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This study examined anterior chain whole body linkage exercises, namely the body saw, hanging leg raise and walkout from a push-up. Investigation of these exercises focused on which particular muscles were challenged and the magnitude of the resulting spine load. Fourteen males performed the exercises while muscle activity, external force and 3D body segment motion were recorded. A sophisticated and anatomically detailed 3D model used muscle activity and body segment kinematics to estimate muscle force, and thus sensitivity to each individual's choice of motor control for each task. Gradations of muscle activity and spine load characteristics were observed across tasks. On average, the hanging straight leg raise created approximately 3000 N of spine compression while the body saw created less than 2500 N. The hanging straight leg raise created the highest challenge to the abdominal wall (>130% MVC in rectus abdominis, 88% MVC in external oblique). The body saw resulted in almost 140% MVC activation of the serratus anterior. All other exercises produced substantial abdominal challenge, although the body saw did so in the most spine conserving way. These findings, along with consideration of an individual's injury history, training goals and current fitness level, should assist in exercise choice and programme design.

Background: Repeated flexion of an intervertebral disc has been identified as a mechanism to produce posterior herniations. Repeated extension under certain conditions has also been shown to cause the nucleus of partially herniated discs to reverse and migrate anteriorly. While research shows that the nucleus pulposus migrates anteriorly in extension and infiltrates the annulus posteriorly in flexion, it is not known if a cycle of flexion followed by a cycle of extension produces more or less annular damage compared to pure flexion alone., Methods: Two groups of porcine spinal motion segments were exposed to either repeated flexion with extension or just repeated flexion. Digitized photographs of dissected specimens enhanced with a radio-opaque blue dye enabled the quantification of the area of annulus infiltrated with nucleus pulposus., Findings: Specimens exposed to both flexion and extension showed significantly more annular damage and axial creep compared to those exposed to flexion alone., Interpretation: It would appear that while flexion alone can still cause nucleus pulposus to track through the annulus of an intervertebral disc, the effects are compounded when it is followed by a subsequent cycle of extension. Thus, movements which require both repetitive flexion and extension, have the potential to produce more annular damage than those which require merely flexion., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

The aim of this study was to quantify joint compression and shear forces at L4/L5 during exercises used to overload the hips. Nine men performed 36 "walking" trials using two modalities: (1) sled towing and (2) exercise bands placed around the ankles. Participants completed forward, backward, and lateral trials with bent and straight legs at three separate loads. Surface electromyography (EMG) was recorded bilaterally from eight torso and thigh sites, upper body and lumbar spine motion were quantified, and hand forces were measured. An EMG-driven musculoskeletal model was used to estimate the muscular contribution to joint compression and shear. Peak reaction, muscle and joint compression and shear forces, and peak gluteus medius and maximus activity were calculated. Significant differences were noted in each dependent measure; however, they were dependent on direction of travel, leg position, and load. The highest joint compression and shear forces for the sled and band conditions were 4378 N and 626 N, and 3306 N and 713 N, respectively. In general, increasing the band tension had little effect on all dependent measures, although a load-response was found during the sled conditions. Before using any exercise to improve hip function, the potential benefits should be weighed against "costs" to neighbouring joints.

Objective: To determine if spine stability would be affected by the competing demands of simultaneous challenged ventilation and supporting a hand-held load., Design: Subjects were their own controls in a repeated measures design where a single task was repeated, once in a different condition, in a random order., Background: Muscle stiffness influences spine stability. The same muscles that contribute to spine stability assist in challenged breathing. We hypothesized that a challenged ventilation task would place low back pain (LBP) sufferers at risk of spine instability., Methods: Subjects (14 normal; 14 with low back pain) performed two trials with a 22kg hand-held weight and the trunk angled forward at 30 degrees . One trial was of 60s duration while breathing ambient air, the other of 70s duration, while breathing 10% carbon dioxide. Spine stability and compression were quantified, using an EMG assisted optimization model in both trials., Findings: Contrary to expectation, spine stability increased during the challenged breathing trials compared to the ambient air condition for subjects with a history of low back pain when abdominal muscle activity was accounted for as a covariate., Interpretation: Subjects with a history of low back pain had higher stability in challenged breathing trials, indicating that some active mechanism protects the spine for the LBP groups in challenging situations. This may be to provide some margin of safety for damaged passive tissues but could be adversely affected by fatigue in the longer term.

