Influence of the mechanical properties of a manipulandum on human operator dynamics

An active servo-system was used to change the stiffness of a manipulandum used in a positioncontrol pursuit-tracking task. The elastic stiffness of the manipulandum connected to the forearm was set by a computer at one of five levels ranging from 0 N/m to 2000 N/m. Subjects were required to track, either by moving their forearm or by generating a force isometrically, a visually presented target whose position changed randomly every second for 100 s. Nonparametric and parametric impulse response functions were calculated between the input (target) and output (force or position) in each tracking condition, and revealed that for all subjects force control was faster than position control when the stiffness of the manipulandum was set at 0 N/m. Subjects were also consistently faster in reaching the target when the stiffness was greater than zero, and were more accurate (steadystate response) when the stiffness of the manipulandum was set at lower rather than higher amplitudes. The parametric impulse response functions revealed that the human operator system was underdamped (0.7) with a natural frequency of approximately 8 rad/s. These findings were interpreted in terms of the responses of the various subsystems (visual, cognitive, contractile, limb mechanics) that comprise the human operator's response.     

Strategies used to stabilize the elbow joint challenged by inverted pendulum loading

The stiffness of activated muscles may stabilize a loaded joint by preventing perturbations from causing large displacements and injuring the joint. Here the elbow muscle recruitment patterns were compared with the forearm loaded vertically (a potentially unstable inverted pendulum configuration) and with horizontal loading. Eighteen healthy subjects were studied with the forearm vertical and supinated and the elbow flexed approximately 90 degrees. In the first experiment EMG electrodes recorded activity of biceps, triceps, and brachioradialis muscles for joint torques produced (a) by voluntarily exerting a horizontal force isometrically (b) by voluntarily flexing and extending the elbow while the forearm was loaded vertically with 135N. The relationship between the EMG and the torque generated was quantified by the linear regression slope and zero-torque intercept. In a second experiment a vertical load increasing linearly with time up to 300N was applied.In experiment 1 the EMG-torque relationships for biceps and triceps had an intercept about 10% of maximum voluntary effort greater with the vertical compared to the horizontal force, the inverse was found for Brachioradialis, but the EMG-torque slopes for both agonist and antagonistic muscles were not different. In experiment 2 there were 29 trials with minimal elbow displacement and all the three muscles activated on the order of 11% of maximum activation to stabilize the elbow; 19 trials had small elbow extension and 14 trials small flexion requiring altered muscle forces for equilibrium; 7 trials ended in large unstable displacement or early termination of the test. An analysis indicate that the observed levels of muscle activation would only provide stability if the muscles' short-range stiffness was at the high end of the published range, hence the elbow was marginally stable. The stability analysis also indicated that the small elbow extension increased stability and flexion decreased stability.     

Dependence of elbow joint stiffness measurements on speed,angle, and muscle contraction level

Elbow joint stiffness is critical to positioning the hand. Abnormal elbow joint stiffness may affect a person's ability to participate in activities of daily living. In this work, elbow joint stiffness was measured in ten healthy young adults with a device adapted from one previously used to measure stiffness in other joints. Measurements of elbow stiffness involved applying a constant-velocity rotational movement to the elbow and measuring the resultant displacement, torque, and acceleration. Elbow stiffness was then computed using a previously-established model for joint stiffness. Measurements were made at two unique elbow joint angles, two speeds, and two forearm muscle contraction levels. The results indicate that the elbow joint stiffness is significantly affected by both rotational speed and forearm muscle contraction level.     

Life shortening in BALB/c mice following brief, protracted, or fractionated exposures to neutrons

The effects of acute, protracted, or fractionated exposures to fission neutrons on survival times of female BALB/c mice were examined and compared. Mice were given single, brief exposures or exposures given in equal fractions at either 1- or 30-day intervals to doses of 0, 2.5, 5, 10, 20, 50, and 200 rad at the Health Physics Research Reactor (HPRR) or protracted exposures at rates ranging from 0.1 to 10 rad/day using a moderated 252Cf source to doses of 0, 2.5, 5, 10, 20, and 40 rad. The 252Cf source was moderated to have a similar spectron to that of the HPRR facility. After single or fractionated exposures the extent of life shortening increased rapidly over the 0-50 rad range and then began the plateau. No simple model adequately described the dose response over this entire dose range. Over the 0-50 rad dose range for exposures at the HPRR and over the 0-40 rad dose range for protracted exposures the dose response could be adequately described by either a linear model or a square root of the dose regression model except when the dose was fractionated using a 30-day interval. In this instance a linear model provided an adequate fit while a square root of the dose model could be rejected. No increase in effectiveness after fractionation or protraction was observed for neutron-induced life shortening at doses below 50 rad, while at 50 and 200 rad an increase in effectiveness was observed in this and in previous studies. These data were interpreted to suggest that in the dose range below 20-40 rad the dose-effect curve for life shortening may be linear and begins to flatten at higher doses rather than continuously bending at low doses.     

