Stuart M. McGill

Mechanical stability of the in vivo lumbar spine: implications for injury and chronic low back pain

One important mechanical function of the lumbar spine is to support the upper body by transmitting compressive and shearing forces to the lower body during the performance of everyday activities. To enable the successful transmission of these forces, mechanical stability of the spinal system must be assured. The purpose of this study was to develop a method and to quantify the mechanical stability of the lumbar spine in vivo during various three-dimensional dynamic tasks. A lumbar spine model, one that is sensitive to the various ways that individuals utilize their muscles and ligaments, was used to estimate the lumbar spine stability index three times per second throughout the duration of each trial. Anatomically, this model included a rigid pelvis, ribcage, five vertebrae, 90 muscle fascicles and lumped parameter discs, ligaments and facets. The method consisted of three sub-models: a cross-bridge bond distribution-moment muscle model for estimating muscle force and stiffness from the electromyogram, a rigid link segment body model for estimating external forces and moments acting on the lumbar vertebrae, and an 18 degrees of freedom lumbar spine model for estimating moments produced by 90 muscle fascicles and lumped passive tissues. Individual muscle forces and their associated stiffness estimated from the EMG-assisted optimization algorithm, along with external forces were used for calculating the relative stability index of the lumbar spine for three subjects. It appears that there is an ample stability safety margin during tasks that demand a high muscular effort. However, lighter tasks present a potential hazard of spine buckling, especially if some reduction in passive joint stiffness is present. Several hypotheses on the mechanism of injury associated with low loads and aetiology of chronic back pain are presented in the context of lumbar spine stability.

Low back disorders : Evidence-based prevention and rehabilitation

Chapter 1 Introduction to the Issues Chapter 2 Scientific Approach Unique to This Book Chapter 3 Epidemiological Studies on Low Back Disorders (LBDs) Chapter 4 Functional Anatomy of the Lumbar Spine Chapter 5 Normal and Injury Mechanics of the Lumbar Spine Chapter 6 Lumbar Spine Stability: Myths and Realities Chapter 7 LBD Risk Assessment Chapter 8 Reducing the Risk of Low Back Injury Chapter 9 The Question of Back Belts Chapter 10 Building Better Rehabilitation Programs for Low Back Injuries Chapter 11 Evaluating the Patient Chapter 12 Developing the Exercise Program Chapter 13 Advanced Exercises.

The biomechanics of low back injury: Implications on current practice in industry and the clinic

The purpose of this paper is to introduce some concepts of low back injury for use towards developing better injury risk reduction strategies and advancing rehabilitation of the injured spine. Selected issues in low back injury are briefly reviewed and discussed, specifically, the types of tissue loads that cause low back injury, methods to investigate tissue loading, and issues which are important considerations when formulating injury avoidance strategies such as spine posture, and prolonged loading of tissues over time. Finally, some thoughts on current practice are expressed to stimulate discussion on directions for injury reduction efforts in the future, particularly, the way in which injuries are reported, the use of simple indices of risk such as load magnitude, assessment of the injury and development of injury avoidance strategies. This paper was written for a general biomechanics audience and not specifically for those who are spine specialists.

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.

Low back stability: from formal description to issues for performance and rehabilitation.

McGILL, S.M. Low back stability: from formal description to issues for performance and rehabilitation. Exerc. Sport Sci. Rev. Vol. 29, No. 1, pp. 26–31, 2001. The concept of stability, together with notions of design and the application of stabilization exercise, is briefly synthesized. The objective is to challenge muscle systems to achieve sufficient functional stability but in a way that spares the spine of excessive exacerbating load.

A myoelectrically based dynamic three-dimensional model to predict loads on lumbar spine tissues during lateral bending

This work describes a dynamic model of the low back that incorporates extensive anatomical detail of the musculo-ligamentous-skeletal system to predict the load time histories of individual tissues. The dynamic reaction moment about L4/L5 was determined during lateral bending from a linked-segment model. This reaction moment was partitioned into restorative components provided by the disc, ligament strain, and active-muscle contraction using a second model of the spine that incorporated a detailed representation of the anatomy. Muscle contraction forces were estimated using both information from surface electromyograms, collected from 12 sites, and consideration of the modulating effects of muscle length, cross-sectional area and passive elasticity. This modelling technique is sensitive to the different ways in which individuals recruit their musculature to satisfy moment constraints. Time histories of muscle forces are provided. High muscle loads are consistent with the common clinical observation of muscle strain often produced by load handling. Furthermore, the coactivation measured in muscles on both sides of the trunk suggests that muscles are recruited to satisfy the lateral bending reaction torque in addition to performing other mechanical roles such as spine stabilization. If an estimate of the intervertebral joint compression is desired for assessment of lateral bends in industry, then a single equivalent lateral muscle with a moment arm of approximately 3.0-4.0 cm would conservatively capture the effects of muscle co-contraction quantified in this study.

