Optimal position of PNF stretching for improving flexibility: Knee flexion in prone position versus hip extension in supine position

Abstract

Background

In clinical practice, knee flexion in the prone position is proposed for manual muscle testing (MMT) of the hamstrings, while hip extension in a supine position is suggested for hamstring stretching. Since the effect of proprioceptive neuromuscular facilitation (PNF) stretching on flexibility is greater during maximal isometric contractions, the MMT position exhibiting greater muscle activity may be more effective. The purpose of this study was to determine the effects of PNF stretching performed in the straight leg raise (SLR) and MMT positions on lower extremity flexibility.

Methods

Thirty healthy young adults participated in this study. Lower extremity flexibilities of the hip, knee, and ankle were measured before and after PNF stretching. PNF stretching was performed in one leg in the SLR position and the other leg in the MMT position.

Results

PNF stretching on both SLR and MMT positions significantly increased hip, knee, and ankle flexibilities. The increase in flexibility was higher for the SLR position compared to the MMT position, especially in knee and ankle flexibility.

Conclusions

PNF stretching in the SLR position exhibited significantly higher flexibility, especially in the knee and ankle. The SLR position can be recommended in most clinical settings. MMT position using knee flexion could be beneficial when hip joint motion is limited.

Keywords

hamstring muscles joint flexibility muscle stretching exercise range of motion

1. Introduction

Flexibility is a fundamental aspect of physical fitness, impacting both performance and injury prevention. Flexibility refers to the range of motion (ROM) available at a joint, which is determined by the extensibility of the surrounding muscles and connective tissues. The muscle is not an isolated entity but rather functions as part of a myofascial unit, a concept introduced by Antonio Stecco and colleagues.¹ This unit includes muscle fibers, connective tissue such as fascia, nerves, and blood vessels, all working together to facilitate movement and maintain structural integrity. Understanding flexibility on an anatomical level involves recognizing the interaction between these components, particularly the role of the intramuscular connective tissue in passive muscle properties, which primarily determine flexibility.

To prevent potential strain injuries caused by hamstring tightness, stretching has been widely recommended before exercise. Several stretching techniques are currently available, but static stretching using an external force and proprioceptive neuromuscular facilitation (PNF) stretching using an internal force are the most widely used.^2,3 PNF stretching was more effective at improving flexibility and thus is sometimes preferred to static stretching.^4,5 In PNF stretching flexibility, improvements are correlated with the intensity of isometric contractions.^6–8 In other words, less improvement in flexibility is attained with submaximal contractions compared to maximal voluntary isometric contractions (MVICs).⁶ Furthermore, the improved flexibility is retained over longer periods from high intensity contractions. In essence, greater contractions of the target muscle is crucial during PNF stretching in order to increase joint flexibility and maintain its effects.

Muscle activity is known to be greatest in the manual muscle testing (MMT) position. The standard position for MMT for the hamstrings is the 60° flexion of the knee joint in a prone position.^9,10 In a study by Worrell (2001), electromyography (EMG) of hamstrings during knee flexion or hip extension in the prone position were examined.¹¹ First, EMG activity ranged from 68.0–82.5% of MVIC depending on the angle of knee flexion. However, it ranged from 13.2–15.7% of MVIC depending on the angle of hip extension.¹¹ In other words, the hamstrings have higher muscle activity during knee flexion than hip extension. However, the standard position for stretching in clinical practice is the straight leg raised in the supine position.⁷ In previous studies comparing EMG activities in prone and supine positions, EMG activity of hamstrings during knee flexion in a prone position (MMT position) was more than twofold higher than that during hip extension in a supine position (SLR position).^12,13 That is, in both supine and prone positions, the hamstrings are activated more during knee flexion than during hip extension. As PNF stretching is based on isometric contraction of the target muscle, stretching in the MMT position may be more effective than the SLR position.

