Abdominal muscles dominate contributions to vertebral joint stiffness during the push-up

The goal of this study was to quantify the relative contributions of each muscle group surrounding the spine to vertebral joint rotational stiffness (VJRS) during the push-up exercise. Upper-body kinematics, three-dimensional hand forces and lumbar spine postures, and 14 channels (bilaterally from rectus abdominis, external oblique, internal oblique, latissimus dorsi, thoracic erector spinae, lumbar erector spinae, and multifidus) of trunk electromyographic (EMG) activity were collected from 11 males and used as inputs to a biomechanical model that determined the individual contributions of 10 muscle groups surrounding the lumbar spine to VJRS at five lumbar vertebral joints (L1-L2 to L5-S1). On average, the abdominal muscles contributed 64.32 +/- 8.50%, 86.55 +/- 1.13%, and 83.84 +/- 1.95% to VJRS about the flexion/extension, lateral bend, and axial twist axes, respectively. Rectus abdominis contributed 43.16 +/- 3.44% to VJRS about the flexion/extension axis at each lumbar joint, and external oblique and internal oblique, respectively contributed 52.61 +/- 7.73% and 62.13 +/- 8.71% to VJRS about the lateral bend and axial twist axes, respectively, at all lumbar joints with the exception of L5-S1. Owing to changes in moment arm length, the external oblique and internal oblique, respectively contributed 55.89% and 50.01% to VJRS about the axial twist and lateral bend axes at L5-S1. Transversus abdominis, multifidus, and the spine extensors contributed minimally to VJRS during the push-up exercise. The push-up challenges the abdominal musculature to maintain VJRS. The orientation of the abdominal muscles suggests that each muscle primarily controls the rotational stiffness about a single axis.     

Position sense in the lumbar spine with torso flexion and loading

Proprioception plays an important role in appropriate sensation of spine position, movement, and stability. Previous research has demonstrated that position sense error in the lumbar spine is increased in flexed postures. This study investigated the change in position sense as a function of altered trunk flexion and moment loading independently. Reposition sense of lumbar angle in 17 subjects was assessed. Subjects were trained to assume specified lumbar angles using visual feedback. The ability of the subjects to reproduce this curvature without feedback was then assessed. This procedure was repeated for different torso flexion and moment loading conditions. These measurements demonstrated that position sense error increased significantly with the trunk flexion (40%, p < .05) but did not increase with moment load (p = .13). This increased error with flexion suggests a loss in the ability to appropriately sense and therefore control lumbar posture in flexed tasks. This loss in proprioceptive sense could lead to more variable lifting coordination and a loss in dynamic stability that could increase low back injury risk. This research suggests that it is advisable to avoid work in flexed postures.     

The relationship between flexibility and EMG activity pattern of the erector spinae muscles during trunk flexion–extension

BackgroundMovements in the lumbar spine, including flexion and extension are governed by a complex neuromuscular system involving both active and passive units. Several biomechanical and clinical studies have shown the myoelectric activity reduction of the lumbar extensor muscles (flexion–relaxation phenomenon) during lumbar flexion from the upright standing posture. The relationship between flexibility and EMG activity pattern of the erector spinae during dynamic trunk flexion–extension task has not yet been completely discovered.ObjectiveThe purpose of this study was to investigate the relationship between general and lumbar spine flexibility and EMG activity pattern of the erector spinae during the trunk flexion–extension task.MethodsThirty healthy female college students were recruited in this study. General and lumbar spine flexibilities were measured by toe-touch and modified schober tests, respectively. During trunk flexion–extension, the surface electromyography (EMG) from the lumbar erector spinae muscles as well as flexion angles of the trunk, hip, lumbar spine and lumbar curvature were simultaneously recorded using a digital camera. The angle at which muscle activity diminished during flexion and initiated during extension was determined and subjected to linear regression analysis to detect the relationship between flexibility and EMG activity pattern of the erector spinae during trunk flexion–extension.ResultsDuring flexion, the erector spinae muscles in individuals with higher toe-touch scores were relaxed in larger trunk and hip angles and reactivated earlier during extension according to these angles (P < 0.001) while in individuals with higher modified schober scores this muscle group was relaxed later and reactivated sooner in accordance with lumbar angle and curvature (P < 0.05). Toe-touch test were significantly correlated with trunk and hip angles while modified schober test showed a significant correlation with lumbar angle and curvature variables.ConclusionThe findings of this study indicate that flexibility plays an important role in trunk muscular recruitment pattern and the strategy of the CNS to provide stability. The results reinforce the possible role of flexibility alterations as a contributing factor to the motor control impairments. This study also shows that flexibility changes behavior is not unique among different regions of the body.     

