IN SITU THERMAL MEASUREMENTS OF SKELETAL MUSCLE IN ORDER TO ESTIMATE RELATIVE METABOLIC UTILIZATION

Northwestern University 05/31/01

ME 224 – Experimental Engineering Professor Horatio Espinosa

Ayan Bagchi Ahmad Dialdin Junhyck Kim Laura Evans

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TABLE OF CONTENTS

1) Summary…………………………………………………….. 3

2) Biological detail………………………………………………3

3) Instrumentation………………………………………………..7

4) Experimental information……………………………………10

5) Results………………………………………………………..12

6) Discussion…………………………………………………….13

7) References cited………………………………………………15

8) Biographical sketches…………………………………………16

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Problem definition and research objective Currently there are many problems and inconveniences associated with cardiac assist devices. To eliminate some of these problems, a variety of systems to drive cardiac assist devices with power from muscle have been proposed and are under development. The power available from a fixed mass of muscle is metabolically limited and maximizing sustained power is required for the successful application of such devices. The purpose of this study is to develop an approach that can yield a metabolic utilization relation through relative myothermic measurements from single contractions of whole muscle. With that relation, one can constrain the power optimization or predict maximum mechanical to metabolic efficiency. The gastrocnemius and soleus muscles and the achilles tendon of two anesthetized New Zealand white rabbits were exposed and the lower leg heel attached to a movable arm to allow changes in muscle length. A Telectronics model 7220NE myostimulator was used to stimulate the muscle with 0.6 ms duration stimuli at a frequency of 50 Hz and voltage of 1.6 V. A Thermometrics Edison NJ: GC11 thermistor and a Dexter Research S60 infared radiation sensing thermopile detector were used to perform heat measurement. An A.L. DESIGN, INC force transducer generated tension in the achilles tendon. As a result of the information gained in this study it is hoped that skeletal muscle powered cardiac assist devices can be implemented. Biological Detail Skeletal muscle work and power could be used for cardiac assistance by energizing pacemakers. In order to consider this idea, however, one must first consider the properties of muscles. Contraction takes place in many steps. ATP formed by cellular respiration is broken down to energize the contraction. First, the nervous system sends electrical impulses in order to stimulate the muscle fiber to contract. The nerve and muscle contact at a neuromuscular junction, where acetylcholine is secreted to initiate the changes in the muscle that cause contraction. Myosin and actin filaments work together to cause contraction. Actin filaments are made up of globular protein that is twisted helically in chains, forming microfilaments in muscle and other contractile elements in cells. Myosin filaments are made of protein and interact with actin to cause cellular contraction. After ATP energizes the myosin heads, the heads bind to actin, forming cross-bridges. This bending pulls the actin filaments to the center of the sarcomere, causing contraction. The myosin heads release after binding to ATP. In the resting state, some fibers maintain a state of partial contraction in order that the entire muscle doesn’t have to overcome the inertia of total relaxation. See fig. 1 for a detailed picture of muscle contraction on the filament level. Fig. 2 shows a detail picture of muscle contraction. In muscles, most of the energy for contraction is stored in phosphagens such as creatine phosphate, which provide a phosphate group to ADP to make ATP. The actin potential is trigger for contraction, while the elevation of calcium (Ca) concentration controls the duration of contraction. Sarcoplasmic reticulum regulates the amount of Ca concentration (See fig. 3 for a

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detail picture of muscular contraction control). In summary, ATP undergoes hydrolysis; thereby creating chemical energy that is converted to mechanical energy by the interaction of myosin cross-bridges with actin filaments. This energy then creates a mechanical output.

First, myosin head is bound to ATP and is in its low energy configuration (1) ATP is hydrolyzed to ADP and inorganic phosphate, is in high-energy configuration (2) myosin heads bind to actin, forming a cross-bridge (3) ADP and phosphate is released, myosin relaxes to low-energy state, causing thin element to slide (4) Myosin head released by the binding of ATP As ATP is hydrolyzed, the myosin head returns to high-energy configuration and begins a new cycle Biology, pg 1016

