Adequate flexibility, which is the ability to move though a full range of motion (ROM), may aid in the prevention of injuries and enhance athletic performance (1–3,7,12,17,19,23,24,26,28). Of the different types of stretching techniques to improve ROM, proprioceptive neuromuscular facilitation (PNF) using the slow-reversal-hold-relax (SRHR) has been shown to be the superior stretching technique in many studies (3,5,6,11,23–26). Cornelius et al. (5) and Rosenberg et al. (23) found PNF stretching to be more effective than static stretching. And when comparing PNF techniques, Rosenberg et al. (23) found SRHR to be more effective than the PNF contract relax method. The increased flexibility with PNF stretching is provided by reciprocal and autogenic inhibition (1,3,5,7,8,23,24,26). Reciprocal inhibition occurs from an agonist muscle contraction facilitating a concurrent relaxation of the antagonist muscle, producing a further stretch (24). Autogenic inhibition facilitates relaxation in the antagonist immediately after a contraction of the antagonist muscle (24,26).
While stretching alone may aid in increasing flexibility, particularly PNF stretching, improvements in ROM also have been facilitated with cryotherapy (2,6,9,10,15,16,18,21–23,29). Cryotherapy decreases superficial tissue temperatures (3,4,9,10,13,15,16,21,22,28,29) causing inhibition of muscle spindles (2,3,5,16,22,26,28,30) resulting in a compensatory vasodilation in the deep intramuscular tissue (3,6,10,23). Certain cryotherapy treatments have the potential to cause varying temperature gradients because of different cutaneous temperatures reached at the end of the treatment. The temperature gradient between the tissue layers results in decreased muscle spasms allowing the muscle to be further stretched (3,6,23).
Many forms of cryotherapy in conjunction with stretching to improve ROM have been used (2,5,9,15,16,18,21–23,29). Another specialized form of ice, wetted ice, which is water added to ice, has not been tested. Based on previous research (9), it is believed that wetted ice will yield greater improvements in hamstring ROM compared with crushed ice. This is because of its greater ability to reduce tissue temperature quickly and longer lasting cooling effects. However, the aforementioned studies measuring ROM have not used wetted ice as a treatment option. Thus, when cryotherapy and PNF stretching techniques are combined, it is unclear which type, crushed or wetted ice, produces the greatest gains with an SRHR PNF stretching technique. Therefore, the purpose of this study was to determine which type of cryotherapy, crushed or wetted, would produce the greatest gains in hamstring flexibility using an SRHR PNF stretching technique. The results are intended to provide evidence regarding the relative effectiveness of combined cryotherapy and PNF stretching that can be used by athletic trainers (ATs) during rehabilitation and strength and conditioning specialist during reconditioning or to improve performance.
Experimental Approach to the Problem
A within-group experimental design was to test the hypothesis and used to limit the amount of variability of testing measures. The independent variables for this study design were the time periods (baseline, pre-PNF, and post-PNF) and cryotherapy conditions (no ice, crushed, or wetted). The dependent variable was hamstring ROM. Subjects had their hamstring ROM measured at baseline, received a treatment condition for 20 minutes, followed by another ROM measure, then the PNF stretching technique, and another ROM measure. The testing sessions were randomized to reduce the order effects of the cryotherapy conditions. Three days were allowed between each testing session to ensure that there were no prolonged effects from the previous session and all sessions occurred in the morning. Hamstring ROM was measured with a goniometer by the same investigator.
Fifteen healthy subjects, 8 men and 7 women (age, 22.6 ± 1.6 years; range, 20–26 years; height, 169.87 ± 7.11 cm; weight, 80.67 ± 30.46 kg; skinfold, 22.4 ± 7.80 mm) with no history of lower leg pathologies or sensitivity to extreme temperatures volunteered to participate. The university's Human Subject's Internal Review Board approved the study and informed consent was obtained from each subject.
Subjects reported to the Human Performance Research Laboratory for 3 separate sessions. During the first session, the subject's demographics (including age, height, weight, leg dominance, and skinfold measurement) were taken. The dominant leg was determined by asking the subject which leg they would prefer to kick a ball. A skinfold measurement was taken by a certified skinfold assessor from Michigan High School Athletic Association, Inc. with 2 years of experience as a certified AT. Skinfold assessment was tested for inter and intrarater reliability at 90% agreement with 2 other ATs during pilot testing on 2 non-participants. Subjects were asked to lie prone on a treatment table, and the midline between the distance from the ischial tuberosity and the popliteal crease was marked. Three skinfold measurements were taken using a Lange Skinfold Caliper (Cambridge Scientific Corp., Cambridge, MD, USA) at this mark and the average was used. The thickness of the superficial adipose layer of each subject's hamstring was then estimated by dividing the average skinfold measurement by 2 (9,18). If the thickness calculation was greater than 20 mm, the subject was excluded from the study to ensure the layer of adipose tissue did not adversely affect the cooling technique used in the study.
