Muscular Activity

1. Muscular Activity • Metabolism • Supplies the energy needed to slide the actin filaments over the myosin filaments. It is a chemical process of converting food into mechanical work and heat. • Some mechanical work is consumed by the body while other is consumed by physical activity • Basic source of energy for contraction of the muscle is glycogen or glucose which is abundant in the blood • Sources of energy (next time)

2. Energy • Metabolism - Sources of energy (see Figure 8-2 Sanders & McCormick (7th ed) • First 3-5 secs • adenosine triphosphate (ATP)-a high energy phosphate compound is mobilized. It breaks down to adenosine diphosphate (ADP) which releases energy. • ATP  ADP + P (phosphate radical) + free energy • ATP Regenerated • To continue muscular activity, ATP must be regenerated • creatine phosphate + ADP  creatine + ATP • creatine phosphate is high energy existing in small amounts in muscles • Depletion of creatine phosphate occurs in about 15 sec • Blood glucose or glycogen is mobilized. Glucose is a blood sugar which is converted by various stages first into pyruvic acid.

3. Energy • Metabolism – further breakdown may be • Anaerobic work – if O2 is not supplied to the muscle, pyruvic acid is converted into lactic acid while ATP is regenerated. Lactic acid accumulation causes muscle fatigue and pain glucose + 2 phosphate + 2 ADP  2 lactate + 2 ATP • Aerobic work – if O2 is supplied, pyruvic acid is broken down into water and carbon dioxide, releasing large amounts of ATP glucose + 38 phosphate + 38 ADP + 6 O2 6 CO2 + 44 H2O + 38 ATP • Oxidation of pyruvic acid in aerobic work involves enzymes, co-enzymes, and fatty acids (Krebs cycle, figure 3.4 – Pulat) • O2 is key to efficient work. Its supply requires more blood be pumped to muscle per unit time as well as heavier breathing to oxygenate blood • Kilocalorie (kcal) – most common measure of energy requirement for physical activity • Resting energy  0.3 kcal per minute for man of about 154 lbs • Resting male (laying down and no digestive activity)  1700 kcal/day • Resting female (laying down and no digestive activity)  1400 kcal/day

4. Energy • Metabolism • Basal metabolic rate (BMR) – amount of energy needed per unit of time to sustain life • Total metabolism equals sum of: • Basal metabolism • Activity metabolism • Digestive metabolism (10 % sum of basal + activity)

5. Supporting Systems • Respiratory Response • Nose and mouth, pharynx, larynx, trachea, lungs (consisting of bronchi and bronchioles, alveolar ducts, alveolar sacs, and alveoli) • Primary function is to expose a large volume of water-saturated air to a large volume of blood to facilitate exchange of gases between air and inhaled air • Gas exchange 70-90 m2 for average adult (surface area) • At rest, 250 ml of O2 is absorbed and 200 ml of CO2 expelled • Lung capacity • 7-8 L for tall athletic young male • Women – 10% less • Untrained – 60-80 % of volume

6. Supporting Systems • Circulation • Heart acts as double pump: blood vessels outside the heart can be categorized into 2 classes • Pulmonary vessels – transport blood from the right ventricle of the heart through the lungs and back to the left atrium of the heart • Systemic vessels – transport blood from the left ventricle of the heart to all the other parts of the body including the head and lower body back to the right atrium • Pulmonary and systemic  peripheral circulation • Kidneys – remove waste (toxic) from the blood • Liver, lungs, skin & intestines also eliminate waste • Arteries – carry blood away from the heart • Veins – carry blood to the heart • Nutrition • Ingestion, digestion, absorption, and utilization of nutrient substances • Provides necessary fuel and chemicals for physical activity

7. Muscular Activity • Dynamic activity – characterized by rhythemic contraction and relaxation of the muscles involved • Example: turning a handwheel to open a valve • Alternating tension and relaxation allows more blood to circulate • Static activity – characterized by a prolonged state of contraction, which restricts blood flow to muscles • Example: holding a box in static posture • Since no glucose or oxygen are being received – activity won’t last long • Design Element – Dynamic vs. Static • Compared to dynamic effort, static effort will require longer rest periods • Static effort could result in employee complaints and turnover • Designers should minimize job elements in a process with static loading

8. Energy Cost of Work • Energy demands increase with the onset of physical work. Increase depends on: • Physical conditioning • Intensity of activity • Gender • Body weight • Cardiovascular response • At rest • Heart rate (HR) – 60-85 beats per minute • Normal resting blood leaves lungs 97 % saturated –  breathing harder will not increase O2 in muscles alone • At work • Increase cardiac output – to increase O2 to muscles, more blood must flow thereby increasing cardiac output • Heart rate increases - # pumping actions • Stroke volume – volume per beat (rest 5 L/min; hard work – 25 L/min or more) • At 40% of a person’s max capacity stroke volume stabilizes while HR increases

9. Energy Cost of Work • Cardiovascular response • Oxygen lag • Fig 3.5, p. 34, Pulat • O2 and heart rate are linear under submaximal work (fig 3.6, 3.7, & 3.8 – Pulat) • Violated when: • Arms over head • Hot environments • Increased blood pressure (BP) – consequence of increased cardiac output • Adrenal glands release catecholamines (hormones) that strength HR and increase BP • BP is necessary to fill heart (note: @ 120 beats/min, heart has ½ sec to fill between strokes)