Accessory muscle activity contributes to the variation in time to task failure for different arm postures and loads.

Time to failure and electromyogram activity were measured during two types of sustained submaximal contractions with the elbow flexors that required each subject to exert the same net muscle torque with the forearm in two different postures. Twenty men performed the tasks, either by maintaining a constant force while pushing against a force transducer (force task), or by supporting an equivalent load while maintaining a constant elbow angle (position task). The time to failure for the position task with the elbow flexed at 1.57 rad and the forearm horizontal was less than that for the force task (5.2 +/- 2.6 and 8.8 +/- 3.6 min, P = 0.003), whereas it was similar when the forearm was vertical (7.9 +/- 4.1 and 7.8 +/- 4.5 min, P = 0.995). The activity of the rotator cuff muscles was greater during the position tasks (25.1 +/- 10.1% maximal voluntary contraction) compared with the force tasks (15.2 +/- 5.4% maximal voluntary contraction, P < 0.001) in both forearm postures. However, the rates of increase in electromyogram of the accessory muscles and mean arterial pressure were greater for the position task only when the forearm was horizontal (P < 0.05), whereas it was similar for the elbow flexors. These findings indicate that forearm posture influences the difference in the time to failure for the two fatiguing contractions. When there was a difference between the two tasks, the task with the briefer time to failure involved greater rates of increase in accessory muscle activity and mean arterial pressure.     

Muscular torque generation during imposed joint rotation: torque-angle relationships when subjects' only goal is to make a constant effort

It is a reasonable expectation that voluntarily activated spinal motoneurons will be further excited by increases in spindle afferent activity produced by muscle stretch. Human motor behavior attributed to tonic stretch reflexes and to reflexes recruited by relatively slow joint rotation has been reported from several laboratories. We reinvestigated this issue by rotating the elbow joint over the central portion of its range while subjects focused on keeping their elbow flexion effort constant at one of three different levels and made no attempt to control the position, speed or direction of movement of their forearm. There is evidence that subjects' voluntary motor status is constant under these conditions so that any change in torque would be of involuntary origin. On average, torques rose somewhat and then fell as the elbow was flexed through a range of 80 degrees at 10, 20 and 60 degrees/s and a similar pattern occurred during elbow extension; i.e., both concentric and eccentric torque-angle profiles had roughly similar shapes and neither produced consistent stabilizing cross-range stiffness. The negative stiffness (rising torque) during the early part of a concentric movement and the negative stiffness (falling torque) during the later part of an eccentric movement would not have occurred if a stabilizing stretch reflex had been present. Positive stiffness rarely gave rise to torque changes greater than 20% in either individual or cross-subject averaged data. When angular regions of negative stiffness are combined with regions of low positive stiffness (torque change 10% or less), much of the range of motion was not well stabilized, especially during eccentric movements. The sum of the EMGs from biceps brachii, brachioradialis and brachialis showed a pattern opposite to that expected for a stretch reflex; there was an upward trend in the EMG as the elbow was flexed and a downward trend as the elbow was extended. There was little change in the shape of this EMG-angle relationship with either direction or velocity. The individual EMG-angle relationships were distinctive for each of these three elbow flexor muscles in four of the six subjects; in the remaining two, biceps was distinctive, but brachioradialis and brachialis appeared to be coupled. Although the EMGs of individual muscles were modulated over the angular range, no consistent stretch reflexes could be seen in the individual records. Thus, we could find no clear evidence for stretch reflex stabilization of human subjects maintaining a constant effort. Rather, muscle torque appears to be reflexly modulated across a much used portion of the elbow's angular range so that any appreciable stabilizing stiffness that is sustained for more than fractions of a second is associated with a change in effort.     

