Long-term effects of diagonal active stretching versus static stretching for cervical neuromuscular dysfunction,disability and pain: An 8 weeks follow-up study

Abstract

BACKGROUND:

A high prevalence and incidence of neck and shoulder pain are common problems that require more attention from health service providers and researchers. Recent findings have indicated that the neck stretching is the one of the most effective physical therapy interventions. Although the recovery of the pain through the stretching exercise has been described in a previous study, functional advantages and changes of the neuromuscular control has not been suggested. Additionally, there was a lack of studies that considered methods of stretching.

OBJECTIVE:

The purpose of the present study was to examine the long-term effects of stretching methods for cervical neuromuscular dysfunction, disability and pain in a sample of sedentary workers with neck pain.

METHODS:

Twenty-four sedentary workers with neck pain participated in this study and were assigned to the following two groups: static stretching (SS), and diagonal active stretching (DS). Active cervical range of motion (ROM), pressure pain threshold (PPT), self reported questionnaire (Neck Disability Index, NDI), and the flexion – relaxation ratio (FRR) from the cervical erector spinae muscles were measured at pre-treatment, post-treatment (after 4 weeks) and follow-up (after 6 and 8 weeks).

RESULTS:

In both groups, the values obtained for the cervical ROM as flexion, extension, lateral flexion, were significantly increased in comparison to pre-treatment ( $p<$ 0.05). Significant increases of the left and right neck rotation, left and right FRR were only observed in the DS group ( $p<$ 0.05). The NDI score of the SS group was significantly lower than that of the DS group ( $p<$ 0.05).

CONCLUSIONS:

Our findings suggest that both static and diagonal stretching exercises were effective for reducing disability of the neck and increasing cervical range of motion. Compared to the SS, in addition, present results suggest that changes of the activation patterns as FRR, and cervical rotational movement could be accomplished with the DS.

Keywords

Stretching computer workers PIR

1. Introduction

In industrialized countries, the high prevalence and incidence of neck and shoulder pain are common problems that require more attention from health service providers and researchers [1, 2]. Sedentary workers and those whose work involves extended static contraction, prolonged static loads, or abnormal working postures involving the neck/shoulder muscles are especially subject to increased risk for neck/shoulder musculoskeletal disorders [1]. Although numerous factors may contribute to neck and shoulder pain, reduced neck movement and abnormal activation patterns of the paracervical muscles are common in those with neck pain [3, 4]. Reduced cervical movement can disrupt functional activities and may impede corrective and protective reactions and cause loss of mobility in the neck area, which is associated with changes in the passive structures of the cervical spine [4].

By demonstrating the flexion relaxation ratio (FRR), a previous study suggested that there is an association between reduced range of cervical movements and altered muscle activation [5]. The FRR originated from the flexion-relaxation (FR) phenomenon which is defined as an electrical silence response in the erector spinae muscles during a full forward-bending posture [6]. Although the reason of the abnormal FRR value and absence of FRP phenomenon is unclear, increased stretch sensitivity and limited range of motion could contributes the absence of the FR phenomenon, which also affects the numerical values of the FRR [5, 6, 7].

Previous studies have suggested several interventions for reducing neck-related musculoskeletal problems that focus on ergonomic improvements in the environment and posture re-education during seated work [3, 8]. More recently, however, direct interventions aimed at resolving neck pain have been introduced, including manipulation, strengthening, and stretching programs [9, 10, 11]. Recent research has indicated that among the common forms of therapeutic interventions for neck-related musculoskeletal problems, stretching of the posterior neck region is beneficial for reducing symptoms and improving function in patients with neck pain [9, 12, 13]. Ylinen et al. found that performing regular stretching exercises is a low-cost intervention that can effectively reduce neck pain and disability [13]. Häkkinen et al. also suggested that stretching only and combined strength training and stretching were equally effective in achieving long-term improvement [14]. Although several previous studies have reported positive effects of stretching programs on neck pain and disability, research examining specific stretching techniques or investigating the long-term effects of such interventions remains limited.

