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Experiment: EMGs during Muscle Fatigue

You're at your local gym, getting your pump on and lifting dumbbells. You're feeling strong and decide to try a 30 lb curl. 1 rep, 2 rep, 3 rep....Ugh... why is it getting so hard to lift? In this muscle fatigue lab we will introduce you to some more in depth muscle physiology and why your muscles get tired.

What will you learn?

You will use the Muscle SpikerBox to record your bicep or forearm muscles while doing isometric muscle contractions until fatigue/failure occurs. You will measure the EMG amplitude during the contractions to learn about changes in muscle cells and neural signals during fatigue.

Prerequisite Labs

• Muscle SpikerBox - You should read the "Introduction to EMG's" experiment for you to understand what motor units are.

Muscle SpikerBox Dumbbell Hand Strength Meter, or Spring Compressor

Our muscle system is the largest system in our body (40%-50% of our weight). This system includes your heart, which is a pump made of specialized cardiac muscle, and the smooth muscles in your guts, allowing food to move.

But to make voluntary actions such as lifting a soldering iron or kicking a soccer ball, you use your skeletal muscles! Your skeletal muscles allow you to do all the wonderful movements with which you pass your days. Your muscles contract and enable movement by sliding microscopic actin and myosin protein filaments across each other, with a full support cast of other players including proteins (troponin and tropomyosin), ions (Na + , K + , Ca 2+ ), energy carriers (ATP), and blood circulation to deliver O 2 and remove CO 2 .

Each of your muscles is subdivided into functional groups of muscle fibers called motor units (Again, see our Introduction to EMG's experiment). A motor unit is a motor neuron and all of the muscle fibers it innervates. To achieve great things, like lifting a heavy weight, motor units join together in a systematic way to supply the force required to achieve strength. This teamwork among motor units is called "Orderly Recruitment" by scientists, and as stated before, motor units with the smallest number of muscle fibers begin contracting first during a movement, followed by the motor units with the largest number of fibers afterward, to allow for a smooth, strong muscle contraction.

In addition, a motor unit can be recruited to replace an already active motor unit that is experiencing fatigue.

So...how does this all relate to your muscles getting tired?

Muscle Fatigue

When a muscle cell fires an action potential due to a motor neuron command, this causes a release of calcium (Ca 2+ ) inside the muscle fiber from the sarcoplasmic reticulum. The Ca 2+ then flows into the area where the actin and myosin is (the sarcomere), initiating a complex cellular reaction with ATP that allows the myosin to pull on the actin. The movement of myosin pulling on actin in the sarcomeres is called a " sliding filament model " and consists of 4 steps.

As long as calcium and ATP are available, the actin and myosin will continue pulling on each other and the twitching will continue. Note that the calcium is rapidly transported back into the sarcoplasmic reticulum where the process must be initiated again by the muscle firing an action potential to cause another twitch. The summing together of many of these incredibly tiny "pulling events" produces a twitch (a very tiny, very fast force). When many twitches occur in a row, the twitches sum together and produce a larger force. ATP is continually provided in the muscle by breaking down glucose (see our "Oxygen Experiment" for an explanation of this metabolism. If glucose isn't available, fatty acids can be used to make pyruvate and keep the Krebs cycle and the oxidative phosphorylation pathway operating. As long as oxygen (O 2 ) is present and can be readily transported to the muscle cell, the oxidative phosphorylation pathway can produce ATP at incredible rates. This is called aerobic contraction , meaning "using oxygen."

Muscle Fatigue occurs when the muscle experiences a reduction in its ability to produce force and accomplish the desired movement. The factors that explain fatigue are complex and after more than 100 years of investigation are still a topic of active research.

For example, short term fatigue (failure to lift a heavy weight, do more push-ups, etc.) is different than long term fatigue such as as a marathon run, a 100 mile bicycle ride, or a full-day hike through the Rocky Mountains of Colorado.

