Comparison of Controlled Articular Rotations and Static Stretching on Hip Internal Rotation Range of Motion

Introduction

Range of motion (ROM) deficits of the hip can contribute to musculoskeletal dysfunctions throughout the body. Lack of adequate hip internal rotation (IR) ROM has been shown to increase the likelihood of anterior cruciate ligament injuries, fall risk, low back pain, and even upper extremity injuries in baseball pitchers.1–7 One potential cause of hip IR ROM deficits may be limited flexibility of muscles that cross the affected joint, specifically the hip external rotator (ER) muscles.8 These muscles include the gluteus maximus, piriformis when the hip is positioned in less than 90 degrees of hip flexion,9 superior gemellus, inferior gemellus, obturator internus, obturator externus, and quadratus femoris. The iliopsoas, while primarily considered to be a hip flexor, also plays a small role in hip ER due to its insertion point on the lesser trochanter of the femur.10

Stretching is a common intervention utilized by rehabilitation professionals to improve ROM when deficits are thought to result from impaired muscle flexibility.11,12 Two types of stretching commonly used in clinical practice include static stretching and dynamic stretching. Static stretching involves maintaining a lengthened position of the target muscle to a point where a stretch sensation is reached.13 Numerous studies support the effectiveness of static stretching for increasing ROM.12–14 Dynamic stretching involves repeated, controlled active movements through the full ROM of a joint via contraction of the agonist muscles. This allows elongation of the antagonist (target) muscles without holding the stretch in the end-range position.14 Dynamic stretching has also been shown to be effective in increasing joint ROM.15,16 Both static and dynamic stretching have been shown to improve hip IR ROM.8,15,17,18

Controlled Articular Rotations (CARs), a form of dynamic stretching, has gained popularity among fitness and rehabilitation professionals as a treatment strategy for improving joint ROM. The concept of CARs is promoted through the Functional Range Systems^TM approach, created by Dr. Andreo Spina. This form of stretching utilizes slow, controlled, active rotational joint motions performed at the outer limits of a joint’s available ROM. The goal is to move the joint through its full, pain-free ROM in all directions, and through all planes of motion. While typical dynamic stretching involves the use of swinging the limb or momentum to create a stretch, CARs utilize controlled, intentional movements. Despite its growing popularity, empirical evidence supporting the effectiveness of CARs is limited. To date, no studies have examined the effect of CARs on joint ROM. Therefore, the purpose of this study was to compare the acute effects of static stretching and CARs on hip IR ROM in a young adult population with limited hip mobility. We hypothesized there would be no significant difference in hip IR ROM between the two stretching interventions.

Methods

Results

Shapiro-Wilk and Mauchly’s tests were non-significant (p>0.05) indicating that assumptions of normality and sphericity were not violated. Means and standard deviations for hip IR ROM at pre-intervention (T1), immediate post-intervention (T2), and 15-minutes post-intervention (T3) are presented in Table 2.

A 2x3 repeated measures ANOVA revealed no significant group x time interaction (F(2,52)=1.69, p=0.19) and no significant main effect of group (F(2,52)=0.393, p=0.54). There was, however, a significant main effect of time on hip IR ROM (F(2,52)=54.06, p<0.001, η²p=0.68), demonstrating a large effect size. Post-hoc pairwise comparisons indicated that hip IR ROM increased significantly from pre-intervention to immediately post-intervention (p<0.001) and remained elevated at 15-minutes post-intervention (p<0.001). No significant difference was observed between the two post-intervention measurements (p=1.00).

Discussion

The purpose of this study was to compare the acute effects of SS and CARs on hip IR ROM in young adults with limited hip mobility. It was hypothesized that no significant difference in hip IR ROM would be observed between the two stretching interventions. The results of the study support this hypothesis. While significant increases in ROM were seen over time, the improvements occurred regardless of group assignment. These findings suggest that SS and CARS are similarly effective in producing short-term improvements in hip IR ROM.

