Immediate Effect of Grade IV Inferior Hip Joint Mobilization on Hip Abductor Torque: A Pilot Study

Joint mobilization and manipulation stimulate mechanoreceptors, which may influence the joint and surrounding muscles. The purpose of this pilot study was to determine the effect of grade IV inferior hip joint mobilization on hip abductor torque. Thirty healthy subjects were randomly assigned to a control group (grade I inferior hip joint mobilization) or an experimental group (grade IV inferior hip joint mobilization). Subjects performed a pre- and post-intervention test of five isometric repetitions on the Cybex Normö dynamometer; the average torque was determined for both pre- and post-intervention measurements. These data were analyzed using the independent samples t-test with the significance level set at P<0.05. The results showed a statistically significant difference between the two groups for an increase in hip abductor torque in the experimental group (P=0.03). The experimental group demonstrated a 17.35% increase in average torque whereas the control group demonstrated a 3.68% decrease in average torque. These findings are consistent with other studies demonstrating that the use of grade IV non-thrust mobilization improves strength immediately post-intervention in healthy individuals. The results of this pilot study provide physical therapists with further support for the utilization of manual therapy in conjunction with therapeutic exercise to enhance muscle strength.Key Words: Arthrokinetic Reflex, Hip Abductors, Hip Joint, Isometric Torque, Manual Therapy, Non-Thrust Mobilization, Muscle StrengthThe hip joint is a ball-and-socket joint composed of the acetabulum and femur^{1, 2}. The hip has strong muscular support, with the gluteus medius functioning as an important stabilizer with a main function of hip abduction. The anterior portion of this muscle also assists in the secondary function of internal rotation^{1, 3}, and the posterior portion of the gluteus medius assists in external rotation of the hip⁴. The gluteus medius functions to stabilize the hip at mid-stance of gait in the coronal plane; it provides lateral pelvic stabilization at terminal stance^{5 – 7}. Compromise in hip abductor muscle function can lead to a Trendelenburg gait pattern, described as a contralateral pelvic drop during stance phase. This may be compensated for by an upper body shift toward the involved side to maintain the center of gravity over the affected hip^{1, 8}; the contralateral quadratus lumborum then compensates by pulling the pelvis superiorly⁹ or the lumbar spine may compensate with ipsilateral lateral trunk flexion¹⁰.The hip abductors transfer forces from the lower extremity to the spine, explaining their frequent involvement in patients with spinal complaints^{9, 11 – 13}. Beckman and Buchanan¹⁴ demonstrated differences in the firing and strength of the hip abductor muscles in the presence of distal lower extremity involvement. Studies have shown that chronic ankle instability was associated with delayed firing of the hip abductor muscles^{9, 14, 15}. Further studies have correlated weakness with isokinetic testing of hip abductor and adductor muscles with ankle and foot injuries^{16, 17}. Friel et al¹⁸ demonstrated a correlation between chronic ankle sprains and ipsilateral hip abductor weakness. Powers¹⁹ described the influence of altered lower extremity kinematics on the patellofemoral joint by identifying two possible mechanisms leading to patellofemoral pain: femoral rotation and knee valgus. Increased femoral internal rotation—as might be caused by gluteus medius weakness—results in an increased Q-angle. Powers et al²⁰ demonstrated that this femoral rotation was the primary contributor to patellar tilts and displacements. Using pressure-sensitive film, these authors reported that 30° of femoral internal rotation significantly increased patellofemoral stress (force per unit) when the knee was flexed beyond 30°. Hip abductor weakness can also lead to valgus at the knee during dynamic tasks. Knee valgus also leads to an increase in the Q-angle, displacing the patella laterally with respect to the patellar groove of the distal femur. Excessive pronation is the end result of tibial abduction, as it compensates for femoral adduction.Joints influence motor unit activation and, therefore, muscle function. The capability of a joint to alter muscle function is mediated by the articular receptors; the articular receptors can inhibit or facilitate muscle tone²¹. In this paper, the term arthrokinetic reflex is used to refer to the tonic and phasic reflex neuromuscular activity, both facilitating and inhibiting, emanating primarily from the Type I and II articular