1.4 Ergonomic Assessment, Posture, and Manual Handling

Ergonomic Assessment, Posture and Manual Handling

Start by observing the actual job and asking which tasks and times are difficult. Record load, reach, repetition, work duration, breaks and available equipment. Relevant exposures can include high force, repetitive movements, prolonged or awkward positions, contact stress and vibration. A useful change reduces the exposure while allowing the person to do the job.

Postural Observation and Muscle Testing

A plumb line is a vertical visual reference. In a side view, a common Kendall reference places the line just anterior to the lateral malleolus and knee axis and near the greater trochanter, shoulder and ear. In a rear view, note the trunk midline and compare levels at the shoulders, pelvis and knees. Record stance, footwear and the task. A visible landmark and the instantaneous joint axis are different quantities. A plumb line alone cannot determine an individual's force line or joint loading.

Janda's upper crossed model describes a proposed pattern in which shortened or overactive pectoral and upper cervical or scapular muscles coexist with reduced function of deep neck flexors and some scapular stabilizers. Classic examples include the upper trapezius and pectorals on one side of the proposed pattern and the deep cervical flexors and lower trapezius on the other. The lower crossed model pairs a tendency toward shortened hip flexors and lumbar extensors with reduced abdominal and gluteal function. These models suggest what to examine. Inspect movement during work, check length and performance separately, and compare symptoms during relevant tasks. A posture label does not prove inhibition, tissue compression or a diagnosis.

Lumbar Pressure and Load Placement

Early in vivo work by Nachemson and Morris measured lumbar disc pressure under different loading and posture conditions. Their tested seated conditions generally produced higher pressure than standing. In a later study, Wilke and colleagues used an implanted transducer in one participant at L4–L5. Some of their readings illustrate how much the exact task matters.

In this participant, unsupported sitting was about 92% of relaxed standing pressure (0.46 ÷ 0.50), while sitting with maximum flexion was about 166%. The early and later findings differ in subjects, levels, posture and methods. Use the individual measurements as examples, without treating them as a universal posture ranking. Disc pressure is also distinct from spinal shear, pain and tissue injury. If a load is moved closer with other conditions fixed, its external flexion moment decreases because its perpendicular moment arm is shorter.

Revised NIOSH Lifting Equation

The US National Institute for Occupational Safety and Health (NIOSH) equation estimates the Recommended Weight Limit (RWL) for eligible two-handed manual lifting tasks. Use it for task design, including repetitive lifting within the manual's conditions. Consult the manual for scope and exclusions. One-handed lifting, lifting people or unstable loads, carrying, and seated or kneeling lifting require other methods or task-specific assessment. The equation does not establish an occupational legal limit or measure an individual worker's lifting capacity.

RWL = LC × HM × VM × DM × AM × FM × CM

LI = load mass ÷ RWL, with numerator and denominator in the same units. NIOSH recommends a lifting index no greater than 1.0 for task design. A higher value identifies increased physical demand and helps prioritize changes. A zero multiplier gives no positive recommended load. Redesign the task instead of calculating an index by dividing by zero. If significant control is required at the destination, calculate RWL at both ends and use the lower value for the lift. If a job includes multiple lifts, use the applicable multi-task procedure. A multiplier's influence depends on the task. The horizontal factor is one common redesign opportunity.

Worked example. A worker lifts a stable 15 kg carton with two hands. At the origin, H = 50 cm, V = 75 cm, D = 25 cm and A = 0°. Coupling is good. Assume the task falls in a frequency and duration category with FM = 1.00 and requires no significant control at the destination. HM is 0.50 and the remaining multipliers are 1.00. The RWL is 23 × 0.50 = 11.5 kg, and LI is 15 ÷ 11.5 ≈ 1.30. If a redesign reduces H to 25 cm without changing the other factors, RWL becomes 23 kg and LI becomes 0.65. Verify the actual reach, grip, destination and workload after the change.

Workstation Design and Controls

Saudi occupational safety and health guidance addresses repetitive handling and computer workstation exposures. Adjust the work to the individual and follow the workplace's applicable procedures. For computer work, place the main screen in front and start with the top at or slightly below eye level. Adjust height and distance for vision, screen size and lenses. Keep keyboard and pointing device close enough for relaxed shoulders and hands aligned with the forearms. Adjust seat height and depth for foot support and avoid pressure behind the knees. Use a footrest when needed. Provide back support and room to vary posture. For prolonged laptop use, a raised screen and separate keyboard and pointing device allow the screen and hands to be positioned independently.

Apply the hierarchy of controls to the source of exposure. Eliminate an unnecessary manual transfer if possible. Substitute smaller containers where that reduces total exposure. Use engineering controls such as a lift table or powered material handling equipment. Administrative measures include changing work schedules, recovery periods and training. Appropriate personal protective equipment can supplement controls for a remaining exposure. Reobserve the task after the change and check whether it creates a new reach, force or repetition problem.