Trunk Exertions Research Articles

Relationship Between BMI and Fatigability Is Task Dependent.

The objective of the current study was to determine the effect of body mass index (BMI) on fatigability of three different muscle groups at four different work intensities. Forty-nine normal-weight, 50 overweight, and 43 obese adults (32.1 ± 9.2 years; 50% males) performed fatiguing handgrip, shoulder flexion, and trunk extension exertions at 20%, 40%, 60%, and 80% of the associated maximum voluntary contractions. Obese adults demonstrated 22% to 30% shorter endurance times than normal-weight adults, but this was only observed at lower intensities and with larger and more postural muscles of the shoulder and low back. Strength and fatigue-related strength loss remained comparable across BMI groups in both males and females in these task-specific conditions. Obesity was associated with faster progression in perception of effort at low-intensity shoulder and trunk exertions. While males were stronger than females across all muscle groups, females exhibited greater shoulder fatigue resistance than males at lower intensity levels. Findings indicate that the relationship between obesity and fatigability is task dependent. These findings provide initial evidence on the impact of obesity on worker capacity. Future work that extends the current investigation to include more occupationally relevant scenarios are needed to facilitate occupational task (re)design and assessment practices, such that altered work capacities of two-thirds of the working population are accommodated.

Is there a reliable and invariant set of muscle synergy during isometric biaxial trunk exertion in the sagittal and transverse planes by healthy subjects?

It has been suggested that the central nervous system simplifies muscle control through basic units, called synergies. In this study, we have developed a novel target-matching protocol and used non-negative matrix factorization (NMF) technique to extract trunk muscle synergies and corresponding torque synergies. Isometric torque data at the L5/S1 level and electromyographic patterns of twelve abdominal and back muscles from twelve healthy participants (five females) were simultaneously recorded. Each participant performed a total number of 24 isometric target-matching tasks using 12 different angular directions and 2 levels of uniaxial and biaxial exertions. Within- and between-subject similarities were assessed by considering both the data of different pairs of participants, where the activation coefficients of one participant were used in the NMF analysis of another participant, and the Pearson’s correlation coefficients (R) between muscle synergy vectors. The results showed that, for a healthy person, a set of four muscles (overall variance accounted for (VAF) of 97.9±0.53%) and four corresponding torque synergies (overall VAF of 92.2±3.03%) could efficiently decompose the sagittal and transverse torque planes into their main directions. Furthermore, the correlation coefficients were 0.77±0.12, 0.86±0.08, 0.78±0.12, and 0.93±0.04, for all synergies, reflecting the consistency of muscle synergies across participants. Overall, our results suggest that by taking advantage of muscle synergies we could potentially overcome the redundancy inherent to control strategies of the trunk neuromuscular system. In future studies, the synergies identified in patients with low back pain could be compared with those extracted from healthy participants towards various clinical and rehabilitation applications.

Assessment of movement distribution in the lumbar spine using the instantaneous axis of rotation

The position of the torso and the magnitude of exertion are thought to influence the distribution pattern of intervertebral movements within the lumbar spine. Abnormal intervertebral movements have been correlated with the risk of spine injuries. Since the capability to measure movement distribution within the lumbar spine noninvasively is limited, a convenient method to diagnose joint motion function was proposed. The goal of this research was to test the efficacy of the instantaneous axis of rotation for assessment of the distribution of movement within the lumbar spine. The proposed method was evaluated in the biomechanical model. The results showed that the location of instantaneous axis of rotation lowered with increased trunk exertion force, and slightly moved higher with increased trunk angle. Recognizing that abnormal location of the instantaneous axis of rotation correlated with spinal pain, these results suggest potential the location of the instantaneous axis of rotation relates to the risk of low back pain on distributed spinal kinematics.

