The effect of fatigue on multijoint kinematics and load sharing during a repetitive lifting test

The Effect of a Repetitive, Fatiguing Lifting Task on Horizontal Ground Reaction Forces

There are many outdoor work environments that involve the combination of repetitive, fatiguing lifting tasks and less-than-optimal footing (muddy/slippery ground surfaces). The focus of the current research was to evaluate the effects of lifting-induced fatigue of the low back extensors on lifting kinematics and ground reaction forces. Ten participants performed a repetitive lifting task over a period of 8 minutes. As they performed this task, the ground reaction forces and whole body kinematics were captured using a force platform and magnetic motion tracking system, respectively. Fatigue was verified in this experiment by documenting a decrease in the median frequency of the bilateral erector spinae muscles (pretest-posttest). Results indicate significant (p< 0.05) increases in the magnitude of the peak anterior/posterior (increased by an average of 18.3%) and peak lateral shear forces (increased by an average of 24.3%) with increasing time into the lifting bout. These results have implications for work environments such as agriculture and construction, where poor footing conditions and requirements for considerable manual materials handling may interact to create an occupational scenario with an exceptionally high risk of a slip and fall.

Ergonomic identification and biomechanical evaluation of workers' strategies and their validation in a training situation: summary of research

Our goal was to evidence the role of workers in the search for safer manual handling strategies. Different strategies used by expert and novice workers, such as footwork (positioning/displacement), box manoeuvres (handgrips and load tilting) and posture were ergonomically identified. They were biomechanically evaluated to shed light on their potential for safe handling by reducing back loadings, back asymmetries and mechanical work requirements. The experts' strategies were validated in a training situation and evaluated by two independent studies among novice workers. The first, a control study, showed that free practice did not lead to safer handling practices. The second, a training situation based on observing contrasted performances of experts and novices for footwork, box manoeuvres and back posture, when combined with practice and the search for optimal solutions, prompted the novices to adopt new ways of manoeuvring boxes and positioning themselves that appear safer for back efforts, asymmetries, and mechanical work. These elements should be included in educational programs for safe handling.

RELEVANCE

The observation of contrasted strategies of expert and novice workers using an ergonomic approach, supplemented by biomechanical evaluations of these strategies, is a key factor in designing training programs for safe handling.

Differences in multi-joint kinematic patterns of repetitive hammering in healthy, fatigued and shoulder-injured individuals

BACKGROUND

Work-related musculo-skeletal disorders have been previously related to movement repetition, inadequate postures, non-ergonomic environments, muscular imbalance and fatigue. However, no direct link between fatigue and injury has been experimentally shown. To address this problem, we compared the effects of fatigue and injury on the kinematics of repetitive hammering.

METHODS

Healthy subjects (n=30) hammered repetitively both before and after fatigue. Fatigue was induced by a combination of static and dynamic procedures. Shoulder-injured subjects (n=15) hammered for 30s without fatigue. Kinematics of motion was recorded.

FINDINGS

The movement time and shoulder range of motion during hammering were not affected by either fatigue or shoulder injury. When fatigued, the healthy subjects displayed decreased range of joint motion, peak velocity and peak acceleration of elbow motion during hammering as well as reduced grip strength. Shoulder-injured individuals had a smaller hammer trajectory amplitude than healthy controls with or without fatigue. They also had lower wrist range of motion, elbow peak velocity, and peak wrist and elbow acceleration compared to healthy subjects hammering without fatigue but only lower wrist peak acceleration compared to healthy subjects hammering with fatigue.

INTERPRETATION

Results showed that fatigue affects elbow motion while shoulder injury affects both wrist and elbow motions during hammering. However, shoulder kinematics were not changed by either fatigue or shoulder injury. These changes at the wrist and elbow may reflect strategies used by individuals with shoulder injury to maintain constant movement duration and shoulder kinematics during movement.