Although many studies have compared the EMG activity of hamstrings between the supine and prone positions, no study to date has investigated changes in flexibility after stretching in these two different positions.¹⁴ This study aimed to examine the effects of PNF stretching performed in the SLR and MMT positions on flexibility of the lower extremities. Lower extremity flexibility was measured using the active straight leg raise (ASLR) test and active knee extension (AKE) test, and ankle dorsiflexion (ADF) test. The study population included healthy individuals to ensure that the results were applicable to general recommendations for improving flexibility and preventing injuries in a broad demographic.

2. Methods

2.1 Participantss

Individuals in their 20s were included in this experiment since this age group typically achieves peak physical maturity and exhibits higher levels of physical activity, making them an ideal demographic for studying responses to stretching.^15,16 Furthermore, the experiment was conducted exclusively on healthy individuals because PNF stretching, in contrast to static stretching, necessitates the active involvement of the experimenter in the stretching exercises.¹⁷ Individuals with a history of hip and knee joint surgery were excluded because surgery on the joint can significantly decrease joint flexibility and might respond differently to flexibility exercises.¹⁸ Additionally, individuals with hip or knee joint pain in the past six months were excluded from the experiment.¹⁹ A total of 30 subjects, (12 males and 18 females) who met the conditions participated in the experiment (age 21.9 ± 2.4 years, height 167.3 ± 8.3 cm, weight 62.9 ± 11.6 Kg). An a priori power analysis was performed using the G*Power 3.1.9.7 to determine the sample size, with an alpha level of 0.05 and a desired power of 0.8. This study was approved by the Institutional Review Board of Woosong University (IRB No. 1041549-221011-SB-149) and informed consent was obtained from all individuals prior to the start of the experiments.

2.2 Procedures

2.2.1 Test procedure

In this study, flexibility is discussed in terms of changes in ROM. Before PNF stretching, the flexibility of the individuals’ lower extremities was assessed (Figure 1). Hip flexibility was evaluated using the ASLR test, knee flexibility with the AKE test, and ankle flexibility with the ADF test, with the order of tests randomized. Each test was conducted twice, and the average was recorded as the $RO M_{pre}$ value. Taking two measurements and averaging them enhances the reliability and precision of the data by reducing the impact of any single outlier or measurement error, thereby increasing the psychometric quality of the research.

Figure 1.

CONSORT flow diagram.

For the ASLR test, individuals laid in a supine position on a treatment table, with their pelvis and the unmeasured leg secured with a strap. The individuals extended their hip until discomfort or pain was felt.²⁰ The hip ROM at this endpoint was measured with a clinometer (Plaincode Software Solutions, Stephanskirchen, Germany). The clinometer has demonstrated excellent reliability and validity when compared to the goniometer.^21–23 To minimize measurement error, the clinometer was calibrated using the following process: the device was held steady in one direction (Figure 2(a)), then turned by 180° to complete the calibration (Figure 2(b)). All experimental procedures were conducted by a single physical therapist with over 10 years of experience.

Figure 2.

First (a) and second step (b) of clinometer calibration.

The AKE test, also performed in the supine position with the pelvis and the other leg strapped, began with the individual's hip and knee flexed at 90°. Individuals then extended their knee until discomfort or pain was felt.²⁴ When myoclonus, which refers to sudden, involuntary muscle jerks, occurred, the knee was flexed slowly until it subsided.²⁵ The ROM was measured at the endpoint.

For the ADF test, individuals sat with the unmeasured leg strapped. Individuals slowly performed ADF while maintaining 90° flexion of the hip. The ROM at the endpoint was measured.²⁶

2.2.2 Intervention

After taking the $RO M_{pre}$ measurements for the hip, knee, and ankle, PNF stretching was performed.⁶ Each leg was alternately stretched in the SLR and MMT positions, in a randomly selected order. During PNF stretching in the SLR position, individuals laid in the supine position with strapped pelvis and unmeasured leg (Figure 3(a)). Individuals performed hip flexion as much as possible on their own (Figure 3(b)). The examiner provided resistance in the opposite direction to induce a maximal isometric contraction for six seconds per trial, repeated five times (Figure 3(c)). In the MMT position, PNF stretching was performed at 60° flexion of the knee joint in the prone position. The individual lay prone on a treatment table and the pelvis and the unmeasured leg were securely strapped (Figure 4(a)). With the knee flexed 60° (Figure 4(b)), they performed an isometric contraction against the examiner's resistance for 6 s (Figure 4(c)). This was repeated a total of five times.²⁷

Figure 3.