Comparison of isometric exercises for activating latissimus dorsi against the upper body weight

Because there is little agreement as to which exercise is the most effective for activating the latissimus dorsi, and its intramuscular components are rarely compared, we investigated the intramuscular components of the latissimus dorsi during both trunk and shoulder exercises. Sixteen male subjects performed four isometric exercises: inverted row, body lifting, trunk extension, and trunk lateral bending. Surface electromyography (sEMG) was used to collect data from the medial and lateral components of the latissimus dorsi, lower trapezius, and the erector spinae at the 12th thoracic level during the isometric exercises. Two-way repeated analysis of variance with two within-subject factors (muscles and exercise conditions) was used to determine the significance of differences between the muscles and differences between exercise variations. The inverted row showed the highest values for the medial latissimus dorsi, which were significantly higher than those of the body lifting or trunk extension exercises. For the lateral latissimus dorsi, lateral bending showed significantly higher muscle activity than the inverted row or trunk extension. During body lifting, the % maximum voluntary isometric contraction (MVIC) of the erector spinae showed the lowest value, significantly lower than those of the other isometric exercises. The inverted row exercise was effective for activating the medial latissimus dorsi versus the shoulder depression and trunk exertion exercises. The lateral bending and body lifting exercises were favorable for activating the lateral component of the latissimus dorsi. Evaluating trunk lateral bending is essential for examining the function of the latissimus dorsi.

Trunk strength, muscle activity and spinal loads in maximum isometric flexion and extension exertions: A combined in vivo-computational study

Determination of the trunk maximum voluntary exertion moment capacity and associated internal spinal forces could serve in proper selection of workers for specific occupational task requirements, injury prevention and treatment outcome evaluations. Maximum isometric trunk exertion moments in flexion and extension along with surface EMG of select trunk muscles are measured in 12 asymptomatic subjects. Subsequently and under individualized measured harness-subject forces, kinematics and upper trunk gravity, an iterative kinematics-driven finite element model is used to compute muscle forces and spinal loads in 4 of these subjects. Different co-activity and intra-abdominal pressure levels are simulated. Results indicate significantly larger maximal resistant moments and spinal compression/shear forces in extension exertions than flexion exertions. The agonist trunk muscles reach their maximum force generation (saturation) to greater extent in extension exertions compared to flexion exertions. Local lumbar extensor muscles are highly active in extension exertions and generate most of the internal spinal forces. The maximum exertion attempts produce large spinal compression and shear loads that increase with the antagonist co-activity level but decrease with the intra-abdominal pressure. Intra-abdominal pressure decreases agonist muscle forces in extension exertions but generally increase them in flexion exertions.

Stationarity test of electromyographic signals during isokinetic trunk exertions

The aim of this study is to investigate the stationarity of the electromyographic (EMG) signals during isokinetic exertions of the trunk muscle. Six healthy subjects participated in this study. The erector spinae muscles were used to collect the signals. In order to simulate the isokinetic exercises of the trunk, a Cybex system with a trunk modular component was used. All subjects performed sustained exertions with a combination of load (25%, 50% MVC) and angular velocity (0, 15, 30 degree/s). The stationarity of the signals was tested by the reverse arrangements test and modified reverse arrangements test. In the results, the signal stationarity was high in all conditions in the reverse arrangements test. However, in the modified reverse arrangements test it was relatively low at about the 30 degree/s angular velocity. In the analysis of variance, the effect of angular velocity was statistically significant at the 0.05 significance level in the modified reverse arrangements test and Tukey's HSD grouping showed that the stationarity of 30 degree/s angular velocity was significantly different from 0, 15 degree/s condition. These results indicate that the stationarity of the EMG signals showed no difference between isometric and isokinetic conditions when the angular velocity was equal to 15 degree/s.

A comparison of a maximum exertion method and a model-based, sub-maximum exertion method for normalizing trunk EMG

The problem with normalizing EMG data from patients with painful symptoms (e.g., low back pain) is that such patients may be unwilling or unable to perform maximum exertions. Furthermore, the normalization to a reference signal, obtained from a maximal or sub-maximal task, tends to mask differences that might exist as a result of pathology. Therefore, we presented a novel method (GAIN method) for normalizing trunk EMG data that overcomes both problems. The GAIN method does not require maximal exertions (MVC) and tends to preserve distinct features in the muscle recruitment patterns for various tasks. Ten healthy subjects performed various isometric trunk exertions, while EMG data from 10 muscles were recorded and later normalized using the GAIN and MVC methods. The MVC method resulted in smaller variation between subjects when tasks were executed at the three relative force levels (10%, 20%, and 30% MVC), while the GAIN method resulted in smaller variation between subjects when the tasks were executed at the three absolute force levels (50 N, 100 N, and 145 N). This outcome implies that the MVC method provides a relative measure of muscle effort, while the GAIN-normalized data gives an estimate of the absolute muscle force. Therefore, the GAIN-normalized data tends to preserve the differences between subjects in the way they recruit their muscles to execute various tasks, while the MVC-normalized data will tend to suppress such differences. The appropriate choice of the EMG normalization method will depend on the specific question that an experimenter is attempting to answer.

