Acute effects of contract-relax proprioceptive neuromuscular facilitation stretching of hip abductors and adductors on dynamic balance

Participants

The research protocol was approved by the Senate Commission for Ethics of Scientific Research at the University School of Physical Education in Wrocław. Criteria for exclusion were: impaired vision or hearing, central nervous system disorders, any balance impairment, lower extremity joint or sacroiliac dysfunction, or joint hypermobility syndrome. All included participants (n = 45, age 19–23 years) signed informed consent and they were assigned to the intervention group (IG) or the control group (CG). Participant characteristics are presented in Table 1.

Procedures

All subjects visited the laboratory 24 h before testing session to undergo anthropometric measures and familiarize with the balance test and stretching exercises. They performed the balance test twice and then three repetitions of CRS (only intervention group). On the next day, in the testing session, the balance test was carried out before (Pre) and immediately after CRS in the intervention group or after 5-minute rest in sitting position in the control group (Post).

Contract-relax proprioceptive neuromuscular facilitation stretching (CRS)

Stretching was administered to both limbs targeting the adductor and abductor muscles of the hip, in which the order of treatment was reversed in half of the sample (right and then left limb or vice versa). The CRS involved passive stretch (to the sense of discomfort) of the target muscles, then the 10-second of 50% maximal voluntary isometric contraction (MVIC) of the same muscles, followed by relaxation (5 s) and passive movement into further stretch (5 s). Feland & Marin (2004) found that submaximal contractions (20–60% MVIC) administered using the CRS method are just as effective as techniques that apply maximal contractions. There were performed three repetitions of the CRS on each muscle group.

Dynamic balance assessment

Mediolateral dynamic balance was measured using the stabilometric platform (Libra; EasyTech, Salerno, Italy). The Libra platform is an electronic oscillating balance board that measures mediolateral tilt from −15° to +15° to an accuracy of 1°. The platform was placed on a level surface using the manufacturer-supplied mat and a computer with dedicated software was connected to the device to register mediolateral tilt. All testing was performed in a quiet, well-lit, temperature-controlled room to minimize external influences. The participant stood barefoot on the balance board and was monitored by personnel for safety precautions in case of loss of balance. Feet were maintained parallel to one another and the upper limbs rested freely along the trunk. In addition, the participants kept their knees extended throughout the test in order to exclude the effects of the knee joint on stability. The task was to maintain balance on the device according to a pre-set level of deviation. During the trial, the participant observed a monitor that showed a sway line indicative of the amount of deflection from the center line (indicative of zero sway). Two borderlines paralleled the center line and marked the sway threshold (Fig. 1). If the participant did not maintain balance on the board, the sway line would deviate from the center line and an audio signal was sounded if one of the borderlines was crossed. The level of difficulty was set to maximum on the device permitting only minimal sway. Dynamic balance was quantified based on the amount of sway registered during the 30-s trial. The angular and temporal values of sway displacement were graphically represented on a stabilogram and used to extract stability outcomes:

• Total area of sway (TA)—the summed internal (within the threshold limit) and external (outside the threshold limit) area created by sway line amplitude that deviated from the center line [°s]

• External area of sway (EA)—the summed external area created by sway line amplitude outside the threshold limit [°s]

• External time (ET)—the summed time when sway line amplitude exceeded the threshold limit [s]

• Reaction time (RT)—the longest interval in which sway line amplitude exceeded the threshold limit [s]

• Global index (GI)—a weighted measure of the variables described above calculated on a relative 100-point scale in which the lower the score the better functional balance.

