UMass Amherst KIN 470 - docx (7 pages)

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School:
University of Massachusetts Amherst
Course:
Kin 470 - Exercise Physiology
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INTRODUCTION Muscle strength endurance and fatigue are all very highly studied areas of exercise physiology Muscle strength is defined by the ability to produce force Muscular endurance is defined by the ability of a muscle or groups of muscles to continuously exert force against resistance or an object for a period Muscle fatigue is defined as the decline in ability of a muscle to produce force A muscle can produce force due to a process called excitation contraction coupling Excitation contraction coupling is when electrical stimulus is converted into a mechanical response For example an action potential in skeletal muscle triggers muscle contraction A common way to examine muscle activity is by use of electromyography EMG EMG is a way to record electrical activity of muscles that is converted into a visual image that can be interpreted This lab uses surface EMG which requires electrodes to be placed on the surface of the skin to record activity Maximal voluntary contraction or maximal force varies between individuals The reason behind this is because of the size principle The size principle states that motor units are recruited from smallest to largest based on force needed For small forces small motor units are recruited to fire For large forces large motor units are recruited When intensity increases rate coding occurs which is when the neurons fire more rapidly Someone who produces a large force uses large motor units and the neurons fire rapidly to sustain the large force Someone who produces a small force uses small motor units and the neurons don t fire as rapidly METHODS The first part of this lab was to obtain maximal voluntary contraction for each subject There were 15 college aged subjects who participated in this study 4 males and 11 females A hand grip dynamometer was connected to a computer which had a biopac program used to record force Before the lab began the hand grip dynamometers were calibrated Each subject took turns holding the hand grip dynamometer and squeezing it as hard as possible for 3 4 seconds This step was completed twice by each subject and the highest value was recorded Also each subject s forearm circumference was measured in centimeters at the widest part of the forearm The second part of this lab was to look at muscle fatigue and recovery The subject with the lowest force was chosen to perform this part of the lab The subject performed twelve 10second contractions by squeezing the hand grip dynamometer as hard as possible for 7 seconds and then relaxing for 3 seconds These contractions were done one after another After the 12th contraction 5 recovery maximal voluntary contractions were measured by squeezing the handgrip dynamometer The subject squeezed the dynamometer immediately after the 12th contraction 30 seconds after the 12th contraction 60 seconds after the 12th contraction 120 seconds after the 12th contraction and 240 seconds after the 12th contraction All the forces were recorded based on the data provided by the biopac program The last part of this lab was to look at muscle endurance The two strongest males and two strongest females were chosen to perform this part of the lab EMG sticky pads were attached on the radius proximal region of the ulna and the back of the elbow of the subject s dominant arm Before this part began 5 and 30 of the subject s MVC was calculated The subject was to squeeze the hand grip dynamometer at 30 of their MVC for as long as they could until the force dropped to 25 of below of their MVC Initial and final EMG was recorded to examine muscle endurance and motor unit activity RESULTS Table 1 Recovery from Fatigue Peak Baseline N Force 230 11 Fatigue index Post MVC 30s 60s 120s 240s 175 97 159 59 170 18 186 57 169 12 76 47 69 35 73 95 81 07 73 49 N Table 2 Peak Force Results Name Brendan Kyle Kayleen Lauren Kalley Karina Jackie Greg Uy Lauren Hannah Yvonne Emily Claire Sophie Forearm circumference cm 26 32 25 2 24 6 26 8 25 23 28 5 29 25 75 23 25 23 5 29 25 25 24 13 Force N 339 1 517 86 309 5 362 8 230 8 257 1 230 11 417 42 239 09 272 3 288 55 272 2 260 18 299 73 276 9 Table 3 Endurance Results Name Training Status Max MVC Time to Initial Final N Failure seconds EMG mV EMG mV Kyle resistance 5x wk 517 86 147 1 01 2 5 Kayleen HIIT resistance 5x wk 309 5 292 1 78 2 79 Lauren cardio 6x wk resistance 5x wk 362 8 402 1 25 1 43 Greg cardio resistance 6x wk 417 42 394 1 29 3 88 Figure 1 Peak Force vs Forearm Circumference Peak Force vs Forearm Circumference 550 500 Force N 450 400 350 300 250 200 22 23 24 25 26 27 28 29 30 31 32 Forearm Circumference cm Calculations Fatigue Index Post MVC Peak Baseline MVC x 100 Post MVC Fatigue Index 175 97 230 11 x 100 76 47 30s Fatigue Index 159 59 230 11 x 100 69 35 60s Fatigue Index 170 18 230 11 x 100 73 95 120s Fatigue Index 186 57 230 11 x 100 81 07 240s Fatigue Index 169 12 230 11 x 100 73 49 DISCUSSION During the fatigue test force decreases due to an increase in inorganic phosphate The increase in inorganic phosphate causes an increase in proton accumulation The increase in inorganic phosphate inhibits cross bridge function Inorganic phosphate rebinds to the actinmyosin complex which slows down the power stroke The slower the power stroke the smaller the force being produced High proton accumulation causes a decrease in pH The decrease in pH causes myosin to bind to actin but adenine diphosphate is not released so there is no ATP for energy There is also less calcium being released which means there are less actin myosin binding sites open There is a linear relationship between force and muscle size Larger muscle has more crosssectional area which has more myosin and actin being bound compared to smaller muscle Having more actin and myosin cross bridges allows for a greater power stroke The greater the power stroke the greater the force The relationship seen in lab was not linear This could be due to subcutaneous fat People with more subcutaneous fat may have larger forearm circumferences but not be able to produce as much force as someone with a small amount of subcutaneous fat Kyle produced the most force during the MVC trial Lauren held her force for the longest duration during the endurance test Kyle is a male who does resistance training 5x week Lauren is a female who does cardio 6x week and resistance training 5 week Based on both of their training statuses and their gender it makes sense that although Lauren did not produce largest force she was able to hold her


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