런 검정을 사용한 근전도 신호의 안정성 평가 시 분할 크기가 신호의 안정성에 미치는 영향

Runs test is a mathematical tool to test the stationarity of electromyographic (EMG) signals. The purpose of this study is to investigate the effects of segmentation size on the stationarity of EMG signals in runs test. Six subjects participated in this experiment and performed isometric trunk exertions for twenty seconds at the load level of 25% and 50% MVC. The signals extracted from the erector spinae muscles were divided into the intervals of 1000ms and the stationarity of the signal in each interval was tested by the runs test. In this test, seven segmentation sizes such as 1.0, 2.0, 3.9, 7.8, 15.6, 31.3 and 62.5ms were applied. Additionally, two stationarity tests of reverse arrangements test and modified reverse arrangements test were used to verify the results of the runs test. In results, the segmentation size of 62.5ms showed the similar results with the other stationarity tests. However, the stationarity values among there tests were different each other when segmentation sizes other than 62.5ms were used. These results indicated the effect of segmentation size in runs test that needs to be considered to have consistent and sensitive result in stationarity test.

Offering proper feedback to control for out-of-plane lumbar moments influences the activity of trunk muscles during unidirectional isometric trunk exertions

The assessment of trunk muscle activation and coordination using dynamometric measurements made in one anatomical plane has hardly minimized the production of out-of-plane coupled moments (CMs). This absence of control may add much variability in moment component partition as well as in recorded muscle activation. The aim of the study was to assess whether providing these CMs as visual feedback efficiently reduces them and whether this feedback influences trunk muscle activation. Twenty men performed five 5-s static ramp submaximal contractions, ranging from 0% to 55% of the maximal voluntary contraction (MVC), in six different directions while standing in a static dynamometer measuring L5/S1 moments. For each direction, four feedback conditions were offered, ranging from simple 1D-feedback in the primary plane of exertion, to full 3D-feedback. Surface electromyographic signals were collected for eight back and six abdominal muscles. Muscle activation amplitudes and CMs were extracted at each 10% force level from 10% to 50% maximum voluntary contraction (MVC). Providing 3D-feedback significantly reduced the CMs, at 50% MVC, by about 1–6%, 1–8% and 2–10% MVC in the sagittal, frontal and transverse planes, respectively. Providing 3D-feedback produced relatively small systematic effects (2–7%) on trunk muscle activation. However, the subjects responded differently to adequately control the coupled moments, leading in some cases to relatively high inter-individual differences in muscle activation. Interestingly, the statistical differences, and size of the effects, were mainly observed when the primary exertions were performed in the frontal and transverse planes. The implications of these findings are discussed.

The Influence of Pain-Related Fear on Peak Muscle Activity and Force Generation During Maximal Isometric Trunk Exertions

A cross-sectional study of peak activation of trunk muscles in 20 participants with chronic low back pain. To determine how pain-related fear influences peak activation of abdominal and trunk extensor muscles during maximal isometric trunk exertions. The pain adaptation model and the pain-spasm-pain model have been proposed to explain differences in trunk muscle activation between healthy participants and those with chronic low back pain. However, there is no clear consensus in the literature in support of either model, and there are no studies that have examined the role of pain-related fear on muscle activity in the context of these 2 models. Participants with chronic low back pain performed a series of maximal isometric exertions into extension, flexion, rotation right and left, and side-bend right and left from a tall kneeling position with the spine in a neutral posture. A doubly multivariate MANCOVA was conducted with a between subject factor of group (high pain-related fear, low pain-related fear), a within subject factor of direction (extension, flexion, rotation left and right, side bend left and right), McGill Pain Questionnaire ratings entered as covariates, and peak EMG for all 10 trunk muscles entered as dependent variables. Averaged across all pull directions and all 10 trunk muscles assessed, participants with high pain-related fear had peak EMG that was only 49.5% of participants with low fear. Additionally, we found significantly smaller peak force for isometric exertions in flexion, side-bend right, and side-bend left directions. Pain-related fear is associated with increased disability, in theory, because of avoidance behavior and disuse. Our data suggest that participants high in pain-related fear specifically avoid activation of the abdominal muscles during maximal isometric trunk exertions.

