Effect of PNF stretching performed in the AKE position on hip,knee,and ankle flexibility

Abstract

BACKGROUND:

To measure hamstring flexibility, the active knee extension (AKE) test is preferred over the straight leg raise (SLR) test as it can be used to measure hamstring flexibility more selectively. However, hamstring stretching is primarily conducted in the SLR position (maximal hip flexion in the supine position) as it allows for maximal hip flexion in the supine position.

OBJECTIVE:

This study evaluates the effects of proprioceptive neuromuscular facilitation (PNF) stretching in the AKE position (maximal knee extension with 90^∘ flexion of the hip in the supine position) on hip, knee, and ankle flexibility.

METHODS:

SLR, AKE, and active dorsiflexion (ADF) tests were used to determine the range of motion (ROM) before (pre-ROM) and after (post-ROM) stretching. PNF stretching consisted of maximal isometric knee flexion at the end range with external resistance to prevent knee flexion. One set of PNF stretches (five trials of six seconds each) was conducted.

RESULTS:

The post-ROMs of hip, knee, and ankle measured via the SLR, AKE, and ADF tests, respectively, were significantly higher than the pre-ROMs.

CONCLUSIONS:

The improvement in knee flexibility was greater than the improvement in hip and ankle flexibility. The AKE position is recommended in clinical settings during PNF stretching for individuals with hamstring tightness. Furthermore, PNF stretching in the AKE position increases the ADF ROM.

Keywords

Ankle flexibility hip flexibility knee flexibility proprioceptive neuromuscular facilitation stretching

1. Introduction

As very reliable tests for assessing hamstring flexibility, the straight leg raise (SLR) and active knee extension (AKE) tests are the two most commonly used in clinical settings [1, 2, 3]. However, during the SLR test, hamstring tightness can cause the pelvis to rotate posteriorly before the end range is achieved [4, 5]. This rotation reduces the accuracy of the hamstring length measurement. Recent imaging studies have reported that flexibility measured via the SLR test is influenced by soft tissues other than the hamstring muscles [4]. In contrast, the AKE test reduces the influence of posterior pelvic rotation and other soft tissues around the hip by flexing the hip to 90^∘, which allows for a more specific measurement of hamstring flexibility [5, 6]. Therefore, although it is difficult to distinguish between the frequency of use of the SLR and AKE test, the AKE test is scientifically preferred [7, 8, 9].

Table 1
Participant characteristics

Gender	Age (year)	Height (cm)	Weight (kg)	Body Mass Index (kg/m²)
Total ( $N=$ 21)	21.4 $\pm$ 2.20	168.0 $\pm$ 9.0	63.8 $\pm$ 12.7	22.4 $\pm$ 2.6
Male ( $N=$ 8)	23.4 $\pm$ 2.26	177.1 $\pm$ 6	74.1 $\pm$ 10.4	23.6 $\pm$ 2.3
Female ( $N=$ 13)	20.2 $\pm$ 1.09	162.4 $\pm$ 4.7	57.5 $\pm$ 9.5	21.7 $\pm$ 2.6

However, the SLR position (maximal hip flexion in the supine position) is typically used to increase the hamstring length [10]. Both passive static stretching by the practitioner and active proprioceptive neuromuscular facilitation (PNF) stretching by the patient are performed in this position [11, 12, 13]. That is, stretching of the muscle is achieved in the SLR position, while flexibility is measured in the AKE position (maximal knee extension with 90^∘ flexion of the hip in the supine position). When performing isometric contractions during hip extension in the SLR position, additional hip extensors, such as the gluteus maximus, can be activated, especially during hold-relax or contract-relax PNF stretching [14, 15]. In a previous study, the electromyography activity of the gluteus maximus was 34.7% and that of the hamstrings was 51.0% during hip extension in the supine position [16]. As the activity of the untargeted muscle increases, the activity of the targeted muscle, the hamstring, decreases. In other studies, the activity in the hamstrings during knee flexion in the prone position is approximately two times greater than that during hip extension in the supine position [17, 18]. Since PNF stretching is based on physiological mechanisms such as autogenic inhibition, reciprocal inhibition, and stress relaxation, the stretching effect can be significantly reduced if the activity of the target muscle decreases [19]. Therefore, PNF stretching in the AKE position may improve hamstring flexibility more specifically than stretching in the SLR position.

The purpose of this study was to examine the effects of PNF stretching performed in the AKE position on hip, knee, and ankle flexibility.