At the beginning of each session, 3 ROM measurements of the hip joint were taken by the same AT using a standard goniometer and averaged for a baseline measurement. The axis of the goniometer was placed over the femoral greater trochanter, whereas the stationary arm was placed parallel with the trunk, and the moving arm aligned with a mark that was drawn over the lateral femoral epicondyle (27). The dominant leg was lifted until an initial stretch was felt by the subject (Figure 1). After ROM measurements, the subject was instructed to lie prone on the treatment table. A 23 × 16-cm template was centered over the marked midline of the hamstring and taped to the hamstring. A surface thermocouple was taped in the middle of the template area to measure skin surface temperature (Figure 2). The subject continued to lie prone for 10 minutes to regulate body temperature. The skin temperature was recorded at the conclusion of the 10-minute baseline, 10 minutes into the treatment, and at the conclusion of the 20-minute treatment period. The specified treatment condition, as determined by a random numbers table, started immediately after the baseline period. The crushed ice contained 2500 ml of ice placed in a 9.5 in by 18 in 1-mil polyethylene Cramer bag with air removed to increase conformity to the hamstring. The wetted ice bag contained 2500 ml of cubed ice along with 150 ml of room temperature water in the same sized ice bag. All ice conditions were applied for 20 minutes. Ice conditions and treatment times were based on a previous study (9).
At the completion of each ice application, the template, ice, and thermocouple were removed, and the subject was asked to lie in a supine position. The subject's hamstring ROM was measured and recorded using the same technique as the baseline measurement. The SRHR stretching technique was then executed. The SRHR procedure consisted of a 10-second passive stretch of the hamstring on the dominant leg with the subject's dominant knee extended. Subjects were then asked to try and push their leg toward the treatment table while the AT resisted the contraction. Subjects held this isometric contraction of the hamstrings for 10 seconds. Subjects were asked to contract at a submaximal level to maximize the amount of stretch while reducing the risk of injury to the hamstrings (25). Subjects then actively contracted their hip flexors to further stretch the hamstrings. Once subjects reached their maximum ROM using the concentric contraction of their hip flexors, the AT applied slight overpressure to passively stretch the hamstring muscle group for 30 seconds. The timing for the 10-second isometric contraction and 30-second passive stretches are based on a previous published protocol (14). The stretching procedure was repeated 5 times with a 10-second rest period between each stretching repetition. At this point, another hip joint ROM measurement was recorded using the same protocol described earlier. Each subject reported for 2 additional treatment sessions to receive all 3 conditions. All subjects were tested in the morning to limit the influence of the circadian cycle on body temperature regulation (13). In addition, all subjects were told to refrain from exercising the night before or morning before the study and to follow their normal dietary habits.
A repeated measures analysis of variance (RM-ANOVA) was used to analyze the differences in the 3 time periods (baseline, pre-PNF, and post-PNF) and the treatment conditions (wetted ice, crushed ice, and no ice) on the hamstring ROM. The second RM-ANOVA was used to analyze the differences in temperature (baseline, 10 minutes into the treatment and at the conclusion of the 20-minute treatment period) by condition. The statistical analysis was performed using Statistical Package for Social Sciences (SPSS version 20; Chicago, IL, USA). The alpha level was set a priori at p ≤ 0.05 to minimize type 1 errors. Assumptions of linearity and correction factors were used when applicable. All post hoc testing was analyzed with the Sidak correction test.
Range of motion descriptive statistics are found in Figure 3. The results of the RM-ANOVA showed that there was a significant interaction effect (F4,56 = 5.32, p = 0.001) between the time periods and the treatment conditions. Post hoc testing of pre-PNF measurements revealed that no ice (75.49 ± 12.19° C) was significantly different than wetted ice (81.73 ± 10.34° C) and crushed ice (81.62 ± 13.19° C). Post hoc testing of post-PNF measurements showed no ice (85.27 ± 13.83° C) was significantly different than wetted ice (89.44 ± 11.31° C) and crushed ice (89.16 ± 13.78° C). However, post-PNF ROM between wetted ice and crushed ice were not significantly different.
