Hamstring injury is one of the most common sport injuries, and stretching exercises are commonly recommended on the basis that they may help prevent these injuries (13,23,25). Although some doubt has been cast on the prophylactic effect of stretching exercises (29), hamstring flexibility is considered important in clinical rehabilitation including the maintenance of muscular and postural balance (32) and rehabilitation in people with osteoarthritis (30).
Several different stretching techniques have been advocated and employed to improve flexibility of the hamstring muscles (12,15,29). Proprioceptive neuromuscular facilitation stretching (PNF) techniques and static stretching (SS) are 2 of the more popular stretches used in clinical practice (14,24). There are a number of research studies that have compared these modalities, with conflicting evidence regarding their relative effectiveness (4,10,14).
To date, research has focused on increased hamstring flexibility by employing several repetitions of the stretch (1,5,10,19,21,22,27). Feland et al. (9) have compared PNF and SS; however, this involved multiple repetitions of the PNF technique in elderly subjects using a straight leg raise technique that failed to isolate the hamstring muscle group (9). Some existing literature has included poorly defined warm-up protocols that make interpretation of results less clear (10), and furthermore, a warm-up before stretching is not always possible in a clinical setting. A single stretching session is common in clinical practice or sporting environments, where often a limited warm-up period is available. To date, there has been no research addressing the impact of a single repetition of PNF in comparison to a single repetition of SS on hamstring flexibility.
The purpose of this study was to compare the effectiveness of a single repetition of hamstring (agonist) contract PNF, SS, and no stretching on hamstring flexibility as measured by changes in available passive knee extension range of motion (ROM). It is hypothesized that there would be a significant difference in the flexibility of the hamstring after a single stretching session in SS, PNF, or control groups.
Experimental Approach to the Problem
The efficacy of a single session of PNF and SS hamstring stretches was evaluated using an independent group design in 45 healthy university students. Participants were randomly allocated to 1 of 3 groups. Randomization was achieved by participants picking a number out of a hat, as established by Schuback et al. (27). One group received an SS, whereas another group received a PNF technique, both applied by the same researcher, and the control group received no intervention (Figure 1). Effectiveness was determined by measuring the available range of passive knee extension at baseline and after intervention. The difference between the 2 was the outcome measure. The same protocol was implemented with the control group with a 30-second gap between measurements.
A convenient sample of 45 healthy university students (22 men and 23 women, age range of 21-35 years) was included in the study (Table 1). Participants had no known neurological or orthopedic disorders, or previous surgery, known fractures, bony abnormalities and had normal active hip and knee mobility. There were no significant between-group differences with respect to age, height, weight, or gender (Table 1). Institutional review board approval was obtained before recruitment of subjects. Informed consent was obtained from all subjects.
Subjects wore shorts to expose the lower limb and allow for palpation of the greater trochanter. They were then asked to lie supine on a height adjustable plinth. A towel was placed between the spine (ca., between T12 and L5) and plinth to help maintain the curvature of the lumbar spine and avoid compensatory lumbar movements influencing the results. Lumbar movements were further limited by the use of 5-cm wide straps positioned across the anterior-superior iliac spines and secured to the plinth (27). Because muscle architecture may differ between dominant and nondominant legs (16), only the dominant legs were tested in this study. The dominant leg was determined by each subject kicking a football 5 times (5). The foot most often used was classed as the dominant leg. To increase reliability, the same researcher instructed and prepared each subject to be measured.
To measure the flexibility of the hamstring, the available range of passive knee extension with the hip in 90° flexion was measured using 2 universal goniometers (Whitehall G300, California, USA) connected via a length-adjustable arm (Figure 2). The stationary arm of the proximal goniometer was fixed along the lateral midline of the trunk, parallel with the edge of the plinth. The moveable arm of the proximal goniometer was aligned along the lateral aspect of the thigh, with the fulcrum overlaying the greater trochanter. The fulcrum of the distal goniometer (DG) was aligned with the lateral portion of the tibiofemoral joint, with 1 arm connected to the proximal goniometer and the other aligned along the lateral aspect of the lower leg (Figure 2).
The subject's hip was passively placed in 90° flexion. To ensure that this position was maintained, the thigh was kept in contact with a secure right angled box attached to the plinth. Before taking any measurements, the researcher ensured the lumbar spine was in contact with the plinth by checking that the towel could not be removed. Subjects were instructed to keep the limb relaxed as the researcher passively raised the lower leg. Full knee extension was considered achieved when subjects reported a strong but tolerable stretch and when no further knee extension was possible because of tissue inextensibility.
The degree of knee extension was measured using the DG. To reduce potential bias, a piece of circular card covered the DG scale during the stretch. Subsequently, the goniometer angle was recorded by a researcher blinded to group allocation.
Bandy et al. (2) have previously shown that the duration of a stretching maneuver does not influence the effectiveness of SS and established that a 30-second SS is as efficient as a 60-second SS in amplifying flexibility. Therefore, a duration of 30 seconds was chosen for this study.
Subjects in the control group relaxed for 30 seconds after the baseline measurement, before a final measurement was obtained. This rest period was chosen to equal the total maximum stretch time used in any intervention.
Subjects in the SS group underwent the following stretching procedure. With the hip remaining at 90° flexion, the researcher passively raised the lower leg, extending the subject's knee to full extension. Subjects were instructed to inform the researcher when a strong but most tolerable and discomfort stretch was felt. A constant stretch was maintained for 30 seconds, during which the researcher continuously monitored the subject. A final measurement of hamstring length was then recorded.