Differentiating lifting technique between those who develop low back pain and those who do not

BACKGROUND

No research to date has been able to discriminate differences in lifting technique for healthy individuals who eventually develop low back pain compared to those that do not while employed in a manual materials handling industry. The purpose of this study was to demonstrate the ability of principal component analysis to identify differences in lifting technique.

METHODS

Principal component analysis was applied to sixteen kinematic and kinetic waveforms describing the two-dimensional motion of the trunk and load. The principal component scores for each variable were used as the dependent measures in a one-way ANOVA to determine group differences.

FINDINGS

Significant group differences (P<0.05) were found for five of the principal component scores capturing associated kinematic waveform patterns related to the control and placement of the box on the shelf, and associated kinetic waveform patterns related to the relative timing of extension moment generation in the sacral and thoracic regions. A related waveform pattern for trunk compression was also found.

INTERPRETATION

Due to the coordinated movements involved in tasks such as lifting, differences among clinical populations have been difficult to demonstrate empirically. We were able to identify different characteristics in lifting kinematics and kinetics prior to the development of low back pain. Principal component analysis was able to identify important biomechanical differences where traditional analyses failed. This is the first study to identify such lifting differences prior to the development of low back pain.

A new method to quantify demand on the upper extremity during manual wheelchair propulsion

OBJECTIVE

To use an ergonomics-based rating that characterizes both demand on, and capacity of, upper-extremity muscle groups during wheelchair propulsion to help identify the muscle groups most at risk for pain or overuse injury in a relatively demanding wheelchair propulsion task.

DESIGN

Case series.

SETTING

Biomechanics research laboratory.

PARTICIPANTS

Sixteen manual wheelchair users with complete (American Spinal Injury Association grade A) T6-L2 paraplegia.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Internal peak joint moments required by each of the major upper-extremity muscle groups for propelling a wheelchair up a ramp; isometric strength of each of the muscle groups in positions simulating wheelchair propulsion; and wheelchair propulsion strength rating (WPSR) for each muscle group, calculated by normalizing the joint demands to their capacity.

RESULTS

The largest joint moment was for shoulder flexion, at 39.7+/-13.9Nm. Shoulder flexion also accounted for the peak WPSR value of 66.5%+/-20.3%. Supination and pronation movements had low peak moment requirements (3.4Nm, 5.0Nm, respectively) but high WPSR values (41%, 53%, respectively).

CONCLUSIONS

Even a relatively benign ramp (2.9 degrees ) places a large demand on the musculature of the upper extremity, as assessed by using the WPSR to indicate muscular demand.

Repeated spinal flexion modulates the flexion-relaxation phenomenon

OBJECTIVES

To determine if repeated spinal flexion and loading modulate the deactivation of lumbar muscles near full flexion (flexion-relaxation).

DESIGN

Repeated measures experimental study of the effect of repetitive trunk flexion and added mass on the flexion-relaxation phenomenon.

BACKGROUND

Repeated flexion causes muscular fatigue, creep of passive tissues and diminished protective reflexes. However, flexion-relaxation has not been studied in repeated trunk flexion, and could be related to the increased risk of low-back disorders.

METHODS

Thirty healthy young subjects performed 100 trunk flexion movements between standing and full flexion. Erector spinae electromyography and lumbar spine flexion were measured during cycles 1-10 (no load), 11-20 (performed holding a mass in the hands), 81-90 (mass in the hands) and 91-100 (no load). The spinal flexion angle at myoelectric silence and full flexion were extracted from each movement cycle.

RESULTS

Twenty-three of the 30 subjects showed flexion-relaxation throughout the repeated trunk flexion. The flexion-relaxation and maximum flexion angles increased at the end of the experiment; the flexion-relaxation angle relative to the maximum flexion angle also increased. This effect depended on the load condition; the flexion-relaxation and maximum flexion angles showed a greater increase in the unloaded than loaded condition.

CONCLUSIONS

The flexion-relaxation phenomenon was changed due to repeated trunk flexion. The increases in flexion-relaxation angle likely involve changes to the neuromuscular control system.