We do understand though some of the basic reasons that muscles become fatigued during high intensity exercise, most notably that the demand for oxygen can be greater than the supply. The blood flow to the muscle can be reduced because of 1) muscles intensely contracting can reduce blood flow and thus oxygen availability, or 2) the muscle is simply working so intensely that there literally is not enough oxygen to meet demand (a sprint at top speed).

If such O 2 isn't available as an electron acceptor , the Krebs cycle and electron transport chain cannot operate, and the muscle must gain ATP from other sources. For example, for rapid, intense activity, phosphocreatine (synthesized from amino acids) can serve as a phosphate donor to allow ATP formation. This is called anaerobic contraction , meaning "not using oxygen."

However, anaerobic contraction can lead to build-up of metabolites and waste products, and a significant increase in the acidity (decreased pH) inside the muscle cell, which can interfere with the many biochemical reactions necessary for the actin and myosin to produce force and slide against each other. This chemical change is thought to be the cause of the "stinging" or burning sensation you feel in your muscles as you become fatigued (such as in arm wrestling or in the last few reps of a difficult weight lifting set).

We can observe the effects of these fatigue processes, albeit indirectly, by examining the amplitude of the EMG signal during a muscle contraction. As fatigue progresses, 1) the firing rate of motor neurons drops, which in turn drops the number of action potentials the muscles themselves then fire, leading to a reduction in strength, and 2) muscles can often also continue generating action potentials due to neural drive, but the muscle is unable to contract due to molecular fatigue events in the muscle fibers, which in turn leads to a reduction in strength.

Before you begin, make sure you have the Backyard Brains Spike Recorder installed on your computer/smartphone/tablet. The Backyard Brains Spike Recorder program allows you to visualize and save the data on your computer when doing experiments. We have also built a simple lab handout to help you tabulate your data. Spike Recorder Computer Software

[Note: In truth you can use any muscle you like for this experiment, as long as you can figure out how to produce fatigue in that muscle in a controlled fashion.]

• Hook up your EMG patch electrodes to your bicep, plug the electrodes into your Muscle SpikerBox, and hook up your SpikerBox to either your mobile software or PC.
• Select a dumbbell that is at about 60% of your maximum lifting weight. Depending on your strength, this will be ~10-25 lbs (~5-12 kg). With your back to a wall to control your posture and arm position, hold the weight in your hand for as long as you can, with your elbow at a 90 degree angle. This is called an " isometric " contraction since your muscles are working, but your joints are not moving. [Note: you will probably find that your wrist gets tired faster than your bicep. You can avoid this problem by hanging a weight off your wrist rather than holding the dumbbell in your hand (see video above).]
• Record your EMG during this task using SpikeRecorder on tablet/smartphone or your computer.

• Connect the EMG electrode patches to your inner forearm and hook up the cables and SpikerBox as previously noted.
• Use a hand-dynamometer or hand gripper (you should buy one in the 50-100 lb (25-45 kg) range), and squeeze the grip for as hard as you can for as long as you can.

Science Fair Project Ideas

• Sometimes, when hiking in your favorite park (like the Wonderland Trail or Torres del Paine ), you find, even if you are not very fit, you can hike for 6-10 hours. However, if you tried to lift a 100 pound (45 kg) barbells repeatedly, you would rapidly get tired within 5-30 reps over a couple minutes depending on your athletic ability. Why is the time scale of fatigue so different in these two activities?
• Try the biceps and forearms fatigue tests on both arms and hands to see if you observe anything different. As you know, you have a dominant arm/hand (left-handed vs right-handed). Is your dominant arm/hand stronger or more fatigue resistant than the other?
• How can two muscles that are about the same size be so different in their fatigue properties? We didn't cover it here, but you can begin reading about slow twitch and fast twitch muscle fibers to learn more.
• Are there muscles that are very fatigue resistant? Can you give us some examples?
• Work out your biceps for a month at your school gym. Measure your fatigue time and EMG changes before the period of training and after the period training using the same test load/force.

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Elastic band exercise induces greater neuromuscular fatigue than phasic isometric contractions

Affiliations.

