Optimizing PNF stretching strategies for hamstring flexibility: Comparing end- and mid-range stretching effects by muscle tightness severity

Abstract

Background

When the hamstring muscles are tight, the risk of injury during physical activity increases, prompting the frequent use of active interventions, such as stretching, in clinical settings. However, most previous proprioceptive neuromuscular facilitation (PNF) stretching studies have been conducted at the end-range of motion, which does not correspond with the point of maximal force output according to the length–tension relationship. The purpose of this study was to compare the effects of PNF stretching performed at the mid-range, where optimal force production is theoretically possible, with those performed at the end-range. Additionally, it aimed to investigate whether flexibility gains differed according to the degree of hamstring tightness.

Methods

Hamstring tightness was evaluated using the active knee extension (AKE) test. Participants were categorized into severe and mild tightness groups. PNF stretching was performed at either 100% of maximal range of motion (MaxROM) or 50% MaxROM. Flexibility changes were assessed post-intervention using the AKE test.

Results

In the severe hamstring tightness group, significant flexibility improvements were observed after PNF stretching at both 100% and 50% MaxROM. In contrast, the mild hamstring tightness group showed significant improvement only at 100% MaxROM.

Conclusions

The effectiveness of PNF stretching in increasing flexibility varies according to joint position (100% vs. 50% MaxROM) and the initial severity of tightness. Notably, individuals with severe tightness experienced significant gains even at mid-range, suggesting that mid-range PNF stretching may serve as a viable alternative for patients who experience discomfort or pain at maximal ranges.

Keywords

joint flexibility muscle stretching exercises proprioception range of motion

1 Introduction

The hamstring muscles control the hip and knee joints simultaneously; therefore, they are involved in various functional movements of the lower extremities and in maintaining overall postural stability.¹ Hamstring tightness can restrict joint range of motion (ROM), leading to inefficient movement patterns and an increased risk of musculoskeletal injury.^2–4 To address these issues, clinicians commonly employ a range of interventions; stretching is the most widely used. Stretching has garnered both clinical popularity and extensive scientific validation over the years.^5,6

Among the various stretching techniques aimed at alleviating hamstring tightness, static stretching and proprioceptive neuromuscular facilitation (PNF) stretching are frequently applied. PNF stretching has been shown to be particularly effective in improving muscle length and flexibility, not only through alterations in the mechanical properties of muscles and connective tissues, but also via neurophysiological mechanisms.^7,8 Although some studies have suggested that PNF stretching may lead to flexibility improvements comparable to, or greater than, those achieved with static stretching, recent meta-analyses have reported no significant differences between the two techniques in range of motion gains following both acute and chronic stretching interventions.^5,9 Currently, research on PNF stretching has certain limitations.

First, most studies have focused primarily on the type, intensity, and duration of stretching, with limited attention given to the joint positions at which stretching is performed.^8,10,11 Most existing studies have applied PNF stretching exclusively at the end-range joint angles (Maximal ROM), and there is a notable absence of research investigating its effects at mid-range joint angles (i.e., 50% of Max ROM).^6,12,13 Since the effectiveness of PNF stretching is thought to derive primarily from neurophysiological inhibition mechanisms rather than direct external force applied to the muscle-tendon unit, it is reasonable to expect that some degree of effectiveness may also be observed at mid-range joint angles.^14–16 In fact, previous studies have shown that submaximal-intensity PNF stretching can yield flexibility gains comparable to those achieved at maximal intensity.^17–20

Second, most prior research has evaluated hamstring tightness based on a single criterion, without considering how varying degrees of tightness may influence stretching outcomes. The mechanical properties of muscle and connective tissue can differ depending on the level of tightness, which may in turn affect the tensile stress applied to the muscle-tendon unit, and subsequently alter the stretching response. For example, even in the absence of muscle shortening, healthy individuals have demonstrated significant increases in hip flexion angles following flexibility training.²¹ Moreover, groups subjected to different flexibility training protocols have shown varied acute stretching effects.²²

This study was conducted to compare the effects of PNF stretching performed at end- and mid-range joint positions and to ascertain whether differences in flexibility gains are influenced by hamstring tightness i.e., mild vs. severe. This approach aimed to elucidate the mechanisms and clinical implications of PNF stretching across different joint angles and baseline tightness levels, thereby providing a scientific basis for more individualized application in clinical settings.