These findings are consistent with previous literature demonstrating that both static and dynamic stretching interventions can improve joint ROM. SS has been extensively studied and shown to be effective for increasing joint mobility.14,26–28 Recent meta-analyses by Shah et al28 and Takeuchi et al12 further support the use of acute SS for improving ROM. Dynamic stretching (DS) has also been shown to be effective for enhancing joint mobility.13,15,29–31 However, studies comparing SS and DS have reported conflicting results regarding their relative effectiveness. Several investigations have found no significant differences between the two approaches,29,32–35 whereas others have reported greater improvements with DS.36,37 In contrast, a meta-analysis by Konrad et al concluded that SS may produce greater ROM gains than DS.27 While no previous studies have specifically examined the effect of CARs on joint ROM, CARs can be conceptually categorized as a form of dynamic stretching. As such, the lack of difference observed between groups in the present study is consistent with the mixed findings reported in the literature comparing SS and DS interventions.

The retention of hip IR ROM gains at 15-minutes post-intervention are also consistent with previous studies. Hubley et al demonstrated that static stretching and stationary cycling were equally effective for increasing hip flexion and extension ROM and retaining the ROM gains for a 15-minute period of inactivity.38 Iwata et al reported increased passive knee extension ROM following DS of the hamstrings with ROM gains maintained for 90 minutes post-intervention.30

The absence of a difference between groups may be explained by shared underlying mechanisms through which both SS and CARs influence joint ROM. Shah et al28 and Takeuchi et al12 suggest that acute improvements in ROM following SS are primarily attributed to an increase in stretch tolerance, as well as reduced muscle-tendon unit (MTU) mechanical properties (e.g. decreased stiffness of muscle and connective tissues). Iwata et al proposed similar mechanisms for improved joint ROM when examining the effects of DS on hamstring flexibility and knee extension ROM.30 The sustained gains in hip IR ROM at 15-minutes post-intervention are potentially due to these same mechanisms.30,39

While the mechanisms appear similar, the physiological basis for the reduced MTU stiffness may differ between stretching interventions. During SS, reductions in MTU stiffness are thought to be related to the viscoelastic stress relaxation that occurs when the muscle is held in a static, lengthened position.40–42 In contrast, reductions in MTU stiffness during DS are likely associated with increasing muscle temperature that occurs as a result of repeated, active contractions. Increased temperature may reduce the viscous resistance of the muscles, thereby enhancing tissue extensibility.43–⁴⁵ Given that CARs involve repeated, controlled active movement at end-range, it is plausible that they produce similar increases in stretch tolerance and temperature-mediated reductions in MTU stiffness, which may explain the comparable improvements in ROM observed between groups.

The current study is not without limitations. First, the study examined only a single session of the stretching interventions. It is therefore unclear whether similar results would be observed following repeated or long-term application of the stretching protocols. Second, while the study focused on participants with limited hip IR ROM, the sample consisted of young adults with a mean age of 23.5 years. This may limit generalizability to other populations, including children, adolescents, and older adults. Finally, although efforts were made to standardize performance of the stretching interventions, variation in participant effort, as well as variation in movement quality during the CARs may have influenced the results.

Future research should investigate the long-term application of CARs on joint ROM and explore the impact of CARs on functional performance measures. Additionally, studies should compare CARs to other mobility interventions across different clinical populations, such as individuals with femoroacetabular impingement or low back pain. This would help to further clarify the role of CARs in rehabilitation and performance settings.

In conclusion, the findings of this study have several important clinical implications. Both SS and CARs resulted in significant short-term improvements in hip IR ROM. These results suggest that clinicians may select either intervention when the goal is to acutely increase hip IR, allowing flexibility in exercise prescription based on patient preference or tolerance. Additionally, CARs may provide an alternative to traditional SS for individuals who prefer active movement-based interventions. Given that improvements were maintained at 15-minutes post-intervention, either stretching approach can be utilized shortly before activities requiring greater hip mobility.