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Biomedical subjects

Stuart M McGill

Publications and source records attributed to Stuart M McGill.

At least 19 recordsLinked to original sources
PubMed2007

Quantification of lumbar stability by using 2 different abdominal activation strategies.

OBJECTIVE: To determine whether the abdominal hollowing technique is more effective for lumbar spine stabilization than a full abdominal muscle cocontraction. DESIGN: Within-subject, repeated-measures analysis of variance was used to examine the effect of combining each of 4 loading conditions with either the hollow or brace condition on the dependent variables of stability and compression. A simulation was also conducted to assess the outcome of a person activating just the transversus abdominis during the hollow. SETTING: Laboratory. PARTICIPANTS: Eight healthy men (age range, 20-33y). INTERVENTIONS: Electromyography and spine kinematics were recorded during an abdominal brace and a hollow while supporting either a bilateral or asymmetric weight in the hands. MAIN OUTCOME MEASURES: Spine stability index and lumbar compression were calculated. RESULTS: In the simulation "ideal case," the brace technique improved stability by 32%, with a 15% increase in lumbar compression. The transversus abdominis contributed .14% of stability to the brace pattern with a less than 0.1% decrease in compression. CONCLUSIONS: Whatever the benefit underlying low-load transversus abdominis activation training, it is unlikely to be mechanical. There seems to be no mechanical rationale for using an abdominal hollow, or the transversus abdominis, to enhance stability. Bracing creates patterns that better enhance stability.

Abdominal Muscles↗
PubMed2006

Effects of abdominal stabilization maneuvers on the control of spine motion and stability against sudden trunk perturbations.

Much discussion exists about which is the most effective technique to improve spine stability. The purpose of this study was to evaluate the effectiveness of abdominal bracing and abdominal hollowing maneuvers to control spine motion and stability against rapid perturbations. Eleven healthy males were posteriorly loaded in different experimental conditions: resting with no knowledge of the perturbation timing; performing each of the stabilization maneuvers at 10%, 15% and 20% of internal oblique maximum voluntary contraction with no knowledge of the perturbation timing; and naturally coactivating the trunk muscles when perturbation timing was known. An EMG biofeedback system was used to control the pattern and intensity of abdominal coactivation. The muscular preactivation of seven trunk muscles (bilaterally registered), the applied force, and the torso muscular and kinematic responses to loading were measured; and the spine stability and compression were modeled. The hollowing maneuver was not effective for reducing the kinematic response to sudden perturbation. On the contrary, the bracing maneuver fostered torso cocontraction, reduced lumbar displacement, and increased trunk stability, but at the cost of increasing spinal compression. When the timing of the perturbation was known, the participants were able to stabilize the trunk while imposing smaller spine compressive loads.

Abdomen↗
PubMed2006

Pushing and pulling: personal mechanics influence spine loads.

This study assessed several mechanical issues related to low back loading during pushing and/or pulling tasks. Nine male participants performed two-handed pushing and pulling tasks at two handle heights with three loads, using a cable pulley system. Four of these men were professional firefighters trained in performing pushing and pulling tasks while the other five were graduate students who lacked manual work experience. The more experienced firefighters produced less spinal compression and shearing forces when compared to the less experienced students under the same conditions. The firefighters were able to create less muscle activation as compared to the students, which indicated a more efficient technique. The main contributing factors to the forces produced on the low back were the quantity of the load being pushed or pulled, handle height, experience level and the technique of the participant. Thus, attempts to set load limits for pushing and pulling tasks are difficult, since technique has such a large influence on back loading. In order to create safer working environments, education on proper pushing and pulling techniques is very important--more important than the physical variables in many cases.

Biomechanical Phenomena↗
PubMed2006

Effects of abdominal muscle coactivation on the externally preloaded trunk: variations in motor control and its effect on spine stability.

