Physical fitness enables the body to respond to the stimuli of physical effort. One of the 5 health-related fitness components, flexibility is the ability to move and bend joints through a full range of motion (ROM). An appropriate amount of flexibility can lead to both a reduced risk of injury and an increase in performance by decreasing the resistance of tissue structures surrounding the joint (13). This decrease in resistance in turn allows the joint to work more efficiently by reducing the energy needed to contract the muscle (13). Furthermore,stretching increases coordination by improving the nerve impulse velocity (2) that allows the muscles to coordinate better by recruiting certain muscle fibers that previously failed to contract (7).
Physiologically, stretching elongates the skeletal muscle and associated connective tissues surrounding a joint. Dynamic stretching involves moving joints through a full ROM and is often used in pre-exercise warm-up. Static stretching does not involve motion and is usually performed postexercise because it is known to reduce power output (8). Static stretching can be broken down into passive resistance and isometric resistance. During passive resistance stretching, one assumes a position and then holds that position with the aid of another body part or with the help of a partner. Isometric resistance stretching involves the resistance of muscle groups through fixed-length contractions of the stretched muscle. The resistance during isometric stretching can be applied in 3 ways: (a) manually, where resistance is applied by oneself, (b) partner assisted, where resistance applied by a partner, and (c) fixed object, where resistance is applied by an apparatus such as a wall or floor.
Proprioceptive neuromuscular facilitation (PNF) is a form of static stretching that combines passive with isometric stretching (6). There are several different PNF techniques, although the most common technique (hold-relax) is performed when a muscle is statically stretched, then isometrically contracted, and finally statically stretched again. The PNF stretching is often perceived as partner-assisted, such that the first and third steps are passive static stretching. The PNF can be performed successfully and safely without a partner (1).
There is a general agreement that partner PNF is more effective to static stretching to increase flexibility (10,12,15). Unfortunately, there are 2 primary disadvantages to partner PNF that include the need for a partner and the doubled time commitment if both individuals are to be stretched. Self-PNF removes these 2 limitations and also places full control of the stretching process to the individual. From a practical standpoint, if self-PNF stretching is more effective than static stretching in improving ROM, then self-PNF could possibly be used in place of static stretching and without the limitations of partner PNF. Therefore, the goal of this study was to determine if self-PNF stretching is more effective when compared with static stretching in increasing the hip ROM and hip, back, and shoulder flexibility (HBSF). It was hypothesized that self-PNF will result in a significantly greater increase in both hip ROM and HBSF compared with static stretching and therefore should replace static stretching.
Experimental Approach to the Problem
The effectiveness of self-PNF compared with static stretching was performed using a within-subject design to remove interindividual factors that may influence changes in flexibility from stretching. In addition, a crossover design was used, such that half the subjects were placed in the one stretching group for the first session and the other half in the other stretching group. After the completion of first session, a 1-week break was given and subjects then switched stretching groups and the process was repeated. This crossover design was used to minimize the residual effects of the stretching from the first session. Prestretching and poststretching intervention measures were determined for each session to ensure that changes could be examined without the influence of residual effects from the initial session. For the independent variables, self-PNF stretching was compared with static stretching because both are more appropriately used after a training session (7) or for rehabilitation purposes. The dependent variables of hip range of motion and sit-and-reach (to measure HSBF) were chosen to examine overall changes within the hip and the hip, back, and shoulders rather than direct effects on the muscle being stretched. It was felt that the chosen design provided an alternate but still practical application. A goal in stretching is often to improve overall flexibility rather than improving the length of a particular muscle. Overall, all subjects were measured for changes in the 2 dependent variables after a bout of static stretching and a bout of self-PNF stretching to determine which stretching technique is most effective.