Figure 1 – the interaction between myosin and actin in muscle contraction

The thick filaments represent myosin, while the thin filaments represent actin. Notice that the lengths of both filaments stay the same during contraction. (a) in the relaxed muscle, the length of each sarcomere is greater than in contracting/contracted muscle (b) as contraction occurs, myosin and actin slide past each other, shortening the sarcomere (c) shown here is a fully contracted muscle. Notice the shortened sarcomere, the overlapping thin filaments, and the small distance between thick filament ends and the Z lines Biology, pg 1015

Figure 2 – sliding-filament model of muscle contraction

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The thin filament consists of two strands of actin in a helix formation (a) muscle at rest: Tropomyosin molecule blocks myosin binding sites that are used to form cross bridges (b) troponin binds calcium ions, exposing the binding sites, which allows cross-bridges to form thereby allowing the muscle to contract Biology, pg 1017

Figure 3 – control of muscle contraction The heat generation of muscular contraction depends on contraction parameters, fiber type, and species. In isometric contraction, two muscles work in opposition for strong, motionless action. Isotonic contraction occurs during mechanical work. Here, as a constant tension is applied the length of the muscle decreases. Tetanic contractions are the maximum contraction possible without injury (see fig. 4).

Dashed lines show the response that would have occurred if only the first action potential had occurred. Biology, pg 1018 Figure 4 – muscle contractions related to time

There are two basic types of muscle fibers: fast twitch (phasic) and slow twitch (tonic) fibers. Phasic fibers are used for rapid, powerful contractions and dynamic activity. They consume ATP at a greater rate and their ability to contract and relax rapidly accommodates the need to move limbs rapidly and precisely. Tonic fibers are mechanically slow and have low rates of force development, relaxation, and unloaded shortening. Mostly postural, they consume ATP at a lower rate and are able to maintain low levels of action for long periods of time with little energy expense.

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Biology, pg 782

Figure 5 – different kinds of vertebrate muscle The paper by Barclay, Constable, and Gibbs studies the energetics of the muscles of mice. Measuring rates of thermal and mechanical energy liberation assessed the energetic cost of work performance. The information presented in this paper is valuable to discovering the possibilities of muscle powered cardiac assistance. In their studies, both slow and fast twitch muscles were studied under isometric tetanic contractions. Their results showed that the rate of heat production from phasic muscles was greater than that of tonic muscles. The maximum shortening velocity and peak power output was also greater for phasic muscles than tonic muscles. However, during shortening the rate of heat output from slow-twitch muscles increased greatly over the isometric heat rate, while that for fast-twitch muscles only increased a small amount. The total rate of energy liberation (heat rate + power) also had a greater increase over that for isometric rate in the case of tonic muscles. The peak mechanical efficiency (power/total energy rate) of both muscles was approximately 30%. This information was gathered by examining biological changes with sufficient temporal resolution to follow changes in the rate of ATP breakdown during brief periods of shortening. Myothermic (heat measurement) techniques were used to calculate the rate of ATP breakdown. From the enthalpy change resulting from the chemical breakdown in muscle fibers, the amount of heat and work can be obtained. These measurements of the output change in heat and work per unit time allow one to get the rate of ATP breakdown. Heat production was found from the rise of the temperature of the preparation during a contraction. There were three sources of heat for calculating the rate of enthalpy liberation: mechanical energy, thermal energy, and stimulus heat. The total rate of enthalpy liberation during shortening (mechanical work) is the sum of the rates of work (i.e. power output) and heat production. The numerical integral of force with respect to distance shortened determined the work performed and the average power output was found by dividing the total work by the shortening period. Some adjustments to the data had to be considered due to the damping of the force response (approximately 20 ms passed before force declined at a steady level). This probably occurred because of the series elastic elements of muscle. To keep this from skewing the results, the first 20 ms of data was omitted and the force of that time period was assumed to be the force measured at 20 ms. As isometric contractions