Proprioceptive Neuromuscular Facilitation Stretch
Subjects in the PNF group underwent the following stretching procedure: Using the same method as for the SS group to obtain full knee extension, the subjects were instructed to contract the hamstrings by flexing the knee against a resistance, applied by the researcher. This contraction was held for a total of 6 seconds (3). The knee was then passively extended to full extension and immediately afterward a final measurement of hamstring length was recorded.
Statistical analysis was performed using Statistical Package for the Social Services (SPSS) version 15 for windows. A mixed factorial 2 × 3 analysis of variance was used to determine the interaction effect with the independent variables including pre-post repeated measure factor and a between-groups factor. Post hoc Bonferroni test was used to test for pairwise differences, and all significance levels were set at 0.05.
The results showed that there was a significant interaction (pre-post * group) effect. Post hoc Bonferroni test showed that there was a significantly greater increase in knee extension range in both stretching groups when compared to the control group (p < 0.05, Table 2 and Figure 3). The PNF group demonstrated a significantly greater gain in passive knee extension when compared to the SS group (mean difference 4.27°; p < 0.05, Table 2 and Figure 3).
The results demonstrated that both stretching techniques (SS and PNF) produced a significant increase in passive knee extension compared to the control group when applied as a one-off maneuver. When comparing the 2 stretching techniques, the PNF technique produced a significantly greater average increase in passive knee extension of 4.27° compared to SS. These findings are consistent with those of previous studies where SS increased ROM when compared to a control (5,31). Proprioceptive neuromuscular facilitation stretching increased ROM when compared with a control (3,27,28) and PNF increased ROM when compared to SS (9,10). However, there are 3 previous studies whose findings appear to conflict with ours (6, 8, 11). Davis et al. (6) found SS to be more effective at increasing ROM when compared to a PNF technique, whereas Ford and McChesney (11) found no differences between stretching interventions. More recently, Fasen et al. (8) demonstrated significant benefits at 4 weeks using active stretches (including PNF) compared to passive stretches, but this effect was reversed at 8 weeks. In previous research, a repetition of a stretch was performed in either 1 session or over a period of several weeks. This study has demonstrated that given the constraints of a single repetition, both stretching techniques are effective, with PNF being the most effective mode of stretching.
Although the results of Chan et al. (5) and Whatman et al. (31) with respect to the effectiveness of SS are consistent with our findings, both previous studies employed several repetitions of the stretch. Chan et al (5) used 10 30-second stretches, 3 times per week, for 4 weeks, and Whatman et al. (31) employed 4 20-second stretches in a single bout. The findings of our study suggest that a 30-second one-off SS will provide immediate results similar to those of a repeated stretching regime.
With respect to the relative effectiveness of PNF vs. SS, the contrasting findings in some previous studies (6,8,11) could be explained by differences in specific PNF approach used. Ford and McCheseny (11) employed active hip flexion contraction, and Davies et al. (6) and Fasen et al. (8) used active quadriceps contraction as their PNF technique. None of these studies therefore employed the hamstring (agonist) contract technique followed in our study and in previous studies demonstrating superiority of agonist-contract PNF techniques compared to SS. This would suggest that hamstring (agonist) contract is a more effective technique than (antagonist) contract-relax methods.
The agonist-contract method of PNF has been hypothesized to facilitate the inverse myotatic reflex that inhibits muscle activation (autogenic inhibition) leading to muscle relaxation and improved flexibility. However, it has been argued that the principle mechanism by which PNF enhances flexibility is because of higher levels of discomfort experienced when the technique is applied compared to SS (4). This theory has some support from evidence indicating that stretching has an analgesic effect by increasing pain thresholds (20). We believe it is unlikely that this latter mechanism was responsible for the relative differences in effectiveness of PNF and SS in our study because participants received the same strong but tolerable stretch in both one-off SS and PNF. Therefore, our findings do not discount autogenic inhibition as a factor in the positive effect of one-off PNF.
The relative homogeneity of our study sample group may limit the application of results to the general population. It has been shown that adults past the fourth decade of life exhibit age-related changes to muscle architecture (7,17,18,26), and it may be the age-related structural or physiological changes that may modulate the response to PNF or SS. However, 1 previous study employing the hamstring (agonist) contract technique in adults aged from 50 to 57 years demonstrated similar superiority of PNF compared to SS (10).
This study has shown that a one-off PNF or SS produces similar short-term gains to repetitive stretching. Future studies may wish to investigate how long these gains in ROM persist and determine the optimum longevity of single bout muscle stretches. Further investigation could also determine the effectiveness of single bout PNF in populations with different activity levels (e.g., sedentary vs. elite athletes).
The purpose of this study was to compare the effectiveness of one-off static stretching and hamstring (agonist) contract PNF on hamstring flexibility as measured by available passive knee extension ROM. The results showed that both techniques significantly increase hamstring flexibility when a strong but tolerable stretch was applied, but that the PNF technique provides a significantly greater increase. The findings suggest that a single bout of hamstring (agonist) contract PNF without warm-up is more effective than SS for producing an immediate effect on hamstring flexibility.
These findings are particularly relevant to physiotherapists (physical therapists) or trainers working in clinical and sporting environments where an immediate effect is desired without warm-up.
No benefits in any form have been or will be received from a commercial party or grant body related directly or indirectly to the subjects of this study. The results of the present study do not constitute endorsement by the authors or the National Strength and Conditioning Association.
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