RELEVANCE

The deactivation of the erector muscles near full flexion occurs at a greater spinal flexion angle and a greater proportion of maximum spinal flexion following repeated spinal flexion. This may be related to the increased risk of injury associated with repeated flexion.

The effects of repetitive motion on lumbar flexion and erector spinae muscle activity in rowers

OBJECTIVE

The purpose of this study was to investigate changes in lumbar flexion together with the pattern and level of muscle activity of selected erector spinae during a rowing trial.Design. Cross-sectional repeated measures design.

BACKGROUND

Low back pain is a common problem in rowers. The amount of lumbar flexion occurring during rowing might influence the possibility of injury.

METHODS

Sixteen young adult school rowers participated in the study. Changes in lumbar flexion and muscle activity were recorded across the drive phase, at three stages of an ergometer based rowing trial. Lumbar flexion was calculated by computerised motion analysis of surface markers attached to the spinous processes of L1 and S1. Surface electromyography techniques were used to examine the magnitude of activity from three erector spinae muscles. The median frequency of the electromyographic signal was examined to quantify fatigue in the erector spinae muscles during isometric maximal effort muscle activation prior to and after the rowing trial.

RESULTS

Lumbar flexion increased significantly (P<0.05) during the rowing trial, as did the magnitude of electromyographic activity from sites over the lumbar multifidus, iliocostalis lumborum and longissimus thoracis muscles. The median frequency decreased significantly (P<0.05) in each muscle examined.

CONCLUSIONS

The findings showed that rowers attain relatively high levels of lumbar flexion during the rowing stroke, and these levels are increased during the course of the rowing trial. Indirect evidence of muscle fatigue in erector spinae muscles was also apparent, and this observation may in part be responsible for the increased levels of lumbar flexion observed.

RELEVANCE

Excessive lumbar flexion may influence the potential for injury to spinal structures. An awareness of increased lumbar flexion and muscle fatigue in the erector spinae muscles may be important for injury prevention programs for rowers.

Body motion during repetitive isodynamic lifting: a comparative study of normal subjects and low-back pain patients

To quantify performance differences between patients with low-back pain (LBP) and a control group during their performance of a repetitive isodynamic lifting task. Case-control study was done. LBP patients were recruited and tested at an outpatient ambulatory chronic pain rehabilitation program before treatment was begun. Fifty-three LBP patients who had prolonged back pain and 53 age and gender matched pain-free control subjects. Overall lifting performance measures included weight lifting and number of lifts completed; kinematic measures of hip and knee movements during lifting were described by hyperbolic tangent models, and included static measures of starting and ending angles, and dynamic measures of midpoint, falltime, and lift speed. Control subjects completed significantly more lifts and lifted more weight than patients. Starting hip flexion was greater for controls and starting knee flexion was greater for patients, indicating that patients used more of a leg lift. Patients and controls also differed significantly on dynamic measures, particularly lifting speed and hip and knee temporal midpoints. Major static and dynamic motion differences were found between LBP patients and controls as they performed repetitive lifting under a constant load. These findings indicate that body motion parameters, in addition to more common strength and endurance measures, are necessary to describe the impact of persistent LBP on a person's lifting abilities.

Muscle fatigue and fatigue-related biomechanical changes during a cyclic lifting task

STUDY DESIGN

Electromyographic and biomechanical methods were utilized to investigate correlations between indexes of localized muscle fatigue and changes in the kinematics and kinetics of motion during a cyclic lifting task.

SUMMARY OF BACKGROUND DATA

Recent advances in time-frequency analysis procedures for electromyographicic signal processing provide a new way of studying localized muscle fatigue during dynamic contractions. These methods provide a means to investigate fatigue-related functional impairments in patients with low back pain.

OBJECTIVES

To study the relationship between localized muscle fatigue and the biomechanics of lifting and lowering a weighted box. Fatigue-related changes in the electromyographicic signal of trunk and limb muscles were evaluated and compared to kinematic and kinetic measures in order to determine whether lifting strategy is modified with fatigue.

