twister is a servo - controlled device for quantifying postural tone in axial and proximal body regions during active , upright stance .
the original device was constructed at oregon health & science university and a similar device is under construction for the university of southampton , uk . in this report
we then provide a detailed description to facilitate its reproduction and show how it can be used to investigate the control of postural tone .
twister consists of a rigid steel frame , rotating platform , torque sensor , counterbalanced suspension system , upper and lower fixations , and body attachments ( fig 1 ) , as well as a servo control system for regulating platform rotation .
twister quantifies postural tone in standing subjects by rotating lower body segments about the vertical axis relative to upper segments .
upper and lower fixations can be attached to impart twisting to the neck , trunk or hip regions ( fig 2 ) . because twister does not provide postural support , tonic activity of skeletal muscle is necessary to counteract gravitational torques .
this tonic regulation is studied by the torsional resistance to twisting as well as electromyography from relevant muscles .
resistance to twisting is assessed by a torque sensor within the upper fixation and reflects the state of all muscles undergoing length changes .
twister uses several different platform rotation profiles to study tonic control , including a triangle profile , step profile , and triangle profile of increasing magnitude ( fig 3 ) .
these rotate the platform at a constant slow velocity , which minimizes inertial effects on the subject and measurement .  
we use a twisting perturbation about the vertical axis because it : 1 ) changes the length of axial and proximal muscles , as these structures are oriented obliquely and have broad anatomic origins and insertions ; 2 ) does not alter the relationship of the whole body and its parts to gravity ; 3 ) rotates the body around an axis of minimal moment of inertia ; 4 ) corresponds to a neutral zone , so that resistance from small displacements from the forward facing position reflects muscular rather than osteo - ligamentous forces ; and 5 ) naturally occurs in everyday activities .
twister is a flexible device that can be used to address various aspects of tonic control .
these include : 1 ) stiffness related to tonic activity ; 2 ) tonic responses to muscular length changes .
3 ) the effect of twisting on remote body regions ; 4 ) tonic effects from kinesthetic information ; 5 ) the effects of disease on postural tone ; and 6 ) perceptual thresholds to slow rotation .  
rotating platform 

 subjects stand on a platform that rotates 20on a bearing about the vertical axis ( figs 1 , 4a ) .
an electric motor powers this rotation at a drive ratio that achieves platform speeds between 0.5/s and 5/s and high torque .
twister rotates the lower body in space , rather than the upper body to eliminate vestibular signals that could disrupt quiet stance .  
a belt and pulley system is used for drive reduction , which dampens vibration and eliminates lash that can interfere with torque measurement .
vibration is minimized because it can cue the subject about platform motion .   for safety ,
hard stops are used to limit maximal platform displacement .   an optical encoder ( hewlett - packard heds-5540 )
fixed to the platform shaft reports rotational displacement for both servo - control and data - analysis . 
 

frame 
 a rigid , steel frame ( 1.5 m x 1.5 m x 3 m ) with diagonal cross bracing creates high torsional stiffness between the platform assembly and torque sensor , necessary for accurate torque measurement . 
 
 upper fixation and suspension system 
 the upper fixation and lightweight , counterbalanced suspension system connect the upper margin of the twisted region to the frame ( fig 4b ) . a torque sensor ( futek tff220 , irvine , ca ) 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 mediolateral axes .
this creates a high stiffness for rotation about the vertical axis ( 590 nm/ ) , 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 ( 0.25 n / cm ) ensures subjects maintain postural stability themselves and prevents the upper fixation from providing a spatial reference .
this also allows each individual to maintain their own , unique vertical posture without affecting postural motion in the horizontal plane .  
a vertical bearing assembly ( fig 1 , 4b ) is used to adjust the upper fixation to subject height . 
 
 lower fixation 

 a lower fixation connects the lower margin of the twisted region to the rotating 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 . 
 

body attachments 
 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 ( fig 2 ) .   to twist the neck ,
attach the helmet above and the shoulders below .   to twist the trunk attach the shoulders above and the pelvis below   to twist the hips attach 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 . 
 

