Msd And Physiology

CUMULATIVE TRAUMA DISORDER SITI ZAWIAH MD DAWAL DEPT OF ENGINEERING DESIGN AND MANUFACTURE UNIVERSITY OF MALAYA

Sometimes called repetitive motion injuries Or work related musculoskeletal disorder Are injuries to musculoskeletal system that develop gradually as a result of repeated micro-trauma due to poor design and the excessive use of hand tools and other equipment

Leading to problems Repetitive motion disorder Carpal tunnel syndrome Tendinitis Tenosynovitis White fingers

Factors lead to development of CTD Excessive force Awkward or extreme joint motions High repetition Duration of work

Common Symptoms Pain Joint movement restriction Soft tissue swelling

Tenosynovitis Inflammation of the tendon sheaths due to overuse or unaccustomed use of improperly designed tools. The wrist is bent especially in palmer flexion or ulnar deviation (or both) Example inserting screw in holes, manipulating rotating control.

Tendinitis If the inflammation spread to the tendons it becomes tendinitis

Carpal Tunnel Syndrome Disorder of hand caused by injury of the median nerve inside the wrist. Repetitive flexion and extension of the wrist under stress may cause inflammation of the tendon sheaths. The sheaths, sensing increased friction, secrete more fluid to lubricate the sheaths and facilitate tendon movement.

The resulting build up of fluid in the carpal tunnel increased pressure which in turn compresses the median nerve. Symptoms include impaired or lost nervous function. Numbness, tingling, pain and loss of dexterity Type of work –job requiring high force and short cycle

Trigger Finger Is the form of tendinitis resulting from the index finger is used excessively for operating triggers. The condition seems to occur most frequently if the handle of the tool or device is so large that the distal phalanx (segment) of the finger has to be flexed while the middle phalanx must be kept straight.

White Finger Results from excessive vibration from tools A similar effect can occur as a result of exposure to cold termed Raynauld’s syndrome.

Design for safe operation Eliminating pinching hazard Sharp corners – should be rounded Sharp edges – should be rounded

How to evaluate the level of CTD Starts with surveying the workers to determine their health and discomfort at work Body discomfort chart

MUSCLE PHYSIOLOGY

Muscles, through the tension they exert when contracting, make physical work possible. There are three types of muscle in the body:

1. Smooth muscle 2. Cardiac muscle 3. Skeletal muscle

Smooth muscle is found in the intestines and makes possible the movements essential for the digestion of food (peristalsis). It is also found in the walls of blood vessels where it is involved in the regulation of blood pressure and blood flow. It is not normally considered to be under conscious control. Cardiac muscle has a special structure and constitutes the bulk of the heart. The energy required for muscle contraction is obtained from phosphate compounds in the muscle tissue. These compounds are formed from the breakdown of food.

Skeletal Muscle Described as striated, strip Voluntary muscle Voluntary because contraction is under conscious control

MUSCLES; STRUCTURE AND FUNCTION

A myofibril

Myofibril Representing series of dark and light bands and consisting of unit called sacromere A sacromere represents the smallest functional unit of a skeletal muscle fibre and consists of  

-thin filament – actin -thick filament -myosin

Sliding filament theory of muscle contraction

The Cardiovascular System

A model of the circulatory system

Industrial Applications of Physiology Improvement of productivity is always a priority, a main goal is to determine acceptable work rate for a given job. Industrial engineers have developed methods for designing manual jobs in a systematic way. These techniques enable them to specify time standards, or standard times for the completion of tasks and to describe the physical load of tasks by means of performance rating. Therefore standards levels of production can be defined. Application of physiological methods in industry is useful for: Measurement of workload Investigating mental stress Nutrition and employee level of fitness

Physiology response to work

Energy costs of some commons daily activities

Physiology methods: Applied in industry to evaluate the physical demands of jobs in terms of energy expenditure. When an individual begins a work task from rest, heart rate and oxygen consumption increase to meet the new demands: Begin working - immediate requirements for energy are met by local energy stores. (ie. muscular) Stop working - HR and OX levels return to initial levels but extra oxygen needed to replenish muscle stores (oxygen debt) during recovery period. In many industrial tasks, physiological response to work: Warm up period Recovery period