• 1 EA4660-C3S Laboratory - Culture, Sports, Health and Society, and Exercise Performance, Health, Innovation Platform, Univ. Bourgogne Franche-Comté, Besançon, France. Electronic address: [email protected].
• 2 EA4660-C3S Laboratory - Culture, Sports, Health and Society, and Exercise Performance, Health, Innovation Platform, Univ. Bourgogne Franche-Comté, Besançon, France.
• 3 EA3920-Prognostic Markers and Regulatory Factors of Heart and Vascular Diseases, and Exercise Performance, Health, Innovation Platform, Univ. Bourgogne Franche-Comté, Besançon, France; Tomsk Polytechnic University, Tomsk, Russia.
• 4 EA4660-C3S Laboratory - Culture, Sports, Health and Society, and Exercise Performance, Health, Innovation Platform, Univ. Bourgogne Franche-Comté, Besançon, France; Department of Biomedical Sciences for Health, University of Milan, Italy.
• PMID: 30554941
• DOI: 10.1016/j.jelekin.2018.12.003

This study investigated the neuromuscular fatigue following an elastic band exercise (EB) of the plantar flexors, compared to an intermittent phasic isometric exercise (ISO). Eleven young healthy males (age: 24.2 ± 3.7) took part in the study, consisting of one experimental session involving two 5-min fatiguing protocols separated by 20 min rest and performed randomly. Both exercises were performed at maximal motor output of the plantar flexor muscles, EMG being used as a feedback signal. Neuromuscular fatigue was assessed through changes in maximal voluntary contraction (MVC) and in evoked responses of soleus and gastrocnemii muscles to posterior tibial nerve stimulation (H-reflex, M-wave, V-wave). Both conditions induced significant decrease in MVC force, but to a greater extent after EB (-20.0 ± 5.1%, P < 0.001) than after ISO (-12.3 ± 4.6%, P = 0.037). While no effect was observed in M-wave amplitude after both exercises, EB resulted in greater decrease of normalized H-reflexes compared to isometric condition. Normalized V-wave significantly decreased only after EB. As a conclusion, the greater fatigability found after EB as compared to ISO was underlain by muscular as well as nervous factors. This higher impact was attributed to the dynamic nature of elastic band exercise as compared to isometric contractions.

Keywords: Electromyography; Force; H-reflex; M-wave; Triceps surae; V-wave.

Copyright © 2018 Elsevier Ltd. All rights reserved.

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Shop Experiment Grip Strength and Muscle Fatigue Experiments​

Grip strength and muscle fatigue.

Experiment #17 from Human Physiology with Vernier

Introduction

Skeletal muscle is composed of bundles of individual muscle fibers and has unique properties which allow it to respond to stimuli by contracting. Individual muscle fibers respond to a stimulus (e.g., nerve impulse) with an all or none response, meaning the muscle fiber contracts to its maximum potential or not at all. Once a muscle has contracted, relaxation must occur before it can contract again. There are three basic types of muscle fibers: slow fibers , fast fibers , and intermediate fibers . Fast fibers contract quickly but for a relatively short duration. Slow fibers respond less rapidly, but are capable of a more sustained contraction. The strength of contraction of a whole muscle is dependent on the number of muscle fibers involved.

In this experiment, you will

• Obtain graphical representation of the force exerted by your hand while gripping.
• Observe the change in hand strength during a continuous grip over time.
• Observe the change in hand strength during rapid, repetitive gripping.

Sensors and Equipment

This experiment features the following sensors and equipment. Additional equipment may be required.

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This experiment is #17 of Human Physiology with Vernier . The experiment in the book includes student instructions as well as instructor information for set up, helpful hints, and sample graphs and data.

Skeletal muscle experiments

Length tension

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The length-tension relationship

For muscles to contract, the muscle proteins called actin and myosin must interact with each other. This occurs when they are optimally aligned opposite each other. If they are too far away from each other, they can't interact optimally, and the same thing happens if they are too close.

Think of two trapeze artists. One of them will be caught by the other - but only if they jump at the right moment. If they jump when they are too far apart, then that artist will fall. If the artist jumps when they are too close, they are likely to collide.