PNF stretching in the straight leg raise position. (1) starting position in the supine position, (2) maximal hip flexion, (3) direction of resistance during PNF stretching.

Figure 4.

PNF stretching in the manual muscle testing position. (1) starting position in the prone position, (2) 60° flexion of the knee joint, (3) direction of resistance during PNF stretching.

Following PNF stretching, the ASLR, AKE, and ADF tests were administered once more, and the results were recorded as the $RO M_{post}$ values. The order of the tests was set randomly.

2.3 Data analysis

The physical therapist read the angle displayed on the clinometer, and the raw data were recorded in an Excel spreadsheet. Before data analysis, the raw data from Excel were transferred into IBM SPSS Statistics 27 (IBM Corp., Armonk, NY, USA) software. Statistical significance was set at p < .05. All results are presented as means ± standard deviation.

Normality of the data was examined using the Shapiro-Wilk test. A two-way ANOVA was conducted to examine the effect of position and joint. Post hoc analysis was performed using paired samples T-test to analyze the differences in $RO M_{pre}$ (before stretching) and $RO M_{post}$ (after stretching). The difference in ROM (ΔROM) after stretching between the two positions was analyzed using the independent samples T-test and Mann–Whitney U-test.

Intraclass correlation coefficients (ICC) were calculated using a two-way mixed-effects model with consistency to assess the reliability of the repeated measurements. The ICC values, along with their 95% confidence intervals and significance levels, were calculated for all 1st and 2nd measurements.

The ROM for the two positions was calculated using the formulas:

\begin{aligned} S L R_{Δ R O M} & = R O M_{p o s t} - R O M_{p r e} \\ M M T_{Δ R O M} & = R O M_{p o s t} - R O M_{p r e} \end{aligned}

The standard error of measurement (SEM) was calculated using the formula:

\begin{aligned} S E M = s t a n d a r d d e i v a t i o n \times \sqrt{1 - I C C} \end{aligned}

The minimal detectable change (MDC) at a 95% confidence interval was calculated using the formula:²⁸

\begin{aligned} M D C_{95 %} = 1.96 \times SEM \times \sqrt{2} \end{aligned}

3. Results

A total of 31 subjects were initially recruited for the study, but one subject did not meet the criteria, resulting in a final sample of 30 participants.

The reliability of the repeated measurements was confirmed through ICC (Table 1). The high ICC values indicate excellent reliability for the ROM repeated measurements. The SEM and MDC values, which were calculated to assess the precision and sensitivity of the measurements, further support the reliability of the measurements. SEM values ranged from 1.6 to 2.3, indicating a low level of measurement error. MDC values, representing the smallest change that can be detected beyond the measurement error, ranged from 4.6 to 6.4.

Table 1.
Intrarater reliability of the repeated measurements.

ICC 95% CI SEM MDC

$ASL R_{pre}$ 0.950 0.908–0.974 2.3 6.4

$AK E_{pre}$ 0.972 0.947–0.985 2.0 5.7

$AD F_{pre}$ 0.951 0.909–0.974 1.6 4.6

$ASL R_{post}$ 0.956 0.919–0.977 2.0 5.5

$AK E_{post}$ 0.960 0.925–0.978 2.3 6.4

$AD F_{post}$ 0.951 0.909–0.974 1.8 4.9

	ICC	95% CI	SEM	MDC
$ASL R_{pre}$	0.950	0.908–0.974	2.3	6.4
$AK E_{pre}$	0.972	0.947–0.985	2.0	5.7
$AD F_{pre}$	0.951	0.909–0.974	1.6	4.6
$ASL R_{post}$	0.956	0.919–0.977	2.0	5.5
$AK E_{post}$	0.960	0.925–0.978	2.3	6.4
$AD F_{post}$	0.951	0.909–0.974	1.8	4.9

ICC: intraclass correlation, SEM: standard error of measurement, MDC: minimal detectable change.