2. Methods

2.1 Subjects

Twenty-one college students (8 male and 13 female, age 21.4 $\pm$ 2.2 years, height 168.0 $\pm$ 9.0 cm, weight 63.8 $\pm$ 12.7 kg) participated in this study (Table 1). Participants with hamstring tightness (AKE $>$ 20^∘) were included in the experiment. Participants who had a history of fracture or surgery in the lower extremity, or who had experienced pain including radiating pain within the last 6 months, were excluded from the experiment. The Institutional Review Board of Woosong University approved the study (1041549-221011-SB-149).

2.2 Procedures

The range of motion (ROM) was assessed prior to stretching via the SLR, AKE, and active dorsiflexion (ADF) tests and this was defined as ‘pre-ROM’. The tests were conducted in a random order. The SLR test was conducted in the supine position. The unmeasured leg and pelvis were strapped to the treatment table to limit unnecessary movement. The participant performed active hip flexion in response to the practitioner’s signal, and when the endpoint was reached, held the posture for approximately three seconds. The endpoint refers to the point at which the leg can be raised without causing pain or discomfort [20]. A clinometer (Plaincode Software Solutions, Stephanskirchen, Germany) installed on an iPhone 11 (Apple Inc, Cupertino, CA, USA) was used to measure the ROM. Two SLR trials were conducted: the first trial was used as a warm-up and was not recorded, while the ROM measured during the second trial was used for the analysis [21]. The practitioner monitored the ankle during the SLR test to ensure no abnormal dorsiflexion or plantar flexion. The AKE test was also conducted in the supine position. A metal frame was placed beneath the measurement leg to ensure a stable posture and a hip flexion of 90^∘. The participant performed knee extension in response to the practitioner’s signal, and when the endpoint was reached, held the posture for approximately three seconds. The endpoint refers to the point at which the individual reaches their maximum ROM, and if myoclonus occurs, slight knee flexion allowed to remove it [22]. During the second trial, a clinometer was used to measure the angle between the horizontal line and the extended lower leg [23]. The ADF test was performed in a sitting position with 90^∘ flexion of the hip and knee and a neutral pelvis. The end point was defined as the maximum range of ADF without experiencing pain or discomfort [24].

PNF stretching was conducted in the AKE position. The practitioner held the hip at 90^∘ flexion and slowly extended the lower leg. The hamstring muscles passively stretched until resistance was felt at the end range. When the participant reached the end of the range, they performed a maximal isometric knee flexion for 6 seconds (Fig. 1). There was a 5-second rest period between trials. The practitioner applied resistance to prevent knee flexion during the maximal isometric contraction. A total of 5 trials were conducted. The SLR, AKE, and ADF tests were conducted after stretching to determine the post-stretching ROM and this was defined as “post-ROM”.

Figure 1.

Proprioceptive neuromuscular facilitation stretching in the active knee extension position.

2.3 Data analysis

Shapiro-Wilk test was used to examine for normality. A two-way ANOVA was conducted to The Shapiro-Wilk test was used to determine the normality of the data. The pre- and post-ROMs were compared using the paired samples T-test. The changes in ROM of the hip, knee, and ankle were also compared using the paired samples T-test.

All statistical analyses were conducted using SPSS Statistics (version 25, IBM Corp., Armonk, NY, USA). Statistical significance was set at $p<$ 0.05. Continuous variables are presented as mean $\pm$ standard deviation.

3. Results

PNF stretching performed in the AKE position increases hip and knee, and ankle flexibility. The post-ROM of the hip was significantly higher than the pre-ROM of the hip, as measured using the SLR test ( $p=$ 0.008) (Fig. 2a). The post-ROM of the knee was significantly higher than the pre-ROM of the knee, as measured using the AKE test ( $p<$ 0.001) (Fig. 2b). The post-ROM of the ankle was significantly higher than the pre-ROM of the ankle, as measured using the ADF test ( $p=$ 0.001) (Fig. 2c).

The improvement in knee flexibility was greater than the improvement in hip and ankle flexibility, but the changes in ROM were not significantly different between the joints (hip and knee, $p=$ 0.019; hip and ankle, $p=$ 0.242; knee and ankle, $p=$ 0.388; Table 2).

Table 2
Changes in range of motion

	Straight leg raise	Knee extension	Dorsiflexion
Change in ROM (^∘)	2.9 $\pm$ 4.5	6.4 $\pm$ 4.1	5.0 $\pm$ 6.2

ROM, range of motion.

Figure 2.

Changes in hip, knee, and ankle range of motion after proprioceptive neuromuscular facilitation stretching.

4. Discussion

The AKE test is preferred over the SLR test for measuring hamstring flexibility in isolation, as it removes the confounding effects of pelvic rotation [25, 26]. However, stretching of the hamstrings is often conducted in the SLR position [27]. In this study, the hip and knee flexibility changes were measured using the SLR and AKE tests after PNF stretching in the AKE position. The ADF test was used to measure the changes in ankle flexibility due to the anatomical link between the knee and the ankle.