Nineteen healthy college-aged participants (age, 19–25 years) were recruited for the study from the Department of Kinesiology at William Paterson University in Wayne, NJ. Of the 19 participants, 8 were female and 11 were male. Mean and SD for weight and height were 61.0 ± 4.2 kg and 166.2 ± 6.3 cm, respectively, for women and 78.6 ± 6.3 kg and 180.5 ± 5.3 cm, respectively, for men. To be selected for the study, participants had to be college age, not pregnant, and have no injuries to the hamstring muscle group, hip, or back area in the past year. Before recruitment, the institution’s Institutional Review Board approved the study, and subjects read and signed an informed consent before participation.
Data collection took place during the fall term of the 2011 university school year. For the preintervention and postintervention measures, participants were asked to perform a 10-minute warm-up consisting of walking at a self-selected pace. Following the initial 10-minute warm-up preceding the first pretest intervention, participants were asked the effort level (on a scale of 1 to 10, 1 being very easy and 10 being very hard) of the warm-up to ensure consistency from one warm-up to the next. Before subsequent warm-ups, participants were provided with this information to maintain consistency from one warm-up session to the next. Immediately after the warm-up, a sit-and-reach test was performed following traditional protocol. Briefly, participants were asked to bend at the hip and reach as far forward as possible until a comfortable (but not painful) stretch was felt while ensuring the knees stay straight (Figure 1). The stretch was conducted with an Acuflex sit-and-reach box (Novel Products, Rockton, IL, USA) measuring how far forward an individual can bend, testing the HBSF. Measurements were only recorded if the subject reached forward and could hold the position for at least 5 seconds. This protocol was included to avoid readings where the subject lunged forward. While in the stretched position, the angle of the hip was also measured using a goniometer (Jamar, Bolingbrook, IL, USA) by 2 individuals on either side of the participant. The measurement via a goniometer was taken with the axis on the greater trochanter, one arm along the longitudinal axis of the femur, and the other arm through a point midway between the anteroposterior cross section of the trunk level with the T12 vertebrae. If the knees flexed or the individual bounced, the procedure was repeated. A total of 2 successful attempts were recorded. All testing sessions were performed on the same day and time for all participants.
The above 2 flexibility measurements were chosen in lieu of the supine active straight leg raise because overall change in flexibility measures were examined rather than solely focusing on the hamstring muscle group. From a practical standpoint, hip ROM and being able to bend over and reach down (i.e., HSBF) is related more to activities of daily living compared with hamstring length, which measures the functionality of a single muscle group.
For the stretching intervention sessions, participants were placed randomly in either the self-PNF or the static stretching group. Before each session, subjects were asked to perform a 10-minute walk at the effort level recorded for their initial warm-up pace during the first testing session. The subjects for this study were upper level undergraduate kinesiology students and familiar with effort levels. For the stretching, both groups were asked to place one foot on a chair, approximately 50 cm in height, while the other remained on the floor (Figure 2). For the self-PNF group, stretching consisted of a 15-second static stretch, a 10-second hamstring isometric contraction at approximately 90% of maximum effort, and a 15-second static restretch. The 90% maximum effort was approximated by asking participants to drive their heel into the chair at approximately 90% maximum effort. No measure of the actual degree of contraction was taken. The static group placed one foot on a chair and held a static stretch for a period of 40 seconds. Participants were instructed only to stretch to the point where it was still comfortable and caused no pain. Both groups performed a total of 2 sets of stretches on each leg, 2 days per week. All stretching sessions were led by 2 of the researchers instructing the subjects during each phase of the stretch (e.g., when to start stretching, when to contract the hamstring muscle group, how much time remain, when to relax). After a 6-week period, postintervention measures were conducted as described above. After a 1-week break, the entire procedure was repeated with participants switching groups.
