Axial stiffness of human lumbar motion segments, force dependence.

This paper addresses the axial stiffness of human lumbar motion segments while subjected to moderate loads. Impacts in axial direction were applied to Functional Spinal Units while they were subjected to weights acting as static pre-load. Accelerations were recorded proximal and distal of the FSU. The transfer function and the resonant frequency were calculated from this data. The stiffness was calculated from the resonant frequency and the load. A simple non-linear model was fitted to the data and a linear relationship was found between stiffness squared and force. The non-linear component in the model strongly affected the stiffness within the chosen load range. The present model may allow in vivo dynamic force determination with improved accuracy, e.g. in experiments where accelerometers have been fixated to pins inserted into the spinous processes of lumbar vertebrae if the static force is known.

A mathematical model of the impact response of the seated subject.

A model of the lumbar spine, pelvis and buttocks is developed, based on linear horizontal and vertical systems along with a rotational subsystem. The model can mimic qualitatively different experimental observations of transmission of vibrations from the seat to L3 in the sitting posture. It is concluded that while the model lacks detailed sophistication, it adds to our qualitative understanding of the biomechanics of seated vibrations by pointing to those subsystems responsible for the observed transfer functions.

Resonant frequency of a pin-accelerometer system mounted in bone.

Invasive measurements of spinal motion using intraosseous metal pins have become common. For this reason, the resonant frequency of intraosseous pins attached with accelerometers was determined using two different methods. It was concluded that plucking the pin is a reliable method for determining the resonant frequency and, in order to accurately measure bone movement at frequencies up to 32 Hz, the pin diameter should be 2.0 mm or more. With a mass of the accelerometer assembly equal to 27 g, the total pin length should not exceed 80 mm with a bone-accelerometer distance of 25 mm and a pin diameter of 2.4 mm.

Measurement of height loss during whole body vibrations.

An experimental, in vivo study was performed to measure height changes in subjects exposed to whole body vibrations while seated. Twelve women, with an average age of 22 years, were exposed to sinusoidal vibrations for 5 mins. The vibration frequency was 5 Hz, and the acceleration was 0.1 g Rms. The height loss stemming from vibration exposure was compared with that experienced while sitting without being subjected to vibrations. The height losses that always occurred from the two exposures were corrected for the effect of posture change. The height loss from vibration was significantly greater than when no vibration was present. Height loss due to posture change was responsible for approximately 50% of the total height loss. From this study it was concluded that whole body vibrations cause increased height loss.

In vivo measurement of intervertebral creep: a preliminary report.

The purpose of this study was to measure the in vivo intervertebral motion in normal human L4-L5 spinal motion segments. The quasi-static sagittal plane rotation, axial translation, and anterior-posterior shear translation were continuously measured in two subjects over a 5-min period, during which time the subjects sat quietly and maintained strict postures. A mechanoelectrical transducer rigidly fixed to the motion segment measured the relative motion. During the test period, the predominant motion occurred in the axial direction whereas coupled rotation and shear translation were not detected. The overall relative axial translations for each of the subjects were the following: -0.2 mm (subject 1),-1.1 mm (subject 2), and-0.9 mm (subject 2 repeat). Distinct modulating effects of respiration were observed superimposed on the intervertebral motion.

The impact response of the seated subject.

An impact method for establishing the dynamic response of the seated subject is introduced. The method employs a pendulum to apply the impact to the suspended seat. Pins are placed in the spinous process at L3. Highly reproducible results are obtained. The results were not affected by the amplitude of impact, implying a linear system. A marked peak of transmissibility is found in the 4-5 Hz range and an attenuation peak is found close to 8 Hz. Both muscle contraction and postural changes affect the dynamic response. A relaxed posture shows greater gain and attenuation peaks. A valsalva stiffens the system and reduces the effective damping. The vertical response of the body probably shows in the 5-6 Hz peak, while the rotational response is probably encompassed in the 8 Hz attenuation peak.

Intervertebral motion during vibration.