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were studied in this project, no external work was performed, therefore the rate of enthalpy liberated (from thermal energy) was equal to the heat production. Allowances must be made in the thermal data also to take account the non-metabolic sources of heat. For example, during shortening the force exerted decreases and the thermoelastic properties of muscle cause heat to be created. The energy used to stretch the elastic component of muscle is liberated as heat while tension decreases during shortening. Heat liberated by the stimulus must also be considered before calculating the total heat production. All of these sources of error were taken into account before analysis of the data occurred. Instrumentation In these studies many different instruments have been used in order to obtain the best data possible. Cardiomyostimulator A sophisticated pacemaker, it is the primary device used in Cardiomyoplasty, a surgical procedure designed to expand a diseased heart’s capacity to pump blood. Physicians detach the latissimus dorsi, a large skeletal muscle in the back, and wrap it around the heart like a blanket, thus connecting its contractions to that of the heart’s. Take note that the heart and muscle represent two different types of muscle fibers. The heart is Type I (slow). Its fibers allow sustained action without muscular fatigue. The back muscle fibers, Type II (fast), allow it faster performance of limited duration. Therefore, a myostimulator is implanted and connected to the heart and back muscle. This pacemaker trains the muscle to beat like the heart. More specifically, one of its leads is anchored to the outside of the heart and senses the heart’s natural electrical activity. It then carries this information to a muscle-pacing channel, which directs these impulses to the second lead, a system of insulated wires. Woven into the skeletal muscle, these wires conduct multiple, precisely timed impulses to it, forcing it to contract and squeeze the heart. This continues until the back muscle fibers complete the chemical transformation necessary to beat like a regular heart without tiring. Thermistor

Figure 6 - thermistors

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Thermistors are temperature sensors based on the resistance change of a sensing element, usually a semiconductor, with the change dependent on temperature. A Negative Temperature Coefficient (NTC) thermistor decreases in resistance exponentially as its temperature increases. The changes in resistance of thermistors are predictable and there is a relatively large per degree change in temperature. Small size allows rapid response to temperature change, while its ruggedness withstands shock and vibration. Thermistors are accurate to 0.5%, are stable, and are reproducible. However, thermistors have a nonlinear output with temperature and only a limited range.

Figure 7 – dimensions of thermistor Characteristics Resistance:

R =R0*e^(β(1/ϕ- 1/ϕ0))

R is resistance of the thermistor at temperature ϕ R0 at reference temperature ϕ0 β is material constant.

Sensitivity:

S = (∆R/R) / ∆T = -β/ϕ2

High S means large output signal and good accuracy

Temperature:

Temperature can be approximated by using Steinhart-Hart relation: 1/ϕ = A + B*ln(Rt) + C(ln(Rt))3 ϕ is temperature (K) A, B, C are coefficients determined through calibration.

Rt is the resistance of the thermistor

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Thermopile Infrared Sensor:

Figure 8 – thermopile infared sensors

A thermopile is a serially interconnected array of thermocouples, each of which consists of two dissimilar materials with large thermoelectric power (α). An electrical tension (thermotension) occurs when a temperature difference develops between the ends of a wire. By connecting a series of these elements, one can add together the thermotensions and produce a measurable output signal. Note that one end of each element is defined as hot and the other as cold. The hot junctions are thermally isolated from the cold ones. While the cold junctions are placed on a surrounding heat sink (silicon substrate), the hot regions are grouped at the center of a thin oxide/nitride membrane and coated with an IR-absorbing material (bismuth black), thus allowing the temperature to rise according to the intensity of the incident infrared.

IR thermopiles are small and give a rapid response time. They have a wide range of temperatures and give an inherently stable response to DC radiation. Thermopiles do not require a source of bias voltage or current. However, the output is nonlinear and linearizing circuits are required to relate Vout to T. They give a very low signal output. Constant movement of the muscle causes incident radiation to not only affect the sensing area, but the sensor housing as well. If the housing heats up as a result, the output signal is affected.

Figure 9 – silicon-based thermopile sensor chip After passing through an IR filter, incident IR radiation is absorbed by the absorptive layer. The absorbed energy leads to a temperature difference between the hot and cold junctions, thus producing a voltage difference across the detector. These sensors are used in this experiment to observe a relationship between the temperature variations and the contractions of the muscle.

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Characteristics Responsivity:

R = ∆Vout / (Qin*At) Q is the power density of the radiation At is the active area.

Voltage Output:

Vout = α*(T1-T2) α is thermoelectric power (a.k.a. Seebeck coefficient), corresponding to (Sa – Sb), the difference of the sensitivities of the materials. T1 is the temperature at the cold junction T2 is the temperature at the hot junction Disadvantage: By using materials with high thermoelectric power (α), like silicon, we find that the resistance is also high. Consequently, thermal noise is high and the signal to noise ratio goes down.