Trunk antagonist co-activation is associated with impaired neuromuscular performance

The goal of this paper was to determine if trunk antagonist activation is associated with impaired neuromuscular performance. To test this theory, we used two methods to impair neuromuscular control: strenuous exertions and fatigue. Force variability (standard deviation of force signal) was assessed for graded isometric trunk exertions (10, 20, 40, 60, 80% of max) in flexion and extension, and at the start and end of a trunk extensor fatiguing trial. Normalized EMG signals for five trunk muscle pairs (RA rectus abdominis, EO external oblique, IO internal oblique, TE thoracic erector spinae, and LE lumbar erector spinae) were collected for each graded exertion, and at the start and end of a trunk extensor fatiguing trial. Force variability increased for more strenuous exertions in both flexion (P < 0.001) and extension (P < 0.001), and after extensor fatigue (P < 0.012). In the flexion direction, both antagonist muscles (TE and LE) increased activation for more strenuous exertions (P < 0.001). In the extension direction, all antagonist muscles except RA increased activation for more strenuous exertions (P < 0.05) and following fatigue (P < 0.01). These data demonstrate a strong relationship between force variability and antagonistic muscle activation, irrespective of where this variability comes from. Such antagonistic co-activation increases trunk stiffness with the possible objective of limiting kinematic disturbances due to greater force variability.

Effects of trunk exertion force and direction on postural control of the trunk during unstable sitting

Background. Pushing and pulling exertions have been implicated as risk factors of low-back disorders. In an attempt to investigate the mechanisms by which pushing and pulling influence risk for low-back disorders, the goal of this study was to investigate the effects of trunk exertion force and exertion direction on postural control of the trunk during unstable sitting. Methods. Seat movements were recorded while subjects maintained a seated posture on a wobbly chair against different exertion forces (0N, 40N, and 80N) and exertion directions (trunk flexion and extension). Postural control of the trunk was assessed from kinematic variability (root-mean-squared amplitude and 95% ellipse area) and non-linear stability analyses (stability diffusion exponent and maximum finite-time Lyapunov exponent). Findings. Kinematic variability and non-linear stability estimates increased as exertion force increased including root-mean-squared amplitude ( P < 0.001), 95% ellipse area ( P < 0.001), stability diffusion exponent ( P = 0.042), and maximum finite-time Lyapunov exponent ( P < 0.001). A subset of measures indicated postural control of the trunk was poorer during flexion exertions compared to extension exertions including root-mean-squared amplitude ( P < 0.001), 95% ellipse area ( P = 0.046), and maximum finite-time Lyapunov exponent ( P = 0.002). Interpretation. Trunk exertion force and exertion direction affect postural control of the trunk. This study may aid in understanding how pushing and pulling exertions can potentially contribute to low-back disorders.

Capability and recruitment patterns of trunk during isometric uniaxial and biaxial upright exertion

Background Work-related risk factors of low back disorders have been identified to be external moments, awkward postures, and asymmetrical dynamic lifting amongst others. The distinct role of asymmetry of load versus posture is hard to discern from the literature. Hence, the aim of this study is to measure isometric trunk exertions at upright standing posture at different exertion level and degree of asymmetry to further delineate the effects of exertion level and asymmetry on neuromuscular capability response. Methods Fifteen healthy volunteers randomly performed trunk exertions at three levels (30%, 60%, and 100% of maximum voluntary exertion and five different angles (0°, 45°, 90°, 135°, and 180°) of normalized resultant moments. During each trial, the normalized EMG activity of 10 selected trunk muscles was quantified. Findings The EMG activity of the 10 trunk muscles was significantly ( P < 0.001) affected by the level of exertion and angle of normalized resultant moment, and their interactions. The controllability of the torque generation was reduced in biaxial exertions. The capability to generate and control the required trunk moments is significantly lowered during biaxial trunk exertions, while all muscles present higher EMG activity. These results suggest that the trunk muscles will be taxed higher while performing biaxial exertion tasks, increasing muscle fatigue possibly leading to a higher probability of low back injury. Interpretation The prediction of biaxial trunk performance based on uniaxial data will result in an overestimation of capability and controllability of the trunk during physically demanding tasks. This study provides a better understanding of the potential mechanisms of injury during asymmetrical and biaxial trunk exertion during work-related tasks.