METHODS

A total of 14 healthy male subjects (26 +/- 5 years) cyclically lifted and lowered a 13 kg box (12 lifts/min) for 4.5 minutes. A 5-second static maximum lifting task was included immediately before and after the cyclic lifting task to measure changes in lifting strength and static electromyographicic fatigue indexes. Electromyographic signals from 14 muscle sites (including paravertebral and limb muscles) were measured. Changes in the electromyographicic Instantaneous Median Frequency, a fatigue index, were computed using time-frequency analysis methods. This index was compared with more standardized measures of fatigue, such as those based on electromyographicic median frequency acquired during a static trunk extension test, subjective fatigue measures, and maximal static lifting strength. Biomechanical measures were gathered using a motion analysis system to study kinematic and kinetic changes during the lifting task.

RESULTS

During the cyclic lifting task, the electromyographic Instantaneous Median Frequency significantly decreased over time in the paravertebral muscles, but not in the limb muscles. Paravertebral electromyographicic Instantaneous Median Frequency changes were consistent with self-reports of fatigue as well as decreases in trunk extension strength. The magnitude of muscle-specific changes in electromyographicic Instantaneous Median Frequency was not significantly correlated with electromyographicic median frequency changes from the static trunk extension task. The load of the box relative to the maximal static lifting strength significantly affected the electromyographicic Instantaneous Median Frequency changes of paravertebral back muscles. Significant changes with fatigue during the task were found in the angular displacements at the knee, hip, trunk, and elbow. These biomechanical changes were associated with increased peak torque and forces at the L4-L5 vertebral segment.

CONCLUSIONS

Our results demonstrate correlation between localized muscle fatigue and biomechanical adaptations that occur during a cyclic lifting task. This new technique may provide researchers and clinicians with a means to investigate fatigue-related effects of repetitive work tasks or assessment procedures that might be useful in improving education, lifting ergonomy, and back school programs. Although both the dynamic and static tasks resulted in spectral shifts in the electromyographicic data, the fact that these methods led to different muscle-specific findings indicates that they should not be considered as equivalent assessment procedures.

The efficacy of training for three manual handling strategies based on the observation of expert and novice workers

OBJECTIVES

To evaluate the efficacy of training for three manual handling strategies, i.e. load tilting/hands positioning, shoulders positioning and feet orientation based on the observation of the contrasted strategies of expert and novice workers and free practice using a search approach.

DESIGN

Ten novice male workers were tested at pre-training (one trial) and post-training with homogeneous boxes (three trials) and heterogeneous boxes (two trials) sampled from two sessions. Training took place with homogeneous boxes whereas heterogeneous boxes were new situations.

BACKGROUND

The effectiveness of training programs in safe handling and the repetition of specified techniques are contested; they should rather be based on expert workers' strategies.

METHODS

Pre-training and post-training trials were analyzed with five video cameras and a large force plate. The biomechanical variables included three safety criteria: net 3D resulting moments at L5/S1, asymmetry of posture/efforts at L5/S1 and mechanical work on load; kinematics and ergonomic variables were used as explicative variables.

RESULTS

Training produced safer strategies by reducing mechanical work and back extensor moments; this occurred in both load conditions, an indication of the transfer of knowledge. These strategies consisted of changes in load maneuvers (tilting/hand positioning) and feet orientation.

CONCLUSIONS

Training programs should be based on observations of workers.

RELEVANCE

These results may guide the specialists involved in training programs. Training based on a search approach by the learner and anchored on observations of contrasted strategies (load tilts/hands positioning and feet orientation) by experts and novices appears promising for safe handling.

The impact of mental processing and pacing on spine loading: 2002 Volvo Award in biomechanics

STUDY DESIGN

The impact of various levels of mental processing and pacing (during lifting) on spine loading was monitored under laboratory conditions.

OBJECTIVES

To explore how mental demands and pacing influence the biomechanical response and subsequent spine loading and, to determine whether individual characteristics have a modifying role in the responses.