An examination of the flexion-relaxation phenomenon in the cervical spine in lumbo-pelvic sitting

The flexion-relaxation phenomenon (FRP) is well documented at end-range lumbar spine flexion in both standing and sitting however, the FRP has been insufficiently investigated in cervico-thoracic musculature. The aim of this study was to determine whether the FRP occurs during forward flexion of the neck, in lumbo-pelvic sitting, amongst a pain-free population. Surface electromyography (EMG) was used to measure muscle activation in 20 (10 men, 10 women) asymptomatic subjects in selected cervico-thoracic muscles during four, 5-s phases (upright posture, forward flexion, full flexion and return to upright) while subjects were positioned in lumbo-pelvic sitting. Spinal kinematics were simultaneously measured using an electromagnetic motion tracking device. No FRP was observed in upper trapezius or thoracic erector spinae (T4). When using visual methods to determine the presence/absence of the FRP, five subjects were believed to show evidence of the FRP in the cervical erector spinae. However, when using various non-visual criteria to determine the existence of the FRP, substantial variations (0–13 subjects) were evident. We recommend that criteria based upon relatively large differences in muscle activation should be considered when defining the FRP. These findings are of significance for future investigations examining specific cervical pain disorders.     

The use of intermittent trunk flexion to alleviate low back pain during prolonged standing

The current study examined of the effect of intermittent, short-term periods of full trunk flexion on the development of low back pain (LBP) during two hours of standing. Sixteen participants completed two 2-h standing protocols, separated by one week. On one day, participants stood statically for 2 h (control day); on the other day participants bent forward to full spine flexion (termed flexion trials) to elicit the flexion relaxation (FR) phenomenon for 5 s every 15 min (experimental day). The order of the control and experimental day was randomized. During both protocols, participants reported LBP using a 100 mm visual analogue scale every 15 min. During the flexion trials, lumbar spine posture, erector spinae and gluteus medius muscle activation was monitored. Ultimately, intermittent trunk flexion reduced LBP by 36% (10 mm) at the end of a 2-h period of standing. Further, erector spinae and gluteus medius muscle quietening during FR was observed in 91% and 65% of the flexion trials respectively, indicating that periods of rest did occurred possibly contributing to the reduction in LBP observed. Since flexion periods do not require any aids, they can be performed in most workplaces thereby increasing applicability.     

Determination of trunk muscle forces for flexion and extension by using a validated finite element model of the lumbar spine and measured in vivo data

Muscle forces stabilize the spine and have a great influence on spinal loads. But little is known about their magnitude. In a former in vitro experiment, a good agreement with intradiscal pressure and fixator loads measured in vivo could be achieved for standing and extension of the lumbar spine. However, for flexion the agreement between in vitro and in vivo measurements was insufficient. In order to improve the determination of trunk muscle forces, a three-dimensional nonlinear finite element model of the lumbar spine with an internal fixation device was created and the same loads were applied as in a previous in vitro experiment. An extensive adaptation process of the model was performed for flexion and extension angles up to 20 degrees and -15 degrees, respectively. With this validated computer model intra-abdominal pressure, preload in the fixators, and a combination of hip- and lumbar flexion angle were varied until a good agreement between analytical and in vivo results was reached for both, intradiscal pressure and bending moments in the fixators. Finally, the fixators were removed and the muscle forces for the intact lumbar spine calculated. A good agreement with the in vivo results could only be achieved at a combination of hip- and lumbar flexion. For the intact spine, forces of 170, 100 and 600 N are predicted in the m. erector spinae for standing, 5 degrees extension and 30 degrees flexion, respectively. The force in the m. rectus abdominus for these body positions is less than 25 N. For more than 10 degrees extension the m. erector spinae is unloaded. The finite element method together with in vivo data allows the estimation of trunk muscle forces for different upper body positions in the sagittal plane. In our patients, flexion of the upper body was most likely a combination of hip- and lumbar spine bending.     

Does prolonged seated deskwork alter the lumbar flexion relaxation phenomenon?

