Psy 552 Ergonomics & Biomechanics Lecture 5

Energy for Muscles  Energy for muscle contractions if provided for by the breaking down of adenosine tri- phosphate to adenosine diphosphate ADP.  When the connecting bond is broken, energy is released. This energy is used to rotate the myosin head.

ATP sources  ADP returns to ATP by a reaction with phosphocreatine – another high energy phosphate.  ATP and phosphocreatine energy stores are depleted in seconds of strenuous exertion.

ATP sources (cont.)  ATP is generated using two processes: Anaerobic metabolism of sugar (glucose) which provides energy for 30 to 90 seconds. This process produces lactate as a by product Aerobic metabolism (aka Krebs or citric acid cycle) which provides energy stores for activity lasting longer than 90 seconds. This process yields water and CO2 as by products

ATP sources (cont.)  During moderate activity levels, the oxygen supply is sufficient to created the needed ATP – a condition called a steady state  At high activity levels, there can be insufficient oxygen that cause ATP production via the anaerobic process yielding an oxygen debt and muscle fatigue.

Types of Muscle Contractions  Isometric (aka a static load): muscle length does not change during a contraction.  Isotonic (concentric shortening) a very rare type of contraction in the workplace. Here the muscle length does change but the load remains constant. While it is true the load is present, changes in geometry change the consistency of the load.

Muscle Contractions (cont.)  Eccentric contraction (aka lengthening contraction) occurs when the external force is greater than the internal force as occurs when lowering a load. The muscle control but does not initiate the movement.

Muscle Contractions (cont.)  Isokinetic (aka constant force) muscle contractions where motion velocity is kept constant  Isoinertial contraction: a contraction against a constant load where the measurement system considers acceleration and velocity.

Muscle response to stimulation  Twitch: occurs when a muscle is stimulated by a single nerve action potential  Latent period: the interval between the stimulation (action potential) and the contraction  Contraction period: the time of muscle shortening  Relaxation period: the time the muscle lengthens to a resting state.

… response to stimulation (cont.)  The response of a muscle depends on: The size and frequency of the stimulus The fiber composition of the muscle The length of the muscle The velocity of the muscle contraction

Size and frequency  As neural stimulation increases, additional motor units will be recruited until a maximum contraction is achieved.  If a second nerve impulse is delivered before the end of the prior impulse a greater contraction force will be created. A process called temporal summation

Size and frequency (cont)  A maximal contraction, call tetanus, occurs when the frequency of impulses reaches its maximum. Max frequencies vary  300/second for the eye muscles  30/second for the soleus, calf muscle

Muscle fiber composition  There are two major muscle fiber types Slow-twitch (Type I) Fast-twitch (Type II)  Fatigue resistant Type IIA  Nonfatigue resistant Type IIB

Type I muscle fibers  Are smaller (e.g., soleus)  Maintain high capacity for aerobic metabolism  Good at low levels of exertion over short periods of time.  Have low peak tensions  Have long rise time to peak tension

Type II muscle fibers  The bicep brachii is a type II muscle fiber  They rely on anaerobic metabolism  Have large peak tensions  Have short rise times to peak tension  Have short peak durations  Are associated with high intensity activity

Muscle length  The ability to contract is directly related to the cross bridging of actin and myosin fibers.  The maximum number of cross bridges exists when the muscle is in approximately the resting position.  No tension is created when there is no overlap

Muscle length (cont.)  Tension reduces when the muscle shortens and there is an overlap between the actin fibers on the opposite side of the sarcomere.  The tension a muscle can produce is also dependent on the stretch of connective tissue.

Velocity  The tension reducing capabilities of a muscle decrease with increased velocity because: Inefficient coupling of cross-bridges as filaments move passed one another Fluid viscosity of the muscle causes viscous friction

Muscle fatigue  Serves as an injury prevention function  Has several causes. The ultimate cause of fatigue, however, is very complex.  Causal factors include: Energy depletion Accumulation of lactates Lack of motivation

Static loads and fatigue  With a static load, blood can be excluded from a contracted muscle The intramuscular pressure associated with a static contraction of the quadricep at 25% of the maximum voluntary contraction exceeds the systolic blood pressure -- no blood flows into the muscles

Static loads and blood flow  At 20 to 30% maximum voluntary contraction (MVC) blood flow will increase in response to the contraction.  At > 30% MVC, blood flow decreases to the muscle  At 70% MVC, blood flow to the muscle stops

Blood flow and fatigue  Without blood flow we get: Increased heat Inadequate oxygen No removal of CO2 Lactate accumulations

Muscle Arrangements  Muscle groups are arranged in groups. Simply put, there are 2 types of muscles functions: Agonists – prime movers Antagonists that relax when the agonist contracts  Several muscles often work together as synergists. One might stabilize a joint while another moves the distal end.

Muscle Arrangements (cont.)  Muscle forces can be divided into 2 force vectors One moving parallel to the bone -- produced by shunt muscles that cause compression at the joint and promote joint stability One moving perpendicular to the bone – produced by spurt muscles that cause rotation of a limb around the joint.