The active stretching techniques using post-isom-etric-relaxation (PIR) have been widely investigated with regard to their effects on tightness or shortness of muscles of the lower extremity, such as the hamstring and quadriceps muscles [14, 15, 16]. However, they have rarely been investigated with regard to their effects on the paraspinal muscles around the neck region. Additionally, clinical literature suggest the interventions emphasize inducing functional movement through diagonal patterns of motion, which include flexion/extension, lateral flexion, and rotational components of motion in the cervical spinal segments [17]. Because the neck muscles contribute moments in three planes of movement [18], performing active stretching techniques in a diagonal pattern might be helpful for patients with neck pain. Despite recent findings demonstrating the positive effects of PIR stretching exercises in patients with neck pain [19], no reported study has compared the effects of static stretching and PIR stretching on neck pain, disability, cervical movement, and neuromuscular dysfunction.

Therefore, the purpose of the present study was to compare the effectiveness of active stretching, which induces post-isometric relaxation and includes multiple planes of movements, with static stretching exercises on cervical range of motion, neuromuscular dysfunction, pain, and disability in subjects with non-specific neck pain.

2. Materials and methods

2.1 Participants

This study was performed on 24 sedentary workers with neck pain who regularly performed seated work for more than 4 hours per day. Information of subject’s recruitment was informed at a local university’s announcement board for a week. The inclusion criteria was the sedentary workers who had neck discomfort or pain in daily life. The exclusion criteria was a history of upper or lower extremity injuries that affected daily life, other than neck pain and discomfort. Data were gathered using a self-administered questionnaire, medical history review, and a clinical examination before participation in the study. Finally, 24 sedentary workers were randomized pair-wise into Group 1 ( $n=$ 12) and Group 2 ( $n=$ 12) by tossing a coin. Table 1 lists the patients’ demographic data. All participants provided written informed consent according to a protocol approved by Kaya University (IRB No. 2017188).

Table 1
Patients’ demographic data

	S.S		D.S
Age	21.75	$\pm$ 0.97	21.67	$\pm$ 0.89
Height (cm)	166.17	$\pm$ 5.20	166.83	$\pm$ 9.42
Weight (kg)	61.58	$\pm$ 7.46	63.92	$\pm$ 17.35
Body mass index (kg/m ${}^{2})$	22.05	$\pm$ 2.06	22.64	$\pm$ 3.67

S.S: Static stretching group, D.S: Diagonal active stretching group.

2.2 Measuring instruments

Outcome measurements were taken at baseline, after 4 weeks, and at 6-week and 8-week follow-ups. The Neck Disability Index (NDI) was used to assess neck pain and discomfort [17]. To measure neck range of motion, a cervical ROM (CROM) instrument (Performance Attainment Associates, St. Paul, MN) was used. Electromyography (EMG) data were obtained by using Free EMG system (Free EMG300, BTS Bioengineering, Italy). EMG signals were amplified, band-pass filtered through 10–500 Hz, and sampled at 1000 Hz. The data obtained were computerized via the EMG system. Surface electrodes were placed on both sides of the cervical erector spinae (CES) muscles, 2 cm lateral to the C4 spinous process [20]. To measure the flexion/relaxation ratio (FRR), subjects were asked to perform a standardized cervical flexion-extension movement in three phases: (1) flexion phase, complete cervical flexion for 5 s; (2) relaxation phase, static period in complete cervical flexion held for 5 s; and (3) extension phase, extension to return to the initial position for 5 s. A digital pressure pain threshold (PPT) device (JTECH medical, Ohio, USA) was also used to identify individual sensitivity to pressure pain.

2.3 Procedure

Patients were assigned to one of the two groups. Group 1 performed static stretching (SS) exercises three times per week, and Group 2 performed diagonal active stretching (DS) exercises using PIR three times per week. NDI scores, cervical ROM, FRR, and PPT were assessed at baseline, after 4 weeks, and at the 6-week and 8-week follow-ups. The blinded examiner conducted the baseline and 6-week follow-up measurements, and the other blinded examiner conducted the 4-week and 8-week follow-up measurements.