external fixation 
 a third , external fixation can be used to twist one body region while measuring torque produced by another . because the latter segment is stationary , the measured torque is not resistive but originates from muscular forces within the segment , potentially induced from remote twisting .  
the external fixation consists of a lightweight telescoping bar that prevents the connected segment from rotating about the vertical axis .
a hinge joint between the bar and frame allows anterior - posterior postural sway .  
alternatively , neck torque in response to hip twisting can be assessed by connecting the pelvis to the external fixation .  
a standard forceplate can be placed between the subjects feet and rotating platform , to simultaneously measure resistive torque in the twisted segment .
servo - control of platform rotation 
 a custom built real - time servo - system controls platform rotation .
this hardware pid controller outputs a motor drive signal based on a platform position signal from the optical encoder and the desired rotation ( see fig 7 ) .
a custom pc program interfaces with the hardware controller to specify the desired temporal profile of platform rotation and initiate a trial .  
select the triangle profile to alternate between constant - speed clockwise and counter - clockwise rotation ( fig .
rotation can also be driven with a triangle profile that increases in amplitude across cycles ( fig .
for all profiles , rotation is smoothed to limit acceleration to 12 /s during movement initiation and directional changes .  
a typical experimental is run as follows : place body attachments ( i.e. helmet , shoulder harness or pelvis orthotic ) on the desired segments , ensuring they are snug and there is no torsional play .  
adjust the height of the linear bearing 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 pre - trial position .
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 .  
torque and platform rotation signals are typically recorded at 50 hz using spike 2 acquisition software ( cambridge electronic devices , cambridge , uk ) .   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 .  
resistive torque typically increases with platform excursion , however the increase slows with larger excursion .
overall resistance is typically quantified by peak - to - peak torque , averaged across cycles .
fig 5a shows single trial responses across subjects for torsional resistance of constant speed ramp for the trunk .
we have observed data reproducible over months within a subject ( fig 5b ; interclass correlation coefficient = 0.89 ) .
mean resistance to twisting differs across body segments , and has been reported to be 0.54  0.24 nm for the neck , 5.11  1.94 nm for the trunk and 3.23  1.67 nm for the hips ( fig 6 ) .
absence of lash is indicated by smooth changes in torque during directional changes and a rapid change in torque at the onset , presumably due to short - range stiffness of muscle ( see fig 3a in gurfinkel et al . ) .
the measured torque reflects both the dynamic changes in tone with twisting as well as the distribution of baseline tonic activity ( which includes co - contraction ) . because of the slow speed of twisting , the increase in torque per degree is equivalent to intrinsic stiffness only when muscle activity is constant .
in general , two types of responses are observed that correspond to constant or modulated tonic activity within the twisted region .
the former is characterized by low cycle - to - cycle variation in torque , high peak - to - peak torque magnitude , and relatively constant emg .
in contrast , dynamic modulation is characterized by high cycle - to - cycle variability , low torsional resistance and emg modulation coherent with twisting . on angle
vs. torque plots unmodulated subjects exhibit a regular hysteresis loop while modulated subjects have an irregular pattern that can reverse direction ( see fig 3a in gurfinkel et al .
dynamic modulation generally consists of increasing tonic activity during muscle shortening and decreasing activity during lengthening ( i.e. sherrington 's lengthening and shortening reactions ) , which are opposite in sign to the stretch reflex .
an integrative measure of the extent of modulation can be obtained by the shift in a subject 's neutral ( zero - torque ) position within a cycle , referred to as torque phase advance . 


components are labeled as follows : 1 ) rotating platform ; 2 ) telescoping bar for lower fixation ; 3 ) hinge joint between lower telescoping bar and rotating platform ; 4 ) helmet attached to upper fixation ; 5 ) torque sensor and counterbalanced suspension system ; 6 ) locking vertical linear bearing ; 7 ) external fixation for measuring induced torque ; 8) hinge joint analogous to 3 ; 9 ) rigid frame ; 10 ) diagonal cross bracing for rigid frame . 


subjects stand on a rotating platform ( yellow ) with upper and lower body attachments affixed to impart twisting to the desired body region .
the upper attachment is connected via a suspension system ( zigzag lines ) to the torque sensor ( t ) , which is fixed with regard to rotation around the vertical axis .
the lower attachment connects to the rotating platform via a hinge joint ( black circle ) that allows rotation in the sagittal plane of the subject .
a : neck twisting is achieved by attaching a helmet to the torque sensor and the shoulders to the platform .
b : trunk twisting is achieved by attaching the shoulders to the torque sensor and the pelvis to the platform .
c : hip twisting is achieved by attaching the pelvis to the torque sensor . 