Experiment Based on the knowledge of biology and instrumentation, an experiment was devised by Kenneth J. Gustafson and Steven H. Reichenbach in order to find a relation between muscle temperature increase and metabolic utilization. In situ thermal measurements were used in this study. The gastrocnemius muscles, soleus muscles, and the achilles tendon of rabbits were examined. The structure of skeletal muscles such as these can be found in fig. 10 and fig. 11 shows the location of these specific muscles. The gastrocnemius muscle is the largest muscle in the calf of the leg. It causes the extension of the foot, rising of the heel, and assists in bending the knee. It is a fast-twitch muscle. The soleus muscle is also in the calf of the leg, behind the gastrocnemius. It helps extend the foot forward and is a slow-twitch muscle. The achilles tendon joins the calf muscles to the heel bone. The greater the contract rate, the greater the power output from the muscle. However, excessive demand can lead to fatigue or damage to the muscle. Therefore a first order approximation for constant metabolic utilization rate was found (contraction / relaxation duration) leading to the optimal contraction duration, which yielded the maximum average power output.

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– skeletal muscle consists of fibers (bundles of long cells), each fiber made up of a bundle of strands (myofibrils), each myofibrils are linear array of sarcomeres (basic contractile units of muscle) – striated (alignment of sarcomere subunits in adjacent myofibrils form light and dark bands) Biology, pg 1015

Figure 10 – skeletal muscle structure

http://www.teleemg.com/Anatomy/QuickAnatomy/leganat.htm Figure 11 – gastrocnemius muscle soleus muscle The setup of the experiment is shown in fig. 12. The lower leg heel was fixed with a moveable arm that changed muscle length. Stimulating electrodes were placed around innervating nerves and connected to a myostimulator. The muscle was then stimulated with 0.6 ms duration stimuli at a frequency of 50 Hz and a voltage of 1.6 V, causing maximum tetanic contractions. Then relative temperature variations were identified with contraction duration.

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To identify the temperature changes, two separate instruments were used. A thermistor, with a time constant of 20ms, was inserted into the medial portion of the soleus muscle. Signal conditioning was implemented by means of a simple bridge amplifier circuit. The second technique involved an infrared radiation thermopile detector, mounted directly above the soleus muscle. It had a 6.5-14 micron band pass filter window and its output signal was low pass filtered through an amplifier. A force transducer was used to record the tension generated by the Achilles tendon. These signals were digitized at 200 Hz using an A/D converter and software and were also filtered with a 50 Hz, 24th order, FIR low pass filter. Temperature variations were obtained by varying pulse train duration and therefore contractions. This was done to identify the temperature variations with contraction duration. The pulse train duration varied from 100 to 400 ms at a fixed muscle length. Muscle length was also varied in a series of 400 ms duration pulse trains between contractions. Four contractions at the rate of 40 bpm were made for each test.

Figure 12 – experimental setup Results Two figures were obtained from the study done by Kenneth J. Gustafson and Steven H. Reichenbach. Figure 13 shows the relative heat production versus isometric contraction duration of the soleus muscle of the rabbit. The relationship shown is as expected: as the contraction duration increases, the heat production of the muscle also increases. Figure 14 shows power optimization versus contraction duration. The solid line shows the graph resulting from a fixed duty cycle constraint (using a goat latissimus dorsi muscle). This means that the ratio of contraction to relaxation duration (the duty cycle) was fixed as a first order approximation for a constant metabolic utilization rate. This assumes that metabolic utilization is proportional to contraction duration. The dashed line represents the graph formed incorporating the metabolic constraint found. This relation increased the predicted contraction duration for optimum standard output. (the vertical lines on the graph show the max power output).

