Practical Applications of Muscle Stimulation

Transcription

1 Practical Applications of Muscle Stimulation William Durfee Department of Mechanical Engineering University of Minnesota Minneapolis, USA 1

2 NON-INVASIVE ASSESSMENT OF MUSCLE FUNCTION External mechanical properties provide window into muscle excitation & contraction mechanisms Electrical stimulation provides controlled input System identification methods reveal muscle parameters Reuires mathematical model of muscle Non-invasive u(t) STIM MUSCULOSKELETAL SYSTEM y(t) FORCE/MOTION 2

3 Most common non-invasive muscle force assessment method Durfee & Iaizzo, Encyclopedia of Medical Devices and Instrumentation, 2006 Encyclopedia of Medical Devices and Instrumentation, 2nd ed. J.G. Webster, ed., Vol 6, pp 62-71, Hoboken, John Wiley & Sons,

4 MATHEMATICAL MODELS OF WHOLE MUSCLE MECHANICS 4

5 Hill models are still the gold standard CE SE PE Proceedings of the Royal Society of London, B. 126(843): , Contractile element force depends on several variables CE Force = f (neural input, length, velocity) F F F Activation Length Velocity 5

6 Multiplicative model for CE force u IRC Activation Dynamics X CE Force-Length X V CE Force-Velocity Fscale Model for isolated muscle u IRC Activation Dynamics (2nd order) Active Element u F X CE Force-Length X x,v V CE Force-Velocity Fscale Force Passive Element X PE Force-Length V PE Force-Velocity 6

7 Model OK for isolated muscle Model Experiment Force (N) Time (s) Durfee & Palmer, IEEE Trans. Biomed. Eng., 41(3):205, 1994 Identification of intact muscle properties more challenging What you want What you have Skeletal geometry 2. Muscle in series with a springy tendon 7

8 Skeletal geometry u(t) STIM MUSCULOSKELETAL SYSTEM y(t) FORCE/MOTION x(t) u(t) MT f(t) GEOM τ(t) LD θ(t) GEOM -1 Some muscles have significant springs Tibialis Anterior 8

9 Tendon spring leads to inaccuracies in estimating muscle force-length curve Zajac, Crit. Rev. Biomed. Eng., 1989, Figure 13 Muscle-tendon model muscle a(t) L M (t) V M (t) CE F M (t)=f T (t)=f(t) tendon SE PE passive 9

10 WHAT'S WRONG WITH THE MUSCLE MODEL Invariant F-A, F-L, F-V (no change with activation) Invariant twitch dynamics (uniform fiber types) Time-invariant (no fatigue) Full model has muscle-tendon acting against limb load K SE CE M m K PE (θ) M J (θ) B PE (θ) 10

11 11 K SE K PE (θ) CE M m B PE (θ) M J (θ) K SE K PE (θ) CE M m B PE (θ) M J (θ) excitation states 5, = = = = = V L CE CE ω θ ) ( 2 ), ( ) ( 1 ) ( ) ( 1 ) ( ) ( 1 ) ( 1 ),, ( t ku a a f M f M f M f M f M PD J PE J T J T m CE m + = = = = + = = & & & & & & Simulation euations CE Force-Length Gordon, J. Physiol., 1966 ( ) shape parameters,, 1 1 exp ) ( = + = ω ρ β ω ρ β L L F FL

12 CE Force-Velocity FORCE F F FV FV ( V ) = ( b a * V ) 1 1 ( V ) = shortening V + b ( b a * V ) V + b 2 lengthening LINEAR FIT VELOCITY LENGTHENING SHORTENING Modeling twitch dynamics Hammerstein model stim Static nonlinearity Linear dynamic system force Identify LDS with impulse response Durfee & Palmer, IEEE TBME, 1994 Identify SL by deconvolution Durfee & MacLean, IEEE TBME,

13 Twitch force varies with stim strength Recruitment Plot Pulse Width Percentage 2 nd -order, critically damped linear system fits well enough Torue BEST FIT k ( s + a) FORCE TWITCH Time (ms) 2 13