Sustained maximum lumbar spine flexion can increase the angle at which the low back flexion relaxation phenomenon (FRP) is observed. This adaptation has been hypothesized to have implications for the control of lumbar spine stability and increase the potential for low back injury. The objective of this study was to investigate if similar changes in the FRP would occur from sub-maximal spine flexion induced by an extended continuous duration of seated office deskwork. Twenty-three participants (12 male and 11 female) performed three bouts of full forward spine flexion interspersed with two 1-h periods of seated deskwork. Lumbar spine angular kinematics and electromyographic activity from the lumbar erector spinae were obtained throughout all trials. The angles at which myoelectric silence occurred (FRP onset) were documented. Lumbar flexion at FRP onset increased by 1.3 ± 1.5° after 1-h of sitting (p < 0.05) with no further increase after 2-h. However, when the angle at the FRP onset was normalized to the total range of flexion, there was no difference in the FRP onset. These results suggest that the seated posture may induce residual deformation in the viscoelastic passive tissues of the low back; this could increase the challenge of controlling spine motion and reduce the load-bearing capacity of the lumbar spine system during activities performed following extended bouts of sitting.     

Influence of chronic low back pain and fear of movement on the activation of the transversely oriented abdominal muscles during forward bending

Introduction: Chronic low back pain (CLBP) and fear of movement (kinesiophobia) are associated with an overactivation of paravertebral muscles during forward bending. This impairs spine motor control and contributes to pain perpetuation. However, the abdominal muscles activation is engaged too in spine stabilization but its modulation with kinesiophobia remains unknown. Our study tested whether CLBP and kinesiophobia affected the activation pattern of abdominal muscles during trunk flexion/extension. Methods: Surface electromyographical recordings of the internal oblique/transversus abdominis (IO/TrA) and external oblique (EO) muscles were analyzed in 12 people with CLBP and 13 pain-free subjects during low-velocity forward bending back and forth from erected posture. Tampa Scale of Kinesiophobia was also administrated. Results: IO/TrA activation, but not EO, was modulated across the phases of movement in both groups, i.e. maximal at onset of flexion and end of extension, and minimal at full flexion. In CLBP group only, IO/TrA activation was increased near to full trunk flexion and in correlation with kinesiophobia. Conclusions: The phase-dependence of IO/TrA activation during trunk flexion/extension in standing may have a role in spine motor control. The influence of kinesiophobia in CLBP should be further investigated as an important target in CLBP management.     

EMG and kinematics of normal subjects performing trunk flexion/extensions freely in space.

Ten normal subjects performed continuous trunk flexion/extensions (F/E) without any restraining apparatus at free, 3, 2.25 and 1.5 s periods and a fatiguing task of F/E at 1.5 s period during 45 s. Kinematics of the trunk was obtained with bilateral electromyographic (EMG) activity of the erector spinae (three levels), the abdominal oblique muscles and the rectus abdominis muscles. The free period chosen by the subjects was found to vary between 3.05 and 1.47 s. Lateral flexion of the spine was similar in each task but rotation about its longitudinal axis increased as the F/E period shortened. When left and right side EMG signals were grouped by level of recording, a significant difference in activity was found. Subjects who produced the slowest free F/E displayed larger fatigue indexes derived from the EMG signals for some of their back muscles than for other subjects. The flexion/relaxation phenomenon was considered present in a muscle if a level <10% of the maximum signal recorded during extension was detected. The phenomenon was found in >50% of the observations and occurred at a similar angle in each task. Kinematics and several characteristics of the EMG signals of the trunk were statistically independent of the speed of motion.     

Viscoelastic creep induced by repetitive spine flexion and its relationship to dynamic spine stability

Repetitive trunk flexion elicits passive tissue creep, which has been hypothesized to compromise spine stability. The current investigation determined if increased spine flexion angle at the onset of flexion relaxation (FR) in the lumbar extensor musculature was associated with altered dynamic stability of spine kinematics. Twelve male participants performed 125 consecutive cycles of full forward trunk flexion. Spine kinematics and lumbar erector spinae (LES) electromyographic (EMG) activity were obtained throughout the repetitive trunk flexion trial. Dynamic stability was evaluated with maximum finite-time Lyapunov exponents over five sequential blocks of 25 cycles. Spine flexion angle at FR onset, and peak LES EMG activity were determined at baseline and every 25th cycle. Spine flexion angle at FR increased on average by 1.7° after baseline with significant increases of 1.7° and 2.4° at the 50th and 100th cycles. Maximum finite-time Lyapunov exponents demonstrated a transient, non-statistically significant, increase between cycles 26 and 50 followed by a recovery to baseline over the remainder of the repetitive trunk flexion cycles. Recovery of dynamic stability may be the consequence of increased active spine stiffness demonstrated by the non-significant increase in peak LES EMG that occurred as the repetitive trunk flexion progressed.     