2 Materials and methods

2.1 Participants

Twenty-six young adults (7 males and 19 females) voluntarily participated in this study.²³ All participants were university students who did not engage in regular competitive sports training at the time of the study. Descriptive characteristics, including age, height, and weight, are presented separately for males and females in Table 1. To quantitatively assess hamstring tightness, the active knee extension (AKE) test was administered, and participants with an AKE angle ≥ 20° were included.²⁴ The exclusion criteria consisted of any history of lower extremity joint surgery or pain within the past six months. These criteria were established to enhance the accuracy and consistency of participant selection. The study was approved by the Institutional Review Board of Woosong University and was registered with the Clinical Research Information Service (CRIS) of the Korea Disease Control and Prevention Agency (https://cris.nih.go.kr). Sample size calculation was performed using G*Power 3.1 for a paired t-test. Based on a previous study reporting a moderate effect size for hamstring-related range of motion (Cohen's d = 0.60), an effect size of dz = 0.6 was selected (alpha = 0.05, power = 0.80).²⁵ This analysis yielded a required sample size of 24. Prior to participation, all individuals were thoroughly informed about the study's purpose and procedures and provided written informed consent in accordance with ethical standards.

Table 1.
Participant characteristics.

Sex Age (years) Height (cm) Weight (kg)

Male (n = 7) 22.4 ± 3.4 175.6 ± 3.5 75.6 ± 8.0

Female (n = 19) 20.3 ± 0.7 161.1 ± 4.5 52.4 ± 8.8

Sex	Age (years)	Height (cm)	Weight (kg)
Male (n = 7)	22.4 ± 3.4	175.6 ± 3.5	75.6 ± 8.0
Female (n = 19)	20.3 ± 0.7	161.1 ± 4.5	52.4 ± 8.8

2.2 Procedures

Participants were positioned supine with their hips and knees fully extended (Figure 1). To assess hamstring flexibility, the AKE test was used. To maintain the starting posture (90° flexion at both the hip and the knee), a metallic support was placed under the lower leg.²⁴ Upon the examiner's cue, participants slowly extended their right knee while maintaining hip flexion, ensuring that no compensatory movement occurred. A slight degree of ankle plantarflexion was permitted. Upon reaching end-range, the knee extension angle was measured using an inertial measurement unit sensor (Physio Link, Physio Inc., Daejeon, Korea). The AKE value was calculated as 180° minus the measured knee extension angle (Pre-AKE).

Figure 1.

Flow diagram for the study.

PNF stretching was applied in a randomized order under two conditions: 100% MaxROM (end-range) and 50% MaxROM (mid-range). The target angles for both conditions were obtained during an active hip flexion. When participants reached their maximal pain-free range, the position was held for approximately 3 s so that the angle could be recorded using a clinometer application (Plaincode Software Solutions, Gunzenhausen, Germany). After the measurement, the limb was returned to the treatment table. For the stretching intervention, the examiner then passively raised the limb to the assigned target angle, which corresponded either to the full measured MaxROM (Figure 2(a)) or to 50% of that value (Figure 2(b)), depending on the condition.

Figure 2.

PNF stretching at end-range representing 100% MaxROM (a) and mid-range representing 50% MaxROM (b).

PNF stretching was applied using the hold-relax technique. The examiner stabilized the limb by placing the participant's lower leg over the examiner's shoulder, which allowed the hip flexion angle to be maintained at the assigned target position, while supporting both the upper and lower leg with the hands to prevent unnecessary movement. On the examiner's instruction, participants performed a maximal voluntary isometric contraction for six seconds. This cycle was repeated five times. After the stretching protocol, hamstring flexibility was reassessed using the same AKE test procedures (Post-AKE).

2.3 Data analysis

All statistical analyses were conducted using IBM SPSS Statistics version 25.0 (IBM Corp., Armonk, NY, USA). The Shapiro—Wilk test was used to evaluate the normality of the data. To compare Pre- and Post-AKE results within each group and at each joint position (100% and 50% MaxROM), the Wilcoxon signed—rank test was used. For between-group comparisons, a nonparametric ANCOVA approach was employed to account for baseline differences. Specifically, residuals from a linear regression of Post-AKE on Pre-AKE were computed across all participants, and a Kruskal–Wallis test was applied to these residuals to assess group differences at each joint position. Statistical significance was set at P < 0.05, and all results are reported as mean ± standard deviation.