Table 2
The average and standard deviation (SD) of range of motion of neck

Variables	Group	Measurement ( ${}^{\circ}$ )
		Pre-test		Post-4-week		Post-6-week		Post-8-week
Flexion	S.S*	36.67	$\pm$ 9.37	43.13	$\pm$ 7.70	44.21	$\pm$ 7.20	44.58	$\pm$ 8.04
	D.S*	34.17	$\pm$ 9.96	42.88	$\pm$ 7.54	45.33	$\pm$ 6.72	45.21	$\pm$ 5.42
Extension	S.S*	57.08	$\pm$ 16.16	64.58	$\pm$ 10.81	62.38	$\pm$ 9.77	62.08	$\pm$ 8.47
	D.S*	61.25	$\pm$ 14.79	71.46	$\pm$ 12.99	69.96	$\pm$ 12.87	69.50	$\pm$ 11.42
Left lateral flexion	S.S*	40.00	$\pm$ 7.98	45.42	$\pm$ 11.47	47.17	$\pm$ 9.98	48.13	$\pm$ 10.09
	D.S*	37.92	$\pm$ 8.38	40.63	$\pm$ 7.08	42.25	$\pm$ 6.44	41.42	$\pm$ 5.73
Right lateral flexion	S.S*	42.50	$\pm$ 7.83	56.67	$\pm$ 15.25	53.17	$\pm$ 11.74	51.42	$\pm$ 9.86
	D.S*	40.50	$\pm$ 6.14	46.75	$\pm$ 7.72	46.63	$\pm$ 9.98	45.46	$\pm$ 10.52
Left rotation	S.S	54.75	$\pm$ 9.67	54.04	$\pm$ 7.76	54.17	$\pm$ 9.03	55.21	$\pm$ 8.31
	D.S*	52.21	$\pm$ 11.06	59.46	$\pm$ 13.15	59.17	$\pm$ 11.58	59.46	$\pm$ 11.04
Right rotation	S.S	62.71	$\pm$ 16.87	60.88	$\pm$ 18.63	63.46	$\pm$ 20.15	63.33	$\pm$ 18.99
	D.S*	61.75	$\pm$ 12.89	67.29	$\pm$ 12.18	69.46	$\pm$ 9.42	69.17	$\pm$ 9.33

${}^{*}$ Significant difference in factor of time. S.S: Static stretching group, D.S: Diagonal active stretching group.

Table 3

The average and standard deviation (SD) of pressure pain threshold (PPT), Neck Disability Index (NDI), and flexion relaxation ratio (FRR)

Variables	Group	Measurement
		Pre-test	Post-4-week	Post-6-week	Post-8-week
Left PPT	S.S	7.33 $\pm$ 2.02	8.10 $\pm$ 1.89	8.61 $\pm$ 2.04	7.99 $\pm$ 1.81
	D.S	6.58 $\pm$ 1.89	9.30 $\pm$ 5.21	9.71 $\pm$ 4.76	9.13 $\pm$ 4.24
Right PPT	S.S	7.86 $\pm$ 2.17	8.32 $\pm$ 2.56	8.22 $\pm$ 2.45	7.99 $\pm$ 2.31
	D.S	6.54 $\pm$ 2.15	8.99 $\pm$ 4.77	8.25 $\pm$ 3.87	8.52 $\pm$ 4.24
Left FRR	S.S	1.21 $\pm$ 0.32	1.31 $\pm$ 0.30	1.37 $\pm$ 0.25	1.31 $\pm$ 0.28
	D.S*	1.10 $\pm$ 0.18	1.42 $\pm$ 0.22	1.44 $\pm$ 0.14	1.47 $\pm$ 0.20
Right FRR	S.S	1.14 $\pm$ 0.26	1.20 $\pm$ 0.25	1.30 $\pm$ 0.28	1.18 $\pm$ 0.22
	D.S*	1.03 $\pm$ 0.20	1.26 $\pm$ 0.18	1.32 $\pm$ 0.35	1.31 $\pm$ 0.31
NDI	S.S* ${}^{{\dagger}}$	3.75 $\pm$ 1.48	1.42 $\pm$ 1.56	0.83 $\pm$ 1.03	1.08 $\pm$ 1.00
	D.S*	5.33 $\pm$ 2.10	3.92 $\pm$ 2.54	2.92 $\pm$ 2.02	1.83 $\pm$ 1.70

${}^{*}$ Significant difference in factor of time. ${}^{{\dagger}}$ Significant difference in factor of group. S.S: Static stretching group, D.S: Diagonal active stretching group.