STUDY DESIGN: A repeated measures biomechanical analysis of the effects of abdominal bracing in preparation for a quick release of the loaded trunk. OBJECTIVES: To quantify the ability of individuals to abdominally brace the externally loaded trunk, and assess their success in achieving and enhancing appropriate spine stability. SUMMARY OF BACKGROUND DATA: Spine stability requires trunk muscle coactivation, which demands motor control skill that differs across people and situations. The quick release protocol may offer insight into the motor control scheme and subsequent effect on spine stability. METHODS: There were 10 individuals who sat, torso upright, in an apparatus designed to foster a neutral spine position. They were instructed to support a posteriorly directed load to the trunk in either their naturally chosen manner, or by activating the abdominal muscles to 10%, 20%, or 30% of maximum ability. The externally applied load was then quickly released, thereby unloading the participant. Muscle pre-activation patterns, spine stability, and kinematic measures of trunk stiffness were quantified. RESULTS: Participants were able to stabilize their spine effectively by supporting the load in a naturally selected manner. Conscious, voluntary overdriving of this natural pattern often resulted in unbalanced muscular activation schemes and corresponding decreases in stability levels. CONCLUSIONS: Individuals in an externally loaded state appear to select a natural muscular activation pattern appropriate to maintain spine stability sufficiently. Conscious adjustments in individual muscles around this natural level may actually decrease the stability margin of safety.

Abdominal Muscles↗
PubMed2006

Effects of different levels of torso coactivation on trunk muscular and kinematic responses to posteriorly applied sudden loads.

BACKGROUND: Studies examining rapid spine loading have documented the influence of steady-state trunk preloads, and the resulting levels of trunk muscle preactivation, on the control of spine stability. However, the effects of different levels of muscle coactivation, and resulting spine loads, on the response to a perturbation of the externally unloaded trunk are unclear. METHODS: Fourteen male subjects coactivated the abdominal muscles at four different levels (approximately 0%, 10%, 20% and 30% of the maximal voluntary contraction) monitored by an electromyography biofeedback system while semi-seated in a neutral lumbar spine position. They were loaded posteriorly in two directions (0 degrees and 30 degrees from the sagittal plane) and with two different loads (6.80 and 9.07 kg). Force perturbation, spine displacement and electromyography activity were measured, and torso compression and stability were modeled. FINDINGS: Abdominal coactivation significantly increased spine stability and reduced the movement of the lumbar spine after perturbation, but at the cost of increasing spinal compression. Preactivation also reduced the frequency and magnitude, and delayed the onset of muscle reactions, mainly for the back muscles and the internal oblique. The higher magnitude load and the load applied in an oblique direction both showed more potentially hazardous effects on the trunk. INTERPRETATION: Torso coactivation increases spinal stiffness and stability and reduces the necessity for sophisticated muscle responses to perturbation. Although further investigation is needed, it appears there is an asymptotic function between coactivation and both stiffness and stability. There also appears to be more hazard when buttressing twisting components of a sudden load compared to sagittal components. Patients with trunk instability and intolerance to spine compression may benefit from low to moderate levels of coactivation.

Abdominal Muscles↗
PubMed2005

The influence of static axial torque in combined loading on intervertebral joint failure mechanics using a porcine model.

BACKGROUND: The spine is routinely subjected to repetitive combined loading, including axial torque. Repetitive flexion-extension motions with low magnitude compressive forces have been shown to be an effective mechanism for causing disc herniations. The addition of axial torque to the efficacy of failure mechanisms, such as disc herniation, need to be quantified. The purpose of this study was to determine the role of static axial torque on the failure mechanics of the intervertebral joint under repetitive combined loading. METHODS: Repetitive flexion-extension motions combined with 1472 N of compression were applied to two groups of nine porcine motion segments. Five Nm of axial torque was applied to one group. Load-displacement behaviour was quantified, and planar radiography was used to document tracking of the nucleus pulposus and to identify fractures. FINDINGS: The occurrence of facet fractures was found to be higher (P=0.028) in the axial torque group (7/9), compared to the no axial torque group (2/9). More hysteresis energy was lost up to 3000 cycles of loading in the axial torque group (P<0.014). The flexion-extension cycle stiffness was not different between the two groups until 4000 cycles of loading, after which the axial torque group stiffness increased (P=0.016). The percentage of specimens that herniated after 3000 cycles of loading was significantly larger (P=0.049) for the axial torque group (71%) compared to the no axial torque group (29%). INTERPRETATION: Small magnitudes of static axial torque alter the failure mechanics of the intervertebral disc and vertebrae in combined loading situations. Axial torque appears to accelerate the susceptibility for injury to the intervertebral joint complex. This suggests tasks involving axial torque with other types of loading, apart from axial twist motion, should be monitored to assess exposure and injury risk.

Animals↗
PubMed2005

Preliminary development of a clinical prediction rule for determining which patients with low back pain will respond to a stabilization exercise program.

