Week 8 Readings Flashcards
Differentiate between the three most common approaches used to assess applied load to the lower back
Biomechanical approach: directly model mechanical stresses placed on the internal structures of the body during lifting. Accurately estimate how work activities stress the bones, muscles, and connective tissues of the body and to predict when these stresses will lead to the damage of these structures
Physiological approach: concerned with the metabolic energy expenditure with repetitive lifting
Useful for repetitive lifting where the load is within the physical strength of the worker
Utility of approach
Determine whether or not the task is within the expected capability of the population
Determine the work/rest schedule for a given task
The subject adjusts weight of load according to their perception of effort such that the task doesn’t result in overexertion or excessive fatigue. Final weight is considered to be the maximum acceptable weight of the lift
Psychophysical approach: study of the relationship between human perception and physical stimuli that lead to that perception
Utility: determine what the subject can handle (capacity) without strain or discomfort
Measure of capacity in this approach is the maximum acceptable weight of lift.
MAW is determined experimentally that a given person could lift repeatedly for long periods of time without undue stress or fatigue
Where is the biomechanical load of the spine typically measured?
Intervertebral disc at L5/S1 spinal segment
How can applied load be estimated?
- Calculate the moment created by force acting at each joint, beginning with joints closest to the hands and ending at the joint of interest
- Hand arm torso model
- Use anthropometric data for equations
What is the difference in estimating static vs dynamic applied load?
Static models: calculations require information on the orientation of the links in the model (subject posture, length of each segment, amss of each segment, and location of COM
Dynamic models: require same information plus angular joint acceleration, linear joint acceleration of each segment at the centre of mass, moment of inertia of each link through the COM
Describe in general, two ways to estimate internal muscle loads in the torso? What are some of the challenges in determining internal muscle forces?
- Two dimensional models: estimate muscle forces by assuming the erector spinae muscle acts to generate force if the external load moment at the torso is acting to increase torso flexion (i.e. lifting activities) and the rectus abdominus is active if the external load moment at the torso is acting to decrease torso flexion (pushing down)
- Relies on use of newtons second law (F=mass * acceleration)
- Use EMG activity to predict how the muscle responds in different situations
- Problems:
- Complexity of human torso - number of muscles is greater than the number of force and moment equations
- Not very accurate for complex dynamic activities - like ones that involve twisting
- Models don’t consider the tendency of muscles to contract both antagonist and agonist muscle to increase stiffnes
How is the failure load estimated?
- Damage load: load that causes first observable damage to specimen
- Damage load equation that uses the compressive strength of the lumbar segments
- Compressive strength equation is calculated using age, gender, body weight, and lumbar motion segment of interest
When should the physiological design approach be used? Why?
- Useful for repetitive lifting where the load is within the physical strength of the worker
- Limitation is not the worker’s strength but the capacity of the oxygen transport system
- Thus, metabolic energy expenditure is the concern
Utility of approach
- Determine whether or not the task is within the expected capability of the population
- Determine the work/rest schedule for a given task
Which physiological measure is considered to be most directly proportional to workload?
Metabolic energy expenditure
Describe 5 factors that limit the physiological models’ predictive power.
- All models are only valid for MMH tasks in the sagittal plane
- They do not take into account the effect of task variables (e.g. frequency, height of handling, box width, and box length) and their interactions
- Subjects were not trained before data collection
- Model developed by Aberg et al. (1968) requires the determination of the body’s centre of gravity which is difficult to perform
- There is a need to measure the individual’s standing metabolic rate in order to apply the models developed by Garg et al.
Which aspects of MMH activites are considered to have unknown or no effect of metabolic energy expenditure?
Object shape: unknown
Load stability/ distribution: unknown
Asymmetrical handling; none
How were the “SNOOK” Tables” developed?
Snook and Ciriello conducted extensive laboratory experiments where they measured the physical capabilities of a large, diverse sample of male and female workers performing various manual tasks.
Participants were asked to perform lifting, pushing, pulling, and carrying tasks. They adjusted the loads until they felt they could perform the task comfortably without undue strain over an eight-hour workday. This approach captured the maximum load individuals believed they could handle safely.
The collected data were analyzed to determine the maximum acceptable loads for different populations, accounting for gender and task parameters. These values were used to create tables representing the percentage of the population capable of performing each task safely.
Describe why the psychophysical approach is considered valid. What are its predictive strengths compared to other techniques
- Study demonstrated that with good subject cooperation and firm experimental control, psychophysical method can identify loads that subject can lift repetitively for an 8 hour work day without metabolic, cardiovascular, or subjective evidence of fatigue
- Combined MMH capabilities can be predicted by simple strength testing
- Combined MMH capacity can be predicted from strength testing directly
- No knowledge of individual capabilities is required
What does the author mean when he says biomechanical, physiological and psychophysical measures are in conflict with one another. What suggestion does the author have for dealing with this issue?
- Criteria for manual material handling limits are not similar
- Proposes different limit values for load and frequencies
- Biomechanical model recommends high weights and high frequency
- Physiological model recommends high weights at low frequency
- Consider all criteria simultaneously to estimate the recommended weight for a lift