Vibration exposure is widely recognized as a risk factor for low back pain. An experimental protocol was designed to quantify the intervertebral motion response in human subjects to sinusoidal vertical vibration at 5 and 8 Hz, and at a variety of acceleration levels. Intervertebral motion in the mid-sagittal plane was measured using a transducer linkage system attached to pins placed directly into the spinous processes of adjacent vertebrae. The postures of the subjects were carefully controlled. The effects of forward flexion, arm support, gravitational load, and sitting on a cushion were evaluated. The rigid body motion of the superior vertebra with respect to the inferior vertebra was expressed in terms of relative sagittal plane rotation, axial translation, and anterior-posterior shear translation. It was found that the lumbar motion segments exhibited coupled periodic behaviour in response to sinusoidal vertical vibration, with up to 1 mm peak-to-peak displacement in the axial direction. The greatest intervertebral motion occurred when the subject was exposed to 5 Hz vibration as compared to 8 Hz. For a constant frequency of 5 Hz excitation, the peak-to-peak amplitudes of the computed motions tended to increase as the acceleration level increased. In the flexed posture, with no arm support, the active trunk musculature helped reduce the intervertebral motion. Additional gravitational load on the shoulders caused increased relative axial displacement. A polyethylene foam cushion placed on the seat reduced vibration transmission at 5 Hz excitation and consequently decreased the intervertebral motion.

Back muscle fatigue and seated whole body vibrations: an experimental study in man.

The electromyography of the erector spinae muscles was studied in the thoracic and lumbar spines of six male subjects when seated in a forwardly bent position (20°) and whilst carrying extra weight (4 kg) on the front of the chest. The electromyographic responses were compared whilst the volunteers were: (a) seated in this position and exposed to whole body vibrations of 5 Hz and 0.2 g acceleration; (b) seated in this position without vibration exposure. Each test period lasted 5 minutes. Vibration exposure increased both the speed and the amount of the development of erector spinae fatigue.

Factors affecting the dynamic response of the seated subject.

An impact method, combined with pins placed into the spinous process at L3, has been used to establish the dynamic response of the spine of the seated subject. The resonant frequency is at 4-5 Hz, due primarily to a vertical response of the buttocks-pelvis system. A maximum attenuation at 8 Hz occurs because of a second resonance due to pelvic rotation. The attenuation is also affected by additional load and by the addition of a helmet. Neck braces have no dynamic effect.

The dynamic response of a subject seated on various cushions.

An impact pendulum was used to examine the dynamic response of the seated subject. The dynamic response is of interest in establishing the relationship between driving and low-back pain. Accelerometers were placed on the seat and in vivo at the L3 vertebra. The transmissibility and phase angle were obtained in the frequency domain for a variety of cushions. Soft cushions were found to increase the gain at the first natural frequency.

Knowledge-based signal processing in the decomposition of myoelectric signals.

The mixture of quantitative and qualitative information utilized in decomposition, which suggests that a purely numerical approach to the task would be inefficient, is discussed. The incorporation of symbolic processing methods into the decomposition system to complement the numerical aspects of the task is discussed. The overall philosophy used in the decomposition system is examined, focusing on the principles of detection used. The organization of the software used by the system is described.

The role of prerotation of the trunk in axial twisting efforts.

The myoelectric activity of selected trunk muscles was recorded during the development of controlled isometric axial torques. Muscle activity was measured bilaterally over the erector spinae, the rectus abdominus, the oblique external and the oblique internal abdominal muscles at the L3 level. Subjects first applied graded isometric torque efforts over a 10 second ramp up to maximum voluntary contraction with the trunk in neutral rotation. They then repeated the effort with the trunk twisted to the left and right. The largest electromyographic activities were found in the agonistic oblique muscles, but considerable antagonistic activity was present also. While the activity of the internal oblique and rectus were bilaterally similar in symmetric standing a difference occurred between the two sides when the trunk was twisted to the right or left. Axial prerotation of the trunk by 30 degrees in the direction of torque development marginally decreased the maximal developed torque, whereas prerotation in the opposite direction increased the developed torque.

The response of the seated human to sinusoidal vibration and impact.

Low back pain has been shown to occur more frequently among vehicle drivers than in representative control groups. Thus the response of the human to vibration and impact is of interest. This study investigated the response of the spine to both impact and sinusoidal excitation in either a relaxed or erect seated posture. The sinusoidal testing apparatus used was a resonating system consisting of two parallel wooden beams, simply supported, and the impact testing apparatus a bearing-guided, spring-suspended platform, struck from below. Ten subjects (5 males, 5 females) were evaluated using both methods. Transfer functions were compared at 2-4 Hz, 4-8 Hz and 8-16 Hz intervals using a sign test. Although in 24 comparisons of either test method (vibration or impact) or posture (erect or relaxed) where eleven showed differences significant at the p less than .05 level, only 2 out of 24 comparisons were the differences distinct enough to be significant (at the p less than .01 level). Both of these latter differences were due to test method while the subjects were sitting erect. In those instances where there were no significant differences due to test method, the impact method may be a viable replacement for the vibration test method. Where the levels of significance are higher (p less than .01 or p less than .05), further study of the magnitude of the differences is indicated and may reveal further insight into the seated individual as a system.