OBJECTIVE: To develop a clinical prediction rule to predict treatment response to a stabilization exercise program for patients with low back pain (LBP). DESIGN: A prospective, cohort study of patients with nonradicular LBP referred to physical therapy (PT). SETTING: Outpatient PT clinics. PARTICIPANTS: Fifty-four patients with nonradicular LBP. INTERVENTION: A standardized stabilization exercise program. MAIN OUTCOME MEASURE: Treatment response (success or failure) was categorized based on changes in the Oswestry Disability Questionnaire scores after 8 weeks. RESULTS: Eighteen subjects were categorized as treatment successes, 15 as treatment failures, and 21 as somewhat improved. After using regression analyses to determine the association between standardized examination variables and treatment response status, preliminary clinical prediction rules were developed for predicting success (positive likelihood ratio [LR], 4.0) and failure (negative LR, .18). The most important variables were age, straight-leg raise, prone instability test, aberrant motions, lumbar hypermobility, and fear-avoidance beliefs. CONCLUSIONS: It appears that the response to a stabilization exercise program in patients with LBP can be predicted from variables collected from the clinical examination. The prediction rules could be used to determine whether patients with LBP are likely to benefit from stabilization exercises.

Adult↗
PubMed2005

Postural control of the lumbar spine in unstable sitting.

OBJECTIVE: To evaluate the neuromuscular strategy adopted during sitting balance on an unstable surface in the frontal plane. DESIGN: Electromyographic evaluation of trunk muscles. SETTING: University spine biomechanics laboratory. PARTICIPANTS: Seventy asymptomatic men (mean age, 34.5 y). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: "Balancers" and "nonbalancers" were identified by principal component analysis of their lumbar spine side flexion angle during sitting balance. Average electromyographic levels were used as a measure of muscle activation. Pearson correlations were used to identify coactivation versus asymmetrical muscle activation of opposite muscle groups. RESULTS: External oblique, internal oblique, and thoracic erector spinae (TES) were most active, and most likely to be used asymmetrically, with other muscles showing low levels of coactivation. Between groups, the average electromyographic levels in the balancers was lower than in the nonbalancers (P<.05), with further differences in the symmetry of external oblique, internal oblique, and TES activation between groups. CONCLUSIONS: Sitting balance in the frontal plane appears to involve a combined feedforward-feedback strategy of muscle activation. Successful balance was characterized by low levels of muscle coactivity, along with higher levels of asymmetric activation in the global trunk muscles, specifically external oblique, internal oblique, and TES.

Adult↗
PubMed2005

The direction of progressive herniation in porcine spine motion segments is influenced by the orientation of the bending axis.

BACKGROUND: It has been shown that disc herniations are a cumulative injury created by repetitive flexion motion while under modest compressive loads. There is a lack of data linking the direction of nucleus tracking to the orientation of the bending motion axis. Our purpose was to determine if the direction that the nucleus tracks through the annulus during progressive herniation is predictable from the direction of bending motion (i.e. a specific side with posterio-lateral herniation). METHODS: Matched cohorts (nu=16) of porcine cervical spine (C3/4 and C5/6) motion segments were potted in aluminum cups and bent at an angle of 30 degrees to the sagittal plane flexion axis while under a sustained compressive load of 1472 N. FINDINGS: The direction of bending motion affected the tracking pattern of the nucleus through the annular fibres in a predictable pattern. Specifically, bending the motion segments at an angle of 30 degrees to the left of the sagittal plane flexion axis biased the movement of the nucleus toward the posterior right side of the disc in 15 of the 16 specimens. INTERPRETATION: Based on this animal model, shown to have similar biomechanical behaviour to humans, the direction that the nucleus tracks through the annular fibres appears to be dependent upon the direction of bending motion. This may have implications on both herniation prevention and rehabilitation of posterio-lateral bulges and herniations.

Animals↗
PubMed2005

Muscle force-stiffness characteristics influence joint stability: a spine example.

BACKGROUND: The muscle force-stiffness relationship has often been modeled as linear, while in situ muscle research has clearly demonstrated non-linearity. Estimation of rotational joint stability relies on both a muscle's instantaneous pre-perturbation force and stiffness. Under conditions of static equilibrium, a muscle's stiffness will function in a stabilizing manner, while its force can function in either a stabilizing or destabilizing manner depending on the muscle's orientation about the joint. METHODS: A single muscle (rectus abdominis) was modeled and its individual direct stabilizing potential about the L4-L5 spine joint was analyzed. Three force-stiffness relationships were examined: (1) linear; (2) non-linear with moderate stiffness magnitudes; (3) non-linear with higher stiffness magnitudes. FINDINGS: With a linear force-stiffness relationship, stability increased proportional to muscle force; with a non-linear relationship, stability peaked and subsequently decreased at submaximal muscle forces. When considering the lower, as opposed to the higher non-linear stiffness magnitudes, the stabilizing potential of the muscle peaked at a lower muscle force level and actually became negative (destabilizing) at a critical stiffness magnitude. INTERPRETATION: It was concluded that a non-linear muscle force-stiffness relationship greatly alters the individual stabilizing potential of the muscle throughout its progression of force development. A muscle's stabilizing contribution may actually peak at and subsequently decrease above a critical submaximal force level. Incorporating this knowledge into stability models may assist in recognizing unstable events that lead to injury at higher levels of muscle activation.

