Lab 5 Human At the beginning of lab you will have the opportunity for 2 bonus points! You must guess which person in the class will have: 1) Maximum Grip Force 2) Longest time to half-max Force (longest time for muscle fatigue!) Objectives: 1) To understand muscle physiology from the perspective of an isolated frog muscle. 2) To understand how isolated muscle physiology relates to whole-muscle performance. 3) To determine the relationship between muscle force and EMG activity. 4) To examine the effect of muscle size and EMG on muscle force by comparing maximum grip force, forearm diameter, and EMG for the entire class. Please read the PhysioEx portion of the lab manual prior to lab! The classic experiment for muscle physiology uses a frog thigh muscle attached to an electrical stimulator and a data-recording computer. However, execution of this lab involves killing many, many, many frogs (~ 60 each semester for BIOL 201). Thus, computer simulations have been developed that allow you to observe a simulated experiment involving the frog thigh muscle. This computer simulation is very good and should effectively teach you some basic principles regarding the physiology of an isolated skeletal muscle. First, you will examine the effect of neural input (action potentials) on the development of muscle force. When a motor neuron delivers a single action potential to a muscle, a mechanical response occurs called a muscle twitch. Graphically, a muscle twitch shows how a single motor unit develops force (i.e. tension) over time in response to a single action potential. A single muscle twitch produces only a very small force; thus a single muscle twitch is not capable of allowing you to do anything. At the level of the motor unit, muscles increase force production to accomplish tasks by adding up individual muscle twitches. Treppe and summation are two ways that individual muscle twitches can be physiologically combined to achieve greater force. Treppe occurs when the frequency of action potential is such that successive NON-OVERLAPPING muscle twitches achieve greater force. Summation occurs when the frequency of action potentials produces OVERLAPPING muscle twitches that summate to produce a larger force. Thus, increasing the frequency of neural signals to muscle cells can increase muscle force. Muscles undergo two basic types of contraction: isometric and isotonic. Isometric contractions occur when a muscle attempts to contract with MAXIMAL force. Under this condition the length of the muscle fiber does not change (Iso = same, metric = length!). Isotonic contractions occur when muscles contract to produce a submaximal force. This is usually associated with muscle MOVING something. When muscles move stuff, they contract with a constant force (i.e. tension, iso = same, tonic = tension). Think carefully about these two types of contraction so that you do not confuse them! In isometric contractions, the muscle attempts to move something that requires more force than it is capable of producing; thus the muscle contracts and cannot shorten any further. Conversely, in isotonic contractions, the muscle MOVES something and the something being moved is of a fixed weight. Thus, the muscle only needs to produce a force slightly greater than the weight of the object to cause it to move! ISOMETRIC CONTRACTION (OBJECT DOES NOT MOVE) Force muscle < = Force object = Muscle cannot shorten and object DOES NOT MOVE ISOTONIC CONTRACTION (OBJECT MOVES) Force muscle > Force object = Muscle shortens (constant force) and object MOVES NOW DO PHYSIOEX EXERCISE 2, Skeletal 1
Electromyogram (EMG) Moving away from isolated muscle, we will next consider how an intact human (or any other animal) uses muscles to produce force. The electrical activity in the motor units during a muscle contraction can be recorded as bursts of muscle action potentials that are collectively known as an electromyogram (EMG). The duration of the bursts of electrical activity in the muscle is about equal to the duration of the muscle contraction. When the electrical activity in the muscle begins, the muscle fibers begin to contract. When the electrical activity stops, the muscle begins to relax. The strength of a muscle contraction is related to the number of motor units in the muscle that are activated during the contraction and the strength is directly proportional to the intensity of electrical activity in the muscle. Two primary factors determine differences in the ability to perform a task using the musculoskeletal system: 1) muscle size and 2) neuromuscular coordination. For example, we all expect that a person with larger arm and chest muscles will be able to lift more weight than a person with smaller arm and chest muscles. This can be explained via muscle physiology studies that show a direct, positive relationship between muscle mass and the ability of muscle to produce force. Thus, individuals with more massive arms and chests will have greater muscle mass in those areas and will ultimately be able to lift heavier weights. However, you may also have noticed that two people that are about equally sized with about equal muscle mass are capable of lifting different maximum weights! What explains this finding? Studies of neuromuscular function have found that different people exhibit different levels of neural coordination of muscle contraction. Remember the STRENGTH of a muscle contraction is directly proportional to the number of motor units recruited, which is directly related to electrical activity. Thus, someone who has trained lifting weights will develop a higher degree of neural coordination, which results in more motor units acting simultaneously to produce a more forceful muscular