14 Using nonlinear force twitch response as a metric 14

15 Torue Time (ms) Single pulse twitch Torue y 2y Time (ms) Double pulse twitch, if ideal linear system 15

16 Torue y Time (ms) Double pulse twitch, real Doublet pulse spacing affects twitch force Force Time (ms) Doublets at 1,2,3,4,5,6,7,8,9,10,20,30,40,50 ms 16

17 Nonlinear summation for TA Normalized Peak Torue Normalized TTI Nonlinear summation depends on which muscle Biceps Quadriceps Tibialis Anterior Mean IPI (ms) 17

18 Normalized twitch parameters same for all recruitment levels L1 L2 L3 L4 L5 L6 L7 L1-L7 L2-L7 Metric F P Fcrit Sig. F P Fcrit Sig. PT TTI ST CT HPW X HDT X WHAT'S NEXT 18

19 Identification strategy 1. Passive length-tension: slowly move 2. Twitch dynamics: isometric 3. Active length-tension: slowly move when active 4. Force-velocity: apply rich velocity perturbations about steady-state Current isometric apparatus 19

20 Improved apparatus Force, EMG, temp, laptop/usb A simple clinical tool to measure limb inertia CE M m K SE K PE (θ) M J(θ) B PE(θ) Force vs. Acceleration Force (lb-f) Acceleration (g) 20

21 Build database of properties for several muscles in non-diseased subjects F-L, F-V, contraction dynamics, doublet properties Nash Avery Search for Hope Fund and the Paul and Sheila Wellstone Muscular Dystrophy Center, University of Minnesota. FES-AIDED GAIT: A NEW APPROACH 21

22 Brain Spinal Cord Stimulator Limb F EXTERNAL F EXTERNAL Inputs CONTROL STIMULATOR Measurements Improve health through weight bearing Brief standing: social and functional Limited ambulation in vicinity of wheelchair No balance, no change in neuro function 22

23 Liberson foot-drop system, 1961 Heel switch triggered peroneal n. stimulation Correction of foot-drop following stroke Started field of FES Several commercial and research embodiments Medtronic implanted foot-drop system 23

24 STIMULATION PLUS SMART ORTHOTICS Muscle stimulation provides power Brakes for locking and control Orthosis provides guidance and support 24

25 CBO Increases speed and distance Gait Speed Gait Distance 50 Speed (m/s) Distance (m) Without CBO With CBO 0 Without CBO With CBO IEEE Trans Rehab Eng, 4(1):13, 1996, IEEE Trans Rehab Neural Eng,

26 CBO has better step-to-step repeatability 120 Without CBO 120 With CBO Knee angle (deg) Knee angle (deg) Time (sec) Time (sec) IEEE Trans Rehab Eng, 4(1):13, 1996, IEEE Trans Rehab Neural Eng, 2003 ENERGY STORING BRACE 26

27 Energy Budget ~30 nm over 60 deg of motion 31.4 J per extension Extract 14 J per cycle 27

28 J. Biomechanical Engineering, 127(6): , ADAMS dynamic model 28

29 Gas springs Cylinders Accumulator 29

30 Hip belt Knee brace Ab/adduction hinge Prismatic joint Placeholders for brakes Medial hinge POWERED HUMAN-ASSIST TOOLS 30

31 Engineering Research Center for Compact & Efficient Fluid Power 31

32 Compact power sources Natural interface and control Portable and/or wearable Replacing muscle And a few other projects 32

33 Muscle metrics Short-stroke, linear actuator 5-20% shortening stroke Pull force: 30 lbs/s. in. 90 W/lb 180 lb athlete w/ 72 lb of muscle puts out 370 W sustained 5 W/lb for human muscle for continuous use 25% efficient Compliant, back-drivable Fatigues Clean Quiet! Vogel (2001), "Prime Mover" Power (W/lb) Muscle--peak Muscle-- sustained Electric motor Automobile engine (Aircraft engine, piston: 700; Aircraft engine, turbine: 2500) Vogel (2001), "Prime Mover" 33

34 Miniature free-piston air-compressor FUTURE MICRO FPAC + 1KPSI TANK 34

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