Biomechanical Phenomena↗
PubMed2004

Quantifying tissue loads and spine stability while performing commonly prescribed low back stabilization exercises.

STUDY DESIGN: A quantitative biomechanical comparison of seven different lumbar spine "stabilization exercises." OBJECTIVES: The purpose of this research was to quantify lumbar spine stability resulting from the muscle activation patterns measured when performing selected stabilization exercises. SUMMARY OF BACKGROUND DATA: Many exercises are termed "stabilization exercises" for the low back; however, limited attempts have been made to quantify spine stability and the resultant tissue loading. Ranking resultant stability together with spinal load is very helpful for guiding clinical decision-making and therapeutic exercise design. METHODS: Eight stabilization exercises were quantified in this study. Spine kinematics, external forces, and 14 channels of torso EMG were recorded for each exercise. These data were input into a modified version of a lumbar spine model described by Cholewicki and McGill (1996) to quantify stability and L4-L5 compression. RESULTS: A rank order of the various exercises was produced based on stability, muscle activation levels, and lumbar compression. CONCLUSIONS: Quantification of the calibrated muscle activation levels together with low back compression and resultant stability assists clinical decisions regarding the most appropriate exercise for specific patients and specific objectives.

Adult↗
PubMed2004

The effect of static torsion on the compressive strength of the spine: an in vitro analysis using a porcine spine model.

STUDY DESIGN: Matched porcine cervical spine motion segments were subjected to two main conditions and compared: axial compression and axial compression combined with varying axial torque. OBJECTIVES: To determine the effect of torsion on the acute compressive strength of the spine. SUMMARY OF BACKGROUND DATA: The spine is often subjected to compression together with axial torque as a component of complex loading, yet there is a lack of documentation on its effect on the compressive strength and injury mechanics. METHODS: Matched cohorts of porcine cervical spine (C5-C6) motion segments were compressed to failure at a rate of 3,000 N/s combined with 0 Nm, 5 Nm, 20 Nm, or 30 Nm of axial torque. Three "failure" points were recorded from the stress/strain association: the first "step" (initial microfracture), the initial slope change (yield point or "slow crush" mechanism), and the ultimate failure point (fracture). Furthermore, resultant injuries were documented using planar radiography and visual inspection following dissection of the motion segments. RESULTS: Axial torque affected the failure characteristics during acute compressive loading. The ultimate strength of the motion segments was significantly reduced with increasing static torques. The compressive load at which initial microfracture occurred, indicated by the first "step" in the load-deformation curve, was increased with 5 Nm, 10 Nm, and 20 Nm of applied torsion in comparison to no torque, but this effect was reduced with 30 Nm of torque. The "slow crush" mechanism of failure was not affected by the addition of axial torque. No radiographic gross injuries to the facet joints were observed. Damage appeared to be confined to the endplate and trabecular network of the vertebral body. CONCLUSIONS: Based on this animal model, shown to have similar biomechanical behavior to humans, axial torque appears to significantly reduce the compressive strength of the spine.

Animals↗
PubMed2004

Determining cavitation location during lumbar and thoracic spinal manipulation: is spinal manipulation accurate and specific?