To measure cervical ROM, each participant was asked to sit on a standard chair with a backrest maintaining a neutral spinal alignment. While in this erect sitting posture, active cervical ROM in the sagittal, coronal, and transverse planes was measured using the CROM instrument. Each subject was asked to move the head until a feeling of muscle tightness or pain occurred. Two trials were performed, with 60-s intervals between the tests. After measuring CROM, EMG data were collected for use in analyzing FRR. The skin was prepared for EMG measurement by cleaning the electrode site with alcohol, shaving it, and lightly abrading the skin with fine sandpaper. Each subject was asked to bend the head slowly, approximating the chin to the manubrium, and then to maintain this position until asked to return to a neutral position. The durations of the three phases were controlled by a metronome. The FRR value was calculated by dividing the maximal muscle activation during the 5-s re-extension phase by the average activation during the 5-s relaxation phase. The mean cervical ROM and FRR values through the two trials for each test were calculated for statistical analysis. The PPT was measured at the posterior neck region. The examiner marked a sight on both side of the upper trapezius, which was 5–8 cm, superior and medial to the superior angle of the scapula. Gradually increasing pressure was applied to the neck area, and each subject was asked to call “stop” when any pain or discomfort was felt.

Figure 1.

Static stretching (A) and diagonal active stretching (B). M: movement, R: resistance, S: stretching.

The stretching exercises were provided by two physical therapists. The physical therapists taught participants the two types of stretching exercises (SS and DS) (Fig. 1), which were then performed under supervision for 2 months. Patients were advised to perform the stretching exercises for four sessions, three times per week, with each session taking about 20 minutes to complete.

In the static stretching group (SS), exercises were performed in the following order: stretching toward lateral flexion of the upper part of the trapezius, scalene, and sternocleidomastoid (SCM); flexion of the extensor muscles; extension of the flexor muscles; and contralateral flexion and rotation combined with scapular depression for the scalene and levator scapula, holding each movement for 10 seconds. This sequence constituted one set. All subjects performed six sets on the right side and six on the left side (Fig. 1A). To prevent muscle injury and fatigue caused by stretching, participants were instructed to allow 30 seconds of rest time between sets.

In the diagonal active stretching group (DS), exercises were performed in the following order: diagonal flexion, right and left; ipsilateral lateral flexion and rotation with extension of the contralateral shoulder; diagonal extension, right and left; and ipsilateral lateral flexion and rotation with extension of the contralateral shoulder. At the submaximal end range, participants were instructed to add self-applied resistance against the movement toward the contralateral direction, and then hold this for 10 seconds. After the self-applied contract-and-relax technique (CRT), the subject performed stretching to the end range for 10 seconds, followed by the same movement in the opposite diagonal direction. This sequence constituted one set. All subjects performed six sets on the right and six on the left side (Fig. 1B). To prevent muscle injury and fatigue caused by CRT and stretching, participants were instructed to rest for 30 seconds between sets.

2.4 Statistical analyses

The SPSS statistical package was used to analyze the significance of differences among the four measurements over time and between two groups. The Shapiro-Wilk test was used to assess for normal distribution among the outcome variables of interest. A two-way repeated-measures ANOVA was used to assess differences in NDI, PPT, cervical ROM, and FRR values. Time was set as a within-subject variable, and group as a between-subject variable. When significant relationships between group and time were identified, a subsequent one-way repeated-measures ANOVA was used to determine the differences for each group. The Bonferroni correction was used as a post hoc test. The level of significance was set at $p<$ 0.05.