1 ) triangle profile : in this case the rotational velocity , maximum excursion and number of cycles are specified .
2 ) discontinuous , step profile : amplitude , velocity , and holding time a step are specified .
3 ) increasing amplitude triangle waves : two cycles of each of 3 , 6 and 9 rotations are shown . in this example
a : configuration for trunk twisting with components labeled as follows : 1 ) rotating platform ; 2 ) motor and servo - control assembly ; 3 ) hinge joint between lower telescoping bar and the rotating platform ; 4 ) lower fixation and pelvis orthotic ; 5 ) upper fixation and shoulder harness ; 6 ) bar connecting suspension to upper fixation ; 7 ) torque sensor and counterbalanced suspension system ; 8) external fixation for measuring induced torque ; 9 ) rigid frame .
b : closeup of torque sensor and suspension system labeled as follows : 1 ) torque sensor ; 2 - 5 ) lightweight hinged aluminum plates .
the hinge between plates 2 and 3 rotates about the anterior - posterior axis , while the hinge between plates 4 and 5 is oriented around the mediolateral axis . 8)
c : configuration for twisting the trunk but measuring the torsional effect on the neck . in this configuration
the pelvis is fixed to the rotating platform ( 1 ) and the shoulders are connected to the external fixation ( 2 ) , which prevents the shoulders , neck and head from rotating , restricting twisting to the trunk .
the head is also attached to the upper fixation ( 3 ) so that any induced neck torque is applied to the torque sensor . 


subjects have consistent torque behavior across cycles , with large variation in resistance between subjects .
traces with highest resistance are typical of unmodulated behavior , while traces with least resistance are typical of high modulation .
peak - to - peak trunk torque shows consistent within - subject behavior across testing sessions but wide inter - subject variation .  


the resistive torque to 10 , 1/s triangle waves for the neck , trunk and hip levels .
the control loop consists of a pid ( proportional , integral , derivative ) controller , which receives input from an optical encoder attached to the platform shaft .
custom software running on a pc is used for selecting the desired platform trajectory , which then downloads this information to the controller .
twister is a servo - controlled device for quantifying postural tone in axial and proximal body regions during active , upright stance .
the original device was constructed at oregon health & science university and a similar device is under construction for the university of southampton , uk . in this report
we then provide a detailed description to facilitate its reproduction and show how it can be used to investigate the control of postural tone .
twister consists of a rigid steel frame , rotating platform , torque sensor , counterbalanced suspension system , upper and lower fixations , and body attachments ( fig 1 ) , as well as a servo control system for regulating platform rotation .
twister quantifies postural tone in standing subjects by rotating lower body segments about the vertical axis relative to upper segments .
upper and lower fixations can be attached to impart twisting to the neck , trunk or hip regions ( fig 2 ) . because twister does not provide postural support , tonic activity of skeletal muscle is necessary to counteract gravitational torques .
this tonic regulation is studied by the torsional resistance to twisting as well as electromyography from relevant muscles .
resistance to twisting is assessed by a torque sensor within the upper fixation and reflects the state of all muscles undergoing length changes .
twister uses several different platform rotation profiles to study tonic control , including a triangle profile , step profile , and triangle profile of increasing magnitude ( fig 3 ) .
these rotate the platform at a constant slow velocity , which minimizes inertial effects on the subject and measurement .  
we use a twisting perturbation about the vertical axis because it : 1 ) changes the length of axial and proximal muscles , as these structures are oriented obliquely and have broad anatomic origins and insertions ; 2 ) does not alter the relationship of the whole body and its parts to gravity ; 3 ) rotates the body around an axis of minimal moment of inertia ; 4 ) corresponds to a neutral zone , so that resistance from small displacements from the forward facing position reflects muscular rather than osteo - ligamentous forces ; and 5 ) naturally occurs in everyday activities .
twister is a flexible device that can be used to address various aspects of tonic control .
these include : 1 ) stiffness related to tonic activity ; 2 ) tonic responses to muscular length changes .
3 ) the effect of twisting on remote body regions ; 4 ) tonic effects from kinesthetic information ; 5 ) the effects of disease on postural tone ; and 6 ) perceptual thresholds to slow rotation .
rotating platform 

