IMPORTANT TERMSIMPORTANT CONCEPTSINTRODUCTIONMUSCLE CONTRACTIONSMUSCLE STRENGTHMUSCLE FATIGUEMUSCLE RECOVERYEXPERIMENTAL PROBLEMPROCEDUREIMPORTANT TERMS1. Muscle strength2. Muscle fatigue3. Muscle endurance4. Excitation-Contraction Coupling5. EMG6. Recruitment7. Rate codingIMPORTANT CONCEPTS1. Factors determining muscle strength2. Muscle fatigue induced by repetitive isometric contractions3. Units of measurement for muscle strength4. Size principleINTRODUCTIONMuscle strength is the ability generate force. This is dependent on the cross sectional area of the muscle. Thus, a larger muscle will be able to produce more force. The larger the muscle, the more sarcomeres exist in parallel. Each group of sarcomeres in series generate some amount of force, so the more sarcomeres in parallel, the more force can be generated. Muscular endurance is the ability of a muscle or group of muscles to sustain repeated contractions against a resistance or sustain a force for an extended period of time. Muscular endurance is quite different than muscular strength, as endurance measures how long a person can maintain a submaximal force, while strength measures how much force a person can produce. One major physiological difference between endurance and strength is fiber type. Type I fibers are more fatigue resistant than type II fibers because they contain greater amounts of myoglobin, mitochondria and blood capillaries. In this lab, we will examine fatigue during a muscle endurance test using force and EMG measurements. Muscle fatigue is defined as a decline in maximal force production and affects the ability to do work. Muscle fatigue can occur in response to short-term, high-intensity exercise or prolonged, submaximal exercise. The mechanisms underlying muscle fatigue are multifactorial, and may be dependent on the type of exercise and the mode of contraction performed.Muscle fatigue can occur as a result of alterations in any step along the force-producing pathway. The central nervous system may play a role in muscle fatigue by altering the neural drive from the motor cortex to the muscle cell. This would be evident as 1) a reduction in the number of functioning motor units or 2) a reduction in motor unit firing rate. This is referred to as ‘central fatigue’. The term ‘peripheral fatigue’ refers to factors associated with inhibition of excitation contraction coupling. These factors may affect the propagation of the action potential across the neuromuscular junction, along the sarcolemma and the transverse tubular system. This may in turn affect the release of calcium from the sarcoplasmic reticulum (SR) to the cytosol, which is critical for cross bridge formation. When energy needs cannot be met by aerobic metabolism and stores of phosphocreatine become depleted, inorganic phosphate (Pi) and H+ start to accumulate. These bi-products of anaerobic metabolism have shown to inhibit maximal force production, along with reduced release of calcium from the SR. MUSCLE CONTRACTIONSThe body is able to produce movement by means of muscle contractions. Muscle contractions occur when tension is generated in a muscle fiber, through the binding of actin and myosin. Actin and myosin are contractile proteins, found in rod-like structures called myofibrils, within skeletal muscle fibers. Each myofibril contains two types of filaments that create a banding pattern. Thick filaments are comprised mainly of myosin, while thin filaments are made up mainly of actin. Binding of these proteinsLABORATORY #6: MUSCLE STRENGTH,ENDURANCE, & FATIGUE1is regulated by intracellular calcium. When calcium is present, actin and myosin are able to bind, resulting in a muscle contraction. The contraction will continue until calcium is re-sequestered back into the sarcoplasmic reticulum.MUSCLE STRENGTHIn this lab, we will be examining muscle strength, or the ability of a muscle or muscle group to generate maximal force opposing a resistance during one repetition. Strength is measured by one maximal effort repetition and is often referred to as a maximal voluntary contraction (MVC). In this lab, absolute values of MVC will be measured in Newtons (N). 1 N = 1 kg·m/s2. There are a number of factors that affect muscle strength. These factors include but are not limited to: training status, type of training, fiber type, age, sex, heredity, and disease/injury. The maximal amount of force a muscle can produce is dependent on the cross-sectional area of the muscle. Thus, as the size of a muscle increases, its force-generating capability also increases. Using muscles actively and strenuously leads to increases in size and strength, while disuse leads to weakness and wasting. Because of this, with all other factors constant, trained individuals often have greater muscular strength than sedentary individuals.Fiber type composition is determined by genetics and to some extent by the type of training an individual is undergoing. Fiber types include type I (slow-twitch) and type II (fast-twitch). Type II fibers tend to have greater cross-sectional area but more fatigable than type I fibers. Power and strength athletes, like sprinters, throwers, and jumpers, have a greater number of type II fibers compared to endurance athletes, like distance runners, swimmers, and cyclists, who usually have a greater number of type I fibers. Strength training is shown to increase type II fiber area more so than endurance training, since type II fibers have the greatest ability tohypertrophy, or increase in size, and excess load is required for a hypertrophy response. Endurance training promotes greater metabolic changes and results in muscles that are smaller in size and more fatigue-resistant than those that are strength trained.For both sexes, muscle strength increases linearly, from early childhood (3-7 yr) through early adolescence (13-14 yr), as body mass increases. From this point, a greater increase in strength is seen in boys, through adolescence and early adulthood (15-20 yr) while girls tend to maintain the linear trend in strength gains or decline in strength after 16 years of age. At the onset of puberty, this gap between sexes progressively increases, due to cultural expectations and changing hormone levels. Testosterone is responsible for increased muscle mass in males, while estrogen influences increased fat mass in females. Throughout adulthood, males, on average, have greater muscle strength than females.