SUMMARY OF BACKGROUND DATA

Modern work often requires rapid physical exertions along with demands of mental processing (both psychosocial stressors). While the effect of physical workplace factors on spine loading has been widely documented, few studies have investigated the impact that interaction of psychosocial factors and individual factors has on spine loads.

METHODS

For this study, 60 subjects lifted boxes while completing two types of mental processing tasks: 1) series tasks with decisions occurring before the act of lifting, and 2) simultaneous tasks with decisions occurring concurrently with the lift. For both of these mental processing conditions, two intensities of mental load were evaluated: simple and complex. Task pacing was also adjusted under slow and fast conditions. Finally, individual characteristics (personality and gender) were evaluated as potential modifiers. An electromyographically assisted model evaluated the three-dimensional spine loads under the experimental conditions.

RESULTS

Simultaneous mental processing had the largest impact on the spine loads, with the complex intensity resulting in increases of 160 N with lateral shear, 80 N with anteroposterior shear, and 700 N with compression. Increased task pace produced greater lateral shear (by 20 N), anteroposterior shear (by 60 N), and compression loads (by 410 N). Gender and personality also influenced loadings by as much as 17%.

CONCLUSIONS

Mental processing stress acted as a catalyst for the biomechanical responses, leading to intensified spine loading. Mental stress appeared to occur as a function of time pressures on task performance and resulted in less controlled movements and increases in trunk muscle coactivation. These adjustments significantly increased spine loading. These results suggest a potential mechanism for the increase in low back pain risk resulting from psychosocial stress caused by modern work demands.

Thoracolumbar proprioception in individuals with and without low back pain: intratester reliability, clinical applicability, and validity

STUDY DESIGN

Repeated measures design of active spinal position sense in individuals with and without low back pain (LBP).

OBJECTIVES

Reproducibility and validity evaluation of thoracolumbar proprioception measurement.

BACKGROUND

Proprioception studies in peripheral joints and the spine suggest that there may be proprioception deficits due to injury, pain, or degeneration. Kinesthetic retraining may be useful in rehabilitation of patients with LBP, but appropriate measures are required to objectively quantify spinal proprioception.

METHODS AND MEASURES

Active-target reproduction in the sagittal, horizontal, and coronal planes was assessed (3 separate occasions for 18 asymptomatic volunteers and 2 occasions for 62 patients with LBP). Repositioning accuracy was expressed as absolute errors (AE) and variable errors (VE). Reliability was analyzed with intraclass correlation coefficient (ICC) and precision with standard error of measurement (SEM) and calculation of the smallest detectable difference (SDD) index. Repeated measures ANOVA and correlations were used for within-group comparisons and discriminant analysis for between-group comparisons.

RESULTS

Reproducibility was better for the asymptomatic group, with AE for flexion and rotation being the most reliable (ICC = 0.76-0.80, SEM = 0.91 degrees-1.34 degrees). SDDs were high for all tests, suggesting limited clinical applicability. Reproducibility for the same tests was poor-moderate (ICC = 0.31-0.64, SEM = 0.45 degrees-3.90 degrees) for the patient group. AE for right-side rotation could discriminate between subject groups with 83.3% specificity but only 54.8% sensitivity.

CONCLUSIONS

Proprioception testing, with the methods employed, did not demonstrate good measurement properties in a sample of patients with recurrent LBP. Neither could it sufficiently discriminate between individuals with and without LBP. Possible reasons for these findings are discussed.

An ergonomic comparison of rowing machine designs: possible implications for safety

OBJECTIVES

Ergometer training is a common cause of injuries in rowers. A randomised crossover study comparing two power head designs was carried out to examine ergonomic risk factors.

METHODS

Six elite male rowers undertook 20 minute fatiguing rowing pieces with both fixed and floating power heads. A CODA MPX infrared telemetric motion analysis detector and the ergometer's interface were used to measure displacement, force, work performed, and power output.

