Work Physiology

Chapter 12

Work Physiology

Physical work is only possible only when there is enough energy to support muscular contractions. In this chapter we will find out how various physiological systems work together to meet the energy-expenditure requirements of work and how these requirements can be measured quantitatively and considered in the analysis of physical work.

Muscle Structure Smooth muscle – stomach, intestines, blood

vessels, & bladder. Digestion of food and regulation of the internal environment of the body.

Cardiac muscle – Heart & like smooth muscle not under conscious control.

Skeletal muscle – operates joint movement & makes up 40% of body weight.– Made up of elongated elements called

myofibrils which are further divided longitudinally into a repeating series of sarcomeres.

– Sarcomeres are made up of a thick filament (myosin) and a thin filament (actin).

– Sliding filament theory of muscle contraction

Muscle Structure

Aerobic & Anaerobic Metabolism

Energy for muscle contraction comes from compounds known as ATP (adenosine triphosphate) and CP (creatine phosphate).

These compounds are derived from metabolism of nutrients in the presence of oxygen (aerobic metabolism) or without oxygen (anaerobic metabolism).

ATP and CP compounds are energy carriers found in all body cells where they fuel the activities of the body and sustain life.

Aerobic & Anaerobic Metabolism

ATP is like a rechargeable battery with a very limited life. It is recharged from CP, aerobic metabolism, and anaerobic metabolism.

CP will provide recharging for about 10 sec for heavy work & 60 sec for moderate work.

When enough oxygen is available ATP can be replaced as it is used up.

It takes up to 3 minutes for the circulatory system to respond to increased aerobic metabolism requirements. During that time anaerobic metabolism is doing the recharging.

When there is not enough oxygen available, anaerobic metabolism creates lactic acid in muscle tissue and results in pain an fatigue.

Circulatory System (Blood) Three types of cells

– Red blood cells – transports oxygen to cells & removes carbon dioxide. They are of most interest in work physiology.

– White blood cells – fights off invading germs and prevents infections

– Platelets – helps stop bleeding Plasma – suspends blood cells (90% water &

10% nutrient & salt solutes) Red blood cells will not be able to efficiently

carry out their function of aerobic metabolism if individual is anemic, in a polluted or poorly ventilated work environment, or at high altitudes.

Circulatory System (Cardiovascular)

Heart - two halves with two chambers each; atrium & ventricle

Two halves work separately – left side pumps vein blood & right side pumps artery blood

Arteries move oxygenated blood and nutrients through arterioles and capillaries to body

Veins return blood back to left side of the heart through venules then to lungs where carbon dioxide is released and re-oxygenation occurs (pulmonary function) before returning to the left side of the heart.

Cardiovascular System Structure

Blood Flow & Distribution Heart generates the pressure to move blood

through contraction & relaxation and at a rate that is adjusted according to physical work requirement temperature, & humidity.

Blood vessels (mainly arterioles) change resistance to blood flow to match oxygen demands of various organs and body tissue.

Systolic pressure – maximum arterial pressure

Diastolic pressure – minimum arterial pressure

Pulse pressure – difference between systolic & diastolic

Cardiac output – increased by beats per minute & stroke volume (range of 5L/min to 25L/min)

Blood Flow Distribution

Respiratory System

Nose, pharynx, larynx, trachea, bronchi, lungs, muscles of chest wall, & diaphragm

Lungs – 200m to 600 alveoli absorb oxygen and expel carbon dioxide

Alveoli ventilation – amount of gas exchanged/min dependent on physical workload

Lung Capacity

Energy Cost of Work

2400 kcal/day for basal metabolism and non-working activities

Working metabolism (WM) is the increase required for work

When WM is increased to point (5 to 7.5 kcal/min) where oxygen deficit occurs, the deficit must be paid for eventually through reduced work or rest.

Working Metabolism Estimates

Expenditure Rates As Activities Change

Measurement of Workload

Oxygen consumption (L/min x 4.8 kcal) – aerobic until oxygen deficit occurs; then anaerobic until exhaustion.

Heart rate - Max rate = 200 – age Blood pressure & minute ventilation

(amount of air breathed out per min)

Subjective measurement of workload – Borg RPE (Ratings of perceived exertion)

MPWC (max physical work capacity) – Short Term (Aerobic)

MPWC (max physical work capacity) – Long Term (Aerobic)

Whole body fatigue occurs at 40-50% of max which requires some anaerobic metabolism. Allow frequent rest periods if workload cannot be adjusted

Rest Periods as Fraction of Work Time (RPFWT)

RPFWT = (PWC – Ejob)/(Erest – Ejob)

NIOSH PWC is 5 kcal/min & 3.5 kcal/min for males and females respectively

RPFWT = (5 – 6.5)/(1.5 – 6.5) = .3

Total rest for 8 hr shift = 8 x .3 = 2.4 hr

Heat Adjustments for Work-Rest Periods

Static Work and Local Muscle Fatigue