The 2 measures tested in this study included hip ROM and HBSF combined using the goniometer and sit-and-reach box, respectively. The goniometer readings and the sit-and-reach box readings were, respectively, averaged across the 2 trials during a testing session. The difference between the preinterventions and postinterventions for all data were then calculated and used for statistical analyses. Therefore, the dependent variables for the analyses were change in hip angle (goniometer reading in degrees) and change in sit-and-reach (in centimeters), and the independent variables were the 2 stretches (self-PNF vs. static). Repeated measures t-tests were used to determine if there was any statistical significance between the means of the 2 groups on the change hip angle and on the change in sit-and-reach. Because 2 t-tests were used, alpha was set to 0.05/2 or 0.025. Eta squared was calculated to determine the effect size of the intervention. Statistical analysis was performed using SPSS version 19 (SPSS, Chicago, IL, USA). As a secondary analysis, the 99% confidence intervals were calculated to determine whether the change in a measure was significantly different from 0. Intraclass correlation coefficients (ICCR) and the associated standard error of measurement were also determined between four separate factors and included agreements between the measurements of raters on right vs. left hip ROM, test-retest within a session for sit-and-reach, and test-retest for ROM of the right hip and left hip separately within a session.
Initially 25 participants were part of this study. However, due to attrition and stretching days missed a total of 19 participants were included for the final analysis. Examination of the individual trial data for each session revealed no difference in goniometer readings of greater than 2 degrees between hips for a single trial and no greater than 4 degrees between trials in a single session. The sit-and-reach box readings did not vary greater than 3 cm across trials in a single session. The ICCR (r) and SEM showed good agreement and minimal error between raters on the right vs. left hip ROM (r = 0.84, SEM = 2.16°), test-retest for sit-and-reach (r = 0.97, SEM = 0.52 cm), intrarater test-retest for the right hip ROM (r = 0.93, SEM = 1.61°), and left hip ROM (r = 0.91, SEM = 1.83°). The hip ROM for the PNF group changed after the 6-week stretching intervention by −6.2 ± 6.6°; a negative value indicates a more acute hip angle and greater ROM (Table 1). The static stretch group minimally lost hip ROM (0.6 ± 4.5°). Both groups experienced an increase in sit-and-reach test after the 6-week intervention, where the PNF group increased their reach by 5.2 ± 3.3 cm and the static stretch group by 2.0 ± 2.6 cm (Table 1).
The 2 t-tests showed a significant difference, p = 0.001 and p = 0.007, between the PNF and static groups for both the changes in hip ROM and sit-and-reach, respectively. Eta squared for the hip ROM measure was calculated to be 0.44, indicating that 44% of the variance between the means of the 2 groups was explained by stretch type. Stretch type also explained 34% of the variance in the means of the sit-and-reach test. Ranges for the 99% confidence interval calculations showed that the PNF group for the hip ROM and both groups for FHBS did not include 0. Therefore, there is a 99% probability that the interventions for these measures were more effective than no intervention (assuming a zero change in these measures with no intervention).
The goal of this study was to examine the change in both hip ROM and HBSF after a 6-week intervention of a form of self-PNF vs. static stretching. The design of this study was unique in the combination of repeated measures (same participants in both groups), the dose (2 times per week bilaterally holding each stretch for 40 seconds for 2 repetitions = 160 seconds), and the specific PNF stretch used.
Examining the static stretching group independently, this group had no gains in hip ROM but experienced gains significantly greater than zero in the sit-and-reach measurement, indicating some gains in flexibility were experienced. These results are consistent with previous studies (3,9,11), although each adopted a different design. The study by O’Hora et al. (9) used a single session, 30-second passive static stretching technique and examined hamstring flexibility via range of passive knee extension while supine. Participants in the study by Davis et al. (3) were also passively stretched for 30 seconds, 3 days per week across a 4-week period. The design results in a weekly dose of 120 seconds, 25% less than what was prescribed in the present study and for a period lasting 2 weeks less than the present study. Results from the study by Davis et al. (3), using the hamstring flexibility test, revealed the static stretch group (N = 5) to have significant gains over a control group at 2 and 4 weeks post-intervention. Ross (11) examined 2 active static stretch routines, relative to stance and swing phases of walking, and found significant differences from preintervention to postintervention on hamstring flexibility. The stretching occurred 5 times per week for 30 seconds, equaling a dose of 150 seconds per week for 2 weeks. The forward phase stretch was similar to the static stretch performed in this study and was found to have less effect than the stance phase stretch. All studies also used a between-subject design, whereas this study included a mixed within-subject design to minimize confounding variables.