You're going to explore this effect of overlap between actin and myosin in the muscle on the amount of useful force (active force) that the muscle can produce.

0:00 One way in which we can change the amount of force that a muscle generates is by changing its length. Now, as soon as I said that you would've realised immediately that's a theoretical issue only, because after all you can't rip your muscle off, stretch it out, stick it back on in the same place and expect it to work.

0:18 However, the data that you will get while you do this experiment requires you to understand the physiology very well to interpret. So although this is purely a theoretical mechanism, it has great educational value in making you think about the underlying physiology.

0:36 And so one of the experiments you will be doing is what's known as the length-tension curve. In essence what you will do is stretch the muscle, apply an electrical stimulus to it, and record the response; the muscle force or muscle tension generated when the electrical stimulus makes the muscle contract.

0:59 You'll find that in fact, that force consists of two components:

1:05 One component is what is known as active tension . Active tension is the tension or force that's useful in doing work. It's the force or tension generated by the interaction of actin and myosin .

1:20 You will recall from the lectures that actin and myosin are found as thick and thin filaments in the sarcomere. When you stretch or compress a muscle, what you're doing is changing the amount of overlap between the thick and thin filaments.

1:36 You may pull the muscle apart so much that the thick and thin filaments do not overlap. In which case, when you apply electrical simulus, they can't interact, and therefore no tension will be generated.

1:49 At the other extreme, you can cause the muscle to compress so much, that the thick and thin filaments interfere with eachother, and now when you stimulate the muscle you will get very little tension. In other words, there's going to be some optimal position where the overlap between the thick and thin filaments allows for the best interaction between the actin and myosin, and then you get the greatest amount of tension that will be produced.

2:16 So that active tension will vary in a very systematic order. I'm going to draw a diagram of it, and this is the sort of data you should expect to see with the active tension.

2:30 So you have a plot of muscle length in millimeters, versus tension.

2:41 Now for active tension, as I've just indicated, you would expect to see that if the muscle is very highly compressed, there's very little active tension that's generated. In contrast, equally if the muscle has been stretched too much, there's very little tension that's generated.

2:57 So you should expect to see something like this bell-shaped curve, where there is some particular optimal muscle length, where you get the greatest amount of active tension.

3:16 And this is the tension that is useful for carrying out work, because it's the tension that can be used to move things.

3:23 However, it's not the only form of tension that a muscle can generate. Muscle doesn't just consist of the contractile elements of thick and thin filaments. Muscle also contains a number of tissues that hold these elements in place.

3:38 These include connective tissues, they include proteins such as Titin , which act very much like rubber bands. And you know the rubber band, when you stretch it, and stretch it and stretch it, each time you stretch it you're developing more tension in the rubber band. That tension is known as passive tension.

4:01 This is the sort of effect that you would expect to see for passive tension. When the muscle is highly compressed obviously there's going to be no stretching of those passive elements, the connective tissue elements, the titin, so there will be no passive tension generated.

4:18 If you keep stretching the muscle, that process will still continue, until finally when you start reaching the optimal muscle length is where you will start to see increases in the passive tension as you keep stretching the muscle.

4:34 This is the sort of curve that you would expect to see for passive tension. Very little passive tension at short muscle lengths. When you start reaching the optimal muscle length where active tension is highest, thereafter you will start developing greater passive tension.

4:54 Unfortunately, passive tension itself is not useful for doing work. Think of the rubber band. If you keep stretching the rubber band, eventually you're going to get to a point where the rubber band is going to stretch and snap. That's what will happen if you're going to carry on stretching the muscle.

5:12 In effect, this is the sort of data that you would be generating. An active tension curve, which is a bell shaped curve. A passive tension curve, which is an exponentially increasing curve. And finally of course, you can sum up the two to generate a total muscle tension curve, which will look exactly like this. To join up with the passive tension curve.