PNF stretching resulted in a significant increase in the ROM of the hip, though the increased ROM may not be clinically meaningful. However, this improvement occurred after only one set of stretches (five stretches for six seconds each). Therefore, the flexibility may increase more after more repetitions. In a previous study, the passive resistance of the hamstrings significantly decreased after repeated stretching for up to 45 seconds and was associated with greater flexibility [28]. In another study, increasing the duration of stretching from 15 seconds to 30 seconds increased the gain in ROM by approximately three times, from 3.8^∘ to 12.5^∘ [29]. Hip flexibility is influenced by two-joint muscles, including the hamstrings, as well as one-joint muscles, including the gluteus maximus [30]. The AKE test should be conducted to further analyze hamstring flexibility. In this study, the ROM significantly increased when measured using the AKE test, supporting the experimental hypothesis that the AKE position is superior to the SLR position for selective stretching of the hamstrings. Previous studies using PNF stretching reported an increase of 7.8^∘ when five seven-second stretches were conducted [31]. When static stretching was applied to the range of passive resistance, the ROM increased by 4.3^∘, and when the resisting force was increased by approximately 30%, the ROM increased by 6.9^∘ [32]. A direct comparison of previous study results is difficult as the studies include a range of participants. However, as a similar stretching protocol was followed in each study, including similar stretching application times, participant age, and participant height and weight, an indirect comparison is possible. In this study, PNF stretching in the AKE position has similar or superior effects as stretching in the SLR position.

This study also measured ankle flexibility. The ADF test revealed a significant increase in ankle ROM after stretching. Although the participant’s position changed from supine to sitting, the AKE and ADF positions are similar. Co-contraction occurs in the surrounding muscles during PNF, which may positively affect the ROM after PNF stretching [33, 34, 35]. In PNF stretching, if the contraction of an antagonist muscle is limited, the contraction of an agonist or synergist muscle may be induced to improve the joint’s flexibility [36, 37]. However, additional factors also affect ankle flexibility. The hamstring muscles originate at the ischial tuberosity and insert into the medial or lateral condyle of the tibia, with the exception of the short head of the biceps femoris that originates at the linea aspera of the femur [38]. In clinical settings, the calf muscle is often stretched to increase the ankle ADF ROM [39, 40]. As the four bodies of the hamstrings are attached to the proximal tibia, the significant increase in ADF cannot be solely explained by the hamstring morphology. This may be due to the differences in properties between non-contractile and contractile proteins. Gelach and Lierse’s research revealed that several layers of fascia connect the hip and ankle [41, 42]. Furthermore, the trunk superior to the hip is linked to the ankle via the thoracolumbar fascia and the posterior oblique sling. The ADF ROM is affected by trunk movement [43, 44]. Furthermore, when static stretching of the posterior trunk is conducted, the ADF ROM significantly increases [44]. Distal joints that are not connected by muscles are connected via passive connective tissues, affecting muscle activity [45, 46].

This study is not without limitations. To ensure the generalization of the experimental results, therefore, it is necessary to increase the sample size while maintaining a balanced gender ratio and ensure a diverse range of participants, including people of different ages, races, and physical abilities. In addition, the scope of the study is limited as the stretching was not conducted in the traditional SLR position. In SLR position, PNF stretching was applied to the plantar flexors to improve the ADF ROM [47]. In future studies, change in flexibility after the same stretching is performed in the SLR and AKE positions should be compared.

5. Conclusions

The findings of this study support the effectiveness of PNF stretching in the AKE position for improving knee flexibility. Additionally, PNF stretching significantly increased ADF ROM, suggesting a positive effect on ankle flexibility. Physical therapists and athletic trainers should consider these findings when developing personalized stretching protocols to optimize hamstring flexibility.

Ethical approval

The study was approved by the Institutional Review Board of Woosong University, Daejeon, Republic of Korea (1041549-221011-SB-149).

Funding

This research was supported by 2022 Woosong University Academic Research Funding (no fund number was assigned).

Informed consent

Informed consent was obtained from all participants prior to their participation.

Footnotes

Acknowledgments

The author has no acknowledgments.

Conflict of interest

The author has no conflicts of interest to report.

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Effect of PNF stretching performed in the AKE position on hip,knee,and ankle flexibility

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

Table 1 Participant characteristics

2.1 Subjects

2.2 Procedures

3. Results

Table 2 Changes in range of motion

5. Conclusions

Ethical approval

Funding

Informed consent

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Participant characteristics

Table 2
Changes in range of motion