There was a statistically significant interaction between the effects of position and joint on ROM (p < .001). In both SLR and MMT positions, stretching increased the flexibility of the lower extremities (Table 2). In the SLR position, ROMs measured by ASLR (p = .001), AKE (p < .001), and ADF (p < .001) increased significantly after stretching. In the MMT position, ROMs measured by ASLR (p = .004), AKE (p = .002), and ADF (p = .035) were also significantly increased after stretching.

Table 2.

Lower extremity flexibility after stretching performed in the straight leg raise position and manual muscle testing position.

	ASLR test		AKE test		ADF test
	${ROM}_{pre}$	${ROM}_{post}$	${ROM}_{pre}$	${ROM}_{post}$	${ROM}_{pre}$	${ROM}_{post}$
SLR position	76.3 ± 9.9	81.1 ± 9.6*	26.3 ± 11.4	18.5 ± 11.3*	27.1 ± 7.7	36.4 ± 8.1*
MMT position	77.6 ± 10.9	81.7 ± 9.5*	22.0 ± 12.8	17.9 ± 11.9*	29.6 ± 7.1	32.0 ± 7.4*

ASLR: active straight leg raise, ADF: active dorsiflexion, ROM: range of motion, SLR: straight leg raise, MMT: manual muscle testing.

* significantly different compared to $R O M_{p r e}$ .

In comparing the ROMs in SLR and MMT positions, there was no significant difference in ASLR ( $RO M_{pre}$ (p = .695) and $RO M_{post}$ (p = .857)), in AKE ( $RO M_{pre}$ (p = .265) and $RO M_{post}$ (p = .855)), and in ADF ( $RO M_{pre}$ (p = .293) and $RO M_{post}$ (p = .081)).

However, the improvement in flexibility (ΔROM) was significantly different between SLR and MMT positions. ΔROM measured by AKE (p = .016) and ADF (p = .001) in the SLR position were higher than those in the MMT position (Figure 5).

Figure 5.

Improvement in flexibility after stretching performed in two different positions. ASLR: active straight leg raise, AKE: active knee extension, ADF: active dorsiflexion. SLR: straight leg raise, MMT: manual muscle testing, ROM: range of motion. * significant difference between two positions.

4. Discussion

This study tested the hypothesis that PNF stretching in the MMT position, which is associated with higher hamstring activation, would be more effective in improving flexibility compared to the SLR position. However, contrary to the hypothesis, the findings indicated that PNF stretching in the SLR position led to greater improvements in flexibility, particularly in the knee and ankle, than in the MMT position. The effect of PNF stretching in SLR position on hip and knee flexibility has been widely demonstrated.²⁹ In this study, an increase in ankle flexibility was also observed after stretching. This might be due to anatomical and functional connections involving the hip, knee, and ankle. Anatomically, the pelvis is connected to the ankle via active contractile tissues such as muscles and passive connective tissues such as the fascia latae, femoral intermuscular fascia, and crural fascia.³⁰ This anatomical property affects functional movement at the joint.^31,32 In fact, many studies have shown that different ankle positions (dorsiflexion, plantar flexion, or neutral position) are significantly linked to the hip flexion angle.^33,34 Palmar et al. clearly demonstrated using an isokinetic dynamometer that when hip flexion is performed with the ankle in dorsiflexion, hip ROM decreases and passive torque increases compared to when the ankle is in plantarflexion.³⁵ These anatomical and functional properties are observed in the effects of stretching as well. Previous studies examining the effect of upper-back stretching on lower extremity flexibility found that the ROM of the knee and ankle joints increased by 12.0° and 11.8°, respectively, suggesting a potential connection between the trunk and ankle.^36,37 In this study, flexibilities of the hip, knee, and ankle were also increased in the MMT position. Unlike single-joint muscles, two-joint muscles such as the hamstrings require different functional roles for each joint, and their quantitative involvement may vary. Because hip joint movement is restricted during MMT in the prone position, the hamstrings primarily functions as a knee flexor. On the other hand, in the supine position, the knee joint is at full extension, so the hamstrings primarily functions as a hip extensor. In both positions, the hamstrings is activated during isometric contractions, providing a reasonable explanation for the increased flexibility observed.

