Head repositioning errors in normal student volunteers: a possible tool to assess the neck's neuromuscular system

BACKGROUND: A challenge for practitioners using spinal manipulation is identifying when an intervention is required. It has been recognized that joint pain can interfere with the ability to position body parts accurately and that the recent history of muscle contraction can play a part in that interference. In this study, we tested whether repositioning errors could be induced in a normal population by contraction or shortening of the neck muscles. METHODS: In the experimental protocol, volunteers free of neck problems first found a comfortable neutral head posture with eyes closed. They deconditioned their cervical muscles by moving their heads 5 times in either flexion/extension or lateral flexion and then attempted to return to the same starting position. Two conditioning sequences were interspersed within the task: hold the head in an extended or laterally flexed position for 10 seconds; or hold a 70% maximum voluntary contraction in the same position for 10 seconds. A computer-interfaced electrogoniometer was used to measure head position while a force transducer coupled to an auditory alarm signaled the force of isometric contraction. The difference between the initial and final head orientation was calculated in 3 orthogonal planes. Analysis of variance (1-way ANOVA) with a blocking factor (participants) was used to detect differences in proprioceptive error among the conditioning sequences while controlling for variation between participants. RESULTS: Forty-eight chiropractic students participated: 36 males and 12 females, aged 28.2 ± 4.8 yrs. During the neck extension test, actively contracting the posterior neck muscles evoked an undershoot of the target position by 2.1° (p <0.001). No differences in repositioning were found during the lateral flexion test. CONCLUSION: The results suggest that the recent history of cervical paraspinal muscle contraction can influence head repositioning in flexion/extension. To our knowledge this is the first time that muscle mechanical history has been shown to influence proprioceptive accuracy in the necks of humans. This finding may be used to elucidate the mechanism behind repositioning errors seen in people with neck pain and could guide development of a clinical test for involvement of paraspinal muscles in cervical pain and dysfunction

Relationship between Biomechanical Characteristics of Spinal Manipulation and Neural Responses in an Animal Model: Effect of Linear Control of Thrust Displacement versus Force, Thrust Amplitude, Thrust Duration, and Thrust Rate

High velocity low amplitude spinal manipulation (HVLA-SM) is used frequently to treat musculoskeletal complaints. Little is known about the intervention’s biomechanical characteristics that determine its clinical benefit. Using an animal preparation, we determined how neural activity from lumbar muscle spindles during a lumbar HVLA-SM is affected by the type of thrust control and by the thrust's amplitude, duration, and rate. A mechanical device was used to apply a linear increase in thrust displacement or force and to control thrust duration. Under displacement control, neural responses during the HVLA-SM increased in a fashion graded with thrust amplitude. Under force control neural responses were similar regardless of the thrust amplitude. Decreasing thrust durations at all thrust amplitudes except the smallest thrust displacement had an overall significant effect on increasing muscle spindle activity during the HVLA-SMs. Under force control, spindle responses specifically and significantly increased between thrust durations of 75 and 150 ms suggesting the presence of a threshold value. Thrust velocities greater than 20–30 mm/s and thrust rates greater than 300 N/s tended to maximize the spindle responses. This study provides a basis for considering biomechanical characteristics of an HVLA-SM that should be measured and reported in clinical efficacy studies to help define effective clinical dosages

Position Sensitivity of Feline Paraspinal Muscle Spindles to Vertebral Movement in the Lumbar Spine

Muscle spindles contribute to sensorimotor control by supplying feedback regarding muscle length and consequently information about joint position. While substantial study has been devoted to determining the position sensitivity of spindles in limb muscles, there appears to be no data on their sensitivity in the low back. We determined the relationship between lumbar paraspinal muscle spindle discharge and paraspinal muscle lengthening estimated from controlled cranialward movement of the L6 vertebra in anesthetized cats. Ramp (0.4 mm/s) and hold displacements (0.2, 0.4, 0.6, 0.8, and 1.2 mm for 2.5 s) were applied at the L6 spinous process. Position sensitivity was defined as the slope of the relationship between the estimated increase in muscle length and mean instantaneous frequency at each length. To enable comparisons with appendicular muscle spindles where joint angle was measured, we also calculated sensitivity in terms of the L6 and L7 intervertebral flexion angle (IVA). This angle was estimated from measurements of facet joint capsule strain (FJC) based on a previously established relationship between IVA and FJC strain in the cat lumbar vertebral column during lumbar flexion. Single-unit recordings were obtained from 12 muscle spindle afferents. Longissimus and multifidus muscles contained the receptive field of 10 and 2 afferents, respectively. Mean position sensitivity was 16.3 imp·s−1·mm−1 [10.6–22.1, 95% confidence interval (CI), P < 0.001]. Mean angular sensitivity was 5.2 imp·s−1·°−1 (2.6–8.0, P < 0.003). These slope estimates were more than 3.5 times greater compared with appendicular muscle spindles, and their CIs did not contain previous slope estimates for the sensitivity of appendicular spindles from the literature. Potential reasons for and the significance of the apparently high position sensitivity in the lumbar spine are discussed

Neural Response During a Mechanically Assisted Spinal Manipulation in an Animal Model: A Pilot Study

Introduction: Mechanoreceptor stimulation is theorized to contribute to the therapeutic efficacy of spinal manipulation. Use of mechanically-assisted spinal manipulation (MA-SM) devices is increasing among manual therapy clinicians worldwide. The purpose of this pilot study is to determine the feasibility of recording in vivo muscle spindle responses during a MA-SM in an intervertebral fixated animal model.Methods: Intervertebral fixation was created by inserting facet screws through the left L5-6 and L6-7 facet joints of a cat spine. Three L6muscle spindle afferents with receptive fields in back muscles were isolated. Recordings were made during MA-SM thrusts delivered to the L7 spinous process using an instrumented Activator IV clinical device.&nbsp;Results: Nine MA-SM thrusts were delivered with peak forces ranging from 68-122N and with thrust durations of less than 5ms. High frequency muscle spindle discharge occurred during MA-SM. Following the MA-SM, muscle spindle responses included returning to pre-manipulation levels, slightly decreasing for a short window of time, and greatly decreasing for more than 40s.Conclusion: This study demonstrates that recording in vivo muscle spindle response using clinical MA-SM devices in an animal model is feasible.&nbsp; Extremely short duration MA-SM thrusts (&lt;5ms) can have an immediate and/or a prolonged (&gt; 40s) effect on muscle spindle discharge. Greater peak forces during MA-SM thrusts may not necessarily yield greater muscle spindle responses. Determining peripheral response during and following spinal manipulation may be an important step in optimizing its’ clinical efficacy. Future studies may investigate the effect of thrust dosage and magnitude.Pubmed link: https://www.ncbi.nlm.nih.gov/pubmed/26618202 </p