3. Results

3.1 Factors of time effects

Compared to the baseline measurements, the cervical ROM values for flexion and right lateral flexion were significantly increased after 4, 6, and 8 weeks in both groups (Flexion: $F=$ 8.08, $p<$ 0.05; Right lateral flexion: $F=$ 7.85, $p<$ 0.05). A significant difference in cervical ROM for extension was observed between the baseline and post-treatment values, with cervical ROM being significantly greater at post-treatment ( $F=$ 13.27, $p<$ 0.05). In both groups, left lateral flexion was significantly greater at follow-up (after 6 and 8 weeks) than at baseline ( $F=$ 4.37, $p<$ 0.05). Both groups had gradual decreases in NDI scores over time ( $F=$ 21.75, $p<$ 0.05).

3.2 Factors of group effects

A significant group effect was observed only for NDI, with scores of the SS group being significantly lower than those of the DS group ( $F=$ 8.83, $p<$ 0.05). The factors of time and group were significantly related in measures of left and right rotation and of left and right FRR. Further statistical analysis revealed that significant increases in left and right neck rotation and in left and right FRR were observed only in the SS group (left rotation: $F=$ 5.11, $p<$ 0.05; right rotation: $F=$ 9.30, $p<$ 0.05; left FRR: $F=$ 29.55, $p<$ 0.05; right FRR: $F=$ 18.28, $p<$ 0.05).

4. Discussion

This study compared the effect of two stretching techniques on active cervical ROM, muscular response, pressure pain threshold, and neck disability in a sample of sedentary workers with neck pain. Because the present study aimed to reveal the effects of stretching exercises, we could not include a control group. In pre-treatment measurements in the present study, the average active cervical ROM was about 35.51 degrees for flexion, 59.16 for extension, 39.95–41.50 for lateral flexion, and 54.75–62.23 for rotation. These values were lower than those reported in a previous study with asymptomatic sedentary workers [21, 22]. According to a previous study, reduced cervical ROM was highly associated with musculoskeletal symptoms, including neck pain [23]. Except for the rotational component of cervical ROM, both stretching techniques significantly increased cervical ROM at post- compared to pre-intervention. Significant increases in rotational cervical ROM were observed only in the DS group. Although a direct comparison is impossible due to differences in participants’ impairments, Al Dajah (2014) also reported that interventions using the PIR phenomenon with diagonal stretching improved the rotational component of shoulder range of motion [24].

Figure 2.

The changes of the neck range of motion following time and stretching methods. ${}^{*}$ Significant difference between conditions. S.S: Static stretching group, D.S: Diagonal active stretching group.

Classically, the FRR values originated from flexion/relaxation (FR), which is a normal pattern of muscle activation characterized by electrical silence in the erector spinae muscles during a full forward-bending posture [6]. The absence of this silence is associated with neuromuscular impairment and dysfunction [25]. Previous researchers have attempted to represent this phenomenon as a ratio scale, dividing the maximal muscle activation during the re-extension phase by activation during the relaxation phase [15, 20]. Recently, researchers have investigated cervical FRR values in symptomatic populations who have neck pain [9, 22]. In previous studies, the average FRR ranged from 1.2–1.5, similar to the present results of 0.9–1.3. This value was lower than normative FRR, which has been demonstrated as 2.5 in previous study [5].

Figure 3.

The changes of the flexion relaxation ratio (FRR) and Neck Disability Index (NDI) following time and stretching method. ${}^{*}$ Significant difference between conditions. S.S: Static stretching group, D.S: Diagonal active stretching group.

A previous study suggested that cervical FRR was associated with cervical active ROM [5]. In the present study, however, the significant increase in FRR values was only observed in the DS group, although both types of stretching exercises were associated with significant increases in cervical ROM. In contrast to SS, DS data included the voluntary contraction around the paraspinal muscles, which might affect the present results. The active stretching using PIR phenomenon is generally described as using autogenic inhibition to lengthen targeted muscles. Although applied to a different body region than in our study, two previous studies compared the active stretching exercises with static stretching exercises [14, 26]. Both of these previous studies found that interventions using PIR phenomenon could change neuromuscular responses [14, 26]. FRR is expressed as a numerical value and is known as a sensitive marker for measuring neuromuscular changes [20], and the present results showing changes in FRR values were similar to previous results.