3 Results

Participants were classified into two groups based on the severity of hamstring tightness, as determined by the AKE test. The mild hamstring tightness (MHT) group included individuals with AKE angles less than 40° (n = 14), whereas the severe hamstring tightness (SHT) group comprised those with AKE angles greater than 40° but less than 60° (n = 12). Flexibility changes before and after PNF stretching were evaluated at both 100% and 50% MaxROM, and the effects were analyzed both within groups and between groups.

For all participants, flexibility significantly improved after PNF stretching at both intensities. At 100% MaxROM, flexibility increased by an average of 9.4° (Z = −3.89, P < 0.001, dz = 1.42), while at 50% MaxROM, flexibility increased by 6.6° (Z = −3.41, P < 0.001, dz = 0.80). These findings indicate that PNF stretching was effective in enhancing hamstring flexibility regardless of the joint position.

In the MHT group, a significant increase in flexibility was observed following stretching at 100% MaxROM, with an average improvement of 6.4° (Z = −2.32, P = 0.021, dz = 0.97) (Figure 3). However, at 50% MaxROM, the average change was only 2.5°, which did not reach statistical significance (Z = −1.57, P = 0.115, dz = 0.34). In contrast, the SHT group showed significant flexibility gains at both joint positions. At 100% MaxROM, the mean increase was 12.9° (Z = −3.08, P = 0.002, dz = 2.69), and at 50% MaxROM, it was 11.2° (Z = −3.07, P = 0.002, dz = 1.72), both of which were significant.

Figure 3.

Within-group flexibility changes before and after PNF stretching performed at (a) maximum range of motion and (b) 50% maximum range of motion. MHT: mild hamstring tightness; SHT: severe hamstring tightness; Pre-AKE: active knee extension test before PNF stretching; Post-AKE: active knee extension test after PNF stretching.

When comparing flexibility improvements between the two groups, the SHT group consistently demonstrated greater increases than did the MHT group under both stretching conditions—100% MaxROM (Kruskal–Wallis H = 18.76, df = 1, P < 0.001) and 50% MaxROM (H = 18.79, df = 1, P < 0.001). Although the graphical representation (Figure 4) shows changes in AKE (i.e., delta values: Post – Pre), statistical comparisons were performed using a residual-based Kruskal–Wallis test, a nonparametric alternative to ANCOVA that accounts for baseline differences.

Figure 4.

Between-group comparison of flexibility changes following PNF stretching performed at the end- and mid-ranges of motion. MHT: mild hamstring tightness; SHT: severe hamstring tightness; End-range: PNF stretching performed at the maximum range of motion; Mid-range: PNF stretching performed at 50% of the maximum range of motion.

4 Discussion

Consistent with the results of previous studies, when PNF stretching was performed without stratifying participants by hamstring tightness levels, a significant increase in flexibility was observed. Specifically, flexibility improved following PNF stretching performed at both the end-range and the mid-range positions, with greater improvements observed at the end-range. According to the length-tension relationship, skeletal muscles exhibit maximum potential for force generation at intermediate lengths, where the formation of actin-myosin cross-bridges is optimized.^26,27 This phenomenon implies that muscle force output, rather than neural activation, may be greater at mid-range than at end-range. In particular, for the knee joint, it is well established that joint angles substantially influence electromyography (EMG) activity, more so than at the hip. For instance, Worrell et al. (2001) reported higher muscle activation between 30° and 60° of knee flexion compared to that at 0° or 90°, reflecting the impact of positional changes on muscle response.²⁸ Despite this theoretical advantage, the observed increase in flexibility at mid-range was less pronounced than that at end-range. This outcome may be explained by several physiological and mechanical factors. First, there may be lower tensile stress in the mid-range position. In this state, muscle and connective tissues remain relatively slack, which reduces the mechanical tension applied to the muscle-tendon unit. As a result, the potential for plastic deformation, such as realignment of collagen fibers, may be limited, thereby attenuating structural changes and subsequent flexibility improvements.^29,30 Second, the limitations of neurophysiological mechanisms must be considered. One of the primary mechanisms underpinning the efficacy of PNF stretching is the activation of the Golgi tendon organ (GTO), which responds to increased muscle tension by promoting autogenic inhibition and relaxing the agonist muscle.^31–33 In a slackened mid-range position, the activation threshold of the GTO may not be adequately met, potentially diminishing the neuromuscular inhibition effect.³⁴ Conversely, in the end-range position, where the muscle-tendon unit is already tensioned, the GTO can be more effectively stimulated, thereby producing greater flexibility gains.^35,36 Taken together, these findings suggest that each joint position has distinct advantages and limitations for PNF stretching. Based on the current results, end-range stretching appears to be at least as effective, if not superior, to mid-range stretching in producing flexibility gains.