In terms of the increase in flexibility, it increased more in the SLR position than from the MMT position. In particular, knee and ankle flexibility were significantly improved. This might be due to the different muscle lengths in the two positions. PNF stretching is generally performed at a point at which the muscle is fully stretched.² In the SLR position, stretching is performed at maximal hip flexion with full knee extension, so the tissues are lengthened to their limit. On the other hand, stretching in the MMT position is performed at 60° flexion of the knee with full hip extension, so the targeted active and passive tissues are lengthened less. Previous studies have shown stretching a muscle beyond the point of discomfort (POD) leads to an increase in flexibility and a decrease in muscle-tendon unit (MTU) stiffness.^38,39 In terms of mechanical properties, tensile forces inflicted on relaxed tissues will not effectively lengthen the MTU. PNF stretching is affected by neurophysiological properties explained by autogenic and reciprocal inhibition.⁴⁰ During stretching, a relaxed MTU will not help lengthen it.⁴¹ Another potential factor pertains to the different muscles involved in the movement. To understand the differences in increased flexibility, EMG activities must be understood comprehensively along with force and torque, as joint ROM is not affected by the hamstrings alone but is multifactorial.¹⁴ While it may differ depending on the specific conditions, hip extension torque is generally larger than knee flexion torque.^12,13,42 In contrast to EMG, force is its summation of all muscles involved in the movement, not a value representing a single muscle. In addition to the hamstrings, the gluteus maximus and posterior head of adductor magnus are also the primary hip extensors, and the middle and posterior head of gluteus medius as well as the anterior head of adductor magnus are secondary muscles involved in hip extension.⁴³ The hamstrings are the primary muscles involved in knee flexion, while gracilis, sartorius, gastrocnemius, and plantaris are secondary. In other words, several other large muscles are additionally involved in hip extension, contributing to the generation of torque.^44,45

This study had certain limitations. First, it did not include a control group, which would have helped better elucidate the effects of PNF stretching. Furthermore, the study population only consisted of healthy adults, limiting the generalizability of the findings. Additionally, EMG was not performed, so muscle activities could not be quantitatively assessed. Regarding the measurement techniques, the clinometer was positioned parallel to the movable arm of the goniometer. Although the clinometer is known for its reliability and validity, it functions differently than to the goniometer, potentially introducing slight variability in the measurements. Future studies should address these limitations in their experimental design and ensure the standardization of measurement tools.

5. Conclusions

In clinical practice, achieving a significant increase in flexibility in a short period is crucial, making it essential to identify the most effective position. Previous studies have shown that higher flexibility improvements are associated with high-intensity muscle contractions, which prompted the need to examine the effectiveness of PNF stretching performed in the MMT position. The results indicated that stretching in the SLR position was more effective in improving the flexibility of the knee and ankle, while hip flexibility increased equally in both positions. Physical therapists or athletic trainers can apply stretching immediately after MMT without changing the position (from prone to supine) if the goal is to increase hip flexibility. Additionally, for goals specifically aimed at increasing or maintaining hip flexibility, especially when hip joint motion is limited, PNF stretching in the MMT position can be an alternative option.

Footnotes

Acknowledgements

The author has no acknowledgments.

ORCID iD

Wootaek Lim

Ethical considerations

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

Informed consent

Not applicable.

Author contributions

Conception, performance of work, interpretation or analysis of data, preparation of the manuscript, revision for important intellectual content: Wootaek Lim.

Funding

The author disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by 2023 Woosong University Academic Research Funding.

Declaration of conflicting interests

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

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