STUDY DESIGN: Sixty-four asymptomatic participants, ranging in age from 22 to 49 years, volunteered to act as patients for the study. Twenty-eight different clinicians performed thoracic and lumbar spinal manipulative procedures. The range of clinical experience was 1 to 43 years. OBJECTIVES: The purpose of this study is to first locate the joints that produce an audible sound in response to manipulation (cavitation) during spinal manipulative procedures so that the accuracy and specificity of manipulation can be assessed. SUMMARY OF BACKGROUND DATA: Clinicians utilizing spinal manipulative therapy (SMT) claim to be very specific and accurate with the delivery of their dynamic thrust. It has been suggested that the clinical success of SMT is dependent on the accurate delivery of that therapy to the target spinal joints. METHODS: Asymptomatic participants received SMT to either the thoracic or lumbar regions of their spine. Accelerometers were secured to the skin over the spinal column, and the relative time at which each accelerometer detected the vibration from the cavitation associated with the SMT was used to calculate the source of the vibration. The site of cavitation was then compared with the target location. RESULTS: For lumbar SMT, the average error from target of 124 cavitations in lumbar procedures was 5.29 cm (at least one vertebra away from target), with a range of 0 to 14 cm. Of these cavitations, 57 were deemed to be accurate and 67 were deemed to be inaccurate. The average error from target of 54 cavitations in the thoracic spine was 3.5 cm, with a range of 0 to 9.5 cm. Of these cavitations, 29 were deemed to be accurate and 25 were deemed to be inaccurate. In most cases, individual manipulative procedures were associated with multiple cavitations ranging from 2 to 6. CONCLUSIONS: In the lumbar spine, SMT was accurate about half the time. However, because most procedures were associated with multiple cavitations, in most cases, at least one cavitation emanated from the target joints. In the thoracic spine, SMT appears to be more accurate.

Acceleration↗
PubMed2004

Determining the stabilizing role of individual torso muscles during rehabilitation exercises.

STUDY DESIGN: A systematic biomechanical analysis involving an artificial perturbation applied to individual lumbar muscles in order to assess their potential stabilizing role. OBJECTIVES: To identify which torso muscles stabilize the spine during different loading conditions and to identify possible mechanisms of function. SUMMARY OF BACKGROUND DATA.: Stabilization exercises are thought to train muscle patterns that ensure spine stability; however, little quantification and no consensus exists as to which muscles contribute to stability. METHODS: Spine kinematics, external forces, and 14 channels of torso electromyography were recorded for seven stabilization exercises in order to capture the individual motor control strategies adopted by different people. Data were input into a detailed model of the lumbar spine to quantify spine joint forces and stability. The EMG signal for a particular muscle was replaced either unilaterally or bilaterally by a sinusoid, and the resultant change in the stability index was quantified. RESULTS: A direction-dependent-stabilizing role was noticed in the larger, multisegmental muscles, whereas a specific subtle efficiency to generate stability was observed for the smaller, intersegmental spinal muscles. CONCLUSIONS: No single muscle dominated in the enhancement of spine stability, and their individual roles were continuously changing across tasks. Clinically, if the goal is to train for stability, enhancing motor patterns that incorporate many muscles rather than targeting just a few is justifiable.

Adult↗
PubMed2004

Linking latest knowledge of injury mechanisms and spine function to the prevention of low back disorders.

While several sophisticated scientific approaches have been employed to understand low back function and injury mechanisms, very few have been broadly used to develop and justify injury prevention strategies. This paper looks beyond the linked segment model, and the lessons learned from this biomechanical approach, to consider the application of more sophisticated approaches. These include modelling approaches with greater anatomical and biological fidelity, fusing the lessons learned from the areas of tissue mechanics and concepts of spine stability, together with some studies that have examined several characteristics including psychosocial, physiological and personal variables. The objective is to better link recently discovered mechanisms of injury and spine tissue health with injury risk reducing approaches.

Humans↗
PubMed2003

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.

Biomechanical Phenomena↗
PubMed2003

Relationships between lumbar flexibility, sit-and-reach test, and a previous history of low back discomfort in industrial workers.

The sit-and-reach (S&R) test is often included in standard fitness tests (e.g., Canadian Physical Activity, Fitness and Lifestyle Appraisal [CPAFLA]), justified on the assumption that it is an indicator of low back health. Two issues were examined here: Is low back flexibility linked to having a history of low back disorders? And is the S&R test an indicator of low back flexibility? The relationship between S&R test scores, lumbar range of motion, and having a history of low back discomfort was examined in 72 asymptomatic (at test time) industrial workers (70 M, 2 F; mean age 35 ys; height 1.79 m; mass 84.7 kg). The S&R test, among many collected, was performed according to the CPAFLA guidelines. History of low back discomfort (LBD) was categorized based on whether or not time was lost from work. The S&R test was unable to distinguish between those with a history of LBD and those without. Specific lumbar sagittal range of motion could make this distinction. A moderate correlation (r = 0.42) surfaced between S&R and lumbar flexibility. This study suggests that the value of S&R as an indicator of previous back discomfort is questionable and there may be better indicators for inclusion in the CPAFLA.

Adult↗