1. Muscle fibers are stimulated at different frequencies, producing different types of contractions. Low frequencies cause individual twitches, while higher frequencies produce temporal summation and then incomplete and complete tetanus. 2. ATP is required for muscle contraction and is synthesized through different pathways including creatine phosphate breakdown, glycolysis, and the Krebs cycle. Glycolysis produces ATP rapidly but also lactic acid, while the Krebs cycle provides more sustained energy production. 3. Muscle fibers come in two main types - white/fast twitch fibers that contract rapidly but fatigue quickly using glycolysis, and red/slow twitch fibers that contract slowly but are fatigue resistant using the Krebs cycle.

1. Degree of muscle stretch (The Effect of Sarcomere Length on Tension)

2. Change the strength of stimulus The stimulus intensity increases , more and more motor units are stimulated, and thus the strength of muscle contraction increases

3. Effects of frequency of Stimulation

4. Twitch and Treppe Contractions Muscle stimulation at variable frequencies low frequency (up to 10 stimuli/sec)- each stimulus produces an identical twitch response moderate frequency (between 10-20 stimuli/sec) each twitch has time to recover but develops more tension than the one before (treppe phenomenon) calcium was not completely put back into SR

5. Incomplete and Complete Tetanus Higher frequency stimulation (20-40 stimuli/second) generates gradually more strength of contraction each stimuli arrives before last one recovers temporal summation or wave summation Maximum frequency stimulation (40-50 stimuli/second) muscle has no time to relax at all twitches fuse into smooth, prolonged contraction called complete tetanus

7. TEMPORAL (wave) SUMMATION IF TWO IDENTICAL STIMULI ARE DELIEVERED TO A MUSCLE IN RAPID SUCCESSION, THE SECOND CONTRACTION WILL SUMMATE WITH THE FIRST. THE RELAXATION PHASE HAS BEEN SHORTENED.

8. INCOMPLETE TETANUS IF THE STIMULUS STRENGTH IS HELD CONSTANT AND THE MUSCLE IS STIMULATED AT AN INCREASINGLY FASTER RATE, INCOMPLETE TETANI RESULTS.

9. COMPLETE TETANUS WHEN ALL EVIDENCE OF MUSCLE RELAXATION DISAPPEARS AND THE CONTRACTIONS FUSE INTO A SMOOTH, SUSTAINED CONTRACTION, COMPLETE TETANUS RESULTS.

10. Fatigue Fatigue is defined as the decrease in muscular activity due to repeated stimuli. When stimuli are applied repeatedly, after some time, the muscle does not show any response to the stimulus. This due to built up of acidic compound which affect protein functioning, lack of ATP and ionic imbalances

11. Functions of ATP in Skeletal Muscle Contraction Hydrolysis of ATP by myosin - energizes the cross-bridges, providing energy for force generation. Binding of ATP to myosin - dissociates cross-bridges bound to actin. Energizes Capumps that actively transport Ca back into the sarcoplasmic reticulum - Lowers cytosolic Ca level leading to relaxation Runs the Na+-K+ pump in the sarcolemma - maintains the resting membrane potential of the sarcolemma Muscle Metabolism

12. ATP: Compose of Adenine nucleotide, two phosphate groups, attached by high energy phosphate bond

13. ATP Synthesis Muscle cells uses three processes to synthesize ATP Hydrolysis of creatine phosphate Glycolysis Krebs cycle and oxidative phosphorylation

14. ATP Synthesis (hydrolysis of creatine phosphate) Creatine phosphate (CP): Organic compound in muscle cells serves as source of energy Most rapid method of ATP generation ADP + CP creatine kinase C + ATP The amount of CP is limited and rapidly depleted Only 1 ATP per CP used CP (creatine phosphate) regenerated during resting conditions (ATP + C  CP + ADP)

15. ATP Synthesis (Glycolysis) Glucose is the major source of energy for synthesizing ATP. Glucose available to muscle from two different sources Glucose enter the body directly from the blood Glucose is produced by hydrolysis of glycogen that stored in muscle cells

16. ATP Synthesis (Glycolysis) Glucose is broken down in glycolysis End product of glycolysis include: 2 ATP molecule Pyruvic acid In the absence of O2, pyruvic acid is converted into lactic acid (the end product of anaerobic pathway) Excess lactic acid builds up and caused muscle fatigue In the presence of O2, aerobic pathway (Krebs cycle) proceeds, pyruvic acid converted to acetyl CoA The end products: CO2, H2O and 36 molecule ATP

17. O2 enter the muscle cells from two sources: O2 enter the muscle cells directly from the blood O2 stored in myoglobin, an oxygen binding proteins

18. Muscle Metabolism: Energy for Contraction Figure 9.18

20. Others use mainly anaerobic respiration.

21. Most use a combination of the two.100% Marathon 95% 10,000 m 83% 5,000 m 60% 1,500 m 50% 800 m 20% 400 m Basketball players use both systems 10% 200 m Less than 1% 100 m

22. Oxygen Debt Oxygen Dept: The oxygen required to completely oxidized lactic acid formed and to replenish the depleted stores of ATP and creatine phosphate For a muscle to return to a resting state: Additional oxygen to rebind to myoglobin Lactic acid must be converted to pyruvic acid which enter krebs cycle producing ATP ATP use to rephosphorylate creatine into CP Glycogen synthesize from glucose

23. Oxygen Debt How do sprinters pay back their oxygen debt at the end of a race? Sprinters will continue to breathe more deeply and rapidly for a number of minutes at the end of their race. This will enable them to pay back the oxygen debt, and allow lactic acid levels to fall.

24. Variations in muscle fiber type There are two types of muscle cells which differ in size and coloration White muscle cells Red muscle cells These structural differences relate to their method of metabolism

25. Features of white muscle fiber Large in diameter Light in color due to reduced myoglobin Few mitochondria Few capillaries High glycogen content Mainly use glycolysis to synthesize ATP Rapid cross bridge cycling results in fast contractions Called fast twitch glycolytic fiber Fatigue rapidly due to build up of lactic acid and depletion of glycogen

26. Features of red muscle fiber Half diameter of white muscle fiber Dark red in color due to myoglobin Surrounded by many capillaries Numerous mitochondria Low glycogen content Cross bridge occur slowly Called slow twitch oxidative fiber Synthesize ATP by krebs cycle Fatigue resistant