contraction. This allows the person to lift heavier weights, even if their muscle is the same size or smaller than another person! The strength of a muscle contraction is related to the number of motor units in the muscle that are activated during the same time period. The electromyogram (EMG) recorded during a muscle contraction is seen as a burst of spike-like signals, and the duration of the burst is about equal to the duration of the muscle contraction. However, it is difficult to quantify the amount of electrical activity in a muscle unless the raw EMG data is mathematically transformed. One of the most common transformations used is the integration of the absolute values of the amplitudes of the EMG spikes (called the absolute integral). The area under the graph of the absolute integral of the EMG is linearly proportional to the strength of the muscle contraction.. In today s experiment, you will use a hand dynamometer to measure a subject s grip strength while recording the EMG of the forearm muscles used to generate grip. The EMG activity will be related to the grip strength by plotting the maximum grip strength as a function the area under the absolute integral of the EMG activity during the muscle contraction. Data recordings will be made from your dominant forearm and the relative strength and electrical activity of the muscles in the forearm will be compared to its diameter (muscle size!). EACH PERSON IN THE CLASS MUST PERFORM THE EXPERIEMTN ON THE DYNAMOMETER! 2
NOW, DO THE IWORX EXPERIMENT The binder contains the information for setting up the iworx equipment for these experiments. Once you have set up the equipment, refer back to these instructions. Experiment 1. MEASURING EMG AND MAXIMUM GRIP STENGTH Procedure 1 Sit quietly with your dominant forearm resting on the table top. Explain the following procedure to the subject: You will squeeze the hand dynamometer four times, with each contraction last two seconds followed by two seconds of relaxation. Each successive contraction should be approximately two, three, and four times stronger than the first contraction. The last contraction should be a maximum effort, the hardest you can possibly squeeze! 2 Click the Record button to begin the recording; then, press the Enter key on the keyboard to mark the beginning of the recording. After the recording is marked, tell the subject to begin squeezing the hand dynamometer following the procedure outlined in the step above. 3 In the relaxation period after the last contraction, click the Stop button, 4 Click the AutoScale buttons for the EMG and Muscle Force, and EMG Integral channels. Your data should look similar to the example below. Data Analysis 1 Use the Display Time icons to adjust the Display Time of the Main window to show the four progressive muscle contractions on the Main window. 2 Click on the Analysis window icon in the toolbar. 3 Look at the Function Table that is above the uppermost channel displayed in the Analysis window. There should be three columns of mathematical functions: 1) Abs. Area, 2) V2-V1, and 3) T2-T1. The values for Abs. Area, V2-V1, and T2-T1 on each channel are seen in the table across the top margin of each channel. 4 Use the mouse to drag the cursors to the beginning and end of the first muscle contraction. The values for Abs. Area on the EMG and Muscle force channels are the relative amount of the electrical activity causing the contraction and relative strength of the muscle, respectively. Record this value in the table on the next page. Repeat for each squeeze. 5 Use cloth measuring tape to measure the circumference of the dominant forearm at approximately 3 centimeters below the elbow. 3
Dominant Forearm Diameter (mm): Relative Grip Strength Absolute Area of EMG Activity Absolute Area under Force Curve Lowest Higher 1 Higher 2 Highest 6 Record each person s data in the excel spreadsheet at the front of the classroom. When everyone is done with this experiment I will plot individual EMG and Arm Diameter vs. Grip Force, so we can see relationships between these variables. The graph will be posted in WebCT. Questions 1. For each person, individually, plot the absolute area of muscle contraction as a function of the absolute area of the EMG signals for each muscle clinch. Is there a linear relationship between the absolute area under EMG signals and the absolute area under the muscle contraction? 2. Does the amplitude of the EMG signal and the force of contraction, as measured by the absolute areas, increase because a finite number of fibers are firing more often, or because more fibers are recruited to fire as the intensity of signals in the motor neurons increases, or a combination of these two? 3. Make notes here regarding the relationships between EMG, Arm Diameter, and Muscle force that I plot on the projector at the front of class. Although it will be posted on WebCT, you may want to sketch this graph while taking notes. 4. Should there be a relationship between the circumference of the forearm and the maximum force developed? What relationship would you expect? Does the class data support this? Is the difference in the circumference of the forearms between people in the class caused by a difference in the number of muscle fibers in the forearm or the diameter of each muscle fiber in the forearm? 4