Lumbar back muscle activity of helicopter pilots and whole-body vibration

Several studies have attributed the prevalence of low back pain (LBP) in helicopter pilots mainly to poor posture in-flight and whole-body vibration, with the latter hypothesis particularly related to a cyclic response of the erector spine (ES) muscle to vibration. This work aims to determine if helicopter vibration and the pilot's normal posture during flight have significant effects on the electromyogram (EMG) of the ES muscle. The bilateral surface EMG of the ES muscle at the L3 level was collected in 10 young pilots before and during a short flight in UH-50 helicopters. The vibration was monitored by a triaxial accelerometer fixed to the pilots' seat. Prior to the flight, the EMG was recorded for relaxed seated and standing postures with 0 degrees (P0) and 35 degrees (P35) of trunk flexion. The effect of the posture during the flight was tested by comparing left and right EMG (normalized with respect to P35). The in-flight muscle stress was evaluated by histograms of EMG activity, and compared to P0 values. Only one pilot in ten showed significant (p<0.05) correlation between the vibration and the EMG over cycles of vibration, and no consistent causal effect was found. The pilots' posture did not show significant asymmetric muscular activity, and low EMG levels were observed during most of the duration of the flight. The results do not provide evidence that LBP in helicopter pilots is caused by ES muscle stress in the conditions studied.     

EMG in rotation–flexion of the torso

The objective of this study was to determine the magnitude and phasic relationship of the torso muscles in rotation–flexion of varying degree of asymmetries of the trunk. Nineteen normal young subjects (7 males and 12 females) were stabilized on a posture stabilizing platform and instructed to assume a flexed and right rotated posture. A combination 20°, 40° and 60° of rotation and 20°, 40° and 60° of flexion resulted in nine postures. These postures were assumed in a random order. The subjects were asked to exert their maximal voluntary isometric contraction (MVC) in the plane of rotation of the posture assumed for a period of 5 s. The surface EMG from the external and internal obliques, rectus abdominis, latissimus dorsi and erector spinae at the 10th thoracic and 3rd lumbar vertebral levels was recorded. The abdominal muscles had the least response at 40° of flexion, the dorsal muscles had the highest magnitude.With increasing right rotation, the left external oblique continued to decrease its activity. The ANOVA revealed that rotation and muscles had a significant main effect on normalized peak EMG (p < 0.02) in both genders. There was a significant interaction between rotation and flexion in both genders (p < 0.02) and rotation and muscle in females. The erector spinae activity was highest at 40° flexion, due to greater mechanical disadvantage and having not reached the state of flexion–relaxation. The abdominal muscle activity declined with increasing asymmetry, due to the decreasing initial muscle length. The EMG activity was significantly affected by rotation than flexion (p < 0.02).     

The effect of a knee support on the biomechanical response of the low back

Stooping and squatting postures are seen in a number of industries (e.g., agriculture, construction) where workers must work near ground level for extended periods of time. The focus of the current research was to evaluate a knee support device designed to reduce the biomechanical loading of these postures. Ten participants performed a series of sudden loading tasks while in a semisquat posture under two conditions of knee support (no support and fully supported) and two conditions of torso flexion (45 and 60 degrees ). A weight was released into the hands of the participants who then came to steady state while maintaining the designated posture. As they performed this task, the EMG responses of the trunk extensors (multifidus and erector spinae) were collected, both during the "sudden loading" phase of the trial as well as the steady weight-holding phase of the trial. As expected, the effects of torso flexion angle showed significant decreases in the activation of the multifidus muscles with greater torso angle (indicating the initiation of the flexion-relaxation response). Interestingly, the results showed that the knee support device had no effect on the activation levels of the sampled muscles, indicating that the loss of the degree of freedom from the ankle joint during the knee support condition had no impact on trunk extensor muscle response. The a priori concern with regard to these supports was that they would tend to focus loading on the low back and therefore would not serve as a potential ergonomic solution for these stooping/semisquatting tasks. Because the results of this study did not support this concern, further development of such an intervention is underway.     