Statistical analysis

All data in the text and figures are presented as mean ± SD. Normal distribution of the data was examined using a Kolmogorov–Smirnov normality test. Initially, mixed-model ANOVA, repeated-measures analysis with one between factor (gender) and one within factor (time) was conducted in the intervention group (IG) to determine if there were differing responses because of gender. There was no significance found between genders, so the male and female scores were combined, and this group’s performance was compared with that of the control group (CG) using mixed-model ANOVA, repeated-measures analysis with one between factor (group; IG and CG) and one within factor (time; Pre and Post). Bonferroni post-hoc multiple comparison was performed if a significant main effect was observed. For each ANOVA, partial eta-squared was calculated as measures of effect size. Values of 0.01, 0.06, and above 0.14 were considered as small, medium, and large, respectively. In the case of non-normality of experimental data (EA and ET), we applied alternative, non-parametric statistical analysis. In that case, a Wilcoxon signed-rank test was utilized to analyze relationships between pairs of related samples (time) and a Mann–Whitney U test was used for analysis of pairs of independent samples (groups). Statistical analyses were performed using Statistica 13.1 software (Dell, Round Rock, TX, USA). The significance level was set at alpha = 0.05.

Global index

Mixed-model ANOVA indicated a significant main effect for groups (F = 5.02, p = 0.03, partial eta squared ${\eta }_p^2=0.11$, observed power = 0.6), time (F = 6.96, p = 0.01, ${\eta }_p^2=0.14$, observed power = 0.7) and the group x time interaction (F = 5.74, p = 0.02, ${\eta }_{\mathrm{p}}^2=0.12$, observed power = 0.7). Bonferroni test revealed that GI in the intervention group was significantly lower after CRS (Pre 3.45 ± 1.34 vs. Post 2.42 ± 0.73; p = 0.0001). There was no significant difference between Pre and Post in the control group (3.62 ± 0.98 vs. 3.57 ± 1.38; p = 0.9). IG values measured in Pre did not differ (p = 0.7) between IG and CG. IG values in Post condition were lower for IG than CG (p = 0.0007) (Fig. 2).

Total area

Mixed-model ANOVA showed a significant main effect for groups (F = 4.83, p = 0.03, ${\eta }_p^2=0.10$, observed power = 0.6), time (F = 7.23, p = 0.01, ${\eta }_p^2=0.14$, observed power = 0.8) and the group ×time interaction (F = 6.72, p = 0.01, ${\eta }_p^2=0.14$, observed power = 0.7). Bonferroni test established a decrease of TA in Post condition in the intervention group (Pre 69.18 ± 16.45° s vs. Post 56.42 ± 9.69° s; p = 0.0001) but not in the control group (Pre 71.16 ± 12.96° s vs. Post 70.93 ± 16.20° s; p = 0.9). There was no significant difference in TA between IG and CG in Pre (p = 0.7), however in Post the TA values were lower for IG than CG (p = 0.0005).

External area

In the intervention group, a Wilcoxon test demonstrated a reduction of EA after the stretching procedure (Pre 4.61 ± 5.49° s vs. Post 1.03 ± 1.30° s; p = 0.00004). In the control group there was no significant difference between Pre and Post condition (Pre 5.86 ± 5.94° s vs. Post 4.11 ± 5.09° s; p = 0.7). Mann–Whitney test revealed that EA was significantly lower in IG as compared to CG (p = 0.002) only in Post but not in Pre (p = 0.4).

External time

Wilcoxon test showed that ET decreased in the intervention group after stretching (Pre 2.57 ± 1.97 s vs. Post 1.06 ± 1.15 s; p = 0.0004); however, in the control group ET remained almost unchanged (Pre 2.71 ± 1.64 s vs. Post 2.71 ± 2.12 s; p = 0.9). Mann–Whitney test indicated that ET values measured in Pre did not differ (p = 0.6) between IG and CG. ET values in Post condition were lower for IG than CG (p = 0.003).

Reaction time

Mixed-model ANOVA demonstrated a significant main effect for groups (F = 10.29, p = 0.003, ${\eta }_p^2=0.19$, observed power = 0.9) and the group ×time interaction (F = 5.69, p = 0.02, ${\eta }_p^2=0.12$, observed power = 0.7). Bonferroni test revealed a decrease of RT in Post condition in the intervention group (Pre 1.14 ± 0.68 s vs. Post 0.53 ± 0.49 s; p = 0.0003) but not in the control group (Pre 1.33 ± 0.74 s vs. Post 1.44 ± 1.09 s; p = 0.7). There was no significant difference in RT between IG and CG in Pre (p = 0.4), however in Post condition the TA was lower for IG than CG (p = 0.003).