Interestingly, our results revealed a significant group difference in NDI scores, which were higher in the DS than in the SS group. Although gradual decreases in the NDI were seen in both groups, this result indicates that subjects with DS reported relatively more neck discomfort or disability. There are two possible explanations for these results. One is the hypermobility of the neck region generated by DS exercises. A previous report warned that the active stretching exercise could generate hypermobility of the involved joints, and recommended careful application of the exercise protocol [26]. The other possible explanation is related to increases in the neuromuscular response against cervical movements. Olivo and Magee reported that application of relaxation techniques in their study did not induce relaxation of the targeted muscles [27].

Although the present study included 8 weeks of follow-up, no significant changes were observed in the pressure pain threshold and no dramatic changes were observed in the cervical FRR. Additionally, some of participants reported continuing neck discomfort at the 8-week follow-up. Some characteristics of the participants might have influenced the present results. The subjects were sedentary workers who performed prolonged computer tasks on a daily basis. Although the exercises were performed three times per week during the study, the researchers could not control sources of external bias such as the working environment and subjects’ daily activities.

The present study has some limitations that should be considered in future research. Our results cannot be generalized because of the small number of subjects and the absence of a control group. Furthermore, the internal validity may not be sufficient. The inability to control participants’ environments could cause history effects. Future research should consider these limitations and proceed with a composite study to examine the effects of cervical stretching exercises on cervical musculoskeletal problems in a more objective manner.

5. Conclusion

This study had three main findings. First, both static stretching and DS were effective for reducing disability of the neck and increasing cervical range of motion. Second, DS changed the pattern of neuromuscular activation and improved cervical rotational movement. Third, the increased cervical flexion/relaxation ratio and rotational movement after the exercises were not associated with subjective comfort or with pain parameters.

Footnotes

Acknowledgments

This research was supported by the National Research Foundation of Korea (NRF) grant funded by Ministry of Science and ICT (no. 2017R1C1B5074795).

Conflict of interest

The authors have no conflict of interest to report.

References

Bernard

. Musculoskeletal disorders and workplace factors: a critical review of epidemiologic evidence for work-related musculoskeletal disorders of the neck, upper extremity, and low back. US, Department of Health and Human Services. 1997: 97-141.

Hoy

March

Woolf

Blyth

Brooks

Smith

, et al. The global burden of neck pain: estimates from the global burden of disease 2010 study. Annals of the Rheumatic Disease. 2014; 73(7): 1309-15.

IJmker

Huysmans

Blatter

van der Beek

van Mechelen

Bongers

. Should office workers spend fewer hours at their computer? A systematic review of the literature. Occupational and Environmental Medicine. 2007; 64(4): 211-22.

Szeto

Straker

O’Sullivan

. A comparison of symptomatic and asymptomatic office workers performing monotonous keyboard work – 2: Neck and shoulder kinematics. Manual Therapy. 2005; 10(4): 281-91.

Yoo

Park

Lee

. Relationship between active cervical range of motion and flexion-relaxation ratio in asymptomatic computer workers. Journal of Physiological Anthropology. 2011; 30(5): 203-07.

Floyd

Silver

. The function of erector spinae muscles in flexion of the trunk. Lancet. 1951; 1(6647): 133-4.

Salamat

Talebian

Bagheri

Maroufi

Jafar Shaterzadeh

Kalbasi

O’Sullivan

. Effect of movement control and stabilization exercises in people with extension related non-specific low back pain- a pilot study. Journal of Bodywork & Movement Therapy. 2017; 21(4): 860-865.

Korhonen

Ketola

Toivonen

Luukkonen

Häkkänen

Viikari-Juntura

. Work related and individual predictors for incident neck pain among office employees working with video display units. Occupational and Environmental Medicine. 2003; 60(7): 475-82.

Lee

Park

Kim

. Immediate Effects of Dynamic Neck Training Combined with the Hold-Relax Technique for Young College Students with Video Display Terminal Syndrome. International Journal of Athletic Therapy and Training. 2016; 21(1): 50-5.

