Flexibility has been defined as the ability of a muscle to lengthen and to allow one joint (or more than one joint in a series) to move through a range of motion (ROM) without limitation or pain.^1,2 Flexibility of muscles is an essential component for injury-prevention and rehabilitation.³ Muscle tightness is caused by a decrease in the ability of the muscle to deform, resulting in decreased ROM of the joint on which it acts.⁴ Plans to maintain the flexibility of muscles are of concern to physical therapists, physical educators, and rehabilitation specialists.^1,2

The length of the hamstrings muscles is considered to play an important role in both the efficiency of basic human movements, such as walking and running, and their effectiveness.⁵ Hamstrings shortening may affect the fundamental features as well as walking-posture changes associated with various musculoskeletal disorders related to the lumbar spine, such as low back pain.^5,6 Therefore, it is necessary to increase the flexibility of the hamstrings by stretching for people with shortened hamstrings.

Clinically, flexibility has been considered one of the most important goals in rehabilitation, and stretching techniques are mainly used in exercises for preparing and finishing athletes for competition, and for the general public.^7,8 To increase hamstrings flexibility, static stretching (SS), ballistic stretching, proprioceptive neuromuscular facilitation (PNF), and dynamic stretching have mainly been used in previous stretching interventions.^9,10 Of those, SS has been the most widely used because it is easy to apply and results in less soft-tissue damage while maintaining maximum muscle length.^1,11 Many previous studies found that SS was a very effective technique.^1,11-13

When stretching exercises are done for muscles with reduced flexibility, pain occurs in the soft tissues, which is defined as ‘stretch pain’.¹⁴ Commonly, transcutaneous electrical nerve stimulation (TENS), hot packs, and ice are used for the purpose of reducing pain.¹⁵ Conventional TENS changes the pain sensation using a counter-stimulation effect based on the gate-control theory.¹⁶ TENS is widely used clinically, mainly for the purpose of pain control, because it is a non-invasive, nonpharmacological intervention that is easy to use.^14,17 The duration and effect of hamstrings flexibility gains after SS with TENS has undergone limited research. The current study evaluates the effect of SS with TENS and compares it to normal SS in patients with limited hamstrings flexibility.

1. Subjects

The subjects were 24 individuals (15 men, 9 women) with limited hamstrings flexibility. The average age of subjects was 23.4±1.6 years, the average height was 169.0±7.3 cm and the average weight was 61.9±8.1 kg. All subjects had hamstrings tightness, defined as a loss of 30° or more from full extension with the hip at 90°.⁷ All subjects had no history of orthopedic or neurologic disorders in the hips, knees, or lower back for the previous 3 months. The subjects were informed about the purpose of the present study and its procedures, and written informed consent was obtained from all subjects.

2. Instruments and measurement

To evaluate the effect of stretching, active knee extension (AKE), passive knee extension (PKE), sit-and-reach, and straight-leg raise (SLR) have traditionally been used.⁷ Clinically, the AKE test is often recommended because it eliminates pelvic rotation and measured hamstrings length selectively than others. In the supine position, the ipsilateral hip is positioned at 90° of flexion and the subject fully extends the knee joint. The AKE test has a high test-retest reliability (0.99).¹⁸

The hamstrings flexibility measurement procedures in this study were as follows: in the supine position, the subject placed his or her dominant leg on a fixed metal experimental frame that maintained the 90° flexion position, and a towel was placed between the T12 and L5 vertebral levels to minimize the effect of other movements on the results. The non-dominant leg was fixed by a strap in the middle of the thigh.^7,19 The angle of knee extension was measured with an electronic inclinometer (Dualer IQ the smarter inclinometer, JTECH medical, Salt lake, USA), which is a reliable measurement with high intraclass correlation coefficients (ICC-3, k) of >0.95.²⁰ AKE angle (degree) and increased flexibility percentage from the initial angle was compared of each time.

3. Intervention

The subjects, who all had limited hamstrings flexibility, received 3 different intervention sessions in random order: SS alone, SS with motor TENS, and SS with sensory TENS. AKE was used to evaluate the flexibility of the hamstrings, with measurements at the beginning of the study and 0, 3, 6, 9, and 15 minutes after each stretching program.⁷ Between each treatment, a break was taken for at least 24 hours to minimize carry-over effects. Each stretching intervention was performed for 30 seconds with rest intervals of 15 seconds, and repetitions were performed for 10 minutes.¹⁹

4. Statistical analyses

All statistical analyses were conducted using SPSS version 18.0 (SPSS Inc., Chicago, IL, USA). The general characteristics of the subjects were reported as the mean and standard deviation of the descriptive statistics. To determine the normal distribution of the measured values, the Kolmogorov-Smirnov test was used. One-way repeated ANOVA was used to compare between the three interventions. The three stretching interventions’ time measurements were analyzed by one-way ANOVA. A post-hoc Tukey test was used for the statistical significance of results. Statistical significance was defined as a p-value of <0.05.

1. Comparison AKE in within-group

There were significant changes in the measurement points regardless of group (p<0.05), but there were no differences in hamstrings flexibility of the three groups before the intervention (p<0.05). Measurements at 3, 6, and 9 minutes after the intervention per the angle of flexibility were different from before SS intervention (p<0.05). Also, in SS with sensory- and motor-level TENS, measurements of flexibility of the angle had significant differences at 3, 6, 9, and 15 minutes after the intervention, compared to before the intervention (p<0.05) (Table 1).