 subjects stand on a platform that rotates 20on a bearing about the vertical axis ( figs 1 , 4a ) .
an electric motor powers this rotation at a drive ratio that achieves platform speeds between 0.5/s and 5/s and high torque .
twister rotates the lower body in space , rather than the upper body to eliminate vestibular signals that could disrupt quiet stance .  
a belt and pulley system is used for drive reduction , which dampens vibration and eliminates lash that can interfere with torque measurement .
vibration is minimized because it can cue the subject about platform motion .   for safety ,
hard stops are used to limit maximal platform displacement .   an optical encoder ( hewlett - packard heds-5540 )
fixed to the platform shaft reports rotational displacement for both servo - control and data - analysis . 
 

frame 
 a rigid , steel frame ( 1.5 m x 1.5 m x 3 m ) with diagonal cross bracing creates high torsional stiffness between the platform assembly and torque sensor , necessary for accurate torque measurement . 
 

upper fixation and suspension system 
 the upper fixation and lightweight , counterbalanced suspension system connect the upper margin of the twisted region to the frame ( fig 4b ) . a torque sensor ( futek tff220 , irvine , ca ) positioned within
the suspension system consists of four rectangular aluminum plates that are alternately hinged along the anterior - posterior and mediolateral axes .
this creates a high stiffness for rotation about the vertical axis ( 590 nm/ ) , 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 ( 0.25 n / cm ) ensures subjects maintain postural stability themselves and prevents the upper fixation from providing a spatial reference .
this also allows each individual to maintain their own , unique vertical posture without affecting postural motion in the horizontal plane .  
a vertical bearing assembly ( fig 1 , 4b ) is used to adjust the upper fixation to subject height . 
 
 lower fixation 

 a lower fixation connects the lower margin of the twisted region to the rotating 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 . 
 

body attachments 
 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 ( fig 2 ) .   to twist the neck ,
attach the helmet above and the shoulders below .   to twist the trunk attach the shoulders above and the pelvis below   to twist the hips attach 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 . 
 