5:48 These are the three sorts of tensions that you will be measuring. In your virtual component, the practical class itself separates out the three tensions. So when you setup the experiment to do the virtual component, when you apply stimulus the data that will be shown on the data graph will be data from the active tension, passive tension, and finally the total tension.

6:14 We want you to collect all three sets of data, enter it into the table each time you apply a stimulus, and subsequently print out that table or extract that table in a way which allows you to generate a graph like this.

6:28 To understand this, you're going to need to understand the physiology that I've given you a very brief sketch for, and that's why this sort of experiment is very useful. It's pedagogically quite a valuable experiment.

6:43 So this first screen grab that you can see is a contraction of the muscle obtained with the muscle at its optimal length. In this part of the experiment we're going to vary the length of the muscle going down to much shorter muscle lengths, non-optimal lengths, as well as going up beyond the optimal length to longer, non-optimal lengths, to look at what happens when we interfere with the overlap between thick and thin filaments.

7:11 So this screen grab that you've got here shows you seven muscle contractions done from different muscle lengths. The one right at the bottom was obtained with a very short muscle length of about 53mm on our scale here, and then progressively increasing it by about 2mm steps.

7:32 You can see two different things. First and foremost, you can see that the size of the contraction, the active tension in other words, increases initially, until you look at the very last two where you'll see that the last one appears to be smaller than the one preceding it.

7:49 So you've got to the optimal length, and you've gone beyond it. So you've produced the maximum amount of active tension, and then it's decreased. The second thing you can see is if you look at the left, right at the start of the recording, you will notice that the baseline prior to muscle contraction itself is changing.

8:11 We're going to do a zoom of that, so that we can see this in greater detail where we will simply expand the baseline to show you the shift in this baseline. And that is the passive tension I was talking about. Each time we stretch the muscle, we stretch the elastic elements of it, and thereby increase the passive tension.

8:33 You can see this increases well beyond the optimal muscle length. It would continue increasing, because as we said in the rubber band analogy, you can do this until eventually the rubber band (muscle) will snap.

The purpose of this experiment is to examine how Passive and Active forces developed by the muscle change with muscle length. The video above explains some of the physiology involved in this process, and discusses expected results in the simulation and if you were doing a wet practical on this aspect of muscle function.

Video instructions

For this simulation, the voltage has been pre-set to achieve the peak muscle contraction response. This simulation looks at the effect of muscle length on contraction strength.

0:00 As you saw in the background video explanation, when the muscle length is too short, when you cause compression of the muscle, you can actually cause active interference between the actin and the myosin , reducing the number of cross-bridges that they can form, and thereby reducing the amount of tension that the muscle could optimally perform.

0:20 As you then progressively stretch the muscle, you take the muscle to the position where there is optimal overlap between the actin and myosin, allowing for the best possible number of cross-bridges to be formed, and muscle tension to be at its optimal.

0:38 As you then progressively increase the muscle beyond this length, what you should expect to see is that in fact there will be an increase in one form of tension, and a decrease in the other form.

0:52 The one form of tension where you will get an increase, is the form known as passive tension. As you saw in the preceding video, this is like when you were stretching a rubber band, you're stretching the elastic elements of the muscle, and you will see an increase in the resting muscle tension.

1:09 However, because now you've stretched the muscle to the point where there is non-optimal overlap between the actin and myson, the active tension (the tension formed by the cross-bridge interaction) will actually start to decrease, because now there's less overlap.

1:26 Let's demonstrate this with a few test lengths. Let's start off with the baseline length of 42 millimeters. Press simulate.

1:38 And you can see that there's no muscle tension generated. Increase it in 0.5 millimeter lengths, but I'm only going to try a couple.

1:45 Let's go to 43.5 for the sake of this demonstration. Notice now that you've got muscle contraction, but you'll notice also there's a small shift in the baseline. That's the shift of the passive tension that I was talking about.

2:00 The difference between that baseline, and the peak, represents the active tension of the tension that's produced by the muscle contraction.

2:09 Let's go up a little bit more to 47 millimeters. Simulate. And here you can see very clearly the increase in passive tension, as well as the increase in active tension between baseline and peak, compared to the active tension here. So we're getting a more optimal overlap between the actin and myosin.