Table 1

A comparison of AKE angle after static stretching interventions (degree (°))

	SSA	Increment (%)	SS + sTENSB	Increment (%)	SS+mTENSC	Increment (%)	F	Post-hoc
Baseline	48.96±7.56		49.42±6.52		50.04±7.09		0.74	-
Post	57.21±6.57†	16.85	58.71±5.20†	18.80	60.42±6.61†	20.74	3.12	-
After 3 m	54.79±6.91†	11.91	55.92±5.34†	13.15	57.54±5.86†	14.99	2.81	-
After 6 m	53.04±6.46†	8.33	54.63±5.00†	10.54	56.75±6.02†	13.41	9.09‡	A,B<C
After 9 m	51.83±6.26†	5.86	53.42±5.29†	8.09	55.67±5.42†	11.25	10.08‡	A,B<C
After 15 m	50.21±7.84	2.55	52.29±5.99†	5.81	54.42±5.52†	8.75	6.33‡	A,B<C
F	41.51*		50.04*		48.46*

^*significant different within group (p<0.05);

^†significant different compared from Baseline (p<0.05);

^‡significant different between intervention group (p<0.05).

SS, static stretching; SS+sTENS, static stretching with sensory level TENS; SS+mTENS, static stretching with motor level TENS.

2. Comparison AKE in between-groups

There were no significant differences before the intervention or 3 minutes after the intervention among the three groups. After 6, 9, and 15 minutes, hamstrings flexibility was different in the three stretching groups (p<0.05). After 6 and 9 minutes, each group (SS alone compared with SS with motor TENS, and SS with sensory TENS compared to SS with motor TENS) had significant differences in hamstrings flexibility (p<0.05). After 15 minutes, only the SS with motor TENS group had significant differences in hamstrings flexibility (p<0.05) (Table 1).

This study aimed to evaluate the effects of three stretching interventions and the subsequent duration of increased flexibility over time, targeting young men and women with limited hamstrings flexibility. TENS applied intensity of this study was different depending on individual perception. There was only one study that combined stretching exercise and TENS without distinction of TENS intensity level.²² However, this study was divided into sensory-level TENS and motor-level TENS to evaluate the result. Approximately, sensory level TENS intensity was 16-38 mA and motor level TENS intensity was 55-65 mA.

In a previous study, the SS technique helped to increase ROM, and SS combined with TENS or cold was more effective than SS alone.^1,24 This study also showed a significant increase in hamstrings flexibility with SS alone and SS applied with TENS, compared to before the intervention. However, the SS combined with TENS group showed a greater incremental increase than did the SS group. Of these, the SS with motor TENS group was the highest (20.74%) when compared to the SS with sensory group (18.80%) and the normal-SS group (16.85%).

To induce the contraction of muscle to increase flexibility, the techniques of hold-relax, eccentric contractions, etc. have been reported to influence flexibility through muscle-neurological mechanisms.^8,10 Based on the proprioceptive neuromuscular facilitation (PNF) technique, the hold-relax technique induces relaxation after a maximum isometric contraction of the muscles that need increased flexibility.³ Eccentric contraction is a technique that applies dynamic extension of the agonist and contraction of the antagonist. In this way, metabolism is increased and there is improved flexibility of muscles via the local temperature increase in the deep muscles, which reduces the viscoelasticity of the muscle-tendon unit.⁷ SS with motor-level TENS, used in this study, is similar to the muscle- neurological mechanism of the above techniques when the TENS is applied at a level that causes visible contractions of the hamstrings.

Compared to the holding time between the three groups, the normal-SS group maintained a growth rate of 5.81% to 9 minutes, and the SS with sensory TENS and the SS with motor TENS groups’ growth rates were 8.57% and 5.81%, respectively, after 15 minutes. Previous studies have described the use of conventional TENS, originally based on the gate-control theory of pain, which suggested that counter-stimulation of the nervous system could modify the perception of pain.^16,25 This is a mechanism for adjusting the level of pain in the spinal cord. The pain-transmission path from the outside of the body through the skin occurs via the primary afferent nerves, passing to the brain through the dorsal column or the anterolateral fasciculus, at which point the individual becomes aware of the pain. Stimulation of large-diameter afferent fibers inhibits second-order neurons in the dorsal horn and blocks pain carried by small-diameter fibers from reaching the top.²⁶ Compared with normal SS, the SS with TENS technique may have influenced the effect, similar to the gate-control theory, adjusted to reduce pain when stretching the hamstrings. The results of the present study show that SS with TENS is more effective in increasing flexibility compared to SS alone, and the groups applying TENS maintained increased flexibility for up to 15 minutes. This result may be due to the muscle-neurological mechanisms of muscle contraction and the gate-control theory of TENS acting in combination.

The effects of muscle stretching are known to include reduced muscular soreness, enhanced physical fitness, reduced risk of joint sprain or muscle strain, and promotion of the development of body awareness.²⁷ However, stretching pain, defined as pain associated with stretch stimulations in muscles, accompanies stretching exercises. Therefore, stretch-pain control is necessary in order to increase flexibility. By stimulating a peripheral nerve, SS with TENS may help prevent muscle fatigue and alleviate muscle spasms, and it could be utilized clinically.

The limitations of this study were that hamstrings flexibility was measured only with the AKE test, and the TENS setting of 40 μs, 100 Hz was fixed, so that different frequencies and pulse wave periods were not known. In future studies, the appropriate frequency and pulse for TENS, as well as various measurement techniques, will need to be researched.