Steady state

Measurement of the physiological cost of work Classic method of determining energy expenditure at work involves the measurement of oxygen uptake using the Douglas bag: Oxygen content of the in the bag can be compared to that of the atmosphere to determine the amount of oxygen metabolized by the subject. Rate of oxygen uptake can be calculated with the time the subject takes to fill up the bag. From the rate of oxygen uptake, the rate of energy expenditure can be calculated. Above method is well established but inconvenient and causes interferences. Many compact instruments now available: Oxylog & Respiration Monitor Belt

Indirect Measures of energy expenditure Heart Rate likened to a signal, which integrates the total stress on the body and can be used as an index of the physiological cost of work. Heart beat rate is known to increase as a function of workload and oxygen uptake. Individuals can have the same rate but completely different levels of oxygen because the maximal oxygen uptake varies between individuals. cannot be used to estimate energy expenditure requirements of a job therefore is often used as an indirect measurement of energy expenditure. easier to measure then oxygen uptake

Relationship between HR and oxygen uptake must be determined when evaluating physiological workload using heart rate. Both have to be measured simultaneously in the lab at a number of different sub-maximal workloads. This is to calibrate the heart rate – VO2 relationship for a worker. Workers hearts rate measured in the field can be converted to an estimate of oxygen uptake with reference to the lab data. (Linear relationship between the two variables) Estimates of energy expenditure can then be calculated. Close correlations between these two methods of estimation suggest that HR measurement of previously calibrated subjects can give

Subjective measures – Borg RPE Scale (Ratings of perceived exertion) Well-known rating scale for subjective measures. Subjective measures worthwhile from cost benefit and practical view Workers are asked to rate the level of exertion they perceive when carrying out a task on a scale from 6 to 20 (corresponding to minimum and maximum heart rates of 60 and 200). Such ratings are often used in conjunction with objective measures. High positive correlation between HR and RPE are usually found Subjective measure should be used with caution because of proneness for distortion experimenter effect (biasness)

Physiological methods have been used to evaluate physical workload in many jobs both in industrialized and developing countries. Physiological methods reflect the effort that the worker puts into the work system rather than the output of the system itself. They are indexes of the effect of work on the worker rather than the effect of the worker on the output of the work system. Workload measurements are important to identify unduly heavy tasks, to evaluate traditional work methods and to arrive at more efficient methods of work. (Especially in developing countries, which rely heavily on manual labor.) There are many studies where physiological methods are used to evaluate and redesign manual tasks of all kinds. It is also used to optimize the design of tools. Physiological methods can also be applied to the investigation of light work, to detect the presence of mental stress. (heart rate increases when under mental

Calculation of rest periods in manual work

Physical Work Capacity Physical work capacity refers to: Worker’s capacity for energy output this capacity depends primarily on energy and available to worker in form of food and oxygen for continuous

The sum of energy provided by oxygen dependent and oxygen independent processes

work at moderate intensities, oxygen dependent processes usually make the major contribution to energy output.

Oxygen dependent system can usually function for as long as nutrients are available. Oxygen is obtained from the air ventilating the lungs and is transported to all parts of the body by the blood but the body has a very limited capacity to store oxygen. When there is insufficient oxygen available the oxygen independent system would produce energy. It is a valuable system, which enables work to be carried out at a high for short periods interspersed with time. This backup system is inefficient and produces less energy per glucose energy than the oxygen dependent system. It also produces waste products, which cause acidity of muscles cells to increase. Work capacity depends on the ability to take up oxygen and deliver it to the cells for use in the oxidation of foodstuffs. Thus ability to work at a high rate is associated with high oxygen uptake.