Purpose: This study was conducted to quantify the normalized amplitudes of the abdominal wall and back extensor musculature during a variety of push-up styles. We also sought to quantify their impact on spinal loading by calculating spinal compression and torque generation in the L4-5 area., Methods: Ten university-age participants, nine males and one female, in good to excellent condition, volunteered to participate in this study. All participants were requested to perform a maximum of 12 different push-up exercises, three trials per exercise. Surface electromyographic data (EMG) were collected bilaterally on rectus abdominis, external oblique, internal oblique, latissimus dorsi, and erector spinae muscles, and unilaterally (right side) on pectoralis major, triceps brachii, biceps brachii, and anterior deltoid muscles. Spine kinetics were obtained using an anatomically detailed model of the torso/spine., Results: This study revealed that more dynamic push-ups (i.e., ballistic, with hand movement) required more muscle activation and higher spine load, whereas placing labile balls under the hands only resulted in modest increases in spine load. Right rectus abdominis (RA) activation was significantly higher than left RA activation during the left hand forward push-up and vice versa for the right hand forward push-up (P < 0.001). External oblique (EO) demonstrated the same switch in dominance during staggered hand push-ups (P < 0.01). The one-arm push-up resulted in the highest spine compression. Skilled participants showed greater synchronicity with peak muscle activation (plyometric type of contractions) during ballistic push-ups., Conclusion: These data will help guide exercise selection for individuals with differing training objectives and injury history.

Quantifying the stability of the spinal column offers a perspective on the effectiveness of the motor control strategy to ensure a stable spine--and minimize the risk of injury from experiencing an unstable event. There are essentially three energy based methods of calculating a stability index for the lumbar spine. All three methods involve mathematical manipulation of an 18 x 18 Hessian matrix. The purpose of this paper was to consider the mathematical implications for the three methods of determining a single stability index, and examine the effects of biological factors such as muscle activation in each of these methods. The first approach computes the Hessian's determinant and is thought to represent a more global or "average" perspective on stability. A second approach computes the smallest eigenvalue of the Hessian matrix to determine the weakest link of the spine. The final method determines an average critical stiffness difference for the spine and is intended to effectively determines how far a human spine is from instability, and allows comparison between tasks. This study shows that the same interpretation of stability is achieved via all three computational approaches--they agree as to whether the spine is stable or not. However they appear to differ in their sensitivity to the effect of muscle activation patterns.

Background: Several recent studies in animal models of spinal pain have shown changes in sensory processing and in reflex muscular responses. One group of researchers reported consistent electromyographic responses in the paraspinal muscles of healthy men after spinal manipulation, and they speculated that such responses may underlie some of the observed clinical effects of spinal manipulation (namely, reduction in pain and muscular hypertonicity)., Objectives: To determine whether a painful mechanical stimulus applied above a spinous process influences paraspinal electromyographic amplitude and whether this response is modulated by a spinal manipulation., Study Design: Analytic cohort with a convenience sample in a research clinic., Methods: Seventeen subjects with back pain (9 men and 8 women) were recruited. Electromyographic signals were recorded from the paraspinal musculature during the following procedures before and after manipulation: quiet stance and prone during the application of a mechanical pain stimulus. A 2-way repeated-measures analysis of variance was used to compare the effect of the force application on electromyographic amplitude. A second 2-way repeated-measures analysis of variance investigated whether the muscular response to a painful stimulus at either segment was influenced by the manipulative procedure., Results: A statistically significant increase in bilateral electromyographic activity was observed at the painful motion segment; however, no such statistical increase occurred at the segment that was not painful. It appears that manipulation results in a decrease in bilateral local electromyographic activity in the painful motion segment during the application of the mechanical stimulus; however, a statistically significant decrease was not found in the control segment. It was also found that while the subjects were quietly standing, the left erector spinae at a painful segment was the only muscle group to show significant differences before and after manipulation., Conclusion: This study suggests that motion segments identified as a problem in subjects with chronic low back pain have an exaggerated local muscular response to a painful stimulus compared with that observed in problem segments. In addition, spinal manipulation appears to attenuate the electromyographic response to a painful stimulus.

