By: Rebecca Dgien and Alivia Heivly 7-5 By: Rebecca Dgien and Alivia Heivly

Sarcomere shortening and muscle fiber stimulation produce tension Individual muscle cells surrounded and tied together by connective tissue Tension when muscle cells contract, they pull on collagen fibers, producing an active force Before Movement can occur: Applied tension must overcome resistance Compression a push applied to an object Muscle cells can only contract Amount of tension produced by an individual muscle depends on the number of pivoting cross bridges it contains A passive force that opposes movement

Frequency of Muscle Fiber Stimulation Twitch- single stimulus-contraction-relaxation sequence in a muscle fiber Duration can be 7.5 msec or 100 msec Myogram- graph of tension development in muscle fibers during a twitch Latent Period- starts during stimulation and usually last about 2 sec Contraction Phase- Tension rises to a peak

Summation and Incomplete Tetanus Summation- addition of one twitch to another Incomplete Tetanus- Muscle producing almost peak tension during rapid cycles of contraction and relaxation

Complete Tetanus Occurs when the rate of stimulation is increased until the relaxation phase is completely eliminated, producing maximum tension Action potentials are arriving so fast that the sarcoplasmic reticulum does not have time to reclaim calcium ions

Number of Muscle Fibers Activated Motor Unit- single motor neuron and all the muscle fibers it innervates Recruitment- activation of more and more motor units Peak tension production occurs when all motor units in the muscle are contracting in complete tetanus

Muscle Tone Motor units within particular muscle are always active Their contractions tense and firm the muscle Resting tension in Skeletal muscle A skeletal muscle that is not regularly stimulated by a motor neuron will atrophy

Isotonic and Isometric Contractions Isotonic Contraction- tension rises and skeletal muscle length changes Isometric Contraction- muscle as a whole does not change length and tension produced doesn’t ever exceed load

Muscle Elongation Following Contraction Contraction is active, but elongation is passive After contraction a muscle fiber usually returns to its original length through a combination of elastic forces, the movements of opposing muscles, and gravity Generated when a muscle fiber contracts and tugs on the flexible extracellular fibers of the endomysium

Checkpoint Questions 10. The frequency of motor unit stimulation and number of motor units involved 11. Gross, the greater number of fibers the more powerful contraction 12. Yes, during isometric contraction