Oxygen uptake at the onset of during and after work. A = oxygen debt, B= repayment of oxygen debt during rest. A=B

The oxygen uptake or metabolic activity does not increase suddenly at the onset of work. There is gradual smooth increase in oxygen uptake. During these first few minutes of work the muscle use energy that does not require oxygen. (Called anaerobic metabolism or oxygen independent process)*. Oxygen uptake does not reach a stable level until several minutes after work has begun depending on how hard the work is, this usually takes about 5 minutes. Eventually a steady state level is reached. This steady state represents the body’s aerobic (oxygen fuelled process) response to the demands of increased workload. When the work stops, the oxygen uptake returns slowly to the resting level prior to work. During this slow return after work, the oxygen debt incurred during the onset of work (area A) is repaid (area B).

* The anaerobic metabolism is inefficient because: It uses nearly 20 times more fuel that aerobic metabolism. Produces waste products (lactic acid), which may accumulate in the working muscles rather than being carried away by the blood. Lack of energy supplies, lack of fuels and accumulation lactic acid in the muscles involved will lead to fatigue and cessation of work. This will also results in aching of muscles.

Fatigue is due to lack of carbohydrates or fluids or the accumulation of waste products. The most important factor in the prevention of fatigue is the maintenance of blood flow to the active muscles. Jobs should be designed to reduce the requirements of static muscle loading, such as gripping, extended reaches and awkward postures. 

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The lowest possible percentage of the muscle’s maximum isometric, or static, effort should be designed for in situations where static effort is part of the job, such as in tool use. Lower tension or effort in muscles results in less impairment of blood flow, the muscles will take longer to fatigue Dynamic or rhythmic muscle contractions are preferred because they allow blood to flow between contractions. However, if the intensity and rate of contraction is high enough, the inter mitten muscle blood flow may still be inadequate Recovery or rest period is needed to replenish the energy stores. The duration of rest will depend on how much fatigue has developed.

Maximum oxygen uptake VO2 max is a term been used to describe an individuals capacity to utilize oxygen (maximal aerobic capacity). it is usually observed that oxygen uptake increases as the work rate is increased. relationship is approximately linear. Oxygen consumption and heart rate do not continue to increase indefinitely. A point is reached where increases in work rate is not accompanied by increases in oxygen uptake – the individual is assumed to reach his maximum level of oxygen uptake and cannot sustain a harder pace. There are limitations to human activity and the factor believed to limits a person’s work rate is the inability of the heart and lungs to supply oxygen to the muscles at a sufficiently fast rate to meet the requirements of the work. Increases in work rate can be met if energy is provided by oxygen independent processes. However, these can only be maintained for a short period because they

The relationship workload and oxygen uptake

One main reason for the interest in work physiology is to consider variations in work capacity between individuals. There is great variability in VO2 max between individuals and therefore there is a distribution of VO2 max in the population. It is clear that tasks must be designed using VO2 max in a way similar to that of other anthropometric variables. One important difference is physical condition.

Factors affecting work capacity (Personal)

Shows how the capacity for sustained physical work depends upon the amount of physical conditioning.

A highly trained individual (runners) can sustain 50% of the maximal aerobic capacity for an 8 hour working day, an average individual can sustain 35% and an untrained individual 25% 

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Cyclist, distance runners have a high VO2 max levels of 70-80 ml of oxygen per kg of body weight. These individuals lie in the extreme range of the population data. The average value is around 40 ml per kg per minute It is generally believed that individuals can work continuously over an 8-hour shift at a rate of 30 to 50 percent of their maximum capacity. In heavy manual work (eg tasks requiring expenditure of 2L of oxygen per minute), individuals with a high VO2 max (6 L of oxygen per minute) will be at an advantage because they will require fewer rest breaks. Individuals with a lower VO2 max (2L of oxygen per minute) will be able to work only for short periods and will require frequent rest breaks.

Variability between individuals also increases with age. The table below shows the maximal oxygen uptake for two individuals from the age of 35 years onwards. The oxygen uptake is given as a percentage of the greatest value attained for that individual. It can be observed that by the age of 65 years individual A was as fit as ever, whereas individual B had a maximal oxygen uptake of 65 % of his high value at the age of 35 years.