Jacek Cholewicki

Stabilizing function of trunk flexor-extensor muscles around a neutral spine posture

Study Design. This study examined the coactivation of trunk flexor and extensor muscles in healthy individuals. The experimental electromyographic data and the theoretical calculations were analyzed in the context of mechanical stability of the lumbar spine. Objectives. To test a set of hypotheses pertaining to healthy individuals: 1) that the trunk flexor‐extensor muscle coactivation is present around a neutral spine posture, 2) that the coactivation is increased when the subject carries a load; and 3) that the coactivation provides the needed mechanical stability to the lumbar spine. Summary of Background Data. Theoretically, antagonistic trunk muscle coactivation is necessary to provide mechanical stability to the human lumbar spine around its neutral posture. No experimental evidence exists, however, to support this hypothesis. Methods. Ten individuals executed slow trunk flexion‐extension tasks, while six muscles on the right side were monitored with surface electromyography: external oblique, internal oblique, rectus abdominis, multifidus, lumbar erector spinae, and thoracic erector spinae. Simple, but realistic, calculations of spine stability also were performed and compared with experimental results. Results. Average antagonistic flexor‐extensor muscle coactivation levels around the neutral spine posture as detected with electromyography were 1.7 ± 0.8% of maximum voluntary contraction for no external load trials and 2.9 ± 1.4% of maximum voluntary contraction for the trials with added 32‐kg mass to the torso. The inverted pendulum model based on static moment equilibrium criteria predicted no antagonistic coactivation. The same model based on the mechanical stability criteria predicted 1.0% of maximum voluntary contraction coactivation of flexors and extensors with zero load and 3.1% of maximum voluntary contraction with a 32‐kg mass. The stability model also was run with zero passive spine stiffness to simulate an injury. Under such conditions, the model predicted 3.4% and 5.5% of maximum voluntary contraction of antagonistic muscle coactivation for no extra load and the added 32 kg, respectively. Conclusions. This study demonstrated that antagonistic trunk flexor‐extensor muscle coactivation was present around the neutral spine posture in healthy individuals. This coactivation increased with added mass to the torso. Using a biomechanical model, the coactivation was explained entirely on the basis of the need for the neuromuscular system to provide the mechanical stability to the lumbar spine.

Impaired postural control of the lumbar spine is associated with delayed muscle response times in patients with chronic idiopathic low back pain

Study Design. Balance performance in unstable sitting and trunk muscle response to quick force release were measured in 16 patients with chronic low back pain and 14 matched healthy control subjects. Objectives. To determine whether patients with low back pain will exhibit poorer postural control, which will be associated with longer average muscle response times. Summary of Background Data. Larger postural sway during standing and delayed trunk muscle response times for patients with low back pain have been reported in several independent studies. Methods. Unstable sitting test was accomplished by attaching different sized hemispheres to the bottom of a seat. Subjects performed trials with eyes open and closed while the displacements of the center of pressure were measured with a force plate underneath the seat. Response to a quick force release was recorded from 12 major trunk muscles with surface electromyography. Subjects performed isometric trunk exertions in a semi-seated position when the resisted force was suddenly released with an electromagnet. Average muscle response times and balance performance were correlated using a linear regression analysis. Results. Patients with low back pain demonstrated poorer balance performance than healthy control volunteers, especially at the most difficult levels. Patients also had delayed muscle response times to quick force release. Average muscle onset times together with age and weight correlated significantly with balance performance with closed eyes (R2 = 0.46), but not with eyes opened (R2 = 0.18). Conclusions. Patients with chronic low back pain demonstrated poorer postural control of the lumbar spine and longer trunk muscle response times than healthy control volunteers. Correlation between these two phenomena suggests a common underlying pathology in the lumbar spine.

Muscle response pattern to sudden trunk loading in healthy individuals and in patients with chronic low back pain.

Study Design. A quick-release method in four directions of isometric trunk exertions was used to study the muscle response patterns in 17 patients with chronic low back pain and 17 matched control subjects. Objectives. It was hypothesized that patients with low back pain would react to sudden load release with a delayed muscle response and would exhibit altered muscle recruitment patterns. Summary of Background Data. A delay in erector spinae reaction time after sudden loading has been observed in patients with low back pain. Muscle recruitment and timing pattern play an important role in maintaining lumbar spine stability. Methods. Subjects were placed in a semiseated position in an apparatus that provided stable fixation of the pelvis. They exerted isometric contractions in trunk flexion, extension, and lateral bending. Each subject performed three trials at two constant force levels. The resisted force was suddenly released with an electromagnet and electromyogram signals from 12 trunk muscles were recorded. The time delay between the magnet release and the shut-off or switch-on of muscle activity (reaction time) was compared between two groups of subjects using two-factor analysis of variance. Results. The number of reacting muscles and reaction times averaged over all trials and directions showed the following results: For healthy control subjects a shut-off of agonistic muscles (with a reaction time of 53 msec) occurred before the switch-on of antagonistic muscles (with a reaction time of 70 msec). Patients exhibited a pattern of co-contraction, with agonists remaining active (3.4 out of 6 muscles switched off) while antagonists switched on (5.3 out of 6 muscles). Patients also had longer muscle reaction times for muscles shutting off (70 msec) and switching on (83 msec) and furthermore, their individual muscle reaction times showed greater variability. Conclusions. Patients with low back pain, in contrast to healthy control subjects, demonstrated a significantly different muscle response pattern in response to sudden load release. These differences may either constitute a predisposing factor to low back injuries or a compensation mechanism to stabilize the lumbar spine.