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Gustafson et. al, figure 2 Figure 13 – heat production vs. contraction duration

Gustafson et. al, figure 1 Figure 14 – power and work vs. contraction duration

Discussions It was found that the thermistor outperforms the thermopile in measuring the temperature. This is due to several factors. The thermopile has low signal to noise ratio, i.e. poor response and sensitivity. Also, the constant movement of the muscle causes incident radiation to not only heat up the sensing area, but the sensor housing as well, thus the output signal is affected. The thermistors, on the other hand, are much simpler to use, are rugged, cost less, and can provide an adequate and accurate signal. The one drawback is that they are invasive, whereas the thermopiles are not. In terms of the data, previous works in this field back up the linear relation between the heat production and the muscle contraction. The data also showcases the instrumentation and demonstrates their feasibility to develop a metabolic relation based on the thermal measurements. As can be seen from fig. 13 and 14, the longer the contraction duration the greater the heat production of the muscle. This is a fairly linear relation, as is the relation between work and contraction duration. The longer the contraction duration, the greater the maximum work rate is. Therefore, the greater the temperature, the greater the maximum work rate. However, the maximum power occurs only at a contraction duration of 0.3 seconds. At larger times the power slowly decreases. This research is important scientifically and as a service to humankind. Current cardiac assist devices present many problems and inconveniences. As they are powered by battery, patients have to be monitored several times a year by specialists in pacemaker monitoring. When battery

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depletion is detected, the patient must undergo surgery again to replace the battery. The leads of pacemakers can malfunction or cause infection to the body. Certain signals can also disrupt pacemakers. For example, currently research is being performed on the interference of some digital cellular phones with the operation of pacemakers. If a way were found to power cardiac assist devices with the patient’s own muscle, many of these problems would be eliminated. Muscle power would not need to be replaced, and frequent monitoring would not be as necessary. With such natural sources of power, there would also be less chance of outside signals interfering with the device’s operation.

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REFERENCES CITED

Associated Press. “Some Cellular Telephones can disrupt Pacemakers”. Copyright © 1997:USA TODAY, a division of Gannett Co. Inc. http://www.usatoday.com/life/health/heartdis/pacemkr/ lhhpa001.htm (May 2001). C.J. Barclay, J.K. Constable, and C.L. Gibbs. “Energetics of Fast- and Slow-Twitch Muscles of the Mouse”. Journal of Physiology. Vol. 472, pg 61-80. 1993. Neil A. Campbell, Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th edition. pg. 1014 – 1019, 782. 1999. “Cardiomyoplasty”. The Heart Hospital at Allegheny General. http://www.asri.edu/AGH/cardio/SERV/Services_Cardiomyo.html (2001)

The Columbia Electronic Encyclopedia. Copyright © 1994, 2000, on Infoplease.com. © 2000 Learning Network. http://www.infoplease.com/ce5/CE047236.html (May 2001). James W. Dally, et al. “Instrumentation for Engineering Measurements". John Wiley & Sons. pg. 421-425, 565-568. Inc., 1993. Kenneth J. Gustafson and Steven H. Reichenbach. “IN SITU Thermal Measurements for Estimation of Relative Metabolic Utilization in Skeletal Muscle”. Advances in Bioengineering. Vol 39, pg 375-376. 1998. “Internet Medical Education, Inc.: Pacemakers”. Copyright © 1996-2000: Internet Medical Education, Inc. http://www.med-edu.com/patient/arrhythmia/pacer.html#problems (May 2001). “NTC Thermistors: Type GC32” Copyright © 2000 Thermometrics. http://www.thermometrics.com/assets/images/gc32.pdf (2000)

Wolfgang Schmidt and Dr. Jörg Schieferdecker. “Understanding Thermopile Infrared Sensors”. Copyright © 1998-2001 PerkinElmer Optoelectronics, Inc. http://opto.perkinelmer.com/library/papers/tp6.htm (2000)

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Biographical Sketches

Ahmad Dialdin: Northwestern University, Mechanical Engineering, Bachelor of Science 2001 Junhyck Kim: Northwestern University, Mechanical Engineering, Bachelor of Science 2001 Laura Evans: Northwestern University, Mechanical Engineering, Bachelor of Science 2002 Caltech/Jet Propulsion Lab, June 1999 – August 1999 Daniel Woodhead Company, June 2000 – September 2000 Northwestern University Mechanical Engineering Department, June 2001 - September 2001 Ayan Bagchi: Northwestern University, Mechanical Engineering, Bachelor of Science 2002 Oak Ridge National Laboratory, summers 1997-1999 University of Tennessee, June 2000 Bechtel National Inc., August 2000