2:29 Let's go a bit further to 49.5, again an increase in the passive tension, clearly here the active tension appears to be greater than the previous length.

2:44 Let's try something like 51 millimeters. Notice now, there's a massive increase in the passive tension, but this time the active tension (between baseline and peak) is now clearly significantly smaller than previously, indicating you've stretched the muscle beyond its optimal length.

3:03 And if we go to 52 millimeters and press, we will see an even further increase in passive tension, and a further decrease in the active tension. These data are plotted below in the graph, where you can see here the active tension (shown in blue) increases, reaches a peak and then declines.

3:24 Passive tension on the other hand, more slowly begins to increase but then increases exponentially, and so the total tension (which is simply a sum of these two), shows in increase, probably a small inflection around here when you collect the data in finer detail, and then a further increase.

3:43 Once you've finished this part of the experiment and collected the data, press "Clear Data" to go to the next experiment.

Please note that although this video demonstrates an older version of the simulation, it should function the same.

Instructions:

• Begin by setting the muscle at its shortest length. (42.0mm)
• Apply a single electrical stimulus and observe the passive (baseline) force, and the active tension (the difference between the peak force and the baseline).
• Increase the muscle length by 0.5mm to 42.5mm. Stimulate the nerve supplying the muscle, observing the changes to both passive and active tensions.
• Continue this until you get three successive recordings where you have increased voltage with no increase in muscle twitch size.
• Systematically increase the muscle length by 0.5mm for the remaining values presented in the scroll box. Press stimulate, then observe the results.

Once you have finished, look at the second graph which plots the active, passive and total tensions for each muscle length. What trends do you observe? How does this relate to the physiology?

Simulating the length-tension relationship

Full instructions can be found on the previous tab. In short:

• Muscle length can be selected from the scroll box on the left.
• First, stimulate the muscle's nerve with the muscle set at 42.0mm.
• Systematically increase the length of the muscle by 0.5mm at a time. Stimulate the nerve at each length.
• Observe the changes in active and passive tension.

Muscle Length:

Actin Myosin Visualisation:

Once you've completed the data collection, you can use the visualisation below to understand the processes underlying the active tension curve in the length-tension graph.

Interact with the slider or select buttons A through E to see how different lengths correspond to points on the tension graph.

Explanation in next tab

Explanation of the Actin Myosin Visualisation

The active tension produced by muscle is the force that can be used to do useful work. It is produced by the cross bridge interaction between the muscle contractile proteins, myosin and actin. During this cross-bridge cycle, actin combines with myosin and ATP to produce force, resulting in the ATP being broken down into adenosine diphosphate and inorganic phosphate.

In each muscle fibre (myofibril), these proteins are organized into repeating segments, called sarcomeres (one of which is shown in the visualization above). Each sarcomere consists of overlapping thick filaments (myosin; dark blue in the illustration above) and thin filaments (actin; red in the illustration).

Muscles contract by the thick and thin filaments interacting and sliding along each other. This interaction occurs when certain actin-binding-sites (the spiky "twigs" on the blue myosin filaments) bind to sites on the actin. Notice that these actin-binding heads are not found along the full length of the myosin filaments, but only at the ends. That means that there will a certain sarcomere length where these actin-binding heads will overlap optimally with the actin filaments - at longer lengths the overlap will be less, and at shorter sarcomere lengths the actin filaments will overlap with each other, so that there will less than optimal actin sites for the myosin actin-binding heads to bind to, to produce the cross bridge cycle.

This will become clearer when you play with the visualization in the previous tab.

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Muscles 101: Comparing muscles to a rubber band might be stretching it….

July 19, 2010 by Lauren Warnecke

Teachers often use the image of a rubber band to describe muscles.  When you stretch a rubber band, it usually returns to its normal size; if you continuously pull it beyond the maximum that it can be stretched, the rubber band stays stretched out.  Using this metaphor, if you continuously stretch your muscles beyond their maximum range of motion (ROM), eventually they will stay stretched out…..