Effect of a personal weight transfer device on muscle activities and joint flexions in the stooped posture

Repetitive work in the stooped posture is a known risk factor for developing low back disorders (LBDs); regardless, the stooped posture is widespread throughout the world in the agriculture, construction, and mining industries. An on-body weight transfer device was tested as a possible intervention for reducing the risk of developing LBDs.Eighteen subjects (11 male and 7 female), with no history of LBDs, performed stooped posture tasks in the laboratory. Surface electromyograms of the erector spinae, rectus abdominis, biceps femoris, and tibialis anterior muscles were recorded. Bodily joint flexions were measured with a combination of inclinometers and electrogoniometers.When wearing the device in the stooped posture, biceps femoris activity and lumbar flexion were significantly reduced. Subjects who did not experience flexion–relaxation had a significant reduction in lumbar erector spinae activity.By reducing back muscle activity, and by limiting lumbar flexion, the device could reduce the risk of developing LBDs for those who work while adopting the stooped posture. The device may also be beneficial for those with existing LBDs. Follow up field studies are needed to confirm the long-term potential benefits of such an intervention approach.     

An empirical approach to characterizing trunk muscle coactivation using simulation input modeling techniques

Accurately describing trunk muscle coactivation is fundamental to quantifying the spine reaction forces that occur during lifting tasks and has been the focus of a great deal of research in the spine biomechanics literature. One limitation of previous approaches has been a lack of consideration given to the variability in these coactivation strategies. The research presented in this paper is an empirical approach to quantifying and modeling trunk muscle coactivation using simulation input modeling techniques. Electromyographic (EMG) data were collected from 28 human subjects as they performed controlled trunk extension exertions. These exertions included isokinetic (10 and 45°/s) and constant acceleration (50°/s/s) trunk extensions in symmetric and asymmetric (30°) postures at two levels of trunk extension moment (30 and 80 Nm). The EMG data were collected from the right and left pairs of the erector spinae, latissimus dorsi, rectus abdominis, external obliques and internal obliques. Each subject performed nine repetitions of each combination of independent variables. The data collected during these trials were used to develop marginal distributions of trunk muscle activity as well as a 10×10 correlation matrix that described how the muscles cooperated to produce these extension torques. These elements were then combined to generate multivariate distributions describing the coactivation of the trunk musculature. An analysis of these distributions revealed that increases in extension moment, extension velocity and sagittal flexion angle created increases in both the mean and the variance of the distributions of the muscular response, while increases in the rate of trunk extension acceleration decreased both the mean and variance of the distributions of activity across all muscles considered. Increases in trunk asymmetry created a decrease in mean of the ipsi–lateral erector spinae and an increase in the mean of all other muscles considered, but there was little change in the variance of these distributions as a function of asymmetry.     

Trunk muscle activation during stabilization exercises with single and double leg support

The aim of this study was to analyze trunk muscle activity during bridge style stabilization exercises, when combined with single and double leg support strategies. Twenty-nine healthy volunteers performed bridge exercises in 3 different positions (back, front and side bridges), with and without an elevated leg, and a quadruped exercise with contralateral arm and leg raise ("bird-dog"). Surface EMG was bilaterally recorded from rectus abdominis (RA), external and internal oblique (EO, IO), and erector spinae (ES). Back, front and side bridges primarily activated the ES (approximately 17% MVC), RA (approximately 30% MVC) and muscles required to support the lateral moment (mostly obliques), respectively. Compared with conventional bridge exercises, single leg support produced higher levels of trunk activation, predominantly in the oblique muscles. The bird-dog exercise produced greatest activity in IO on the side of the elevated arm and in the contralateral ES. In conclusion, during a common bridge with double leg support, the antigravity muscles were the most active. When performed with an elevated leg, however, rotation torques increased the activation of the trunk rotators, especially IO. This information may be useful for clinicians and rehabilitation specialists in determining appropriate exercise progression for the trunk stabilizers.     

Trunk muscle electromyography and whole body vibration

By measuring the electromyographic (EMG) activity of the paraspinal muscles, we have estimated the average and peak-to-peak torque imposed on the spine during whole body vibration. Six subjects had surface electrodes placed on their erector spinae muscles at the L3 level. The EMG-torque relationship was estimated by having each subject perform isometric horizontal pulls in an upright seated posture. The subject was then vibrated vertically and sinusoidally in a controlled, flexed, slightly lordotic seated posture, in 1 Hz increments from 3 to 10 Hz at a 0.1 g RMS seat acceleration level. Between vibration readings taken at each frequency, a static reading was also taken with the subject maintaining the same posture. The entire vibration-static 3-10 Hz test was repeated for reliability purposes. Specialized digital signal processing techniques were developed for the EMG signals to enhance the measured cyclic muscle activity and to allow automatic measurement of the time relationship between the mechanical displacement and the estimated torque. We found significantly more average and peak-to-peak estimated torque at almost all frequencies for vibration vs static sitting.     

