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A muscle fibre is a long cylindrical muscle cell held together in bundles to make up an individual skeletal muscle Human skeletal muscles consist of a mixture of each type of muscle fibre The relative proportion of each fibre type varies in the same muscles of different people e.g. Elite marathon runners have a greater proportion of slow twitch fibres and elite sprinters have more fast twitch fibres. The two main types can be distinguished on the basis of their speed of contractions Slow Twitch Fibres (Type I) Slow twitch fibres are designed for aerobic exercise, using oxygen to produce a small amount of tension over a long period of time They are very resistant to fatigue as the speed of the contraction is slow They have a higher capacity for aerobic respiration They are red in colour because of more myoglobin Performers in endurance events tend to have a higher percentage of slow twitch muscle fibres Slow twitch muscle fibre composition of various athletes Athletic group Shoulder (Deltoid) Long distance runners Calf (Gastrocnemius) 79% (m), 69% (f) Canoeists 71% (m) Triathletes 60% (m) Swimmers 67% (m), 69% (f) Sprinters 59% (m) Shot Putters 63% (m) 24% (m), 27% (f) Cyclists Weight Lifters Thigh (Vastus lateralis) 57% (m), 51% (f) 53% (m) 44% (m) 38% (m) Fast Twitch Fibres – Type 2a and 2b Subdivided in to two types; Fast Oxidative Glycolytic fibres (F.O.G.) and Fast Twitch Glycolytic (F.T.G.) They are both designed for anaerobic capacity, producing high contractions in a short period of time This however means that they have a low resistance to fatigue They are white in colour because of low levels of myoglobin Relative percentages of fast and slow twitch fibres in a range of sporting activities 90 80 70 60 50 Type I Fibres 40 Type II Fibres 30 20 10 0 Fast Oxidative Glycolytic Fibres (F.O.G.) – Type 2a These anaerobic fibres are more resistant to fatigue that type 2b FOG fibres generate less force than type 2b fibres though Activities such as a 200m swim or 800m race will rely on type 2a fibres. Fast Twitch Gylcolytic (F.T.G) – Type 2b Type 2b fibres have the greatest anaerobic capacity Generate largest amount of force of contraction Very poor resistance to fatigue Expect a Sprinter or Shot Putter to have large amounts of FTG fibres Summary of muscle fibre types Characteristics Slow Twitch (Type I) Fast Oxidative Glycolytic (Type 2a) Fast Glycolytic (Type 2b) Fibre Size Small Large Large Mitochondria density High Low Lowest No of capillaries Large Moderate Small Myoglobin content High Moderate Low PC stores Low High High Glycogen stores Low High High Triglyceride stores High Moderate Low Speed of contraction Slow (110) (ms) Fast (50) Fastest (<50) Force of contraction Low High Highest Resistance to fatigue High Low Lowest Aerobic capacity High Low Lowest Anaerobic capacity Low High Highest Activity suited to Marathon 1500m 110m Hurdler Past exam question Anatomy and Physiology Question 1 In terms of fibre type, the composition of muscle is largely genetically determined and can influence the activities in which people participate. a. Identify two structural and two functional characteristics of a slow oxidative muscle fibre. b. If a person has a high percentage of slow oxidative fibres what type of physical activity are they more likely to participate in? [5]