This experiment analysed the spinal shrinkage due to repetitive exertions confined to each of three separate axes (twist, lateral bend, flexion). While the experiment was performed twice with small technique modifications in the twisting task (and thus two data collections were performed), the essential components were as follows. A total of 20 subjects were loaded with an equal moment of 20 Nm in each of the three axes, on 3 separate days (one axis per day). Subjects performed each task for 20 min at 10 repetitions min(-1), where stadiometer measurements of standing height were taken prior to and immediately following the 20 min exertion. The twisting task demonstrated significant spinal shrinkage (1.81 and 3.2 mm in the two experiments) between the pre- and post-stature measurements while no clear effect emerged for the other two tasks. These data suggest that repetitive torsional motions impose a larger cumulative loading on the spine when compared with controlled lateral or flexion motion tasks of a similar moment.

Objective: To test the effect of two measurement techniques for repeated measures of spine height using stadiometry following five experimental activity conditions.DESIGN. Six subjects were repeatedly measured while they stepped in and out of the stadiometer for each pair of measures and again on another day when they remained in place in the stadiometer for all 10 measures.RESULTS. There was much greater variability in height measures with the "in-out" method while the "in place" method demonstrated a steady shrinkage over the 3-3.5 min required to obtain the repeated measures. RelevanceContrary to popular practice, leaving a subject in the stadiometer during repeated measures includes the shrinkage that occurs over the 3-3.5 min of measurement when standing and reduces random variation due to posture change.

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

The increased intra-abdominal pressure (IAP) commonly observed when the spine is loaded during physical activities is hypothesized to increase lumbar spine stability. The mechanical stability of the lumbar spine is an important consideration in low back injury prevention and rehabilitation strategies. This study examined the effects of raised IAP and an abdominal belt on lumbar spine stability. Two hypotheses were tested: (1) An increase in IAP leads to increased lumbar spine stability, (2) Wearing an abdominal belt increases spine stability. Ten volunteers were placed in a semi-seated position in a jig that restricted hip motion leaving the upper torso free to move in any direction. The determination of lumbar spine stability was accomplished by measuring the instantaneous trunk stiffness in response to a sudden load release. The quick release method was applied in isometric trunk flexion, extension, and lateral bending. Activity of 12 major trunk muscles was monitored with electromyography and the IAP was measured with an intra-gastric pressure transducer. A two-factor repeated measures design was used (P < 0.05), in which the spine stability was evaluated under combinations of the following two factors: belt or no belt and three levels of IAP (0, 40, and 80% of maximum). The belt and raised IAP increased trunk stiffness in all directions, but the results in extension lacked statistical significance. In flexion, trunk stiffness increased by 21% and 42% due to 40% and 80% IAP levels respectively; in lateral bending, trunk stiffness increased by 16% and 30%. The belt added between 9% and 57% to the trunk stiffness depending on the IAP level and the direction of exertion. In all three directions, the EMG activity of all 12 trunk muscles increased significantly due to the elevated IAP. The belt had no effect on the activity of any of the muscles with the exception of the thoracic erector spinae in extension and the lumbar erector spinae in flexion, whose activities decreased. The results indicate that both wearing an abdominal belt and raised IAP can each independently, or in combination, increase lumbar spine stability. However, the benefits of the belt must be interpreted with caution in the context of the decreased activation of a few trunk extensor muscles.

During muscle contraction, electrical activity necessarily precedes force output, yet models that utilize processed electromyograms sometimes predict force as preceding EMG under rapid ballistic loading conditions. The purpose of this study was to define the frequency response transfer function of the upper and lower erector spinae musculature, at different lengths and tensions, using rectified, low pass filtered EMG. This would enable accurate estimates of force from the processed electromyogram, specifically during impulsive contractions. Abdominal and erector spinae EMG were measured in synchrony with impulsive low back moments in five men. EMG signals were rectified and low pass filtered repeatedly with cut-off frequencies from 1 to 3 Hz at 0.5 Hz increments in order to quantify the frequency response. It was found that EMG signals processed through a simple, Butterworth low pass filter could not produce the measured force output without an additional time shift. These shifts were quantified by cross-correlating EMG and force with increments of 1 ms. In order to define the transfer function of EMG to force, optimal cut-off frequencies were selected two ways: quantitatively by searching for maximum cross correlations coefficients, and qualitatively. Results indicated that the frequency response of both the upper and lower erector spinae can be modelled with a cut-off frequency between 2 and 2.5 Hz and that these values are not significantly modulated by changes in muscle length or tension.