10.

Kim

Kwag

. Clinical effects of deep cervical flexor muscle activation in patients with chronic neck pain. Journal of Physical Therapy Science. 2016; 28(1): 269-73.

11.

Kolberg

Horst

Moraes

Kolberg

Belló-Klein

Partata

. Effect of high-velocity, low-amplitude treatment on superoxide dismutase and glutathione peroxidase activities in erythrocytes from men with neck pain. Journal of Manipulative Physiological Therapeutics. 2012; 35(4): 295-300.

12.

Häkkinen

Kautiainen

Hannonen

Ylinen

. Strength training and stretching versus stretching only in the treatment of patients with chronic neck pain: a randomized one-year follow-up study. Clinical Rehabilitation. 2008; 22(7): 592-600.

13.

Ylinen

Kautiainen

Wirén

Häkkinen

. Stretching exercises vs manual therapy in treatment of chronic neck pain: a randomized, controlled cross-over trial. Journal of Rehabilitation Medicine. 2007; 39(2): 126-32.

14.

Ferber

Osternig

Gravelle

. Effect of PNF stretch techniques on knee flexor muscle EMG activity in older adults. Journal of Electromyography and Kinesiology. 2002; 12(5): 391-97.

15.

Pialasse

Dubois

Choquette

Lafond

Descarreaux

. Kinematic and electromyographic parameters of the cervical flexion-relaxation phenomenon: The effect of trunk positioning. Annals of Physical and Rehabilitation Medicine. 2009; 52(1): 49-58.

16.

Puentedura

Huijbregts

Celeste

Edwards

Landers

, et al. Immediate effects of quantified hamstring stretching: hold-relax proprioceptive neuromuscular facilitation versus static stretching. Physical Therapy in Sport. 2011; 12(3): 122-26.

17.

Alder

Becker

Buck

. PNF in practice: an illustrated guide, 3rd ed. Heidelberg. Springer. 2008.

18.

Gilchrist

Moglo

Storr

Pelland

. Effects of head flexion posture on the multidirectional static force capacity of the neck. Clinical Biomechanics (Bristol, Avon). 2016; 37: 44-52.

19.

Vernon

Mior

. The neck disability index: a study of reliability and validity. Journal of Manipulative and Physiological Therapeutics. 1991; 14(7): 409-15.

20.

Murphy

Marshall

Taylor

. The cervical flexion-relaxation ratio: reproducibility and comparison between chronic neck pain patients and controls. Spine (Phila Pa 1976). 2010; 35(24): 2103-08.

21.

Youdas

Garrett

Suman

Bogard

Hallman

Carey

. Normal range of motion of the cervical spine: an initial goniometric study. Physical Therapy. 1992; 72(11): 770-80.

22.

Yoo

. Comparison of Cervical Range of Motion and Cervical FRR between Computer Users in Their Early and Late 20s in Korea. Journal of Physical Therapy Science. 2014; 26(5): 753-54.

23.

Lee

Nicholson

Adams

. Cervical range of motion associations with subclinical neck pain. Spine (Phila Pa 1976). 2004; 29(1): 33-40.

24.

Al Dajah

. Soft Tissue Mobilization and PNF Improve Range of Motion and Minimize Pain Level in Shoulder Impingement. Journal of Physical Therapy Science. 2014; 26(11): 1803-05.

25.

Mayer

Neblett

Brede

Gatchel

. The quantified lumbar flexion-relaxation phenomenon is a useful measurement of improvement in a functional restoration program. Spine (Phila Pa 1976). 2009; 34(22): 2458-65.

26.

Miyahara

Naito

Ogura

Katamoto

Aoki

. Effects of proprioceptive neuromuscular facilitation stretching and static stretching on maximal voluntary contraction. Journal of Strength Conditioning Research. 2013; 27(1): 195-201.

27.

Olivo

Magee

. Electromyographic assessment of the activity of the masticatory using the agonist contract-antagonist relax technique (AC) and contract-relax technique (CR). Manual Therapy. 2006; 11(2): 136-145.