1) The document presents two models - a muscle synergy model (MSM) and most contributive model (MCM) - to explain how force perception arises from the activation of multiple muscles. 2) An experiment is proposed to test which model better accounts for force perception, involving measuring muscle activation via EMG and relating it to subjects' force judgments. 3) Preliminary results from some subjects show the MSM model fits their force perception better than the MCM model, providing initial evidence the MSM may generally better describe the relationship between muscle control and sensed force.

1. Abstract ID: 1725
Authors: Simone Toma, Marco Santello
Abstract Track: Neural Engineering
Abstract Sub Track: Neural Decoding and Control
Investigating Force Perception By
Means of Muscle Synergies Analysis
Neural Control of
Movement Laboratory

2. FORCE PERCEPTION IS RELATIVE RATHER THAN ABSOLUTE
(Jones, 2003; Gandevia 1990)
Matchingforce(%MVC)
Reference force (% MVC)
Matchingforce(%MVC)
2 5 8 10
Relative to the muscles
used
Relative to muscles
state
Unfatigued matching Muscle
Fatigued matching Muscle
2 5 8 10
Reference force (% MVC)
(Carson et al. 2002 ; Jones 1995)
2 5 8 10
Length Match > Length Ref
MVC Match > MVC Ref
2 5 8 10
Length Match < Length Ref
MVC Match < MVC Ref
Reference force (% MVC)
Relative to muscles
length
(Proske et al., 2012; Cafarelli et al., 1979)
2 5 8 10
Length Match = Length Ref
MVC Match = MVC Ref
elbow (E)
fingers (F)
Reference
(sensed)
Force
Edited from anatomic.us
wrist (W)
Match
(reproduced)
Force
Simone Toma, PhD
October 13, 2017
Abstract ID 1725
BMES 2017

3. FORCE PERCEPTION IS RELATIVE RATHER THAN ABSOLUTE
(Jones, 2003; Gandevia 1990)
Matchingforce(%MVC)
Reference force (% MVC)
Matchingforce(%MVC)
2 5 8 10
Relative to the muscles
used
Relative to muscles
state
Unfatigued matching Muscle
Fatigued matching Muscle
2 5 8 10
Reference force (% MVC)
(Carson et al. 2002 ; Jones 1995)
2 5 8 10
Length Match > Length Ref
MVC Match > MVC Ref
2 5 8 10
Length Match < Length Ref
MVC Match < MVC Ref
Reference force (% MVC)
Relative to muscles
length
(Proske et al., 2012; Cafarelli et al., 1979)
2 5 8 10
Length Match = Length Ref
MVC Match = MVC Ref
elbow (E)
fingers (F)
Reference
(sensed)
Force
Edited from anatomic.us
wrist (W)
Match
(reproduced)
Force
1) Sense of force is directly proportional to muscle contraction (i.e., descending motor command)
2) Sense of force depends on the properties of the target muscles
Simone Toma, PhD
October 13, 2017
Abstract ID 1725
BMES 2017

4. Influenced by spastic synergistic activation
𝐹 𝑚𝑎𝑡𝑐ℎ 𝐹𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒
Non-impairedImpaired
*
0
Overestimation
Underestimation
Nonimpared
Impared
Matching Arm
1
-1
(Yen et al., 2015)
MUSCLES CO-ACTIVATION INFLUENCES FORCE PERCEPTION
Reference +
Concurrent
Match
(Adjusted)
+/-
(Kilbreath et al., 1991; Gandevia et al., 1990)
0 100 200 300 0 100 200 300
75
100
125
150
Estimate(%control)
(g)
Concurrent
Thumb
(g)
Concurrent
Index
*
*
*
*
Match
Index
Match
Thumb
75
100
125
150
Estimate(%control)
control Concurrent
with foot
Match
Index
Relative to muscles pair
1
0.2
0.4
0.6
0.8
1
1.2
EF EE SF SE AB AD ER
StrengthRatio
Hemiparetic (impaired VS nonimpared limb)
Controls (non-dominant VS dominant limb)
(Dewald et al., 1995)
Simone Toma, PhD
October 13, 2017
Abstract ID 1725
BMES 2017