Deficits in Neuromuscular Control of the Trunk Predict Knee Injury Risk A Prospective Biomechanical-Epidemiologic Study

Background Female athletes are at significantly greater risk of anterior cruciate ligament (ACL) injury than male athletes in the same high-risk sports. Decreased trunk (core) neuromuscular control may compromise dynamic knee stability. Hypotheses (1) Increased trunk displacement after sudden force release would be associated with increased knee injury risk; (2) coronal (lateral), not sagittal, plane displacement would be the strongest predictor of knee ligament injury; (3) logistic regression of factors related to core stability would accurately predict knee, ligament, and ACL injury risk; and (4) the predictive value of these models would differ between genders. Study Design Cohort study (prognosis); Level of evidence, 2. Methods In this study, 277 collegiate athletes (140 female and 137 male) were prospectively tested for trunk displacement after a sudden force release. Analysis of variance and multivariate logistic regression identified predictors of risk in athletes who sustained knee injury. Results Twenty-five athletes (11 female and 14 male) sustained knee injuries over a 3-year period. Trunk displacement was greater in athletes with knee, ligament, and ACL injuries than in uninjured athletes (P < .05). Lateral displacement was the strongest predictor of ligament injury (P = .009). A logistic regression model, consisting of trunk displacements, proprioception, and history of low back pain, predicted knee ligament injury with 91% sensitivity and 68% specificity (P = .001). This model predicted knee, ligament, and ACL injury risk in female athletes with 84%, 89%, and 91% accuracy, but only history of low back pain was a significant predictor of knee ligament injury risk in male athletes. Conclusions Factors related to core stability predicted risk of athletic knee, ligament, and ACL injuries with high sensitivity and moderate specificity in female, but not male, athletes.

Trunk muscle activation in low-back pain patients, an analysis of the literature

This paper provides an analysis of the literature on trunk muscle recruitment in low-back pain patients. Two models proposed in the literature, the pain-spasm-pain model and the pain adaptation model, yield conflicting predictions on how low- back pain would affect trunk muscle recruitment in various activities. The two models are outlined and evidence for the two from neurophsysiological studies is reviewed. Subsequently, specific predictions with respect to changes in activation of the lumbar extensor musculature are derived from both models. These predictions are compared to the results from 30 clinical studies and three induced pain studies retrieved in a comprehensive literature search. Neither of the two models is unequivocally supported by the literature. These data and further data on timing of muscle activity and load sharing between muscles suggest an alternative model to explain the alterations of trunk muscle recruitment due to low-back pain. It is proposed that motor control changes in patients are functional in that they enhance spinal stability.

Trunk Muscle Recruitment Patterns in Patients With Low Back Pain Enhance the Stability of the Lumbar Spine

Study Design. A comparative study of trunk muscle recruitment patterns in healthy control subjects and patients with chronic low back pain was conducted. Objective. To assess trunk muscle recruitment in patients with low back pain. Summary of Background Data. Conflicting evidence has been reported on the level and pattern of trunk muscle recruitment in patients with low back pain. The disparities can be explained partly by methodologic differences. It was hypothesized that trunk muscle recruitment patterns may be altered in patients with low back pain to compensate for reduced spinal stability. Methods. For this study, 16 patients with low back pain and 16 matched control subjects performed slow trunk motions about the neutral posture and isometric ramp contractions while seated upright. Ratios of electromyographic amplitudes and estimated moment contributions of antagonist over agonist muscles and of segmentally inserting muscles over muscles inserting on the thorax and pelvis only were calculated. In addition, model simulations were performed to assess the effect of changes in muscle recruitment on spinal stability. Results. The ratios of antagonist over agonist, and of lumbar over thoracic erector spinae electromyographic amplitude and estimated moment contributions were greater in the patients than in the control subjects. The simulation model predicted that these changes would effectively increase spinal stability. Conclusions. Trunk muscle recruitment patterns in patients with low back pain are different from those in healthy control subjects. The differences are likely to be functional with respect to enhancement of spinal stability in the patients.

Coordination of muscle activity to assure stability of the lumbar spine

The intention of this paper is to introduce some of the issues surrounding the role of muscles to ensure spine stability for discussion -- it is not intended to provide an exhaustive review and integration of the relevant literature. The collection of works synthesized here point to the notion that stability results from highly coordinated muscle activation patterns involving many muscles, and that the recruitment patterns must continually change, depending on the task. This has implications on both the prevention of instability and clinical interventions with patients susceptible to sustaining unstable events.