Background and Purpose: There have been no previous studies that quantitatively assessed the load on the spine during extensor exercises. The purpose of our study was to investigate the loading of the lumbar spine and trunk muscle activity levels while subjects performed typical trunk extensor exercises., Subjects: Thirteen male volunteers (mean age = 21.0 years, SD = 1.0, range = 19-23; mean height = 176.0 cm, SD = 6.2, range = 165-188; mean mass = 77.0 kg, SD = 7.0, range = 63-89) participated., Methods: The subjects performed four different back exercises. Electromyographic (EMG) activity was recorded from 14 trunk muscles. The postures that corresponded to the maximum external moment were identified and quantified using rigid body modeling combined with an EMG-driven model to determine joint loading at the L4-5 joint. The exercises were then evaluated based on the lumbar spine loading and peak muscle activity levels. A reference task of lifting 10 kg from midthigh was included for comparison., Results: The exercises involving active trunk extension produced the highest joint forces and muscle activity levels. Exercises involving leg extension with the spine held isometrically demonstrated asymmetrical activity of the trunk muscles, thereby reducing loads on the spine., Conclusion and Discussion: The back extensor exercises examined provided a wide range of joint loading and muscle activity levels. Single-leg extension tasks appear to constitute a low-risk exercise for initial extensor strengthening, given the low spine load and mild extensor muscle challenge. When combined with contralateral arm extensions, the challenge and demand of the exercise were increased. The compressive loading and extensor muscle activity levels were highest for the trunk extension exercises.

Objective: To examine the changes in length of the spine during prolonged bedrest., Design: The height of subjects was recorded over two control days and subsequently during the course of 32 hours of bedrest., Setting: Data were collected in a student dormitory., Participants: Eight male volunteers from an undergraduate student population., Results: Subjects lost approximately 10mm in spine height over the course of each of two control days. When they remained in bed after a usual night's rest, their spine height did not increase further. Even after 32 hours of continuous bedrest, the spine did not increase its length beyond the normal morning height., Conclusions: Bedrest does not allow normal expulsion of fluids from the disc leading to higher stresses associated with fully imbibed discs.

Measurement of spinal shrinkage (stadiometry) has been suggested to be a convenient measure of low-back load in workplace settings. This report documents three separate experiments that collectively form a central theme: Is the measurement of spinal shrinkage a suitable assessment technique to quantify the cumulative effect of loading on the low back given the many sources for variability in the signal? A stadiometer was fabricated to measure both sitting and standing height. The first experiment was to compare sitting with standing stature changes over time in an attempt to locate the major site of shrinkage. There were no statistically significant differences in stature change found between either the sitting or standing posture for any condition suggesting that nearly all height changes occur in the spine. The second experiment compared the cumulative effects from static load holding to dynamic load lifting. Some subjects experienced more shrinkage in the static task while others experienced more in the dynamic task. In the third experiment, subjects performed work-rest cycles consisting of periods of sitting and lifting, and repeated over two days, to examine the recovery phenomenon. No specific pattern emerged owing to unpredictable subject variability. The first general observation obtained from the results of all three experiments is that the response of subjects to a wide array of activities appears to be variable both within each subject and over repeated exposures to identical conditions on different days. While subject variability (and perhaps biological variability) is a liability, it may be feasible to develop load time integrals for load exposure in the future, since the asset of the spinal shrinkage approach appears to be that it is one of the few available techniques to assess cumulative loading for both isometric postures, prolonged sitting, repeated tasks and responds to the positive adaptive changes that occur from periods of rest. However, it would appear that more quantification of the relationships that modulate spinal shrinkage are required to account for the variance in stature measurements.

Muscles of the torso have been implicated to play a role in stabilization of the low back, and to assist in ventilation. This motivated an investigation to combine a load challenge to the low back with a breathing challenge, similar to that which a worker might experience when shovelling snow. Perhaps modulation of muscle activity needed to facilitate breathing may compromise the margin of safety of tissues that depend on constant muscle activity for support. Eight young healthy males dynamically lifted, and isometrically held, large loads (73-95 kg) and breathed a 10% CO2 gas mixture to elevate breathing (both with and without hand-held loads). Individual tissue forces were calculated using an anatomically detailed, dynamic model of the torso that was sensitive to individual variation by utilizing myoelectric signals, intra-abdominal pressure, ventilation rate and spine kinematics, obtained from each subject, as input. For large loads in the hands, most subjects appeared to stabilize the trunk with large muscle forces relegating the responsibility of creating lung air flow to the diaphragm. When reasonably small low-back demands were coupled with a breathing challenge and higher ventilation rates two out of eight subjects demonstrated entrainment of abdominal activity to breathing that resulted in additional cyclic low-back compressive loading of the order of 1000 N. Ergonomists should consider the additional tissue loading from physiologically demanding tasks and the related ventilation challenge, together with the tissue loads required to support external objects and maintain body posture.