RESULTS

There was no significant difference in the total work performed, power per stroke, or metabolic load between the two ergometer designs. Fatigue was shown by a mean (SEM) fall of 9.7 (0.79) W/stroke (95% confidence interval (CI) 8.0 to 11.5) between minutes 8-10 and minutes 16-18 (p<0.001). The stroke length was 53 (13) mm (95% CI 18 to 89) longer with the fixed power head (p<0.02). With fatigue, the stroke with the fixed power head lengthened at the "catch" (beginning of the stroke) by 19.5 mm (p<0.01) and shortened at the finish of the stroke by 7.2 mm (p<0.05). No significant changes in stroke length were seen with the floating power head. The mean force per stroke was 12.1% (95% CI 3.0 to 21.2) (27.3 (8.0) N) higher with the power head fixed versus floating (p<0.02).

CONCLUSIONS

It is postulated that longer stroke lengths and greater forces are risk factors for soft tissue injuries. Further research into whether floating power head rowing ergometers are associated with lower injury rates than fixed power head designs is now needed.

Recent advances in lumbar spinal mechanics and their significance for modelling

Mathematical models are often used to quantify the overall forces and moments acting on the lumbar spine. However, if the purpose of the research is to explain how spinal tissues can be injured, it is necessary to distribute the overall forces and moments between (and within) different spinal structures, because it is the concentration of force which causes injury, and elicits pain. This paper reviews recent experimental evidence concerning the distribution of forces and moments acting on the lumbar spine. Lordotic postures increase loading of the posterior annulus and apophyseal joints, whereas moderately flexed postures tend to equalise compressive stress across the disc, and unload the apophyseal joints. Sustained compression reduces the volume and pressure of the nucleus pulposus, while increasing compressive stresses in the annulus and neural arch. Sustained compression also reduces disc height, giving some slack to collagen fibres in the intervertebral disc and ligaments, and causing them to resist bending less. Disc degeneration has a similar effect on disc height, and stress distributions. On the other hand, discs and ligaments can be subjected to greater bending moments following a period of sustained or repetitive bending, because sustained bending impairs the normal protective reflex from the back muscles, and repetitive bending fatigues the back muscles, reducing their ability to protect the spine. Incorporating this information into mathematical models will make them better able to identify which activities are most likely to injure the lumbar spine in life.

Spinal and supraspinal factors in human muscle fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.

The effect of a fatiguing exercise by the index finger on single- and multi-finger force production tasks

We studied the effects of fatigue, induced by a 60-s maximal isometric force production with the index finger, on multi-finger coordination and force production by the other fingers of the hand. Finger forces were measured during single- and multi-finger maximal voluntary force production (MVC) at two sites, the middle of the distal or the middle of the proximal phalanges. Two fatiguing exercises involving force production by the index finger were used, one at the distal phalanx and the other at the proximal phalanx. The MVC of the index finger dropped by about 33% when it was produced at the site involved in the fatiguing exercise. In addition, large transfer effects of fatigue were observed across sites of force application and across fingers. Force deficit increased under fatigue, especially due to a drop in the recruitment of the index finger. Under fatigue, the index finger was less enslaved during force production by other fingers. During multi-finger tasks, the percentage of total force produced by the index finger was significantly reduced after the fatiguing exercise. The principle of minimization of secondary moments was violated under fatigue. We suggest that the most impaired (fatigued) finger shows less interaction with other fingers or, in other words, is being progressively removed from the multi-finger synergy. Some of the observed changes in finger coordination suggest effects of fatigue at a central (neural) level.