The results of the RM-ANOVA for temperature showed a significant interaction effect (F4,48 = 435.07, p < 0.001). Post hoc testing revealed that mean temperature values at the 10-minute time period were significantly different for all treatment conditions; no ice (31.54° C), wetted ice (7.08° C), and crushed ice (10.62° C). At the conclusion of the treatment, all mean temperature values were significantly different from each other; no ice (31.69° C), wetted ice (5.15° C), and crushed ice (8.53° C) (Figure 4).
Our results showed that both ice conditions produced greater gains in ROM compared with no ice with the SRHR, but the ice conditions were not significantly different. However, our finding showed that cryotherapy facilitates improvements in ROM conflicts with the findings of Cornelius et al. (5) and Rosenberg et al. (23), which showed that cryotherapy and stretching was no better than stretching alone. Our cryotherapy results do support those of Brodowicz et al. (2) who reported a greater increase in ROM when stretching was combined with cryotherapy than when combined with heat or with stretching. These studies that have compared cryotherapy and stretching techniques have used inconsistent parameters, which makes it difficult to test a set protocol for increasing hamstring ROM and to compare results across studies.
In addition to our findings that ice improved ROM, we also showed that both forms of cryotherapy combined with SRHR PNF stretching on hamstring ROM to be equally effective, which is contrary to current evidence. Wetted ice has been shown to reach colder temperatures compared with crushed ice because of increased rate of transfer of thermal energy within the ice pack because water was added (9). This should result in greater inhibition of muscle spindle activity, thereby increasing hamstring relaxation and allowing for a further stretch and greater improvements in ROM than crushed ice or other common forms of cryotherapy (2,3,5,16,22,26,28,30). However, our results showed similar changes in ROM with the 2 ice conditions when combined with stretching. Our results showed that wetted ice increased ROM by an average of 13.73° C vs. crushed ice, which increased ROM by an average of 13° C. More research are needed to determine if these ice conditions will produce ROM differences with longer treatment times, or if ROM gains are maintained for longer periods of time after treatment.
The similar increases in ROM with our 2 cryotherapy conditions may be attributed to the decreased rate of muscle nerve conduction velocity (NCV) that has been shown to occur at skin temperatures of 12.5° C (4,13,20). When the skin temperature reaches this critical value, there is a 10% decrease in NCV. Decreasing NCV increases the latency and duration of the action potentials in motor and sensory nerves, while decreasing the amplitude of the action potentials to produce a greater stretch (5,13). Improvements in ROM could also be related to an increase in the pain threshold because of the analgesic effect and NCV change. Our results showed that wetted ice produced a skin temperature of 5.15° C, whereas crushed ice produced a skin temperature of 8.53° C. While the wetted ice was over 3° C colder, both were well below the 12.5° C threshold, and both ice conditions were equally effective in increasing ROM using the SRHR PNF stretching technique and much greater than stretching alone. Our results suggest that it is not the magnitude of temperature reduction, but whether there is significant cooling to reach the threshold for a facilitation of stretching effectiveness.
Examination of our results led to an important observation in terms of clinical practice for increasing ROM with time of ice application. At the 10-minute time period, wetted ice produced skin temperature of 7.08° C, whereas crushed ice produced a skin temperature of 10.62° C. Since both ice treatments lowered temperature below 12.5° C before the 10-minute mark, cryotherapy treatment times could be 10 minutes or less and still used to improve flexibility. Further research is recommended to examine effective cryotherapy application times and ROM increases. Although we used recreational active subjects, further examination should also be explored with athletes from various competitive sports to determine difference in gains with ice and PNF stretching. In addition, applying the ice condition to the larger surface area of the hamstring may also produce different outcomes than smaller muscles so further exploration of the treatment size is warranted.
Based on the results of this study, cryotherapy combined with SRHR stretching will improve hamstring ROM more than stretching alone in recreational active individuals. There were no significant differences seen between the 2 cryotherapy treatments, therefore, either type of ice may be used with stretching to facilitate improvements in hamstring ROM. Strength and conditioning specialists, ATs, and other allied health professionals should feel comfortable in choosing any form of ice combined with PNF stretching to improve ROM that is frequently lost after an injury, or for those athletes who simply wish to improve their ROM for improved performance. Although ice may not be a conventional tool for the strength and conditioning specialists, it is an easily used and readily available tool. Athletic trainers and strength and conditioning specialists must work together as recovery athletes move from rehabilitation to reconditioning. Sharing effective strategies that can facilitate this process can strengthen this important relationship. Strength and conditioning specialists can use the results of this study to maximize stretching results that may improve performance.
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Keywords:Copyright © 2015 by the National Strength & Conditioning Association.
cryotherapy; range of motion; temperature