Advantages of the Onion-Skin scheme of motor unit …Advantages of the Onion-Skin scheme of motor...
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CONCLUSIONS REFERENCES INTRODUCTION METHODS The model used for the simulaon of the MU firing rate and force behavior is a modi- fied version of that developed by Contessa and De Luca (2013) for the FDI and VL. 1. Eccles et al. J Physiol, 1958. 4. De Luca & Erim. Trends Neurosci, 1994. 2. Kernell. Acta Physiol Scand, 1965. 5. De Luca & Hostage. J Neurophysiol, 2010. 3. De Luca et al. J Physiol, 1982. 6. Contessa & De Luca. J Neurophysiol, 2013. SIMULATION RESULTS (FDI) MU firing rates and force were simulated at constant levels of input excitaon ranging from 5 to 10% and increasing to 100% maximal excitaon in steps of 10% for both schemes. At each input excitaon level, we calculated: a) the number of acve MUs; b) their firing rate; c) their force output; and d) the force output of the whole muscle. Low- vs. High-threshold MU Forces: Onion-Skin) The force generated by the earliest recruited MU is fully fused within 5% input excitaon from its re- cruitment. The force generated by the last recruited MU does not fully fused even at maximal excitaon. AHP) All MUs fuse around maximal input excitaon. This work was supported by Grant# HD-050111 from the Naonal Instute of Child Health and Human Development (NICHD) and by a grant from the Neuromuscular Research Foundaon. ACKNOWLEDGEMENTS Our results for both the FDI and the VL show that the Onion-Skin scheme has disnct advantages: 1) Lower-threshold MUs produce more force at lower excitaon levels. Thus, a relat- evely fewer number of lower-threshold MUs, in most part oxidave and able to sus- tain force for extended me, are required for lower force producon. 2) It produces smoother force, especially at the lower force levels that are used for normal daily acvies. 3) Higher-threshold MUs never fully fuse, maintaining the potenal for a force “re- serve capacity” that might be available in extraordinary circumstances. 4) It provides more sustainable contracons . Over the past five decades, the noon that higher-threshold shorter-aſter-hyperpo- larizaon (AHP) motoneurons have greater firing rates than lower-threshold lon- ger-AHP ones has been commonly accepted. This noon, here named the AHP scheme, derived from observaons in electrically smulated cat motoneurons and supports the assumpon that motor unit (MU) firing rates match their mechanical properes to “opmize” force generaon [1,2]. That is, lower-threshold MUs have wider and smaller force twitches that require lower firing rates to tetanize. In contrast, we have shown that earlier-recruited MUs maitain higher firing rates than later recruited ones during voluntary isometric contracons, resulng in an in- verse orderly hierarchy of firing rate curves named the Onion-Skin scheme [3-5]. The purpose of this study is to use a novel model of muscle force generaon [6] to compare the force characteriscs produced by the two schemes during constant-force contracons in the first dorsal interosseous (FDI) and vastus lateralis (VL) muscles. Advantages of the Onion-Skin scheme of motor unit firing during voluntary contracons Carlo J. De Luca 1-3 , Paola Contessa 1-2 NeuroMuscular Research Center, Boston University, Boston, USA 1 ; Delsys Inc., Nack, USA 2 Department of Electrical and Computer Engineering; Biomedical Engineering; Neurology; Physical Therapy, Boston University, Boston, USA 3 Force Model Schemac: (A) The input excitaon drives the firing behavior of all MUs in the muscle. (B) The firing rate spectrum de- scribes the firing behavior of MUs as a funcon of input excitaon. (C) The force twitch spectrum de- scribes the MU mechanical prop- eres. (D) Muscle force output is the