Muscular torque generation during imposed joint rotation: torque-angle relationships when subjects' only goal is to make a constant effort

It is a reasonable expectation that voluntarily activated spinal motoneurons will be further excited by increases in spindle afferent activity produced by muscle stretch. Human motor behavior attributed to tonic stretch reflexes and to reflexes recruited by relatively slow joint rotation has been reported from several laboratories. We reinvestigated this issue by rotating the elbow joint over the central portion of its range while subjects focused on keeping their elbow flexion effort constant at one of three different levels and made no attempt to control the position, speed or direction of movement of their forearm. There is evidence that subjects' voluntary motor status is constant under these conditions so that any change in torque would be of involuntary origin. On average, torques rose somewhat and then fell as the elbow was flexed through a range of 80° at 10, 20 and 60°/s and a similar pattern occurred during elbow extension; i.e., both concentric and eccentric torque-angle profiles had roughly similar shapes and neither produced consistent stabilizing cross-range stiffness. The negative stiffness (rising torque) during the early part of a concentric movement and the negative stiffness (falling torque) during the later part of an eccentric movement would not have occurred if a stabilizing stretch reflex had been present. Positive stiffness rarely gave rise to torque changes greater than 20% in either individual or cross-subject averaged data. When angular regions of negative stiffness are combined with regions of low positive stiffness (torque change 10% or less), much of the range of motion was not well stabilized, especially during eccentric movements. The sum of the EMGs from biceps brachii, brachioradialis and brachialis showed a pattern opposite to that expected for a stretch reflex; there was an upward trend in the EMG as the elbow was flexed and a downward trend as the elbow was extended. There was little change in the shape of this EMG-angle relationship with either direction or velocity. The individual EMG-angle relationships were distinctive for each of these three elbow flexor muscles in four of the six subjects; in the remaining two, biceps was distinctive, but brachioradialis and brachialis appeared to be coupled. Although the EMGs of individual muscles were modulated over the angular range, no consistent stretch reflexes could be seen in the individual records. Thus, we could find no clear evidence for stretch reflex stabilization of human subjects maintaining a constant effort. Rather, muscle torque appears to be reflexly modulated across a much used portion of the elbow's angular range so that any appreciable stabilizing stiffness that is sustained for more than fractions of a second is associated with a change in effort.     

Activation level of extensor carpi ulnaris affects wrist and elbow acceleration responses following simulated forward falls

The main objective of this study was to measure the acceleration response at the wrist and elbow as a function of different levels of isometric forearm muscle activation during the impact phase of a simulated forward fall.A seated human pendulum was designed to impact the hands of 28 participants while maintaining one of four levels of isometric muscle activation (12%, 24%, 36% and 48% maximal voluntary exertion (MVE)) in the extensor carpi ulnaris muscles. The acceleration responses including peak acceleration (PA), acceleration slope (AS) and time to peak acceleration (TPA) were measured at the wrist and elbow along two axes (axial and off-axis) with two low mass surface mounted accelerometers.At the wrist, significant muscle activation effects were found for PA_off, AS_axial, AS_off, such that they increased as muscle activation increased from baseline to 48% MVE. At the elbow, a similar response was noted, with the acceleration variables increasing as muscle activation level increased, except for AS_off.The results suggest that increases in muscle activation from 12% to 48% MVE stiffen the forearm complex and increase the transmissibility of the impact reaction force shock waves through the forearm.     

Fatigue and muscle activation during submaximal elbow flexion in children with cerebral palsy

The purpose of this study was to investigate whether children with cerebral palsy (CP), like typically developing peers, would compensate for muscle fatigue by recruiting additional motor units during a sustained low force contraction until task failure.Twelve children with CP and 17 typically developing peers performed one submaximal isometric elbow flexion contraction until the task could no longer be sustained at on average 25% (range 10–35%) of their maximal voluntary torque. Meanwhile surface electromyography (EMG) was measured from the biceps brachii and triceps brachii, and acceleration variations of the forearm were detected by an accelerometer. Slopes of the change in EMG amplitude and median frequency and accelerometer variation during time normalised to their initial values were calculated.Strength and time to task failure were similar in both groups. Children with CP exhibited a lower increase in EMG amplitude of the biceps brachii and triceps brachii during the course of the sustained elbow flexion task, while there were no significant group differences in median frequency decrease or acceleration variation increase. This indicates that children with CP do not compensate muscle fatigue with recruitment of additional motor units during sustained low force contractions.     