Muscles have two main jobs: generating power and responding to “perturbations”.

The rubber band/muscle metaphor is great in describing the body’s response to perturbations (such as the bus moving forward when you’re not ready for it, someone handing you something that you think is heavy but is actually light, being bumped into, etc).  In these instances, a reaction called the stretch reflex recoils muscles that have been stretched as a result of a perturbation.  In the example of standing on a bus that moves when you’re not expecting it, you’ll notice that you lean forward and then right yourself.  If the muscle is stretched too far, the muscle can’t recoil and instead you are forced to take a step forward to prevent from falling.

Try this: Stand up with your feet under your hips and eyes closed.  You’ll notice that you sway forward and back slightly.  In order to help you maintain balance, your brain triggers muscle action in the calf and ankle when you start to pitch forward, righting your stance .  This is the stretch reflex in action (in combination with structures in the inner ear that control balance).

While the image of a rubber band is useful in demonstrating the elastic nature of muscles in the stretch reflex, it’s not always as simple as the stretch/recoil and stretch-farther/less-recoil that we gain from thinking of muscles simply as rubber bands.

I’ll explain, but first, a brief anatomy and physiology lesson:

How do muscles work?

A muscle is built of bundles of lots and lots and lots of muscle fibers bundled together by a sheath called the sarcolemma.  One muscle fiber contains lots and lots and lots of myofibrils, and one myofibril contains two types of myofilaments (thick and thin). This is where the magic happens.

The brain sends a signal (a neural impulse called an action potential) to the muscle that says “Hey muscle! Contract!”  Through a complex series of chemical reactions, proteins on the thick and thin filaments bond to one another and create energy in a chemical form.  The chemical energy is converted into a mechanical (tensile) force that generates power to move bone.   Every time you point, jump, bend, etc. your body goes through the same brain-muscle-bone loop called Excitation-Contraction Coupling (in case you want to look it up on Wikipedia ) and it all happens faster than you can snap your fingers.

Wow. That’s amazing. And not at all like a rubber band.

Apart from this complex process, there are a number of variables that impact muscle behavior, such as temperature.  Warming-up increases the body’s core temperature and also helps breaks any leftover bonds (what I refer to as “crunchiness”) that might be hanging around.  As mentioned in my previous post, Is It Okay To Stretch Before Class? , stretching before activity has a short-lasting (acute) effect on range of motion, but the effect of stretching is maximized if you are warmed-up. Warming up also increases the amount of power a muscle can produce, making movements more efficient.

Think about this: What would it feel like to do grand allegro first in a ballet class? I don’t care to find out, but you can imagine that your ability to produce power, and therefore height, in your jumps is much better at the end of class when your muscles are warm.  Muscles also react differently when they are sore, strained or fatigued and all of these topics are complicated enough to deserve their own posts, so I won’t delve into them here…

More than anything else, I want to emphasize that rubber bands don’t have brains. The key point in all of this is that you have a brain, and that your brain drives everything that happens in your muscles.  It senses unexpected events and recoils muscles back into place.  It sends neurological impulses to muscles, causing a series of chemical reactions, producing energy that is converted into force that makes you move.  I said it once, and I’ll say it again:

That is amazing.

Reference: Enoka, R. M. Neuromechanics of Human Movement, 3rd. edition

Lauren Warnecke  is a freelance writer and editor, focused on dance and cultural criticism in Chicago and across the Midwest. Lauren is the dance critic for the  Chicago Tribune,  editor of See Chicago Dance, and founder/editor of Art Intercepts , with bylines in  Chicago Magazine ,  Milwaukee Magazine, St. Louis Magazine  and Dance Media publications, among others. Holding degrees in dance and kinesiology, Lauren is an instructor of dance and exercise science at Loyola University Chicago. Read Lauren’s posts .

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Rubber band experiment

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 Grab a rubber band and stretch your curiosity to discover exothermic and endothermic reactions

Simple household objects are involved in this practical, which shows off a simple principle in a clear and effective way.