Co-activation alters the linear versus non-linear impression of the EMG–torque relationship of trunk muscles

The use of electromyographic signals in the modeling of muscle forces and joint loads requires an assumption of the relationship between EMG and muscle force. This relationship has been studied for the trunk musculature and been shown to be predominantly non-linear, with more EMG producing less torque output at higher levels of activation. However, agonist–antagonist muscle co-activation is often substantial during trunk exertions, yet has not been adequately accounted for in determining such relationships. The purpose of this study was to revisit the EMG–moment relationship of the trunk recognizing the additional moment requirements necessitated due to antagonist muscle activity. Eight participants generated a series of isometric ramped trunk flexor and extensor moment contractions. EMG was recorded from 14 torso muscles, and the externally resisted moment was calculated. Agonist muscle moments (either flexor or extensor) were estimated from an anatomically detailed biomechanical model of the spine and fit to: the externally calculated moment alone; the externally calculated moment combined with the antagonist muscle moment. When antagonist activity was ignored, the EMG–moment relationship was found to be non-linear, similar to previous work. However, when accounting for the additional muscle torque generated by the antagonist muscle groups, the relationships became, in three of the four conditions, more linear. Therefore, it was concluded that antagonist muscle co-activation must be included when determining the EMG–moment relationship of trunk muscles and that previous impressions of non-linear EMG–force relationships should be revisited.

The effects of breath control on maximum force and IAP during a maximum isometric lifting task

Evidence exists linking breath control to increases in intra-abdominal pressure and lumbar stability. Weight-lifting experts use this evidence as a rationale to suggest that increases in lumbar stability afforded by specific forms of breath control can influence the amount of force produced by the trunk. No studies have examined this issue. Therefore, this study determined whether voluntary control of the breath is related to maximal trunk extension force and if maximal force is correlated to intra-abdominal pressure. Thirteen men and 20 women (mean age: 25.6 years (5.5)) performed a maximal isometric trunk exertion in a knee bent posture using voluntary breath conditions: (1) inhalation prior to exertion with hold during exertion; (2) exhalation prior to exertion with hold during the exertion; (3) inhalation prior to the exertion with exhalation during the exertion. A subset of subjects (n=11) were also simultaneously measured for intra-abdominal pressure. Separate repeated measures ANOVA were used to determine the effects of breath conditions on force and intra-abdominal pressure. Pearson coefficients were used to determine the correlation between force and intra-abdominal pressure. Breath control did not significantly affect isometric force production (P=.089) but did affect intra-abdominal pressure (P=.003). Correlations between force and intra-abdominal pressure in each breath condition were low (range: 0.152-0.583). Although breath control was shown to influence intra-abdominal pressure, it does not appear to influence isometric trunk extension force in a knees bent position. Further, the intra-abdominal pressure produced in such efforts appears to be unrelated to the amount of force produced.

The effects of lumbosacral orthoses on spine stability: What changes in EMG can be expected?

Antagonistic trunk muscle activity is normally required to stabilize the spine. A lumbosacral orthosis (LSO) might reduce the need for this antagonistic activity by providing passive stiffness to the trunk and increasing spine stability. The maximum reduction in trunk muscle EMG and in the resultant spine compression force due to the LSO was estimated using a biomechanical model. The lumbar spine stability was first quantified for the average trunk muscle EMG recorded from 11 male subjects performing various isometric trunk exertion tasks. Subsequently, the spine-stiffening effects of the LSO were implemented in the model and trunk muscle forces were reduced iteratively until the original level of spine stability without the LSO was achieved. The upper bound estimates of the reduction in trunk muscle EMG due to LSO ranged from 0.6% to 14.1% of the maximum voluntary activation depending on the task and the muscle. The resultant spine compression force averaged across all tasks decreased by only 355 N. A much larger variance of the experimental data precluded the detection of these effects at statistically significant levels. However, the small effects size does not necessarily exclude the possibility of functional benefits of slightly reducing muscle activity in patients with low back pain.