summaon of the force con- trIbuons of all MUs. (E) A force feedback allows simulaon of force sustained at given targets. Firing Rate Spectra: Onion-Skin) The spectrum was derived from empirical data from voluntary iso- metric linearly-varying/constant contracons in humans [6]. AHP) The spectrum was modeled based on the hypothesis that MU firing rates provide “opmal” force twitch fusion for all MUs [1,2]. Force Generaon Capacity & Force Variability: The Onion-Skin scheme produces more absolute and smooth- er force than the AHP scheme up to 60% input excitaon. When approach- ing maximal excitaon, the AHP scheme can produce greater and smoother force. Maximal Force Generaon Capacity: Onion-Skin) If MU firing rates are ar- ficially doubled from their values at maximal excitaon, the whole muscle force output increases from the value at maximal excitaon by ~ 20% MVC. AHP) MUs are fully fused at 100% exci- taon. Hence, muscle force output cannot increase even if MU firing rates are increased from their vaue at maxi- mal input excitaon. Muscle Endurance Time: During a simulated contracon sustained at 50% maximum voluntary contracon, the forces of the individual MUs, and consequently the force of the whole muscle, decline faster for the high- er-threshold faster-firing MUs in the AHP scheme than in the Onion-Skin scheme. The endurance limit is reached earlier in the AHP scheme than in the Onion-Skin scheme. AHP Scheme vs. Onion-Skin Scheme: As summarized in the figure above, the On- ion-Skin scheme is not designed to maximize muscle force, as proposed for the AHP scheme. Instead, it generates force more quickly and more smoothly when force is ini- ated, and it provides lower maximal force with the capacity to sustain it over longer me. Smoother force producon enables accurate performance of daily tasks. High- er-threshold MUs maintain a reserve capacity that could be accessible in extraordi- nary circumstances. These features support the flight-or-fight reflexive response in the presence of danger and are more conducive to evoluonary survival. Firing Rate Force Twitch Force MU Diameter max A Firing Rate Force Twitch Force MU Diameter max B max A AHP Scheme Onion-Skin Scheme (A) (D) Electrical Aspects - motoneuron level Mechanical Aspects - muscle fiber level OUTPUT FORCE TARGET FORCE controller MU 1 + - error MU k MU 1 MU k Impulse train FIRING RATE SPECTRUM Firing noise (B) Time dependent changes Gain factor FORCE TWITCH (C) INPUT EXCITATION (E) Σ Onion-Skin Spectrum 0 20 40 60 80 100 Excitaon (% max) AHP Spectrum 0 20 40 60 80 100 Excitaon (% max) Firing Rate (pps) 0 27 9 Firing Rate (pps) 36 18 MU 1 MU n MU n MU 1 45 0 27 9 36 18 45 Onion-Skin 0 4 8 Rec thr 5% exc 10% exc Force (AU) 0 600 300 Firing Rate (pps) 0 9 27 45 36 18 Rec thr 80% exc 100% exc MU 1 MU 120 AHP Rec thr 20% exc 40% exc 60% exc 80% exc 100% exc 0 20 40 60 80 100 0 9 27 45 36 18 Excitaon (% max) Firing Rate (pps) 0 4 8 Force (AU) 0 0.2 0.4 0.6 0.8 1 0 600 300 Time (s) Rec thr 80% exc 100% exc MU 1 MU 120 Rec thr 5% exc 10% exc Rec thr 80% exc 100% exc 0 20 40 60 80 100 Excitaon (% max) Force Variability Onion-skin scheme AHP scheme CV (%) 0 20 40 60 80 100 Excitaon (% max) Muscle Force Force (AU) 0 3 9 15 12 6 2 6 12 8 4 10 0 Onion-Skin Firing Rate (pps) 0 9 27 45 36 18 AHP 0 20 40 60 80 100 0 9 27 45 36 18 Excitaon (% max) Firing Rate (pps) Force (% MVC) 0 80 0 0.2 0.4 0.6 0.8 1 Time (s) 40 100 60 120 Doubled firing rates 20 Force (% MVC) 0 80 40 100 60 120 20 MU Forces (AU) Onion-Skin 0 4 8 12 Muscle Force (%MVC) 40 80 100 60 AHP 20 0 0.2 0.4 0.6 0.8 1 Time (s) 0 0.2 0.4 0.6 0.8 1 Time (s) 0 MU 1 MU 13 MU 25 MU 49 MU 97 MU 109
Transcript of Advantages of the Onion-Skin scheme of motor unit …Advantages of the Onion-Skin scheme of motor...