Electromyographic studies of the lumbar trunk musculature during the development of axial torques.

The myoelectric activity of selected trunk muscles at the L3 level was studied during the development of a controlled isometric axial torque. Muscle activity was recorded with surface electrodes bilaterally over the erector spinae muscles, the rectus abdominus, the obliques externus, and the obliques internus. Subjects applied graded isometric axial torque efforts with the addition of a Valsalva. The largest myoelectric activities were recorded over the obliques externus and internus. The side of the muscle developing the torque did show the greatest myoelectric activity, but considerable antagonistic activity was also present. Neither the rectus abdominus nor the internal oblique muscles showed any difference from side to side. The calculated forces in the rectus abdominus and the erectores spinae were high, suggesting that a considerable amount of the muscle contraction was used to control the posture. A Valsalva did not decrease myoelectric activity, but did increase the torque developed.

Mounting of the transducers in measurement of segmental motion of the spine.

A seated female subject was subjected to sinusoidal whole body vibration at 2, 4, 5 and 6 Hz. Accelerations were measured by accelerometers on pins screwed into the spinous process. The pins were also fitted with light emitting diodes (LED). The displacement of these LEDs were compared to LEDs attached directly to the skin. Substantial differences in measured displacements were noted between surface mounted LEDs and those mounted on pins rigidly attached to the skeleton. These differences were more marked further out from the center line.

Motor unit recruitment and firing rates interaction in the control of human muscles.

Muscle contractions are modulated by the number of motor units recruited and their respective firing rates. The work described in this report documents an interplay between recruitment and firing rates of motor units. The recruitment of a new motor unit appears to have a disfacilitatory influence on the firing rates of previously activated motor units. It is speculated that this effect is likely to be mediated, at least partially, via the stretch reflex loop and possibly by the recurrent inhibition of the Renshaw circuit. Such a mechanism would be functionally useful in providing smooth control of muscle output via peripheral circuitry (consisting of proprioceptive reflexes and recurrent inhibition), thus lessening the amount of detailed supervision of the alpha-motoneuron pool required by the central nervous system.

Myoelectric signal conduction velocity and spectral parameters: influence of force and time.

Reports on measurement of muscle fiber conduction velocity in humans are scarce. Inferences on the behavior of conduction velocity have been drawn from the behavior of myoelectric spectral parameters. The present report contains information on conduction velocity and spectral parameters studied at various muscle contraction levels and during and after sustained contractions. The following results have been obtained from measurements on the tibialis anterior muscle. Conduction velocity demonstrated a positive correlation with limb circumference and with muscle force output. Thus we conclude that the diameters of the muscle fibers of high-threshold motor units are, on an average, larger than those of low-threshold motor units. The study of a sustained contraction and on the recovery after such a contraction revealed that conduction velocity consistently decreased during a strong contraction as did various myoelectric spectral parameters. However, the spectral parameters decreased approximately twice as much as did the conduction velocity, and we conclude that factors other than the conduction velocity along the muscle fibers affect the myoelectric signal during a high-level contraction. These other factors appertain to changes in the firing statistics of individual motor units as well as the correlation between the firings of different motor units.

Adaptation of human atrial muscle repolarisation after high rate stimulation.

We analysed the effect of a sudden change from a high rate of stimulation to a "physiological" rate upon the repolarisation of human atrial muscle. Microelectrode technique was used to study the effect upon the action potential (AP) recorded from myocardial specimens obtained during open heart surgery in nine patients. Suction electrodes were used to record monophasic action potentials (MAPs) in 12 patients undergoing electroconversion of atrial fibrillation. The abrupt change from 30 min of high-rate stimulation of the atrial specimen to a pacing rate of 60 per min resulted in a successive prolongation of the atrial myocardial AP duration so that 50% of the prolongation was reached after 3 min according to an exponential analysis. A similar prolongation of repolarisation was seen in the MAP recordings after conversion of atrial fibrillation to sinus rhythm and during regular atrial stimulation at a rate of 100 per min. In these recordings, the time needed to reach 50% of the prolongation of the MAP after DC conversion was about 7 min. The findings demonstrate that human atrial muscle undergoes an adaptation of repolarisation after abrupt slowing from a fast stimulation rate. A steady-state level of the AP or MAP duration is reached 10 to 15 min after the change of rate. Together with earlier studies, these experiments indicate, that when right atrial MAP recording is done for assessment of the likelihood of the patient's remaining in sinus rhythm after conversion of atrial fibrillation, the recording must be made within a few minutes of the conversion.