, 40, 41]. Pieters et al. measured BFlh stiffness with participants in a prone position (the BFlh in shortened position) [42]; similar to the position used for fascicle length measurements, potentially masking training‐induced changes in stiffness. In contrast, we assessed the BFlh stiffness with the knee fully extended and the hip flexed at 70°, placing the BFlh in a more lengthened position. We acknowledge that direct comparison with Pieters et al. remains limited as we did not assess BFlh stiffness in the prone position, and future studies incorporating measurements at multiple muscle lengths would provide more comprehensive insights. Nevertheless, taking all these findings together, it is reasonable to conclude that moderate‐volume NHE training effectively modifies the extracellular matrix properties of the BFlh, and importantly, that these adaptations can be maintained with ultralow‐volume training.

It has been reported that eccentric knee flexor strength gained through NHE training significantly declines after 4 weeks of training cessation [17, 43]. Additionally, a previous study demonstrated that in sprinters who regularly perform hamstring training, eccentric knee flexor strength can substantially decrease after just 2 weeks of training cessation, primarily through impaired neural activation rather than morphological changes [18]. Similarly, it has been shown that the increased BFlh fascicle length achieved through supramaximal NHE training markedly decreases within only 2 weeks of training cessation [17, 25], although the mechanism (sarcomere loss versus changes in extracellular matrix properties) remains unknown. Furthermore, recent findings indicate that in athletes who routinely engage in eccentric hamstring training, passive stiffness of the BFlh increases after a 2‐week cessation of training [22], probably due to alterations in extracellular matrix properties Considering the collective evidence from previous studies [17, 18, 22, 25, 43] along with the results of the present study, it is reasonable to conclude that maintaining the neural, morphological, and mechanical adaptations underlying the increased eccentric knee flexor strength and BFlh fascicle length as well as the reduced passive BFlh stiffness requires at least one session of supramaximal eccentric loading per week. In this context, the NHE training protocol employed in the present study, particularly the ultralow‐volume training protocol (four repetitions per week) performed at supramaximal intensity, holds significant practical value as a solution to the problem of low compliance with NHE training among athletes.

A notable strength of the present study is the inclusion of participants who regularly performed lower‐limb resistance training, including eccentric hamstring exercises, rather than merely recreationally active individuals [13, 17, 25, 43]. While numerous studies have examined the effects of NHE training on eccentric knee flexor strength or BFlh fascicle length [14, 17, 25, 44], most focused on recreationally active populations, and few previous studies included control groups with no additional exercise interventions [13, 44]. Based on this previous literature, it remained unclear whether NHE training would be effective for individuals already engaging in lower‐limb resistance training and whether the observed increases in eccentric knee flexor strength and BFlh fascicle length could be specifically attributable to NHE training rather than other training modalities. The present study successfully addresses these concerns, demonstrating that NHE training provides additional benefits even in resistance‐trained individuals, bridging the gap between sports practice and research and providing evidence directly applicable to athletes.

In addition to the measurement position for BFlh stiffness discussed above, this study has several other limitations that should be acknowledged. First, the ultrasound probe used had a limited field of view width of 50 mm, which was insufficient to capture the entire BFlh fascicle length. Consequently, the BFlh fascicle length was estimated using a trigonometric equation, following established protocols from previous research [13, 14, 45]. However, the validity of this method for assessing BFlh fascicle length remains unverified. Furthermore, this approach may introduce measurement errors compared to extended field‐of‐view methods [45] and carries the potential for systematic over‐ or under‐estimation due to possible alterations in three‐dimensional fascicle trajectory following NHE training. Additionally, fascicle length was measured at a single region (50% of thigh length) despite known regional heterogeneity in BFlh fascicle length [46] and its adaptations [34]. Second, SWE measurements were performed on a small two‐dimensional region, which may not capture global or regional changes in muscle stiffness. Therefore, future studies assessing architectural and mechanical properties of the whole BFlh in three dimensions are warranted.

In conclusion, the present study demonstrated that 2 weeks of moderate‐volume NHE training can induce significant neuromuscular adaptations in resistance‐trained individuals, including increased eccentric knee flexor strength and BFlh fascicle length, along with decreased BFlh stiffness. Importantly, these adaptations can be effectively maintained with markedly reduced training volume as low as four repetitions per week for at least 8 weeks.

4.1. Perspective

The present study demonstrated that a short period of moderate‐volume NHE training followed by ultralow‐volume supramaximal NHE training effectively maintained improvements in eccentric knee flexor strength, BFlh fascicle length, and BFlh stiffness associated with reduced HSI risk. This training strategy may be particularly practical during periods when training compliance is challenging, such as during competitive seasons. However, prospective longitudinal studies with injury surveillance are necessary to determine whether these physiological adaptations actually translate into reduced HSI incidence. Such studies would provide critical evidence regarding the effectiveness of ultralow‐volume NHE training protocols in real‐world athletic settings. Furthermore, identifying which of the three parameters examined in the present study (eccentric knee flexor strength, BFlh fascicle length, BFlh stiffness) has the strongest association with HSI risk would advance our understanding of injury mechanisms.

Ethics Statement

This study was approved by the ethics committee (No. 2024‐44) of Graduate School of Health and Sports Science, Juntendo University.

Consent

A written informed consent was obtained from all participants prior to data collection.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Table S1: Post hoc pairwise comparisons and effect sizes (Cohen's d) for eccentric knee flexor strength, BFlh fascicle length, and BFlh squared‐SWS.

Figure S1: Time‐course changes in eccentric knee flexor strength in response to different Nordic hamstring exercise protocols. *p < 0.05 vs. baseline (Week 0); †p < 0.05 vs. Week 2; #p < 0.05 vs. control group.

Figure S2: Time‐course changes in fascicle length of the biceps femoris long head (BFlh) in response to different Nordic hamstring exercise protocols. *p < 0.05 vs. baseline (Week 0).

Figure S3: Time‐course changes in squared‐value of shear wave speed (SWS) of the biceps femoris long head (BFlh) in response to different Nordic hamstring exercise protocols. *p < 0.05 vs. baseline (Week 0); #p < 0.05 vs. control group.

Miura K. and Miyamoto N., “Effects of Reduced Training Volume of Nordic Hamstring Exercise on Eccentric Knee Flexor Strength, and Fascicle Length and Stiffness of Biceps Femoris Long Head,” Scandinavian Journal of Medicine & Science in Sports 35, no. 8 (2025): e70115, 10.1111/sms.70115.

Data Availability Statement

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