Overall, there seems to be a consensus that over time, static stretching does improve flexibility. However, because of the varying dose and weeks between studies, it is difficult to determine what combination of these factors would result in the most effective passive stretch routine. Furthermore, previous studies examined hamstring flexibility, whereas this study examined hip ROM and HBSF. This discrepancy may be one reason that the gains in this study were not as pronounced. Specifically, static stretching of the hamstring group aids in hip ROM and HBSF, but other joints and tissue also have an influence on these factors.
The PNF stretching technique in this study was specifically a self-administered agonist contract-relax method, meaning the stretching was a combination of active static stretching and the hamstring contraction against a resistance (chair), often called a PNF antagonist stretch. At the time of data collection, there were no known studies that examined this form of self-PNF stretching. Results from this study showed that this form of PNF stretching significantly improves hip ROM and HBSF. Davis et al. (3) found that PNF agonist stretching significantly improved hamstring flexibility from baseline over a 4-week period but not a 2-week period. Improvements were also found immediately after a contract-relax agonist PNF stretching protocol (5). However, the gains found were no better than static stretching or active control stretching.
The primary focus of this study was to examine the change in hip ROM and HBSF within the same group of participants after a static stretch vs. a self-PNF routine. In both measures, there was a significantly greater improvement after self-PNF stretching. The outcome of this study is similar to those using different PNF techniques (3,4,9). However, Davis et al. (3) found that only static stretching resulting in hamstring lengthening over a 2-week period and agonist contract-relax PNF with a partner only showed improvements after 4 weeks. Furthermore, the static stretch group (n = 5) showed greater improvements over the 4-week period vs. the PNF group (n = 5). Several factors could attribute to the differences found in this study and include partner-PNF vs. self-PNF, the length of the study, and the doses. The study by Davis et al. (3) had participants stretch once for 40 seconds, 3 times per week, whereas this study had the participants stretch twice for 40 seconds, 2 times per week. However, the most likely factor for the variance between studies is that this study included a within-subject design of 19 participants compared with a between-subject design with 5 participants in each group in the study by Davis et al. (3).
The study by O’Hora et al. (9) found significantly greater gains in partner agonist contract-relax PNF compared with static stretching on hamstring length. These results were found for immediate gains in hamstring length, suggesting that a single bout of stretching is sufficient per session. However, it is not certain whether these gains transmit to a long-term gain. Still, the number of repetitions per session for long-term gains is something that should be examined.
Comparisons of this study with previous ones should be taken with caution. The present study measured hip ROM and HBSF to examine the overall effects of static stretching vs. self-PNF over a 6-week period. Previous studies focused on hamstring length, which is not appropriately measured using sit-and-reach or hip angle measures (15). Regardless of this discrepancy, self-PNF did show greater improvements compared with static stretching. Still, the effects of self-PNF on individual muscles (e.g., hamstring) and other specific joints (e.g., shoulders) should be investigated.
This study compared static stretching to self-PNF over a 6-week period on overall flexibility in the hip and hip, back, and shoulders in a group of healthy college-aged individuals. Greater improvements using self-PNF for hip ROM and HBSF suggest that this stretching method is more effective and may help individuals adhere to a stretching routine because of the significant gains that will likely be experienced. Most importantly, self-PNF can be performed without a partner, making this form of PNF more convenient while leaving the individual in control of the stretch. Therefore, coaches and practitioners should adopt self-PNF over traditional static stretching to improve hip ROM and HBSF.
No funding was provided for this study.
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