Low back joint loading and kinematics during standing and unsupported sitting

The aim was to examine lumbar spine kinematics, spinal joint loads and trunk muscle activation patterns during a prolonged (2 h) period of sitting. This information is necessary to assist the ergonomist in designing work where posture variation is possible—particularly between standing and various styles of sitting. Joint loads were predicted with a highly detailed anatomical biomechanical model (that incorporated 104 muscles, passive ligaments and intervertebral discs), which utilized biological signals of spine posture and muscle electromyograms (EMG) from each trial of each subject. Sitting resulted in significantly higher (p< 0.001) low back compressive loads (mean±SD 1698±467 N) than those experienced by the lumbar spine during standing (1076±243 N). Subjects were equally divided into adopting one of two sitting strategies: a single ‘static’ or a ‘dynamic’ multiple posture approach. Within each individual, standing produced a distinctly diVerent spine posture compared with sitting, and standing spine postures did not overlap with flexion postures adopted in sitting when spine postures were averaged across all eight subjects. A rest component (as noted in an amplitude probability distribution function from the EMG) was present for all muscles monitored in both sitting and standing tasks. The upper and lower erector spinae muscle groups exhibited a shifting to higher levels of activation during sitting. There were no clear muscle activation level diVerences in the individuals who adopted diVerent sitting strategies. Standing appears to be a good rest from sitting given the reduction in passive tissue forces. However, the constant loading with little dynamic movement which characterizes both standing and sitting would provide little rest/change for muscular activation levels or low back loading.

Intra-abdominal pressure mechanism for stabilizing the lumbar spine

Currently, intra-abdominal pressure (IAP) is thought to provide stability to the lumbar spine but the exact principles have yet to be specified. A simplified physical model was constructed and theoretical calculations performed to illustrate a possible intra-abdominal pressure mechanism for stabilizing the spine. The model consisted of an inverted pendulum with linear springs representing abdominal and erector spinae muscle groups. The IAP force was simulated with a pneumatic piston activated with compressed air. The critical load of the model was calculated theoretically based on the minimum potential energy principle and obtained experimentally by increasing weight on the model until the point of buckling. Two distinct mechanisms were simulated separately and in combination. One was antagonistic flexor extensor muscle coactivation and the second was abdominal muscle activation along with generation of IAP. Both mechanisms were effective in stabilizing the model of a lumbar spine. The critical load and therefore the stability of the spine model increased with either increased antagonistic muscle coactivation forces or increased IAP along with increased abdominal spring force. Both mechanisms were also effective in providing mechanical stability to the spine model when activated simultaneously. Theoretical calculation of the critical load agreed very well with experimental results (95.5% average error). The IAP mechanism for stabilizing the lumbar spine appears preferable in tasks that demand trunk extensor moment such as lifting or jumping. This mechanism can increase spine stability without the additional coactivation of erector spinae muscles.

DYNAMICALLY AND STATICALLY DETERMINED LOW BACK MOMENTS DURING LIFTING

Assessment of the effects of lifting on the low back has most frequently been done with the aid of static models. Many lifting movements appear to have substantial inertial components. It was of interest, therefore, to determine the size of the difference between statically and dynamically calculated lumbar moments during a demanding but not unusual manual lift observed in a metal fabrication industry. The results of several trials by four young men showed that the dynamic model resulted in peak L4/L5 moments 19% higher on average, with a maximum difference of 52%, than those determined from the static model. The technique adopted in the lift could minimize the difference. When the inertial forces of the load itself and the load weight were incorporated into an otherwise static model (quasi-dynamic) then the resulting L4/L5 moments exceeded those of the fully dynamic model by 25%. In many industrial tasks static analyses may severely underestimate the demands of dynamic lifts. These results show that a reasonably inexpensive approach in lifting task analysis is to measure the dynamic forces of the load on the hands and to use these in an otherwise static model. This results in a conservative assessment of the injury risk of lifts at least of the type reported in this study.

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.