Introduction

A variety of injuries that occur during exercise can result in serious muscle damage. Among them, strain injury is the most common injury, and the hamstring muscle is the most commonly injured multi-joint muscle (Safran et al., 1989). Individuals who have limited flexibility are more prone to hamstring injuries by excessive stress, and such injuries can negatively affect physical fitness and running economy (Kim MH 2008; Kim OH 2009). Therefore, hamstring flexibility is considered to be important for improving athletic performance and preventing injuries during exercise. This is applicable for both senior and young athletes. (Feland et al., 2001). Many studies have been conducted to determine effective stretching techniques to increase hamstring flexibility under a variety of conditions. Among the various stretching techniques, static stretching, ballistic stretching, and proprioceptive neuromuscular facilitation (PNF) stretching are widely used these days (Covert et al., 2010; Lim CH 2011; Kirmizigil et al., 2014; Cini et al., 2016; Hill et al., 2016; Rodriguez Fernandez et al., 2016).

In comparison with other stretching techniques, PNF stretching is known to be highly effective in increasing range of motion (ROM) (Etnyre et al., 1986a; Etnyre et al., 1986b; Osternig et al., 1990; Magnusson 1998), but the effectiveness is different by specific conditions, such as stretching time, frequency, and duration (Borms et al., 1987; Sullivan et al., 1992; Bandy et al., 1994; Bandy et al., 1997). During PNF stretching, the ROM is increased by the autogenic inhibition of the stretched muscle as a result of the stimulation of the Golgi tendon organ due to the increased tension (Moore et al., 1980; Bae SS 1993; Yuktasir et al., 2009). Many studies have demonstrated that motor pool excitability was significantly decreased by the PNF technique when compared with static stretching; this inhibitory effect enables effective muscle stretching without stimulating the stretch reflex (Etnyre 1986a; Manoel et al., 2008).

The PNF stretching technique involves the following: hold-relax, contract-relax, and contract- relax-antagonist-contract. However, unlike in the case of passive static stretching, the PNF technique requires voluntary muscle contraction by the subject. For example, in hold-relax, the intensity of stretching is driven by the voluntary contraction effort applied by the subject as the practitioner passively withstands the resistance created as a result of the contraction. However, although the voluntary contraction effort by the subject is an important factor that determines the effectiveness of stretching, it is difficult to quantify the contraction effort in the clinical field or in sports physiotherapy. It is not possible for the practitioner to quantitatively measure the contraction effort through touch. In addition, it is difficult for a subject to determine the amount of his/her own muscle contraction.

In this study, hip extension force was measured with a hand-held dynamometer (HHD) while the subjects performed the hold-relax PNF stretching and its effect on hamstring flexibility was determined. Differences between sexes were also investigated to consider strength variations between men and women.

Materials and Methods

1. Subjects

Thirty-four individuals (17 men; 17 women; mean age=21.2±1.1 years), who were college students, were recruited for this experiment. Informed consent was obtained from all subjects prior to participating in the study.

2. Procedure

Knee extension ROM was measured with a double- arm goniometer with the individual in a supine position. Before beginning the experiment, the lateral epicondyle of the femur, greater trochanter of the femur, and the lateral malleolus of the fibular were palpated to determine the landmarks for ROM measurement. Active knee extension (AKE) was measured after the subjects performed knee extension until the discomfort point was reached while maintaining 90-degree hip flexion. To maintain 90-degree hip flexion, a custom-made metal brace was used (Fig. 1). To warm-up, all subjects performed a total of six AKEs before the experiment and the sixth AKE was recorded as pre-stretch. After the hold-relax technique, AKE was measured again and recorded as post-stretch. The subjects were given a rest break on the table after post- stretch, and additional AKE data were collected at 2, 6, 10, 20, and 30 min.

Fig. 1

Subject performing active knee extension to measure hamstring flexibility.

3. Stretching protocol

During each stretch, subjects performed isometric contraction of the hamstring toward the table while opposing the resistance applied by the practitioner for 5 s. At this time, a HHD (MicroFET3, Hoggan Health Industries Co., UT, USA) was placed under the heel to measure the hip extension muscle force. After performing isometric contraction, the subject’s leg was lifted by the practitioner until the discomfort point was reached. The set involved performing hold-relax PNF stretching for 5 s and repeating this 6 times; hip extension muscle force was measured with a HHD during each session.

4. Statistical analysis

Data analysis was performed using IBM SPSS version 23 for Windows (IBM Corp., Armonk, NY, USA). The repeated measures analysis of variance was used to determine the differences in the mean number of AKEs over the test periods. The Bonferroni test was used to test for pairwise differences. All values are presented as mean±standard deviation. The significance level was set at p<0.05.

Results

There was no significant difference in mean force measured by the HHD (Table 1). When all six trials were considered, the mean force did not decrease, and the range remained constant (155.1 to 161.6). However, the average minimum (min) and maximum (max) values for each subject differed considerably (p<0.001).

Table 1

The mean force measured with hand-held dynamometer

	1st trial	2nd trial	3rd trial	4th trial	5th trial	6th trial	Min	Max
Average (N)	155.3±35.8	155.1±41.8*	160.4±40.5	158.9±30.0	160.9±37.3	161.6±41.0	135.5±30.5	183.7±36.5*

Min: average minimum values, Max: average maximum values.

^* p<0.05, significant difference compared to Min.

The post-stretch AKE value measured immediately after stretching significantly decreased (p< 0.001). and did not differ from the AKE value at 6 min (p=0.451) (Fig. 2). The rapid rise of AKE values was observed until 10 min. The AKE values at 10, 20, and 30 min remained constant and did not differ from the pre-stretch values.