CONCLUSIONS
REFERENCES
INTRODUCTION
METHODS
The model used for the simulation of the MU firing rate and force behavior is a modi-fied version of that developed by Contessa and De Luca (2013) for the FDI and VL.
1. Eccles et al. J Physiol, 1958. 4. De Luca & Erim. Trends Neurosci, 1994.2. Kernell. Acta Physiol Scand, 1965. 5. De Luca & Hostage. J Neurophysiol, 2010.3. De Luca et al. J Physiol, 1982. 6. Contessa & De Luca. J Neurophysiol, 2013.
SIMULATION RESULTS (FDI)
MU firing rates and force were simulated at constant levels of input excitation ranging from 5 to 10% and increasing to 100% maximal excitation in steps of 10% for both schemes. At each input excitation level, we calculated: a) the number of active MUs; b) their firing rate; c) their force output; and d) the force output of the whole muscle.
Low- vs. High-threshold MU Forces: Onion-Skin) The force generated by the earliest recruited MU is fully fused within 5% input excitation from its re-cruitment. The force generated by the last recruited MU does not fully fused even at maximal excitation. AHP) All MUs fuse around maximal input excitation.
This work was supported by Grant# HD-050111 from the National Institute of Child Health and Human Development (NICHD) and by a grant from the Neuromuscular Research Foundation.
ACKNOWLEDGEMENTS
Our results for both the FDI and the VL show that the Onion-Skin scheme has distinct advantages: 1) Lower-threshold MUs produce more force at lower excitation levels. Thus, a relat-evely fewer number of lower-threshold MUs, in most part oxidative and able to sus-tain force for extended time, are required for lower force production. 2) It produces smoother force, especially at the lower force levels that are used for normal daily activities. 3) Higher-threshold MUs never fully fuse, maintaining the potential for a force “re-serve capacity” that might be available in extraordinary circumstances. 4) It provides more sustainable contractions .
Over the past five decades, the notion that higher-threshold shorter-after-hyperpo-larization (AHP) motoneurons have greater firing rates than lower-threshold lon-ger-AHP ones has been commonly accepted. This notion, here named the AHP scheme, derived from observations in electrically stimulated cat motoneurons and supports the assumption that motor unit (MU) firing rates match their mechanical properties to “optimize” force generation [1,2]. That is, lower-threshold MUs have wider and smaller force twitches that require lower firing rates to tetanize.In contrast, we have shown that earlier-recruited MUs maitain higher firing rates than later recruited ones during voluntary isometric contractions, resulting in an in-verse orderly hierarchy of firing rate curves named the Onion-Skin scheme [3-5].
The purpose of this study is to use a novel model of muscle force generation [6] to compare the force characteristics produced by the two schemes during constant-force contractions in the first dorsal interosseous (FDI) and vastus lateralis (VL) muscles.
Advantages of the Onion-Skin scheme of motor unit firing during voluntary contractionsCarlo J. De Luca1-3, Paola Contessa1-2
NeuroMuscular Research Center, Boston University, Boston, USA1; Delsys Inc., Natick, USA2Department of Electrical and Computer Engineering; Biomedical Engineering; Neurology; Physical Therapy, Boston University, Boston, USA3
Force Model Schematic: (A) The input excitation drives the firing behavior of all MUs in the muscle. (B) The firing rate spectrum de-scribes the firing behavior of MUs as a function of input excitation. (C) The force twitch spectrum de-scribes the MU mechanical prop-erties. (D) Muscle force output is the summation of the force con-trIbutions of all MUs. (E) A force feedback allows simulation of force sustained at given targets.
Firing Rate Spectra: Onion-Skin) The spectrum was derived from empirical data from voluntary iso-metric linearly-varying/constant contractions in humans [6]. AHP) The spectrum was modeled based on the hypothesis that MU firing rates provide “optimal” force twitch fusion for all MUs [1,2].
Force Generation Capacity & Force Variability: The Onion-Skin scheme produces more absolute and smooth-er force than the AHP scheme up to 60% input excitation. When approach-ing maximal excitation, the AHP scheme can produce greater and smoother force.
