The overall goal of the following experiment is to quantify postural tone along the body axis. This is achieved by a servo control device called Twister that can impose axial torsion, but does not provide anti-gravity support. As this device twists a selected region of the body axis, there are slow changes in the length of muscles crossing the region.
Next, the resulting resistive torque is measured in order to quantify the tonic control of all muscles undergoing length changes. Results are obtained that show the magnitude and dynamic control of tonic muscle activity based on the peak-to-peak magnitude and phase of the resistive torque, as well as modulation of EMG In.In natural conditions, the most part of the skeletal musculature or musculature of the body is in continuous, low level steady state activity. This mode of activity is called tonic activity, tonic postal activity.
Our goal is to study the physiological mechanism of this mode of tonic control. The twisted device is unique in allowing us to address important questions in neuroscience, such as, how's the brain control normal postural tone? It's also important for addressing questions in neurology such as how does abnormal tone in patients with Parkinson's disease affect their functional performance and how can we improve tone with treatment?
Twister is a servo controlled device for quantifying postural tone in axial and proximal body regions. During active upright stance subject stand on a platform that rotates plus or minus 20 degrees on a bearing about the vertical axis. An electric motor powers this rotation at a drive ratio that achieves platform speeds between 0.5 degrees per second and five degrees per second, and high torque to eliminate vestibular signals that could disrupt quiet stance.
Twister rotates the lower body in space rather than the upper body. In this configuration, the pelvis is fixed in space with an attachment to a frame, an optical encoder fixed to the platform Shaft reports rotational displacement for both servo control and data analysis. A rigid steel frame with diagonal cross bracing prevents rotation of the upper body and creates high torsional stiffness between the platform assembly and torque sensor necessary for accurate torque measurement.
The upper fixation and lightweight counterbalanced suspension system connect the upper margin of the twisted region to the frame. A torque sensor positioned within the upper fixation measures a subject's resistance to rotation. The suspension system consists of four rectangular aluminum plates that are alternately hinged along the anterior, posterior, and medial lateral axis.
This creates a high stiffness for rotation about the vertical axis in order to accurately measure torque without restricting movement in other dimensions. In particular, the low stiffness for translation in X, Y, and Z directions ensures subjects maintain postural stability themselves and prevents the upper fixation from providing a spatial reference spring's act to counteract the weight of the suspension system. A vertical bearing assembly is used to adjust the upper fixation to subject height.
A lower fixation connects the lower margin of the twisted region to the rotating platform so that body segments below the lower fixation rotate with the platform. The lower fixation consists of a lightweight telescoping bar that is connected to the rotating platform. A hinge connects the telescoping bar to the platform to allow anterior posterior postural sway.
Three attachments are used with twister, a lightweight helmet, a shoulder harness, and a pelvis orthotic, which can each be securely fixed to the body. To twist the neck, the head device is adjusted to the proper height After the helmet is attached, attached the device to the shoulders. At this point, the head is immobilized while the shoulders can rotate.
Alternatively to twist the trunk, the shoulders are attached above and the pelvis below. Now the lower body can twist while the upper body is held fixed. As a third option, the hips can be twisted by attaching the pelvis above.
In this case, twisting is localized to internal and external hip rotation. As the feet shank and thigh rotate with the platform, a standard force plate can be placed under the subject's feet on the rotating platform to simultaneously measure resistive torque in the twisted segment. This force plate can also be used to quantify postural sway during twisting.
A custom built real-time servo system controls platform rotation through a custom PC program that interfaces with the hardware controller to specify the desired temporal profile of platform rotation and initiate a trial. The hardware, PID controller outputs a motor drive signal based on a platform position signal from the optical encoder and the desired rotation. The controller generates three profiles for platform rotation.
In one mode, the controller selects a triangle profile to alternate between constant speed, clockwise and counterclockwise rotation. Alternatively, the step profile can be selected to achieve a discontinuous stepping mode of rotation. Rotation can also be driven with a triangle profile that increases in amplitude across cycles.
To begin an experiment place body attachments on the desired segments, ensuring they are snug and there is no torsional play. The experiment shown here will demonstrate trunk twisting after the subject is standing on the rotating platform facing forward, adjust the height of the linear bearings so that the upper fixation is at the same height as the corresponding body attachment. Adjust the lower fixation using the telescoping bar to correspond to the height of the lower body attachment.
Attach the upper and lower fixations to the corresponding body attachments. Positioning adjustments, so zero torque is applied to the subject in the pretrial position. After the subject is blindfolded, instruct the subject to stand relaxed and not to intervene.
Select an amplifier gain for the torque sensor according to which body region is twisted. In order to maximize the dynamic range of this signal, reset the bias on the torque sensor. Begin surface oscillation in jaw and data recording.
Torque and platform rotation signals are typically recorded at 50 hertz. Using Spike two acquisition software initiate twisting with the desired platform rotation profile in general movement should be slow and smooth enough so that subjects do not accurately perceive. Twisting shown.
Here is an example of trunk torque associated with staircase. Mode of platform rotations demonstrated. Here is trunk torque associated with the ramp increasing amplitude of triangular rotations.
Resistive torque typically increases with platform excursion, however, the increased slows with larger excursion. Overall resistance is typically quantified by peak tope. Torque averaged across cycles mean resistance to twisting differs across body segments from the neck to the trunk to the hips.
Shown here are single trial responses across 10 subjects for torsional resistance of the trunk, three cycles of 10 degree, one degree per second triangle waves were used. Note that individual subjects have consistent behavior across cycles with large variation and resistance between subjects. Traces with highest resistance are typical of constant tonic muscle activity.
While traces with least resistance are typical of high EMG modulation. The results show intersubject repeatability in torsional resistance across time. Shown here are two measurements from seven subjects separated by one month.
Peak tope. Trunk torque shows consistent within subject behavior across testing sessions, but wide intersubject variation. The estimation of the state of the tone is very important things in clinical.
In the diagnostic study. Even the mechanism of tone activity is not completely clear. The study of the physiology of muscle tone control can be very helpful to understand the pathology of tone, type of spasticity, rigidity, and something else.
In addition to measuring axial postural tone, twister can be used to study how the brain controls spatial orientation. That is how we use our senses like vestibular information and muscle sense to know where straight ahead is.
