MMH - Health Hazards

Manual material handling (MMH) is the most common cause of occupational fatigue and low back pain. This document focuses on preventing back injuries caused by MMH in the industrial workplace and only covers the handling of inanimate objects.

Immediate and short-term effects include injuries and fatigue. Sharp or rough surfaces, falling and flying objects are common causes of wounds, lacerations (cuts), or bruises during MMH. The worker can also experience these injuries by falling or by colliding with objects.

Fatigue is a common and expected effect of MMH. The effort required to perform MMH tasks uses up muscular energy. Where the pace of work is not too high, workers can find enough time between individual tasks to recover their energy, and work can be resumed and continued safely throughout the whole shift. On the other hand, a fast pace of work shortens the time between tasks and does not allow the workers to restore their energy. As a result, workers who try to maintain a fast pace may become increasingly tired as the shift progresses. Even a moderate pace of lifting, not necessarily at the maximum lifting limit, if maintained for a prolonged time without breaks, can decrease the workers' lifting ability by speeding up their fatigue. Fatigue can also contribute to serious injuries to the musculoskeletal system. These injuries can later develop into chronic conditions that can become difficult to treat effectively. Additionally, fatigue decreases workers' alertness, making them more likely to act without due caution. This reduction in alertness, in turn, increases their risk for incidents.

More serious issues related to MMH are the long-term health effects, such as chronic back pain.

Back pain can result from various causes. The most common causes are strains and cramps in the back muscles. Back pain can also result from tears in the tendons connecting the back muscles to the spine or from sprains and tears in the ligaments interconnecting the vertebrae (bones of the spine). Less frequently, it arises from direct damage to the vertebrae or the discs that separate them.

A worker can sustain a back injury from a single episode, such as lifting too heavy a load, slipping and falling, or receiving a blow to the back. However, most often, it is not the single episode that causes back injury. It is the repetition that contributes most to the occurrence of injuries. Performing MMH tasks continually, even at a moderate intensity, will cause mechanical stress to accumulate in the worker's back, increasing the likelihood of injury. Eventually, even a mild effort in MMH can result in back injury and disabling back pain. Recovery from back injuries can take a long time, and further injury may occur, making the problem worse.

Work-related factors include the weight of the load lifted, the range of the lift, the location of the load in relation to the body, the size and shape of the load, the distance and duration the load is carried, and the number and frequency of lifts performed. Excessive bending and twisting also increase the risk of back injury.

For most workers, lifting loads over 20 kilograms results in an increased number and severity of back injuries. While the weight of the load is the most obvious factor, it is not the only one that determines the risk of injury. The location of the load is also important. A load lifted far from the body imposes more stress on the back than the same load lifted close to the body. A bulky object is harder to lift than a compact one of the same weight because it (or its centre of gravity) cannot be brought close to the body. Lifting a bulky object also forces a worker into an awkward and potentially unbalanced position. The preferred range for lifting is between knee and waist height. Lifting above and below this range is more hazardous.

How often the worker performs MMH tasks and for how long are extremely important factors. Frequently repeated and long-lasting tasks are the most tiring and, therefore, the most likely to cause back injury. Highly repetitive MMH tasks also make the worker bored and less alert. This fact, in turn, can affect safety. Workers need rest breaks to prevent excessive fatigue. Frequency and duration of rest breaks are important factors to be considered.

Poor layout of the workplace also increases the risk of injury. For example, shelving that is too deep, too high, or too low causes unnecessary bending or stretching. Lack of space to move freely increases the need for twisting and bending. Unsuitable dimensions of benches, tables, and other furniture force the worker to perform MMH tasks in awkward positions that add stress to the musculoskeletal system. Similar stressful body movements occur where work areas are overcrowded with people or equipment.

Training is a very important component of back injury prevention. Workers must be educated about correct lifting techniques for the tasks they do. New employees, those changing jobs, or those returning to work especially need to know the correct techniques and the dangers of not lifting correctly before beginning their job tasks.

Sometimes, specific job tasks require wearing personal protective equipment. These items may hinder movement while lifting and cause injuries. Be sure that a job hazard analysis is performed and these restrictions have been addressed.

Tasks that involve manual handling exceeding the worker's physical capability and a poor workplace layout are the most common causes of back injuries. Physical capability includes elements such as sex, gender, physical size, age, pregnancy and any previous injury. Please see the OSH Answers Personal Protective Equipment - Body Type and Gender Considerations for more information. 

Temperature and humidity affect the worker performing MMH. When it is too hot and too humid, the worker tires more quickly and becomes more susceptible to back injury. On the other hand, cold temperatures decrease the flexibility of muscles and joints. This stiffness also increases the likelihood of musculoskeletal injuries.

Inadequate lighting in the work area indirectly affects the worker performing MMH, particularly where the precise placement of handled objects is important. In compensating for poor visibility, the worker often must handle objects in an awkward position for extended periods of time. Poor lighting on steps and stairways, ramps, and loading docks increases the potential for incidents resulting in back injuries. By misjudging distances, the height of steps, or ramp angles, the worker can easily lose balance and fall while carrying a load.

Whole-body vibration alone can cause back pain. It imposes compression on the spine that gradually damages the discs between the vertebrae. Combining MMH with vibration increases the risk of injury.

The capacity to perform MMH varies considerably among individuals. In general, the lifting strength of women as a group is less than that of men. However, the individual ranges of strength are wide. This variation means that some women can safely handle greater loads than some men. Therefore, discrimination against women for MMH is not justified. In a situation where selection is the only way to minimize the possibility of injury, women and men should be given an equal opportunity to be selected for the job. However, there are certain working conditions that, when present alone and more particularly when combined with MMH, create greater health hazards for women. Owing to body composition and structure, women are less tolerant of heat and whole-body vibration. Such hazardous conditions should be fully controlled and not serve as an excuse for gender discrimination.

Ageing diminishes strength. Since the rate of decline varies greatly with the individual, discrimination against older workers solely on this basis is unjustified. Statistics show that back injuries among workers over 45 years of age are less frequent than among those between 20-45 years of age. Experience seems to counterbalance decreasing physical capacity. With experience comes skills, dexterity, and practical know-how for completing tasks, all of which are very important factors contributing to safe MMH. The unskilled, inexperienced worker is at greater risk in tasks that require skills in handling. On the other hand, the older, experienced worker is at risk in tasks requiring sheer physical strength.