external fixation 
 a third , external fixation can be used to twist one body region while measuring torque produced by another . because the latter segment is stationary , the measured torque is not resistive but originates from muscular forces within the segment , potentially induced from remote twisting .  
the external fixation consists of a lightweight telescoping bar that prevents the connected segment from rotating about the vertical axis .
a hinge joint between the bar and frame allows anterior - posterior postural sway .  
alternatively , neck torque in response to hip twisting can be assessed by connecting the pelvis to the external fixation .  
a standard forceplate can be placed between the subjects feet and rotating platform , to simultaneously measure resistive torque in the twisted segment .
servo - control of platform rotation 
 a custom built real - time servo - system controls platform rotation .
this hardware pid controller outputs a motor drive signal based on a platform position signal from the optical encoder and the desired rotation ( see fig 7 ) .
a custom pc program interfaces with the hardware controller to specify the desired temporal profile of platform rotation and initiate a trial .  
select the triangle profile to alternate between constant - speed clockwise and counter - clockwise rotation ( fig .
rotation can also be driven with a triangle profile that increases in amplitude across cycles ( fig .
for all profiles , rotation is smoothed to limit acceleration to 12 /s during movement initiation and directional changes .
a typical experimental is run as follows : place body attachments ( i.e. helmet , shoulder harness or pelvis orthotic ) on the desired segments , ensuring they are snug and there is no torsional play .  
adjust the height of the linear bearing 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 pre - trial position .
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 .  
torque and platform rotation signals are typically recorded at 50 hz using spike 2 acquisition software ( cambridge electronic devices , cambridge , uk ) .  
resistive torque typically increases with platform excursion , however the increase slows with larger excursion . overall resistance is typically quantified by peak - to - peak torque , averaged across cycles .
fig 5a shows single trial responses across subjects for torsional resistance of constant speed ramp for the trunk .
we have observed data reproducible over months within a subject ( fig 5b ; interclass correlation coefficient = 0.89 ) .
mean resistance to twisting differs across body segments , and has been reported to be 0.54  0.24 nm for the neck , 5.11  1.94 nm for the trunk and 3.23  1.67 nm for the hips ( fig 6 ) .
absence of lash is indicated by smooth changes in torque during directional changes and a rapid change in torque at the onset , presumably due to short - range stiffness of muscle ( see fig 3a in gurfinkel et al . ) .
the measured torque reflects both the dynamic changes in tone with twisting as well as the distribution of baseline tonic activity ( which includes co - contraction ) . because of the slow speed of twisting , the increase in torque per degree is equivalent to intrinsic stiffness only when muscle activity is constant .
in general , two types of responses are observed that correspond to constant or modulated tonic activity within the twisted region .
the former is characterized by low cycle - to - cycle variation in torque , high peak - to - peak torque magnitude , and relatively constant emg .
in contrast , dynamic modulation is characterized by high cycle - to - cycle variability , low torsional resistance and emg modulation coherent with twisting . on angle vs. torque plots unmodulated subjects
exhibit a regular hysteresis loop while modulated subjects have an irregular pattern that can reverse direction ( see fig 3a in gurfinkel et al .
dynamic modulation generally consists of increasing tonic activity during muscle shortening and decreasing activity during lengthening ( i.e. sherrington 's lengthening and shortening reactions ) , which are opposite in sign to the stretch reflex .
an integrative measure of the extent of modulation can be obtained by the shift in a subject 's neutral ( zero - torque ) position within a cycle , referred to as torque phase advance . 


components are labeled as follows : 1 ) rotating platform ; 2 ) telescoping bar for lower fixation ; 3 ) hinge joint between lower telescoping bar and rotating platform ; 4 ) helmet attached to upper fixation ; 5 ) torque sensor and counterbalanced suspension system ; 6 ) locking vertical linear bearing ; 7 ) external fixation for measuring induced torque ; 8) hinge joint analogous to 3 ; 9 ) rigid frame ; 10 ) diagonal cross bracing for rigid frame . 


subjects stand on a rotating platform ( yellow ) with upper and lower body attachments affixed to impart twisting to the desired body region .
the upper attachment is connected via a suspension system ( zigzag lines ) to the torque sensor ( t ) , which is fixed with regard to rotation around the vertical axis .
the lower attachment connects to the rotating platform via a hinge joint ( black circle ) that allows rotation in the sagittal plane of the subject .
a : neck twisting is achieved by attaching a helmet to the torque sensor and the shoulders to the platform .
b : trunk twisting is achieved by attaching the shoulders to the torque sensor and the pelvis to the platform .
c : hip twisting is achieved by attaching the pelvis to the torque sensor . 


1 ) triangle profile : in this case the rotational velocity , maximum excursion and number of cycles are specified .
2 ) discontinuous , step profile : amplitude , velocity , and holding time a step are specified .
3 ) increasing amplitude triangle waves : two cycles of each of 3 , 6 and 9 rotations are shown . in this example
a : configuration for trunk twisting with components labeled as follows : 1 ) rotating platform ; 2 ) motor and servo - control assembly ; 3 ) hinge joint between lower telescoping bar and the rotating platform ; 4 ) lower fixation and pelvis orthotic ; 5 ) upper fixation and shoulder harness ; 6 ) bar connecting suspension to upper fixation ; 7 ) torque sensor and counterbalanced suspension system ; 8) external fixation for measuring induced torque ; 9 ) rigid frame .
b : closeup of torque sensor and suspension system labeled as follows : 1 ) torque sensor ; 2 - 5 ) lightweight hinged aluminum plates .
the hinge between plates 2 and 3 rotates about the anterior - posterior axis , while the hinge between plates 4 and 5 is oriented around the mediolateral axis .
c : configuration for twisting the trunk but measuring the torsional effect on the neck . in this configuration
the pelvis is fixed to the rotating platform ( 1 ) and the shoulders are connected to the external fixation ( 2 ) , which prevents the shoulders , neck and head from rotating , restricting twisting to the trunk . the head is also attached to the upper fixation ( 3 ) so that any induced neck torque is applied to the torque sensor . 