5. Influenced by spastic synergistic activation
𝐹 𝑚𝑎𝑡𝑐ℎ 𝐹𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒
Non-impairedImpaired
*
0
Overestimation
Underestimation
Nonimpared
Impared
Matching Arm
1
-1
(Yen et al., 2015)
MUSCLES CO-ACTIVATION INFLUENCES FORCE PERCEPTION
Reference +
Concurrent
Match
(Adjusted)
+/-
(Kilbreath et al., 1991; Gandevia et al., 1990)
0 100 200 300 0 100 200 300
75
100
125
150
Estimate(%control)
(g)
Concurrent
Thumb
(g)
Concurrent
Index
*
*
*
*
Match
Index
Match
Thumb
75
100
125
150
Estimate(%control)
control Concurrent
with foot
Match
Index
Relative to muscles pair
1
0.2
0.4
0.6
0.8
1
1.2
EF EE SF SE AB AD ER
StrengthRatio
Hemiparetic (impaired VS nonimpared limb)
Controls (non-dominant VS dominant limb)
(Dewald et al., 1995)
1) Sense of force is selectively biased by the combination of muscles that are co-activated
2) Force misperception associated to involuntary synergistic activation of upper limb muscles
Simone Toma, PhD
October 13, 2017
Abstract ID 1725
BMES 2017

6.  Not describing more everyday life situations (‘multiple muscle grouping’)
 Very few attempts to link indirect measure of descending motor command (i.e., EMG) to force perception
Most Contributive Model
TWO APPROCHES FOR FORCE PERCEPTION WITH MULTIPLE MUSCLES
How information from multiple group of muscles is integrated for an unique percept of force?
To shed light on the relation between motor control and force perception
To reveal action-for-perception mechanisms that might be generalized for other sensorimotor tasks
Muscle Synergies Model
Force Percept
Motor-Sensory Integration Areas
Σ
Motor-Sensory Integration Areas
Σ
Synergy 1
Motor
Drive
Spinal
cord
EMG 1 (e.g., biceps)
EMG 2 (e.g., triceps)
EMG 3 (e.g., anterior deltoid)
Synergy 2
Synergy 3
Motor
Drive Spinal
cord
EMG 1 (e.g., biceps)
EMG 2 (e.g., triceps)
EMG 3 (e.g., anterior deltoid)
Muscles 1
Muscles 3
Muscles 2
Simone Toma, PhD
October 13, 2017
Abstract ID 1725
BMES 2017

7.  Not describing more everyday life situations (‘multiple muscle grouping’)
 Very few attempts to link indirect measure of descending motor command (i.e., EMG) to force perception
Most Contributive Model
TWO APPROCHES FOR FORCE PERCEPTION WITH MULTIPLE MUSCLES
How information from multiple group of muscles is integrated for an unique percept of force?
To shed light on the relation between motor control and force perception
To reveal action-for-perception mechanisms that might be generalized for other sensorimotor tasks
Muscle Synergies Model
Force Percept
Motor-Sensory Integration Areas
Σ
Motor-Sensory Integration Areas
Σ
Synergy 1
Motor
Drive
Spinal
cord
EMG 1 (e.g., biceps)
EMG 2 (e.g., triceps)
EMG 3 (e.g., anterior deltoid)
Synergy 2
Synergy 3
Motor
Drive Spinal
cord
EMG 1 (e.g., biceps)
EMG 2 (e.g., triceps)
EMG 3 (e.g., anterior deltoid)
Muscles 1
Muscles 3
Muscles 2
1) The two models will predict different muscles combination to describe perception
2) Muscle Synergy model (MSM) will account for force perception better than the Most Contributive Model (MCM)
Simone Toma, PhD
October 13, 2017
Abstract ID 1725
BMES 2017

8. EXPERIMENTAL PROTOCOL FOR MODEL TESTING
UpwardForces(N)
Ascendant stair
Descendant stair
trials
0
15
30
Upward forces (N)
p(‘YES’)
0
0.5
1
15 300
Psychometric curve
Probability of answer yes
Slope: 0.25
PSE: 16.03
Upward forces
Psychometric curve
Muscle-Metric curve
p
ΔSlope: -0.1
Δ PSE: -2.2
R² : .78
𝑃𝑜𝑜𝑙𝐸𝑀𝐺𝑡𝑟 = 𝑛𝑜𝑟𝑚𝑀𝐴𝑉𝑖 ∗ 𝑤𝑖
𝑛
𝑖=1
EMG Mean Absolute Value/MVC across trials
Multiple regression coefficient
EMG8
time
EMG1
…
MAV1
MAV8
1. Brachioradialis
2. Biceps Brachii (long)
3. Triceps Brachii (long)
4. Middle Trapezius
5. Upper Trapezius
6. Latissimus Dorsi
7. Anterior Deltoid
8. Postertior Deltoid
Do I fell an
upward force
on my arm?
Toma S., Lacquaniti F. Plose One (2016)
𝑃𝑜𝑜𝑙𝐸𝑀𝐺
Criterion = PooleEMG
Num.ofTrials
Upward forces
p(PoolEMG>criterion)
Muscle-metric curve
Probability PoolEMG > criterion
Slope: 0.25
PSE: 16.03
Simone Toma, PhD
October 13, 2017
Abstract ID 1725
BMES 2017
0.2
0.4
0.6
0.8
1
R²
Models n. of Muscles
4EXP 7 6 5 BST 2 LST
p(R²|N.ofMuscles)
N. of Muscles
68 7 12345
R² = 0.60
R² = 0.70
R² = 0.80
MCM amount of perception
description