Maximal Force Generation Capacity: Onion-Skin) If MU firing rates are arti-ficially doubled from their values at maximal excitation, the whole muscle force output increases from the value at maximal excitation by ~ 20% MVC.AHP) MUs are fully fused at 100% exci-tation. Hence, muscle force output cannot increase even if MU firing rates are increased from their vaue at maxi-mal input excitation.
Muscle Endurance Time: During a simulated contraction sustained at 50% maximum voluntary contraction, the forces of the individual MUs, and consequently the force of the whole muscle, decline faster for the high-er-threshold faster-firing MUs in the AHP scheme than in the Onion-Skin scheme. The endurance limit is reached earlier in the AHP scheme than in the Onion-Skin scheme.
AHP Scheme vs. Onion-Skin Scheme: As summarized in the figure above, the On-ion-Skin scheme is not designed to maximize muscle force, as proposed for the AHP scheme. Instead, it generates force more quickly and more smoothly when force is ini-tiated, and it provides lower maximal force with the capacity to sustain it over longer time. Smoother force production enables accurate performance of daily tasks. High-er-threshold MUs maintain a reserve capacity that could be accessible in extraordi-nary circumstances. These features support the flight-or-fight reflexive response in the presence of danger and are more conducive to evolutionary survival.
Firing Rate Force Twitch ForceMU Diametermax A
Firing Rate Force Twitch ForceMU Diameter
max B
max A
AHP Scheme
Onion-Skin Scheme
(A) (D)
Electrical Aspects - motoneuron level
Mechanical Aspects - muscle fiber level
OUTPUT FORCE
TARGETFORCE
controller
MU 1
+- error
MU kMU 1
MU k
Impulse train
FIRING RATE
SPECTRUM
Firing noise
(B)
Time dependent
changes
Gain factor
FORCE TWITCH
(C)
INPUT EXCITATION
(E)
Σ
Onion-Skin Spectrum
0 20 40 60 80 100
Excitation (% max)
AHP Spectrum
0 20 40 60 80 100
Excitation (% max)
Firin
g Ra
te (p
ps)
0
27
9Firin
g Ra
te (p
ps) 36
18
MU 1
MU n
MU n
MU 1
45
0
27
9
36
18
45
Onion-Skin
0
4
8
Rec thr5% exc10% exc
Force (AU)
0
600
300Firin
g Ra
te (p
ps)
0
9
27
45
36
18 Rec thr80% exc100% exc
MU 1
MU 120
AHP Rec thr20% exc40% exc60% exc80% exc100% exc
0 20 40 60 80 1000
9
27
45
36
18
Excitation (% max)
Firin
g Ra
te (p
ps)
0
4
8
Force (AU)
0 0.2 0.4 0.6 0.8 1 0
600
300
Time (s)
Rec thr80% exc100% exc
MU 1
MU 120
Rec thr5% exc10% exc
Rec thr80% exc100% exc
0 20 40 60 80 100Excitation (% max)
Force Variability
Onion-skinscheme
AHPscheme
CV (%
)
0 20 40 60 80 100Excitation (% max)
Muscle Force
Forc
e (A
U)
0
3
9
15
12
6
2
6
12
8
4
10
0
Onion-SkinFi
ring
Rate
(pps
)
0
9
27
45
36
18
AHP
0 20 40 60 80 1000
9
27
45
36
18
Excitation (% max)
Firin
g Ra
te (p
ps)
Force (% M
VC)
0
80
0 0.2 0.4 0.6 0.8 1Time (s)
40
100
60
120
Doubled firing rates
20
Force (% M
VC)
0
80
40
100
60
120
20
MU
For
ces
(AU
)
Onion-Skin
0
4
8
12
Mus
cle
Forc
e
(%M
VC)
40
80
100
60
AHP
20
00.2 0.4 0.6 0.8 1
Time (s)0 0.2 0.4 0.6 0.8 1
Time (s)0
MU 1MU 13MU 25MU 49MU 97MU 109