An intriguing aspect of this study lies in the differential outcomes based on the initial severity of hamstring tightness. In participants with mild tightness, flexibility gains differed significantly between end- and mid-range positions. However, in those with severe tightness, similar improvements were observed in both conditions (12.9° at end-range vs. 11.2° at mid-range). These findings indicate that individuals with low baseline flexibility may experience substantial benefits from PNF stretching regardless of the joint position. Furthermore, the effectiveness of mid-range stretching in this population suggests that muscle tightness itself is a determinant of stretching efficacy. It is worth noting that previous studies have reported conflicting results. For instance, studies comparing males and females have shown that women generally exhibit greater joint flexibility,^37,38 and under low-tension conditions such as light warm-ups, women demonstrated greater increases in hamstring flexibility with repeated stretching.³⁹ This could be attributed to lower tissue stiffness, which may enhance ROM gains during stretching interventions.^37,38,40

This study is not without limitations. First, the stretching posture used involved hip flexion while maintaining full knee extension, which may not be the most effective position for isolating the hamstring muscles. Previous research has shown that hamstring muscles exhibit more than twice the EMG activity when functioning as knee flexors compared to hip extensors.^41,42 Therefore, AKE positioning, which involves knee extension, may be more appropriate for targeting hamstring lengthening than the traditional straight-leg raise position.^41–43 Second, this study examined only the acute effects of a single PNF stretching session in a relatively young population; therefore, the findings may not generalize to repeated or long-term interventions or to populations with different age-related or clinical characteristics. Moreover, the absence of a non-PNF stretching control condition limits the ability to distinguish the specific contribution of PNF-related neuromuscular mechanisms from general stretching effects. Finally, the lack of EMG analysis limited the ability to quantitatively assess muscle activity during stretching. Future studies should incorporate EMG assessments in the AKE position and include more diverse populations to further explore the relationship between neuromuscular activation and stretching outcomes.

5 Conclusion

This study demonstrated that the effectiveness of PNF stretching in improving hamstring flexibility varies by joint position (100% vs. 50% MaxROM) and hamstring tightness (mild vs. severe). Notably, individuals with severe tightness exhibited significant flexibility improvements even when stretching was performed at the mid-range position. These findings support the notion that individuals with low baseline flexibility are more likely to experience greater benefits from stretching interventions. Importantly, the comparable effects observed in the mid-range position for participants with severe tightness suggest that mid-range PNF stretching may serve as a suitable alternative for those who experience discomfort or pain at maximal ROM. From a clinical standpoint, these results highlight the value of individualizing stretching protocols based on the degree of hamstring tightness. Adapting therapeutic approaches to match each patient's unique musculoskeletal profile may improve adherence and the overall effectiveness of flexibility interventions.

Footnotes

Ethical considerations

All study procedures were approved by the Institutional Review Board of Woosong University. Informed consent was obtained from all participants before the experiment.

Author contributions

CONCEPTION, PERFORMANCE OF WORK, INTERPRETATION OR ANALYSIS OF DATA, PREPARATION OF THE MANUSCRIPT, REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Wootaek Lim.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by 2024 Woosong University Academic Research Funding and the Regional Innovation System & Education (RISE) program through the Daejeon RISE Center, funded by the Ministry of Education and the Daejeon Metropolitan City, Republic of Korea (2025-RISE-06-009).

Declaration of conflicting interests

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

ORCID iD

Wootaek Lim

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