Experiment 2: Muscle Fatigue Background Physiologically, muscle fatigue means that muscles produce less force! Please review Chapter 12 pgs 411-412 for a general discussion of the factors that contribute to muscle fatigue. Also the following figure illustrates 9 factors that contribute to muscle fatigue (from Enoka 2002, neuromechanics of human movement) Neural Factors: Brain/CNS may be protecting body from injury 1) Reduced activity of the motor cortex in the brain 2) Central Nervous System to Motor Neuron Connections Muscular Factors: Usually result of physical/chemical limitation of muscles ability 3) Motor Unit certain fiber types become quickly fatigued (type 2-b), fewer motor units activated 4) Neuromuscular Junction Acetylcholine synthesis slower than rate of action potentials being sent from CNS (DISEASE STATE) 5) Excitation-Contraction Coupling most common factor, reduction in release of Ca2+ from sacroplasmic reticulum > reduction in Ca2+ binding to troponin > reduced ability of myosin-actin binding > less contraction. 6) Depletion of Metabolic Substrates (Fig 12-13 in your text), Phosphocreatine (needed for ATP, power stroke of myosin head!), ATP Glycogen 7) Blood Supply - muscles increase the pressure during contraction, which can squeeze closed the blood vessels needed to supply metabolites, remove waste, and remove heat. 5
Procedure (Read steps 1-3 before beginning experiment) 1 Sit quietly with your dominant forearm on the table top. The subject will squeeze the bulb of the hand dynamometer as tightly and as long as possible in an attempt to fatigue the muscles of the forearm. As time passes, the subject s muscle force will decrease, but at a rate that is dependent on the fitness of the subject. When the subject s muscle strength drops to a level that is below half of the subject s maximum muscle force at the beginning of the recording, the recording will be stopped. This could take as little as 20 seconds or as long as a few minutes. IMPORTANT: Instruct the subject that he/she must maintain constant pressure for the duration of the experiment. He/she should not briefly relax and re-squeeze. The Muscle Force trace should steadily decline at some rate (which will vary across subjects) during the experiment. There should not be frequent amplitude increases in the Muscle Force recording; such frequent amplitude increases indicate the subject is not maintaining constant pressure on the dynamometer. Such a recording is not demonstrating muscle fatigue. It is your job as the experimenter to monitor the recording and ensure the subject is properly carrying out the experiment as instructed!!!!!! 2 Click the Record button, followed by pressing the Enter key on the keyboard. Record a baseline for ten seconds and then instruct the subject to squeeze and hold the hand dynamometer with as much force as possible. Continue to record. 3 When the subject s muscle strength (based on the Muscle Force trace) decreases to a level that is below 50% of the maximum muscle strength at the beginning of the recording, instruct the subject to release the bulb. Click the Stop button to halt the recording. 4 Click the AutoScale buttons for the EMG and Muscle Force channels. The recording should be similar to below. 6
Data Analysis 1 Use the Display Time icons to adjust the Display Time of the Main window to show the complete muscle fatigue recording on the Main window. 2 Click on the Analysis window icon 3 Look at the Function Table that is above the uppermost channel displayed in the Analysis window. The mathematical functions, Abs. Area, V2-V1, and T2-T1 should appear in this table. The values for Abs. Area, V2-V1, and T2-T1 on each channel are seen in the table across the top margin of each channel. 4 On the Muscle Force channel, use the mouse to click on and drag the cursors to specific points on the recording to measure the following, record the value for each of these in the table below: The maximum muscle force. To measure this force, place one cursor on the baseline before the muscle contraction and the second cursor on the peak muscle force near the beginning of the contraction. The value for V2-V1 on the Muscle Force channel is this amplitude. The half-max muscle force. Divide the maximum muscle force by 2. The half-max fatigue time. This is the time it takes for the muscle force to decrease to 50% of the maximum during the fatigue experiment. Place one cursor on the peak muscle force. Move the second cursor to the right of the peak muscle force until the absolute value for V2-V1, seen on the Muscle Force channel, is equal to the half-max muscle force The Max and half-max EMG amplitudes. 1) Move the cursors so that they bound 1 SECOND AFTER your initial maximum grip. Record the Abs Area from the EMG channel. This is your Max EMG amplitude. 2) Move the cursors so that they now bound 0.5 SECONDS ON EITHER SIDE of your half-max force grip. Record the Abs Area from the EMG channel. This is your Half-Max EMG amplitude. Enter your values in the table below. Max Force Hal-max Force Half Max Time Max EMG Half Max EMG Enter these values in the excel spreadsheet at the front of the classroom. I will generate graphs (Half-max EMG vs. Half Max Force; Half Max time vs. Arm Diameter) of these data for the entire class. Please sketch graphs below, as a study aid for the exam! They will also be posted on WebCT. 7
Questions How is the time to fatigue to half-strength related to the size of the muscles involved (i.e. arm diameter)? What factors could explain the relationship you find using information in this lab handout and from you textbook (chapter 12 ps. 411-12). What is the relationship between half-max EMG and half-max Force? Is this similar to the relationship between max EMG and max Force found in the last experiment? Why should they be similar, i.e. what does EMG tell you about muscle producing force? Aside for muscle size, what other factors contribute to muscle fatigue? Which factors do you think are primarily responsible for differences among people in this class? Could we easily measure these factors in lab on humans? 8