The effect of fatigue on multifinger co-ordination in force production tasks in humans

1. This study investigated the effects of fatigue, induced by production of maximal isometric force for 60 s with four fingers, upon indices of multifinger co-ordination. 2. Measurements of individual finger forces were performed during single- and multifinger maximal force production (maximal voluntary contraction, MVC) for two sites of force application, the middle of the distal or the middle of the proximal phalanxes. Two fatiguing exercises were used, involving force production at the distal phalanxes and at the proximal phalanxes. Fourteen subjects were tested. 3. The total force in four-finger tasks dropped by about 43 % when it was produced at the site involved in the fatiguing exercise. During force production at the other site, MVC dropped by 23 %. During single-finger MVC tests, force drop with fatigue was similar across all four fingers (about -25 % of their corresponding MVCs). 4. Force production by one finger was accompanied by involuntary force production by other fingers (enslaving). Enslaving remained unchanged by fatigue when measured during force generation at the site involved in the fatiguing exercise, but increased during force production at the other site. 5. The total MVC of four fingers acting in parallel was smaller than the sum of the MVCs of these fingers in single-finger tasks (force deficit). The force deficit increased with fatigue. Force-sharing patterns during four-finger tasks showed only minor changes under fatigue. 6. These results indicate that the effects of fatigue were not limited to changes in the force-generating capabilities of the muscles. In particular, fatigue could lead to a reorganisation at a neural level that defines commands to individual fingers.

Fibre type characteristics and function of the human paraspinal muscles: normal values and changes in association with low back pain

This review focuses on the role of the paraspinal muscles in relation to the development and existence of low back pain. It begins with a discussion of the deficits in paraspinal muscle strength and fatigue-resistance observed in low back pain patients and addresses the issue of 'cause or effect' with respect to muscle dysfunction and back pain. Our current knowledge regarding the 'normal' fibre type characteristics of the human erector spinae is then presented and the influence of these fibre type characteristics on the muscle's performance capacity is discussed. Alterations in the 'microanatomy' of the musculature in connection with low back pain, and the associated implications for the performance capacity of the patient, are then considered. Finally, a number of outstanding issues in relation to the clinical significance of back muscle dysfunction are identified, leading to the proposal of areas for future research.

Estimation of trunk muscle forces and spinal loads during fatiguing repetitive trunk exertions

STUDY DESIGN

The effects of human trunk extensor muscle fatigue on the estimated trunk muscle forces and spinal loading were investigated during the performance of repetitive dynamic trunk extension.

OBJECTIVE

To evaluate if alterations in the trunk muscle recruitment patterns resulted in a greater estimated active loading of the spine and, in turn, an increased risk of injury.

SUMMARY OF BACKGROUND DATA

Epidemiologic studies highlight the increased risk of low back injury during repetitive lifting, implicating fatigue of muscles and/or passive tissues as causes of such injury. Increased trunk muscle activity or altered recruitment patterns resulting from fatigue in the primary trunk extensor muscles may indicate an increase in the active loading of the spine, which could contribute to an increased risk of injury.

METHODS

Sixteen healthy study participants performed repetitive isokinetic trunk extension endurance tests at two load levels and two repetition rates, while their net muscular torque output and trunk muscular activity were measured. During each exertion, trunk torque, position, and velocity were controlled, so that any change in muscle activity could be attributed to fatigue. An electromyography-assisted model, adapted to accommodate the decline in maximum muscular tension generation resulting from fatigue, was used to estimate the 10 trunk muscle forces and spinal loading. Linear regression was used to quantify the rate of change in muscle force and spinal loading resulting from fatigue, while analysis of variance was used to determine if the rate of change was dependent on the task conditions (load and repetition rate).

RESULTS

Significant elevations were estimated for the latissimus dorsi and external oblique muscle forces in more than 70% of the endurance tests, whereas significant reductions in the erector spinae muscle force were predicted in 75% of the trials. The magnitude of the range of change of the erector spinae and latissimus dorsi muscle forces was dependent on the load level and repetition rate. The reduction in erector spinae forces offset the augmented force in the other muscles, because the net changes in compression and lateral shear forces on the spine were not significant, and the anteroposterior shear was reduced.

CONCLUSION

The results of the study do not suggest that an increase in the muscular loading of the spine occurs as a result of changing trunk muscular recruitment patterns. Therefore, future studies should focus on injury mechanisms that may occur as a result of a change in the viscoelastic passive tissue responses, muscular insufficiency, or a decline in neuromuscular control and coordination.