9. MUSCLE SYNERGY APPROACH
W
Contribution of Syn. 1
Contribution of Syn. 2
Contribution of Syn. 3
Original EMG
Trials
EMG
*
Local VAF:
93%
VAF
Global VAF:
92%
EMG RECONSTRUCTION
EMG8
time
EMG1
…
30%
MAV1
MAV8
1 ≤ Number of cluster ≤ 8
NNMF
Muscles Synergies (w) Activation Coefficients (c)
0
1
0
1
Activationstrength
0
1
0
1
0
1
0
1
TrialsMuscles
1 3 876542
SYNERGY EXTRACTION
𝑐𝑖
𝑐𝑖𝑐𝑖𝑐𝑖
Upward forces
p(𝑐𝑖𝑡>𝑐𝑖)
Upward forces
Synergy 1
Synergy 2
Synergy 3Increased activation
Reduced activation
Muscles synergy curves
𝑖 = synergy
𝑌𝑖𝑡
∗
= 𝛽0 + 𝛽1 𝐹𝑡 + 𝜀𝑖𝑡 + 𝜇𝑖
Generalized Linear Mixed Model
Nested Model Selection
Full model 3rd
synergy
out
2nd
synergy
out
1st
synergy
out
ΔAIC
p(synergycoeffincrease)
Upward forces
Best Force-Related Synergy Curve
Perceptual data
Best model (Synergy 2 & 3)
p(‘yes’)
R² : .69
ΔSlope: 0.04
ΔPSE: 2.2
Simone Toma, PhD
October 13, 2017
Abstract ID 1725
BMES 2017

10. Deviance ratio Δ 𝑠𝑙𝑝 Δ 𝑝𝑠𝑒 𝑅 𝑀𝑆𝑀
2
𝑅 𝑀𝐶𝑀
2
Subj. 1 1.21 0.01 -0.3 0.83 0.65*
Subj. 2 1.15 0.09 -1.3 0.64 0.16*
Subj. 3 1.01 0.03 2.2 0.69 0.52*
Subj. 4 11.08** 0.62 0.7 0.80 0.67*
Subj. 5 3.09 0.37 1.6 0.84 0.76*
Subj. 6 3.76 0.09 2.3 0.57 0.34*
Subj. 7 1.79 -0.07 -0.8 0.79 0.54*
Subj. 8 9.56** 1.1 0.9 0.64 0.77
Subj. 9 1.92 0.43 0.9 0.69 0.59*
Subj. 10 0.98 -0.16 -0.5 0.65 0.28*
SYNERGY (MSM) & MOST CONTRIBUTIVE (MCM) MODEL PREDICTIONS
Upward forceUpward force
Psychometric curve
Perceptual Syn. curve
p(‘yes’)
Muscle-metric curve
ΔSlope: 0.03
ΔPSE: 2.2
𝑅 𝑀𝐶𝑀
2
: 0.52
ΔSlope: -0.74
ΔPSE: -0.72
MSM MCM
pp
pp
𝑅 𝑀𝐶𝑀
2
: 0.69
PSE SLOPE
0
2
-2
Δparameters
-0.5
1 *
MSM curve
Full synergies set
MCM curve
Subjects
Indexofsharingmuscles
0
0.5
1 num(MSM muscles > 0.5 & MCM muscles)
num(MSM muscles > 0.5)
Simone Toma, PhD
October 13, 2017
Abstract ID 1725
BMES 2017
0
0.5
1R²
**
Full
Synergies
set
MSM MCM
≈20%

11. CONCLUSIONSSimone Toma, PhD
October 13, 2017
Abstract ID 1725
BMES 2017
1) The two models will predict different muscles combination to describe perception
2) Muscle Synergy model (MSM) will account for force perception better than the Most Contributive Model (MCM)
3) We have validated a method to address in depth the relation between motor control, muscles and sense of force

12. THANK YOU!Simone Toma, PhD
October 13, 2017
Abstract ID 1725
BMES 2017
Francesco Lacquaniti, MD
Spec. in Neurology, MAE
Professor of Physiology
Faculty of Medicine and Surgery
Marco Santello, PhD
Director, SBHSE
Harrington Endowed Chair
and Professor