Analyses of myo-electrical silence of erectors spinae

Electromyographic activity of the erector spinae was studied in 25 healthy, young individuals during forward bending and then coming back to erect posture. Sudden onset of electrical silence called the flexion-relaxation phenomenon was seen to occur in all at 57% of the maximum hip flexion and at 84% of the maximum vertebral flexion. Abrupt re-commencement of the activity was seen at almost similar flexion angle while coming back to erect position. The experiment was repeated with the buttocks held against the wall so as to prevent the posterior migration of the pelvis and also the hip flexion to some extent. The effect was to produce inhibition of the electrical activity earlier at 75% of maximum vertebral flexion (p<0.001) while reactivation of erector spinae occurred soon after the extension started from the maximum trunk flexion. Eleven male subjects repeated the experimental task holding 22 lb weight in front and then on their back tied around the iliac crest. In both the instances the myo-electrical silence was found to occur at greater vertebral flexion. It is concluded that the passive equilibrium between gravity induced tensile torque and the extension torque of stretched posterior vertebral ligaments is responsible for the flexion-relaxation phenomenon than the stretch receptors.     

Corrective sitting strategies: An examination of muscle activity and spine loading

The purpose of this study was to quantify the load on the lumbar spine of subjects when they are asked to adjust from a slouched sitting posture into an upright posture with one of three different strategies: “free” (no instruction) and two coached patterns: “lumbopelvic” dominant and “thoracic” dominant. The activity of selected muscles and kinematic data was recorded from 20 volunteers while performing the three movement patterns to adjust sitting posture. Moments and forces at the lumbar spine were computed from an anatomically detailed model that uses kinematics and muscle activation as input variables.The lumbopelvic pattern produces less joint moment on the lumbar spine (on average 31.2 ± 3.9 N m) when compared to the thoracic pattern (43.8 ± 5.8 N m). However, the joint compression force was similar for these two patterns, but it was smaller in the free pattern, when no coaching was given (lumbopelvic: 1279 ± 112 N, thoracic: 1367 ± (125 N, free: 1181 ± 118 N). Lower thoracic erector muscle activity and higher lumbar erector activity were measured in the lumbopelvic pattern in comparison with the other two. In summary the lumbopelvic pattern strategy using predominantly the movement of anterior pelvic tilt results in smaller joint moments on the lumbar spine and also positions the lumbar spine closest to the neutral posture minimizing passive tissue stress. This may be the strategy of choice for people with low back flexion intolerance.     

Validation of lumbar spine loading from a musculoskeletal model including the lower limbs and lumbar spine

Low back mechanics are important to quantify to study injury, pain and disability. As in vivo forces are difficult to measure directly, modeling approaches are commonly used to estimate these forces. Validation of model estimates is critical to gain confidence in modeling results across populations of interest, such as people with lower-limb amputation. Motion capture, ground reaction force and electromyographic data were collected from ten participants without an amputation (five male/five female) and five participants with a unilateral transtibial amputation (four male/one female) during trunk-pelvis range of motion trials in flexion/extension, lateral bending and axial rotation. A musculoskeletal model with a detailed lumbar spine and the legs including 294 muscles was used to predict L4-L5 loading and muscle activations using static optimization. Model estimates of L4-L5 intervertebral joint loading were compared to measured intradiscal pressures from the literature and muscle activations were compared to electromyographic signals. Model loading estimates were only significantly different from experimental measurements during trunk extension for males without an amputation and for people with an amputation, which may suggest a greater portion of L4-L5 axial load transfer through the facet joints, as facet loads are not captured by intradiscal pressure transducers. Pressure estimates between the model and previous work were not significantly different for flexion, lateral bending or axial rotation. Timing of model-estimated muscle activations compared well with electromyographic activity of the lumbar paraspinals and upper erector spinae. Validated estimates of low back loading can increase the applicability of musculoskeletal models to clinical diagnosis and treatment.