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 muscleto 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 reasonbehind this is because of the size principle. The size principle states that motor units are recruitedfrom 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 asmall 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 theforearm.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, 10-second 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 hand-grip 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 recordedto examine muscle endurance and motor unit activity.RESULTS:Table 1: Recovery from FatiguePeakBaseline(N)Post MVC(N)30s 60s 120s 240sForce 230.11 175.97 159.59 170.18 186.57 169.12Fatigue index 76.47% 69.35% 73.95% 81.07% 73.49%Table 2: Peak Force ResultsName Forearm circumference(cm)Force (N)Brendan 26 339.1Kyle 32 517.86Kayleen 25.2 309.5Lauren 24.6 362.8Kalley 26.8 230.8Karina 25 257.1Jackie 23 230.11Greg 28.5 417.42Uy 29 239.09Lauren 25.75 272.3Hannah 23.25 288.55Yvonne 23.5 272.2Emily 29.25 260.18Claire 25 299.73Sophie 24.13 276.9Table 3: Endurance ResultsName Training Status Max MVC Time to Initial Final(N) Failure(seconds) EMG(mV) EMG(mV) Kyle resistance 5x/wk 517.86147 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 3.88Figure 1: Peak Force vs. Forearm CircumferenceCalculations:Fatigue Index = (Post MVC/Peak Baseline MVC) x 100Post 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%22 23 24 25 26 27 28 29 30 31 32200250300350400450500550Peak Force vs. Forearm CircumferenceForearm Circumference (cm)Force (N)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 actin-myosin 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 pHcauses 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 cross-sectional 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 longestduration 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