Limited forearm motion compensated by thoracohumeral kinematics when performing tasks requiring pronation and supination

This study investigates the altered thoracohumeral kinematics when forearm rotation is restricted while performing five activities requiring pronation and supination. Two splints simulated both a fixed-supinated or fixed-neutral forearm in six healthy subjects; the three-dimensional coupled relationship among motion about the forearm, elbow, and shoulder were analyzed. In using a screwdriver, the normal range of forearm rotation of 77.6° (SD = 30.8°) was reduced in the fixed-supinated to 11.3° (SD = 2.9°) and fixed-neutral to 18.2° (SD = 6.2°). This restriction from the fixed-supinated and fixed-neutral forearms was compensated at the shoulder by a significant increase in the total range of (1) ad/abduction by 57.3° and 62.8° respectively (p < .001), (2) forward-reverse flexion (24.3° and 18.2° respectively; p < .05) and (3) internal-external rotation (37.1° and 44.2° respectively; p < .001). A similar result was demonstrated for the doorknob activity. The elbow did not significantly contribute to forearm rotation (p = .14), and is believed to be due to the elbow axis being orthogonal and oblique to the forearm axis. For open kinetic-chain activities, with a fixed-supinated forearm performing there was a significant coupled increase in ad/abduction (p < .05) and int/external rotation (p < .05) for the phone and feeding tasks, with the phone task also having a significantly increased forward shoulder flexion (p < .05). For the fixed-neutral forearm, significant compensatory movement was only seen in the feeding task with increased ad/abduction and internal-external shoulder rotation (p < .05) and the card inserting task with increased ad/abduction and forward-reverse shoulder flexion. Limited forearm function requires compensatory motion from adjacent joints to perform activities that require pronation and supination.     

Simulated active control produces repeatable motion pathways of the elbow in an in vitro testing system.

The purpose of this study was to determine if the repeatability and pattern of elbow kinematics are affected by changing the relative magnitudes of loads applied to muscles around the elbow in vitro. In eight cadaveric upper extremities, passive and three methods of simulated active elbow flexion were tested with the forearm maintained in both pronation and supination. Passive flexion involved moving the elbow manually through a full arc of motion. Simulated active flexion used a custom designed loading system to generate elbow motion by applying loads to various tendons via pneumatic actuators. Three different simulated active loading protocols, with loading ratios based on muscle activity and physiologic cross-sectional area, were tested. Testing was performed initially on an intact elbow, and then an unstable elbow model created by transection of the lateral collateral ligament (i.e. the radial and lateral ulnar collateral ligaments). An electromagnetic tracking device was used to measure rotation of the ulna relative to the humerus. Varus-valgus angulation and internal-external rotation were less repeatable during passive flexion than simulated active flexion, regardless of the loading ratio used, in both the intact (p<0.05) and unstable (p<0.05) elbows. Throughout the arc of flexion, the motion pathways were similar for the three simulated active motion protocols employed in this study (p>0.05). The pathways followed during passive motion were different from those generated with simulated active motion, especially in the unstable elbow with the forearm supinated (p<0.001). These results suggest that using simulated active motion rather than manual passive motion can improve the repeatability of elbow kinematics generated in the laboratory, and that a wide range of muscle loading ratios may produce similar kinematic output.     

Posture and hand load alter muscular response to sudden elbow perturbations

Joint stiffness and stability are reliant on coordinated muscle activity which may differ depending on initial posture and loading during sudden perturbations. This study investigated the effects of arm posture and hand load on muscle activity during perturbations of the arm. Fifteen male participants experienced perturbations to the wrist causing elbow extension using a combination of three body postures (standing, supine, sitting) and three hand load conditions (no, solid, and fluid loads), with known and unknown timing. Surface EMG was collected from eight muscles of the right upper extremity. The response to sudden loading was examined using muscle activities pre (baseline) and post (reflex) perturbation. During the baseline period, known perturbation timing resulted in greater muscular activity than for unknown timing, while the opposite was found for the reflex period. During the reflex period with fluid load, biceps brachii and brachioradialis demonstrated increased activity of 2.4% and 4.0% of maximum respectively, from supine to standing. During the reflex period, the fluid load resulted in forearm co-contraction 23% and 47% greater than the solid and no load conditions. Body orientation and hand loading influenced muscular response to elbow perturbations. Muscle co-contraction at the elbow during known timing suggests a contribution to elbow joint stability that may reduce injury risk caused by sudden elbow loading.     