mechanoreceptors^{21, 22}.The Guide to Physical Therapist Practice ²³ has defined mobilization and manipulation as synonymous terms describing a manual therapy technique comprising a continuum of skilled passive movements to the joints and/or related soft tissues that are applied at varying speeds and amplitudes, including a small-amplitude/high-velocity therapeutic movement. During mobilization/manipulation, the capsuloligamentous tissues of a joint are mechanically stretched²¹. One primary goal of mobilization is to improve extensibility of restricted capsuloligamentous tissue; secondarily, articular mechanoreceptor activation level is affected. Joint mobilization has been demonstrated to improve physiologic and accessory motions to hypomobile structures²⁴. This in turn causes an alteration in the articular mechanoreceptor resulting by way of arthrokinetic reflex activity in enhanced muscle strength^{21, 22}. These arthrokinetic reflex actions have been hypothesized to occur through the down-regulation of inhibitory input on motor unit activity^{21, 22}. Joint mobilization not only has an impact on the motor unit activity in muscles functioning over the joint, but it also has been shown to affect more remote muscles as well, including muscles on the contralateral side of the body²². Herzog et al²⁴ demonstrated that distracting cervical facet joints stimulated the articular mechanoreceptors exerting significant coordinated reflexogenic influences on the activity of the neck and limb musculature. Cibulka²⁵ performed mobilization to a dysfunctional sacroiliac joint and restored the normal length-tension relationship of the hamstrings, thereby increasing the main torque produced. Liebler et al²⁶ demonstrated a significant increase in lower trapezius strength with the utilization of grade IV spinal mobilization: The Cybex Norm® dynamometer recorded a 6% increase in lower trapezius strength in the experimental group as compared to a 0.2% increase in the control group. Cleland et al²⁷ similarly demonstrated improved lower trapezius strength in response to manipulative treatment of the lower thoracic spine (T6-12) using grade V thrust techniques; they reported a statistically significant increase (P<0.01) in peak strength of 14.5% for the experimental group versus 3.9% for the control group. Yerys et al²⁸ demonstrated a significant effect of grade IV mobilization on gluteus maximus strength; the experimental group demonstrated a 14% increase in strength as compared to a 4% increase in the control group. As in the present study, both Liebler et al²⁶ and Yerys et al²⁸ applied a grade IV non-thrust mobilization to the subjects in the experimental group and a grade I to those in the control group.The above studies highlight the role of the joint capsule and its reflexogenic influence on muscles. Failure to recognize the importance of these arthrokinetic reflex circuits may explain the difficulty in neuromuscular re-education and strengthening of muscle groups. This in turn leads to failure of an exercise regime to achieve the desired results with regard to improved muscle function²⁹. Many rehabilitation programs focus on strengthening exercises using resistance regimens; however, few focus on the actual quality and control of movement. Manual techniques may effectively be used in cases of muscle imbalances, which are a form of dysfunction. Bookhout²⁹ suggested that greater success in rehabilitation might be achieved through the use of manual techniques, either before or in conjunction with resistive exercises.Mobilizing a restricted joint may increase muscular strength by removing the reflexogenic inhibition emanating from the joint mechanoreceptors^{21, 22, 24, 26, 29}. For example, a mechanical hip joint disorder associated with ipsilateral adductor muscle contracture, inferior capsuloligamentous hypomobility, and gluteus medius weakness, especially of the posterior fibers³⁰, will theoretically impose inhibitory neural input on the gluteus medius while simultaneously imposing reflex facilitation on the adductor muscles each time that the hip abducts against its restrictive barrier of motion^{21, 22}. This may lead to further functional destabilization of the hip joint. Above we discussed the important role of the gluteus medius muscle not only at the hip but also in the entire lower extremity and in the spine. Considering this important role of the gluteus medius muscle and the information on arthrokinetic reflex circuits discussed above, the research hypothesis in this pilot study is that grade IV inferior hip joint mobilization performed at the end of abduction will result in an immediate increase in hip abductor torque when compared to a grade I inferior hip joint mobilization.