Last week, we looked at four of the six methodologies used by occupational biomechanics practitioners -- kinesiology, modeling methods, anthropometrics and bioinstrumentation. Let's now look at the other two: mechanical work capacity and motion and time evaluation. (Editor's Note: There's a glossary of occupational biomechanics terms in the Tools section of SafetyXChange.)

5. Mechanical Work Capacity

Every person has a different biological capacity for stressful work. Mechanical work capacity measures the amount of working loads that a person can handle without suffering adverse effects. It considers variables such as the specific biomechanical joints that will be utilized for the task and calculates the moments (force vectors) to ensure that the worker's limit or capacity (injury) is not reached before a task is completed.

Joints: The primary consideration should be the measurements of the specific connecting joints within the skeletal system used during the task. Joint motion and muscle strength are important properties in biomechanics in determining the probability of injury.

Posture: Posture plays an important part in muscle strength measuring and must be considered with all test subjects. The strength of the bicep changes based upon the moment arm and angle of flexion and extension. It is best to prepare a strength diagram to determine the effect of the moment arm and peak strength of the muscle under testing. (Moment arm is the rotation produced in a body when force is applied, such as torque.)

Muscle Strength: Measuring muscular strength both from a static and dynamic position requires specific testing criteria. The measuring device must record peak strength and average strength over approximately three-second intervals. The testing must be performed without creating discomfort for the test subject in the muscle being tested. There must be rest periods of specific and consistent length for test subjects to recover. All distractions must be eliminated and standard postures must be adhered to.

Measuring static strength is pretty straightforward; but measuring dynamic strength is far more complicated because of the many variables, including:

• Muscle velocity
• Displacement, and
• Acceleration curves estimates.

Large body segments fluctuate dramatically depending upon the acceleration or deceleration of the movement. If it's assumed that the maximum strength of a lifted load would be performed slowly, then a worker's dynamic lifting strength can be estimated. Of course, during the lifting movement the load should be kept close to the body and the back should be vertical. This posture is the safest when lifting objects from the floor or near the floor. A bent-over posture, keeping the load away from the body, is simply inefficient and has a higher probability of back and spinal injury.

Speed of the exertion motion and strength of the muscle are also correlated. The faster the muscle is in contraction, the more strength it produces. This could be considered by some as muscle momentum since a quick jerk can momentarily increase the muscle strength as compared with muscle strength over a longer time period.

6. Motion and Time Evaluation

The sixth methodology, motion and time evaluation involves:

A Joint Assessment: To achieve biomechanical safety, workers and supervisors must identify and be aware of unsafe stress factors.

Measuring Time: Time refers to the element of finishing a task; for example, for those working on conveyor belts where the product passes in front of the worker, the task has a time value. Each task has a minimum, median and maximum time limit to perform the task safely. An evaluation of the worker and the subject task is required to set safe time limits for a particular job function and predetermine the motion for each task.

Assessing Motion: To analyze safe task performance, you must be able to separate and evaluate each movement of a particular task. Some of the movements analyzed include:

• Reach
• Position for most efficient movement
• Release factor after the motion is completed
• Rebound, as when pulling pieces apart on an assembly line
• Grasping or controlling an object
• Eye movement and focus
• Turning or manipulating tools or parts
• Body segment motion, and
• Motion of all or some of these movements simultaneously.

Conclusion

After gathering as much data as possible, a biomechanical team must analyze the data. The team should create charts, graphs and tables. These tables should classify motion and time so that analysis of the work capacity can be determined. This data will help a company choose the right person for the right task and identify tasks that require position adjustment. Remember that all seemingly similar tasks are not the same; an employer may be unaware of minor adjustments made by a worker.

SAFETY TIP OF THE WEEK

Winter Driving Safety

With spring approaching, it's tempting to get complacent about winter's hazards. But be prepared for the unexpected and make sure you've packed these essentials in your car:

It's also a good idea to toss in an old blanket and any winter clothes you don't wear anymore, especially an old pair of boots.

Car buried beneath an overnight snowstorm? Take a minute to clear away all the snow and ice from your vehicle before you drive off. This will help prevent sheets of ice from flying off your car and hitting the vehicle behind you. And remember to always keep your vehicle's air intake grill free of snow and ice.

Gary W. Brower, Sr.

CSXT Engineering Roadway Mechanics

Fire Extinguishers Recalled

Fire Extinguisher Fails to Discharge.

The US Consumer Product Safety Commission (CPSC) has announced a voluntary recall of the Strike First Dry Chemical Fire Extinguishers, designed for commercial, industrial, multi-residential and vehicle use.

The manufacturer has received three reports of these fire extinguishers failing to discharge properly when activated. Fortunately, there have been no injuries, but the failure to discharge poses a serious fire hazard.

The recalled 2.5 lb and 5 lb extinguishers were sold between December 2002 and April 2004. Products bearing these model and serial numbers should be repaired:

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