Musculoskeletal disorders: anatomy and risk factors

To prevent musculoskeletal disorders, it helps to understand what is being injured and why. This lesson introduces the tissues of the musculoskeletal system, the common disorders that affect them, and the risk factors that cause them. It also explains a simple but powerful idea from biomechanics: the distance between a load and the body multiplies the strain on the back and shoulders. That idea underlies most of the lifting guidance in this course.

The musculoskeletal system

The musculoskeletal system supports the body, protects internal organs and allows movement. Its main parts are:

• Bones, which form the skeleton and act as levers.
• Joints, where bones meet, lined with cartilage and held together by ligaments.
• Muscles, which contract to produce force and movement.
• Tendons, tough cords that attach muscles to bones and transmit muscle force.
• Ligaments, which connect bone to bone and stabilize joints.
• Nerves, which carry signals between the brain, spinal cord and the rest of the body; they are part of the nervous system but are often injured by the same work factors.
• Blood vessels, which supply oxygen and nutrients to working tissues and remove waste.
• Spinal discs, the pads of fibrous tissue with a gel-like centre that sit between the vertebrae of the spine and absorb load.

These tissues can adapt to load. Muscles grow stronger with training, and tendons and bones become more robust. But they also have limits. When load exceeds what a tissue can tolerate, or when there is not enough time to recover between loads, damage occurs. Muscles and tendons have relatively limited blood supply at their attachment points, and discs have almost none in adults, so they heal slowly.

The spine and the lower back

The spine is a column of 33 vertebrae: 7 cervical (neck), 12 thoracic (upper back), 5 lumbar (lower back), and the fused sacrum and coccyx. The lumbar spine carries the weight of the upper body plus anything held in the hands, and it is the most common site of work-related back pain. Between the lumbar vertebrae, the discs are compressed and sheared every time a person bends, lifts or twists. The back muscles that hold the trunk upright and straighten it after bending run close to the spine, just a few centimetres behind the discs.

This geometry creates a mechanical disadvantage. A load held in the hands is usually much farther from the spine than the back muscles are. To balance the load, the back muscles must pull with a force many times greater than the weight of the load, and that muscle force presses the vertebrae together. Most of the compression on a lumbar disc during lifting comes not from the weight of the load itself but from the back muscles working to balance it.

Levers and moments

A moment, or torque, is the turning effect of a force about a pivot point. It equals the force multiplied by its perpendicular distance from the pivot, called the moment arm. For lifting, the pivot is roughly the lower lumbar disc. The weight of the load and the weight of the upper body create forward moments that try to bend the trunk forward. The back extensor muscles create a backward moment to resist. For balance, the two moments must be equal.

Because the back muscles act at a moment arm of only about 5 cm, while a load may be 30 to 60 cm in front of the spine, the muscle force must be roughly 6 to 12 times the weight of the load, plus extra force to hold up the trunk. This is why holding a load close to the body is the single most effective change a worker can make in a lift, and why reaching across a table or into a deep bin is so hard on the back.

The compression on the disc is roughly the muscle force plus the component of the body and load weights along the spine. Biomechanical research used to develop the original NIOSH lifting guide in 1981 set an action limit of about 3,400 N of lumbar disc compression, a level above which some workers are at increased risk of injury, and a maximum of about 6,400 N, above which most workers are at risk. The example shows how easily a bent, extended reach can approach the action limit even with a moderate load.

Common musculoskeletal disorders

MSDs can affect almost any part of the body. They range from short-term strains that heal in days to chronic conditions that last for years. Some are sudden, such as a back strain during a heavy lift; others develop gradually, such as carpal tunnel syndrome after years of forceful gripping.

Early symptoms of MSDs include aching, stiffness, burning, tingling, numbness, swelling, reduced range of motion, weakness and clumsiness. Symptoms that appear during work and fade overnight are an early warning. Symptoms that continue overnight or during days off, that wake a person from sleep, or that interfere with daily activities show that injury is progressing. Early reporting and early action greatly improve outcomes, which is why reporting systems are a key part of every ergonomics program.

The physical risk factors

Research has identified a set of physical risk factors that increase the likelihood of MSDs. The risk rises when factors are combined, for example high force with high repetition and awkward posture, and when exposure lasts a long time without recovery.

• Force: the effort needed to lift, push, pull, carry, grip or press. Heavy loads, stiff tools, tight connectors and slippery or awkward objects all increase force. A pinch grip, using the fingertips, is about four or five times weaker than a full power grip, so the same object demands more relative effort when pinched.
• Awkward posture: positions away from the neutral, mid-range position of a joint, such as bending or twisting the back, reaching overhead or behind the body, bending the neck, bending the wrist, or kneeling and squatting. Awkward postures reduce strength and increase the load on tissues.
• Repetition: performing the same motion many times. High repetition gives tissues little time to recover. Rough guides suggest that cycle times under about 30 seconds, or more than half the cycle spent on the same motion, indicate high repetition for the upper limbs.
• Duration: the length of exposure over a shift, a week and a career. Longer exposure to any factor increases risk.
• Static loading: holding a posture or load without moving, such as holding arms raised or standing in one place. Static muscle contraction restricts blood flow and leads to fatigue quickly.
• Contact stress: pressure from a hard or sharp edge on soft tissue, such as resting wrists on a desk edge, using the palm as a hammer or tool handles that dig into the hand.
• Vibration: hand-arm vibration from power tools and whole-body vibration from vehicles and machines, covered in Lesson 10.
• Cold: cold temperatures and cold air blowing on hands reduce dexterity and sensation, lead people to grip harder and add to vibration effects.

Tissue tolerance and recovery

A useful way to think about MSDs is to compare the load on a tissue with its tolerance. Every task places some load on tissues. If the load is below the tissue's tolerance and there is time to recover, the tissue adapts and may become stronger. If a single load exceeds the tolerance, as in a very heavy lift, an acute injury results. If loads are moderate but repeated many times without recovery, fatigue damage accumulates and the tissue's tolerance itself falls, until a load that would normally be harmless causes an injury. This is why recovery time, breaks and variation in tasks matter as much as the size of each load.

Individual factors

People differ in their susceptibility to MSDs. Age, previous injuries, general fitness, body size, some medical conditions (such as diabetes, arthritis and thyroid disorders), pregnancy and activities outside work can all affect risk. These factors are real, but they should not be used to blame workers or to screen people out of jobs. The most effective prevention is to design work that is within the capacity of a wide range of people. When individual workers need accommodation, for example after an injury or during pregnancy, employers have duties to accommodate them under human rights, disability and return-to-work law, as Lesson 12 explains.

Psychosocial and organizational factors

Research shows that MSDs are also linked to psychosocial and organizational factors: high work demands with little control, time pressure, machine-paced work, monotonous tasks, low support from supervisors and co-workers, job insecurity and incentive pay systems that reward speed. These factors may raise muscle tension, reduce breaks, discourage reporting and make people more likely to take risks such as skipping a lift assist. They also affect how people experience and recover from pain. A complete ergonomic assessment considers how work is organized, not just how it is physically performed.

The neutral posture

Each joint has a neutral posture in which it is strongest and the tissues are under the least strain. For the back, neutral means upright with the natural curves of the spine. For the neck, it means the head balanced over the shoulders, looking straight ahead or slightly down. For the shoulders, it means the upper arms hanging relaxed at the sides. For the elbows, it means bent at roughly 90 degrees. For the wrists, it means straight, in line with the forearm. Working close to neutral postures as much as possible, and changing position often, is a basic ergonomic principle that appears in every lesson that follows.