This work investigated the passive bending properties of the intact human torso about its three principal axes of flexion: extension, lateral bending, and axial rotation. Additionally, the effects of wearing an abdominal belt and holding the breath (full inhalation) on trunk stiffness was investigated. The torsos of 22 males and 15 females were subjected to bending moments while "floating" in a frictionless jig with isolated torso bending measured with a magnetic device. Belts and breath holding appear to stiffen the torso about the lateral bending and axial rotation axes but not in flexion or extension. Torsos are stiffer in lateral bending and capable of storing greater elastic energy. Regression equations were formulated to define stiffness and energy stored for input to biomechanical models that examine low back function and for bioengineers designing hardware for stabilization and bracing or investigation of traumatic events such as automobile collision.

The authors examined the relation between exposure to seated whole-body vibration (WBV) and an increase in the loss of height of the spine over and above normal diurnal changes. A device for measuring sitting height is described (SD, 1.4 mm for repeated measures). The mean change in body height (diurnal reduction) during two normal days in five men aged 23 to 25 years was 10.6 mm, (SD, 3.2 mm). On the third day, the change in sitting height was measured before and after vertical vibration (5 Hz with a peak-to-peak amplitude of 3 mm, and peak acceleration less than 2 m/s2) and again at the end of the day. The mean reduction in sitting height over the half hour of vibration exposure was 9.0 mm versus less than 1 mm for the control condition. The mean height loss over the third day (the day of 30 minutes of vibration exposure) was only 3.6 mm (compared with 10.6 mm lost over a control day with no vibration exposure). Hence, exposure to vibration increased the creep response in all subjects during exposure but, at the end of the day, there was a recovery in height, such that subjects were taller at the end of the day of vibration exposure. It is hypothesized that this "rebound" effect is due to an inflammatory response in the spine.

The purpose of this study was to determine whether abdominal belts such as those prescribed to industrial workers reduced trunk muscle activity and/or increased intra-abdominal pressure (IAP). In this study, six subjects lifted loads (72.7 to 90.9 kg) both with and without wearing a weightlifter belt. In addition, further trial conditions required that subjects lifted both with the breath held or continuously expiring on lifting effort. Dynamic hand loads were recorded together with intra-abdominal pressure (IAP) and abdominal, intercostal and low back EMG. Every subject demonstrated an increase in IAP when wearing the belt during both breathing conditions: 99 mmHg with no belt; 120 mmHg wearing belt (p less than 0.0001). However, it was also found that significant increases in IAP occurred (p less than 0.017) when the breath was held versus exhaling with or without the belt. One would expect that if the belt relieved either the direct compressive load on the spine or assisted IAP to produce an extensor moment then this would be reflected in diminished extensor muscle activity. Erector spinae activity tended to be lower with the breath held suggesting a reduced load on the lumbar spine although wearing a belt did not augment this reduction. In the case studies with subjects wearing an ergogenic corset designed for use by industrial manual materials handlers, perceptions of improved trunk stability were reported. However, the muscle activity and IAP results of this study during short duration lifting tasks make it difficult to justify the prescription of abdominal belts to workers.

An anatomic-mathematical representation of the individual ligaments and disc at L4-L5 was used in a model to examine their role in joint behavior. Geometry of the anatomic components was obtained from cadaveric dissection and the anatomic literature; mechanical properties were obtained from various literature sources. Resistance to flexion of the L4-L5 joint was simulated using various ligament cross-sectional areas, and combined loading modes of compression coupled with bending in an attempt to understand physiologic conditions that may occur during activities such as lifting. Results suggest that the interspinous ligament sustains the greatest stress during flexion at L4-L5, is the dominant component in resistance to flexion, and also creates large shear components that must be resisted by the facet complex. Simulated compressive loads of 6300 N were noted to allow an additional 2 degrees of flexion at the joint.