Rheological properties of suspensions containing cross-linked starch nanoparticles prepared by spray and vacuum freeze drying methods

The rheological behavior of suspensions containing vacuum freeze dried and spray dried starch nanoparticles was investigated to explore the effect of these two drying methods in producing starch nanoparticles which were synthesized using high pressure homogenization and mini-emulsion cross-linking technique. Suspensions containing 10% (w/w) spray dried and vacuum freeze dried nanoparticles were prepared. The continuous shear viscosity tests, temperature sweep tests, the frequency sweep and creep-recovery tests were carried out, respectively. The suspensions containing vacuum freeze dried nanoparticles showed higher apparent viscosity within shear rate range (0.1-100s(-1)) and temperature range (25-90°C). The suspensions containing vacuum freeze dried nanoparticles were found to have more shear thinning and less thixotropic behavior compared to those containing spray dried nanoparticles. In addition, the suspensions containing vacuum freeze dried particles had stronger elastic structure. However, the suspensions containing spray dried nanoparticles had more stiffness and greater tendency to recover from the deformation.     

Dynamic autoregulation of cutaneous circulation: differential control in glabrous versus nonglabrous skin

The purpose of this project was to test the hypothesis that, independent of neural control, glabrous and nonglabrous cutaneous vasculature is capable of autoregulating blood flow. In 10 subjects, spectral and transfer function analyses of arterial pressure and skin blood flow (laser-Doppler flowmetry) from glabrous (palm) and nonglabrous (forearm) regions were performed under three conditions: baseline, ganglionic blockade via intravenous trimethaphan administration, and trimethaphan plus oscillatory lower body negative pressure (LBNP; -5 to -10 mmHg) from 0.05 to 0.07 Hz. Oscillatory LBNP was applied to regenerate mean arterial pressure variability that was abolished by ganglionic blockade. Ganglionic blockade was verified by an absence of a heart rate response to a Valsalva maneuver. Spectral power and transfer function gain between blood pressure and skin blood flow were calculated in this oscillatory frequency range (0.05-0.07 Hz). Within this frequency range, ganglionic blockade significantly decreased spectral power of blood flow in both the forearm and palm, whereas regeneration of arterial blood pressure oscillations significantly increased spectral power of forearm blood flow but not palm blood flow. During oscillatory LBNP, transfer function gain between blood pressure and skin blood flow was significantly elevated at the forearm (0.28 +/- 0.03 to 0.53 +/- 0.02 flux units/mmHg; P < 0.05) but was reduced at the palm (4.7 +/- 0.5 to 1.2 +/- 0.1 flux units/mmHg; P < 0.05). These data show that independent of neural control of blood flow, glabrous skin has the ability to buffer blood pressure oscillations and demonstrates a degree of dynamic autoregulation. Conversely, these data suggest that nonglabrous skin has diminished dynamic autoregulatory capabilities.     

Validation of portable muscle tone measurement device for quantifying velocity-dependent properties in elbow spasticity.

The objective of this study is to develop a portable device for quantifying the velocity-dependent properties of spastic elbow muscles. Based on a motor-driven system, validation tests of the portable system such as accuracy and response of sensors were first examined. Furthermore, simulated modules (inertia, damper and spring) as well as elbow joints (15 control and 15 hemiplegic subjects) were manually stretched under four different frequencies (1/3, 1/2, 1 and 3/2 Hz) through 60 degrees range of motion. Joint resistance and displacement during sinusoidal stretch were collected for further analysis. Two quantitative parameters (i.e., viscous components under each frequency and averaged viscosity across four frequencies) were derived to estimate the velocity-dependent properties of elbow joint. Tests of simulated modules confirm the manual stretch protocol and data analysis are valid in estimating the velocity-dependent component during a sinusoidal stretch. Compared to normal control, viscous component in each stretch frequency and averaged viscosity were significantly higher in subjects with spasticity (P < 0.001). The viscous component and averaged viscosity were found highly correlated with the modified Ashworth scale. These findings suggest that measurements of viscous component and averaged viscosity during manual sinusoidal stretching using the portable device could be clinically useful in evaluating spasticity.     