subjects have consistent torque behavior across cycles , with large variation in resistance between subjects .
traces with highest resistance are typical of unmodulated behavior , while traces with least resistance are typical of high modulation .
peak - to - peak trunk torque shows consistent within - subject behavior across testing sessions but wide inter - subject variation .  


the resistive torque to 10 , 1/s triangle waves for the neck , trunk and hip levels .
the control loop consists of a pid ( proportional , integral , derivative ) controller , which receives input from an optical encoder attached to the platform shaft .
custom software running on a pc is used for selecting the desired platform trajectory , which then downloads this information to the controller .
it is our opinion that twister can be used address many questions in tonic control . to date
probably the most important feature of twister is that it provides an integrated , kinetic measure of tone .
this torque measure of tone is not provided by kinematic , inverse dynamics or electromyographic approaches , and is necessary to answer many questions regarding tone . also , twister is unique in not substantially interfering with natural antigravity or postural behavior , and provides a tonic , rather than a phasic perturbation .
one potential use of twister is the quantification of the tonic effects of disease on postural tone .
while the intrinsic and reflex stiffness has been well studied for many neurological and musculoskeletal conditions using fast perturbations , the quantitative effect of many diseases on postural tone not well characterized . in particular , twister can be used to quantify the effects of disorders such as rigidity , hypotonia , dystonia , and back and neck pain on the magnitude , distribution and symmetry of postural tone along the body axis .
it can also be used to measure axial kinesthesis , eg ; perception of body rotation based on muscle proprioceptors and perceptuomotor symmetry , eg ; representation of straight ahead during axial twisting .
finally , twister can be used to study the effect of intervention on these measures of axial postural tone .
we estimate the cost of hiring an engineering firm to fabricate twister is approximately $ 30,000 us .
this device can likely be manufactured in - house for a fraction of this cost , as the price of raw materials is low , but significant fabrication is required . throughout its use ,
we hope that this report will help other investigators build twisting devices or otherwise stimulate research into this fundamental but poorly understood area .
 the control of tonic muscular activity remains poorly understood . while abnormal tone is commonly assessed clinically by measuring the passive resistance of relaxed limbs1 , no systems are available to study tonic muscle control in a natural , active state of antigravity support . 
 we have developed a device ( twister ) to study tonic regulation of axial and proximal muscles during active postural maintenance ( i.e. postural tone ) . 
 twister rotates axial body regions relative to each other about the vertical axis during stance , so as to twist the neck , trunk or hip regions . 
 this twisting imposes length changes on axial muscles without changing the body 's relationship to gravity . because twister does not provide postural support , tone must be regulated to counteract gravitational torques . 
 we quantify this tonic regulation by the restive torque to twisting , which reflects the state of all muscles undergoing length changes , as well as by electromyography of relevant muscles . 
 because tone is characterized by long - lasting low - level muscle activity , tonic control is studied with slow movements that produce " tonic " changes in muscle length , without evoking fast " phasic " responses . 
 twister can be reconfigured to study various aspects of muscle tone , such as co - contraction , tonic modulation to postural changes , tonic interactions across body segments , as well as perceptual thresholds to slow axial rotation . 
 twister can also be used to provide a quantitative measurement of the effects of disease on axial and proximal postural tone and assess the efficacy of intervention . 
Protocol
1. Introduction
2. Overview
3. Detailed description of device 
4. Experimental protocol
5. Representative results
Discussion
Disclosures