Fig. 2

Longitudinal changes in hamstring flexibility as measured by the active knee extension test. PRE: Pre-stretch, POST: Post-stretch. *p< 0.05, significant difference compared to pre-stretch.

The pre-stretch values differed between men and women, but there was no difference in the sustained effect of stretching on hamstring flexibility over time. A significant difference was only observed in both men (p=0.021) and women (p=0.015) at post-stretch when compared with pre-stretch (Table 2). The AKE values at pre-stretch were not different compared with the AKE values even at 2 min.

Table 2

The difference in active knee extension values between men and women

	PRE	POST	2 min	6 min	10 min	20 min	30 min
Men	27.6±11.4	20.8±10.0*	22.3±8.9	27.2±7.7	29.2±7.2	29.7±8.4	30.7±9.7
Women	16.8±11.6	12.6±9.9*	15.9±10.0	17.8±11.6	20.2±12.6	21.2±13.3	22.0±13.2

PRE: Pre-stretch, POST: Post-stretch.

^* p<0.05, significant difference compared to pre-stretch.

Discussion

Hamstring strain injury is one of the most common exercise-induced injuries, and increased hamstring flexibility can prevent such injuries. Various stretching techniques are used to improve flexibility (Chang et al., 2002), among which the PNF technique is known to be highly effective in increasing ROM; however, there is still some controversy regard this technique. During stretching, resistance is affected by both the viscoelastic properties of the tissues and the contractile components (Condon et al., 1987). The PNF technique is considered to increase flexibility by decreasing active resistance (Osternig et al., 1990). Unlike other stretching techniques, the PNF stretching technique requires voluntary muscle contraction by the subject, and it is hard to quantitatively measure the extent of contraction. In this study, the isometric contraction of the hip extensor was successfully measured and quantified with a hand-held dynamometer during hold-relax PNF stretching.

The mean forces were constant from the 1^st to the 6^th trial, and no significant difference in values was observed between trials. A decrease in strength due to fatigue over time was not observed in this study. However, a significant difference was noted when the total average of each min and max value was calculated for the 6 trials. That is, no difference between trials was observed during the 6 trials if the total average value for the group was considered, but the values of each subject were not constant during the 6 trials. The strength applied by the subjects was not constant between trials, and there was a considerable difference between the max and min values. During isometric contraction, the strength can be changed for each trial. To ensure that PNF stretching has the desired effect, the practitioner should continuously provide verbal encouragement and stimulation to encourage the subject to maintain constant isometric contraction.

The AKE value at pre-stretch did not differ from the value obtained at 2 min. Although there was no significant difference between the values taken at the different time points, the values gradually increased from 6 min. The 5 AKEs performed during warm-up might have temporarily improved hamstring flexibility, decreasing the AKE value at pre-stretch. However, the warm-up effects were diminished because permanent deformity caused by sustained stimulation was not observed in this study. After 10 min, the AKE value returned to the pre-warm-up value and was considerably higher than the pre-stretch AKE value. From pre-stretch to 30 min, two distinct trend lines—one with a rapid increase and the other in a steady state—were observed. The AKE value that decreased due to stretching increased quickly and reached the pre- stretch level within 10 min. The sudden rise in the AKE value remained steady after 10 min. These results were consistent with the results of other previous study (Spernoga et al., 2001).

With regard to differences in hamstring flexibility between men and women, women were observed to have slightly higher flexibility before stretching; however, there was no statistically significant difference in effects of stretching on hamstring flexibility between the sexes. The higher value observed in the women during pre-stretch could be explained by the fact that the women have relatively higher ROM than men (James et al., 1989; Chaparro et al., 2000; Doriot et al., 2006). However, there was no difference in the increase in knee extension ROM after PNF stretching and in the sustained effect between the 2 groups. In conclusion, there was a fundamental difference in flexibility between the men and women; however, the hold-relax PNF stretching performed for a total of 30 s (6 trials×5 s/trial) did not significantly affect this fundamental difference in flexibility between men and women.

This study had limitations. The first one was that the effect of the intensity could not be fully studied owing to the short time and total duration of stretching. A single stretch is not enough to cause a deformity that would lead to permanent tissue change. The increased flexibility due to the viscoelastic properties of the tissues quickly returns to the original condition, and the decrease in stiffness induced by stretching get reversed due to the thixotropic properties of the tissue if there is no additional movement after stretching (Lakie et al., 1984; Hagbarth et al., 1985; Lakie et al., 1988; Walsh 1992). It is known that routine stretching is required to maintain the effect that is transiently acquired after stretching (Tanigawa 1972; Osternig et al., 1990). The other limitation of this study was that the value measured with the HHD was not the mean force for the entire 5-s duration of a trial because the max value is only displayed on the screen of the device after completion of a trial. Therefore, future studies will be required to investigate the relationship between hip extension strength represented by the mean force, and not a max value, and hamstring flexibility during long- duration stretching.

In conclusion, hip extension muscle force was successfully measured with a HHD during hold-relax PNF stretching to increase hamstring flexibility, and it was found that the voluntary effort applied by the subjects was not consistently maintained between trials. Apart from the duration and frequency of stretching, the voluntary effort applied by a subject during PNF stretching is important, and ensuring that a constant intensity is maintained by measuring the subject’s quantitative strength between attempts will be beneficial to achieving a durable stretching effect. Additionally, providing feedback to subjects in real time based on the value displayed on the screen will be very useful in motivating subjects and achieving effective stretching in the clinical field or sports physiotherapy.

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Figure & Data

References

Citations

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