Mechanism of leg stiffness adjustment for hopping on surfaces of different stiffnesses

When humans hopin place or run forward, leg stiffness is increased to offsetreductions in surface stiffness, allowing the global kinematics andmechanics to remain the same on all surfaces. The purpose of thepresent study was to determine the mechanism for adjusting legstiffness. Seven subjects hopped in place on surfaces of differentstiffnesses (23-35,000 kN/m) while force platform, kinematic, andelectromyographic data were collected. Leg stiffness approximatelydoubled between the most stiff surface and the least stiff surface.Over the same range of surfaces, ankle torsional stiffness increased1.75-fold, and the knee became more extended at the time of touchdown(2.81 vs. 2.65 rad). We used a computer simulation to examine thesensitivity of leg stiffness to the observed changes in ankle stiffnessand touchdown knee angle. Our model consisted of four segments (foot,shank, thigh, head-arms-trunk) interconnected by three torsionalsprings (ankle, knee, hip). In the model, an increase in anklestiffness 1.75-fold caused leg stiffness to increase 1.7-fold. A changein touchdown knee angle as observed in the subjects caused legstiffness to increase 1.3-fold. Thus both joint stiffness and limbgeometry adjustments are important in adjusting leg stiffness to allow similar hopping on different surfaces.

     

Dynamics of the head-neck system in response to small perturbations: Analysis and modeling in the frequency domain

The response of the head-neck system to forces of small amplitude (up to 15 N) is described. Sinusoidal (0.6–20 Hz) and impulsive (duration: 100 msec) forces are applied in the sagittal plane to the head of the subject who is instructed to resist the disturbancy. In the case of sinusoidal forces of frequency less than about 2 Hz the active effort to resist the disturbancy results in a largely distorted sinusoidal displacement. Above this frequency the response becomes almost linear. The variations with frequency of the amplitude and the phase of the linear response relative to the applied force (transfer function) are used to characterize the dynamics of the system. The transfer functions evaluated from the impulse response are very similar in shape to those obtained with sinusoidal forces. In both cases the results suggest that the system behaves as a quasi-linear second order system with two degrees of freedom. The most prominent nonlinearities, beyond those present in the low frequency range, are related to the properties of the neck muscles. In particular, the transfer functions clearly show that the rigidity of the system increases as a function of the continuous value of the applied force. On the basis of previous work, both the passive properties of the muscles and those pertaining to the neuronal control system are pooled together in the form of viscoelastic parameters. A simple model of the system is introduced and applied to the experimental results. Its main features are 1) the presence of two centers of rotation. 2) the dependency of the viscoelastic parameters (stiffness and viscosity) on the frequency. It is suggested that both these features are necessary and sufficient to account for the observed behaviour above 2 Hz.     

Control of stiffness by the medium latency electromyographic response to limb perturbation.

It is hypothesized that the medium latency electromyographic (EMG) response (ML) to limb perturbation functions to preset limb stiffness to a constant initial level. Three predictions are derived from this hypothesis: Firstly, in the presence of an instruction calling for opposition to limb perturbation, a control signal, ML, will be observed and will lead to the establishment of a constant level of limb stiffness. Secondly, in the absence of an instruction to oppose, no control signal will be observed and correspondingly a constant stiffness will not be generated. Thirdly, the latency of onset of stiffness control will covary with the latency of onset of ML. These predictions were tested in experiments involving perturbation of the human forearm about the elbow joint, with surface EMG measurements and computation of the limb stiffness function. The results are in accord with these predictions, and support the hypothesis that ML functions in the feedforward control of limb stiffness.     

Mechanical coupling between transverse plane pelvis and thorax rotations during gait is higher in people with low back pain

This study investigated whether people with low back pain (LBP) reduce variability of movement between the pelvis and thorax (trunk) in the transverse plane during gait at different speeds compared to healthy controls. Thirteen people with chronic LBP and twelve healthy controls walked on a treadmill at speeds from 0.5 to 1.72 m/s, with increments of 0.11 m/s. Step-to-step variability of the trunk, pelvis, and thorax rotations were calculated. Step-to-step deviations of pelvis and thorax rotations from the average pattern (residual rotations) were correlated to each other, and the linear regression coefficients between these deviations calculated. Spectral analysis was used to determine the frequencies of the residual rotations, to infer the relation of reduced trunk variability to trunk stiffness and/or damping. Variability of trunk motion (thorax relative to pelvis) was lower (P=0.02), covariance between the residual rotations of pelvis and thorax motions was higher (P=0.03), and the linear regression coefficients were closer to 1 (P=0.05) in the LBP group. Most power of segmental residual rotations was below stride frequency (~1 Hz). In this frequency range, trunk residual rotations had less power than pelvis or thorax residual rotations. These data show that people with LBP had lower variability of trunk rotations, as a result of the coupling of deviations of residual rotations in one segment to deviations of a similar shape (correlation) and amplitude (regression coefficient) in the other segment. These results support the argument that people with LBP adopt a protective movement strategy, possibly by increased trunk stiffness.