Mechanical Properties of the Human Cervical Spine as Shown by Three-dimensional Load–displacement Curves

Study Design. The mechanical properties of multilevel human cervical spines were investigated by applying pure rotational moments to each specimen and measuring multidirectional intervertebral motions. Objectives. To document intervertebral main and coupled motions of the cervical spine in the form of load–displacement curves. Summary of Background Data. Although a number of in vivo and in vitro studies have attempted to delineate normal movement patterns of the cervical spine, none has explored the complexity of the whole cervical spine as a three-dimensional structure. Methods. Sixteen human cadaveric specimens (C0–C7) were used for this study. Pure rotational moments of flexion–extension, bilateral axial torque, and bilateral lateral bending were applied using a specially designed loading fixture. The resulting intervertebral motions were recorded using stereophotogrammetry and depicted as a series of load–displacement curves. Results. The resulting load–displacement curves were found to be nonlinear, and both rotation and translation motions were coupled with main motions. With flexion–extension moment loading, the greatest degree of flexion occurred at C1–C2 (12.3°), whereas the greatest degree of extension was observed at C0–C1 (20.2°). With axial moment loading, rotation at C1–C2 was the largest recorded (56.7°). With lateral bending moments, the average range of motion for all vertebral levels was 7.9°. Conclusions. The findings of the present study are relevant to the clinical practice of examining motions of the cervical spine in three dimensions and to the understanding of spinal trauma and degenerative diseases.

The Effects of Core Proprioception on Knee Injury A Prospective Biomechanical-Epidemiological Study

Background In sports involving pivoting and landing, female athletes suffer knee injury at a greater rate than male athletes. Hypotheses Proprioceptive deficits in control of the bodys core may affect dynamic stability of the knee. Female, but not male, athletes who suffered a knee injury during a 3-year follow-up period would demonstrate decreased core proprioception at baseline testing as compared with uninjured athletes. Study Design Cohort study (prognosis); Level of evidence, 2. Methods Study subjects were 277 collegiate athletes (140 female, 137 male) who were prospectively tested for core proprioception by active and passive proprioceptive repositioning. Athletes were monitored for injury for 3 years. An ANOVA and multivariate logistic regression were used to test whether core proprioception was related to knee injuries in athletes. Results Twenty-five athletes sustained knee injuries (11 women, 14 men). Deficits in active proprioceptive repositioning were observed in women with knee injuries (2.2°) and ligament/meniscal injuries (2.4°) compared with uninjured women (1.5°, P ≤ .05). There were no differences in average active proprioceptive repositioning error between injured men and uninjured men (P ≥ .05). Uninjured women demonstrated significantly less average error in active proprioceptive repositioning than uninjured men (1.5° vs 1.7°, P ≤ .05). For each degree increase in average active proprioceptive repositioning error, a 2.9-fold increase in the odds ratio of knee injury was observed, and a 3.3-fold increase in odds ratio of ligament/meniscal injury was observed (P ≤ .01). Active proprioceptive repositioning predicted knee injury status with 90% sensitivity and 56% specificity in female athletes. Conclusions Impaired core proprioception, measured by active proprioceptive repositioning of the trunk, predicted knee injury risk in female, but not male, athletes.

Delayed trunk muscle reflex responses increase the risk of low back injuries

Study Design. Prospective observational study with a 2- to 3-year follow-up. Objectives. To determine whether delayed muscle reflex response to sudden trunk loading is a result of or a risk factor for sustaining a low back injury (LBI). Summary of Background Data. Differences in motor control have been identified in individuals with chronic low back pain and in athletes with a history of LBI when compared with controls. However, it is not known whether these changes are a risk for or a result of LBI. Methods. Muscle reflex latencies in response to a quick force release in trunk flexion, extension, and lateral bending were measured in 303 college athletes. Information was also obtained regarding their personal data, athletic experience, and history of LBI. The data were entered into a binary logistic regression model to identify the predictors of future LBI. Results. A total of 292 athletes were used for the final analysis (148 females and 144 males). During the follow-up period, 31 (11%) athletes sustained an LBI. The regression model, consisting of history of LBI, body weight, and the latency of muscles shutting off during flexion and lateral bending load releases, predicted correctly 74% of LBI outcomes. The odds of sustaining LBI increased 2.8-fold when a history of LBI was present and increased by 3% with each millisecond of abdominal muscle shut-off latency. On average, this latency was 14 milliseconds longer for athletes who sustained LBI in comparison to athletes who did not sustain LBI (77 [36] vs. 63 [31]). There were no significant changes in any of the muscle response latencies on retest following the injury. Conclusions. The delayed muscle reflex response significantly increases the odds of sustaining an LBI. These delayed latencies appear to be a preexisting risk factor and not the effect of an LBI.