This experiment should take 30 minutes.

• Eye protection
• Rubber band, 0.5 cm wide (one for each participant)
• Weight >1 kg

Health, safety and technical notes

• Read our standard health and safety guidance .
• Always wear eye protection.
• Ensure rubber bands are sterile and clean.
• Ask participant to stand back so that broken rubber bands do not drop weights onto feet.
• Hairdryers should not be brought from home, ensure all electricals used have an up-to-date pat test.
• Take the rubber band. Quickly stretch it and press it against your lips. Note any temperature change compared with the unstretched band.
• Now carry out the reverse process. First stretch the rubber band and hold it in this position for a few seconds. Then quickly release the tension and press the rubber band against your lips.
• Compare this temperature change with the first situation.
• Set up the apparatus as shown in the diagram. Make sure that if the rubber band breaks, the weight cannot drop on feet.
• Predict what happens if this rubber band is heated with a hair dryer.
• Write down your prediction.
• Measure the length of the stretched rubber band.
• Now heat the rubber band using the hair dryer and observe the result.
• Measure the new length.
• The depth of treatment depends on the ability of the students.
• Students should recognise the difference between exothermic and endothermic reactions.
• A rubber band width of 1–1.5 cm and a 2 kg mass works well.
• A ruler standing beside the apparatus is effective as students can see the contraction as it occurs.
• Another alternative is to use a clampstand and adjust the height of the weight until it just touches the bench.

By placing the rubber band against their lips, students may detect the slight warming that occurs when the rubber band is stretched (exothermic process) and the slight cooling effect that occurs when the rubber band contracts (endothermic process).

The equation Δ G = Δ H - T Δ S (where Δ G means change in Gibb’s free energy, Δ H is enthalpy change, Δ S is entropy change and T is the absolute temperature) can be rearranged to give T Δ S = Δ H - Δ G . The stretching process (exothermic) means that Δ H is negative, and since stretching is nonspontaneous (that is, Δ G is positive and -Δ G is negative), T Δ S must be negative.

Since T , the absolute temperature, is always positive, we conclude that Δ S due to stretching must be negative.

This tells us that rubber under its natural state is more disordered than when it is under tension.

When the tension is removed, the stretched rubber band spontaneously snaps back to its original shape; that is, Δ G is negative and -Δ G is positive.

The cooling effect means that it is an endothermic process (Δ H > 0), so that T Δ S is positive. Thus, the entropy of the rubber band increases when it goes from the stretched state to the natural state.

• Based on your initial testing (by placing the rubber band against your lips) decide which process is exothermic (heat given out): stretching or contracting of the rubber band?
• The chemist Le Chatelier made the statement, ‘… an increase in temperature tends to favour the endothermic process’. Explain in your own words how this statement and how your answer to question 1 can account for your observations when heating the rubber band.
• Draw a number of lines to represent chains of rubber molecules, showing how they might be arranged in the unstretched and stretched forms.
• Contraction.
• They should observe that the rubber band contracts when heated, which may well be the opposite of what they have predicted. The most simplistic answer may be that since the endothermic process is favoured when heating occurs, this is a contraction in the case of the rubber polymer since this is the endothermic process.

Rubber band experiment- student sheet

Rubber band experiment- teacher notes, additional information.

This practical is part of our  Classic chemistry experiments  collection.

• 11-14 years
• 14-16 years
• 16-18 years
• Practical experiments
• Thermodynamics
• Physical chemistry

Specification

• Heat energy released by the reaction system into the surroundings increases the entropy of the surroundings, whereas heat energy absorbed by the reaction system from the surroundings decreases the entropy of the surroundings.
• The change in standard entropy for a reaction system can be calculated from the standard entropies of the reactants and products. ΔSᵒ = ΣΔSᵒ (products) – ΣΔSᵒ (reactants).
• The entropy (S) of a system is a measure of the degree of disorder of the system. The greater the degree of disorder, the greater the entropy.
• 4.2.2 recall that the balance between entropy change and enthalpy change determines the feasibility of a reaction;

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