Posture-dependent trunk extensor EMG activity during maximum isometrics exertions in normal male and female subjects

Posture-dependent trunk function data are important for appropriate normalization of submaximal trunk exertions, and is also necessary to define a more precise and specific use for strength testing in the prevention and diagnosis of spinal disorders. The aim of the current study was to quantify maximal effort trunk muscle extensor activity and trunk isometric extension torque over a functional range of sagittal standing postures. Twenty healthy, young adult male and female subjects performed isometric extension tasks over a sagittal posture range of –20° extension to +50° flexion, in 10° increments. Erector spinae muscle activity was recorded bilaterally at the level of L3 using surface EMG electrodes. Isometric trunk extension torque was measured using a trunk dynamometer. EMG and trunk torque differed significantly between genders, but there were no differences between male and female subjects when the data were normalized with respect to the upright posture. For the combined male and female population, upright posture normalized L3 EMG activity (EMG n) and trunk extension torque (T n) increased 1.7-fold and 3.5-fold, respectively, over the 70° range of sagittal postures examined. The ratio (T n/EMG n) increased two-fold (0.83 to 1.67) from –20° extension to +50° flexion, indicating that the neuromuscular efficiency increases with flexion. Trunk extension torque normalized with respect to the upright posture was linearly and positively correlated ( r=0.59, P<0.001) to similarly normalized L3 EMG activity. This relatively weak correlation suggests that trunk muscle synergism and/or intrinsic muscle length-tension relationships are also modulated by posture. This study provides data that can be used to estimate trunk extensor muscle function over a broad range of sagittal postures. Our findings indicate that appropriate postural normalization of trunk extensor EMG activity is necessary for studies where submaximal trunk exertions are performed over a range of upright postures.

Performance of an Artificial Neural Network Model in the Prediction of Lower Torso Muscle Recruitment Patterns

The prediction of muscle forces around the low back is a key aspect in the calculation of spinal loads. Artificial Neural Networks (ANNs) are suited to handle this prediction problem, and have been employed in the past in similar efforts. This paper evaluates the performance of an ANN in predicting trunk muscle forces, whose values were estimated from measurements of myoelectric activity during various high-magnitude static trunk exertions. Sensitivity of the ANN to its two user-adjustable parameters, Muscle Self-Inhibition and Muscle Inhibition, is also evaluated. Results indicate moderate agreement between predicted forces and forces estimated from measured myoelectric activity (average R2s in the 0.50s), although some muscles were well predicted (R2s in the 0.80s). The model iterations resulting from variations in the ANN Inhibition parameters varied only slightly in terms of prediction performance levels, suggesting that this particular ANN model may be limited by other factors, including inter-individual differences in motor control strategies.

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

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. To determine whether patients with low back pain will exhibit poorer postural control, which will be associated with longer average muscle response times. Larger postural sway during standing and delayed trunk muscle response times for patients with low back pain have been reported in several independent studies. 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. 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 (R(2) = 0.46), but not with eyes opened (R(2) = 0.18). 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.

An Electromyography-Assisted Model to Estimate Trunk Muscle Forces During Fatiguing Repetitive Trunk Exertions

During submaximal shortening muscle contraction, fatigue characteristically results in an increase in measured surface electromyography, whereas the maximum force that can be produced by muscle is reduced. This finding compromises researchers' ability to estimate muscle stress in a joint system such as the spine, which is composed of more muscles than degrees of freedom of the joint. A three-dimensional, electromyography-assisted, dynamic biomechanical model of spinal loading was developed and validated for use during fatiguing repetitive trunk extension exertions. A time-varying maximum muscle stress was included to model the effect of a change in the maximum force-producing capacity of the erector spinae muscle. Sixteen men performed submaximal isokinetic trunk extension endurance tests at 15 degrees per second. The exertion level (35% and 70% of their maximum dynamic extension torque) and repetition rate (5 and 10 repetitions per minute) of the tests were varied during four testing sessions. Using trunk muscle electromyography and the measured torque as input, the model predicted significant linear reductions in the maximum muscle stress in 78% of the endurance tests, which resulted in an estimated decrease in erector spinae force in 75% of the tests. Conversely, if the maximum muscle stress was assumed to be constant, the erector spinae force would have been predicted to increase in 73% of the tests. The magnitude of the change in predicted erector spinae maximum muscle stress and force depended on the exertion level and repetition rate. This model will allow researchers to assess the effects of changes in recruitment patterns of trunk muscles during dynamic trunk extension on the estimated spinal loading of the lumbar spine.