10. Energy Cost of Work • Redistribution of blood (Sanders & McCormick, p. 231)

11. Energy Cost of Work • Physical work capacity – worker’s capacity for energy output function of: • Food, oxygen, sum of energy provided by aerobic and anaerobic process • Working at 30-40% of one’s maximum aerobic power in 8 hour shift causes notable muscular fatigue • Energy cost of specific activities • O2 energy consumption have good relationship • Indirect measure of energy consumption • 1 L of O2 4.8 kcal • Table 3.2 – different activities kcal • Factors • As rate of activity increases, energy cost increases • Body weight increase energy cost • Men  3000-3500 kcal/day • Women  2500-3000 kcal/day • At age 65  75 % calories expended compared to those at age 25

12. Energy Cost of Work • Keeping energy cost of work at acceptable limits • For men • Maximum time-weighted average of 5 kcal/min due to activity energy cost of work • Maximum time-weighted HR average of 100 beats/min • For women • 4 kcal/min • 90 beat/min • Table 3.3 is a classification of work • Factors affecting energy consumption • Method of work • Work posture • Work rate or pace • Tool design

13. Energy Cost of Work • Fatigue • Manifested in slight tiredness to complete exhaustion • Produce lactate - which can be a good predictor of fatigue • One subjective measure is the Borg-RPE (rating of perceived exertion) scale – similar to scale being used in MP 1 • Scale of 6-20 are linearly related to the HR expected for that level of exertion • Intended to rate exertion during dynamic work • Motivation – highly motivated underrate their level • Work-rest cycle • If workload cannot be maintained within recommended limits (work standards) there must be rest • Some charts have been developed to determine amount of rest required

14. Rest Allowance (Kodak, 1986) Example: Someone working 10 minutes @ VH level get 80% rest or 8 minutes L M H V EH

15. Rest Allowance • Murrell (1965) • Solve Problem 3 in Pulat • Using Murrell and method in text book

16. Strength and Endurance • Strength – the maximum force that one can exert voluntarily (kilograms using dynamometer) • Static – measured standing or sitting posture • Example: Pushing a button on a cell phone • Design so that 95% of population can exceed these forces • Dynamic – measured during work using dynamic or isokinetic strength testing devices that control for speed of movement • Speed of movement affects lifting ability • Characteristics of strength • Strength peak by late 20s, gradual decline to 75% at age 65 • Women on average have 2/3 strength of men • Exercise can increase by as much as 50% • Maximum grip @ 135 degrees of elbow flex • Endurance – ability to maintain activity over time • Research says that people can only maintain maximum effort only briefly • 20% of peak static over time • 30% of peak dynamic work

17. Nervous System • Coordinates and regulates body activities • Responsible for the initiation and control of muscular activity • The basic element of the nervous system is the neuron. Neuron is composed of a nerve cell along with a minimum of two nerve fibers. • Dendrite brings messages to the nerve cell; multiple dendrites may be associated with a cell therefore multiple paths may send signals • Axon takes messages to another cell; a neuron may not have more than one axon. • Myelin coats the nerve fibers which serves to protect one nerve fiber's messages from another; short-circuiting is prevented • Synapse is the gap between the dendrite of one neuron and the axon of another neuron. • Chemical/electrical process allow an impulse of one neuron to be passed to a succeeding on unaltered in 1/1000 of a second.

18. Nervous System • Three major parts to the nervous system • Central Nervous system which includes the brain and spinal cord • Peripheral Nervous system which controls the voluntary activity • Automatic system which covers the balance of the nervous system • Automatic Nervous System • Controls involuntary body activity including function of glands, smooth muscle tissue, and the heart. Includes the: • Sympathetic or thoracolumbar division stimulating these fibers creates small amounts of thick saliva, depression of gastrointestinal activity, and increased heart rate and occur in emergencies such as freight and consume energy. • parasympathetic or craniosacral division • For example when people are in a health threat, parasympathetic affect blood pressure, constricting of the pupils, etc. • An axon in hand controls 3 -6 muscle fibers; An axon in leg controls 100 -200 muscle fibers

19. Nervous System • Peripheral Nervous System • Controls the voluntary activity of the body • 2 major elements are the sensory and motor systems • Sensory system is responsible for conveying information from your senses to the central mechanisms; sensory neurons have one axon and one dendrite each • Once a message is received and processed through the Central nervous system, signals are passed down the pathways to the appropriate muscles. Motor neurons lie in the central mechanisms, and their axons travel in groups called motor nerves • Speed of transmission • Motor fibers 70-120 m/s • Others 12-70 m/s • Considering the distance between the sensory organs and motor organs to the central mechanism, it is not uncommon to have reaction times in the 300-500 millisecond range

20. Nervous System • Central Nervous System • Two parts: spinal cord and brain • They are connected by the brain stem • Spinal cord is gray matter encased in a bony column called the spinal column. • contain the reflex neurons • contains the neurons of the sensory system • The brain • medulla - connects the spinal cord with the higher centers of the brain; nuclei for the neurons of the automatic nervous system • cerebral cortex is the part of the brain where information is processed; where information is stored; most important with respect to perception and processing of information • Thalamus - relay station for the brain; sorts out messages and directs them to correct motor channels