Delivery workers have one of the highest rates of musculoskeletal injury compared to other occupations. The television show King of Queens might portray Doug Heffernan as a lighthearted, rarely debilitated delivery worker, but in reality, this occupation is physically and mentally demanding, with injuries comparable to those experienced by professional athletes.

The unique biomechanical challenges of this occupation put these workers at significant risk for a host of musculoskeletal complaints, including injuries to the shoulders, neck, back, head, and knees.

Delivery workers have several work-related challenges: long periods of seated driving, repetitive lifting and bending, heavy lifting, reaching, jumping out of vehicles, going up and down stairs, opening and shutting cargo doors, and being on a hectic time schedule. Poor posture during these activities adds to the risk of injury.

Of course, there are particularly high rates of injury and pain in the neck and back, but shoulders, upper extremities, and lower extremities are also at risk. My previous articles on musicians (January/February 2012), dentists (March/April 2012), and hotel workers (May/June 2012) covered shoulder injuries; here I focus on low-back and knee problems.

Massage treatment is very effective for addressing these disorders, but more importantly, massage helps relieve the cumulative stresses that can lead to more debilitating conditions.

Biomechanics of Low-Back Muscle Injury

The primary action of the torso in lifting is lumbar extension, mainly with the lumbar extensor muscles. Other muscles contribute to lumbar extension, but the main extension force is generated by the erector spinae muscle group. Improper lifting routinely puts stress on these muscles, which leads to muscular injuries such as strain, spasm, or chronic hypertonicity.

One of the reasons back injuries are so frequent is that the lumbar muscles are in a poor mechanical position for lifting heavy loads. In relation to the lumbar spine, the lumbar muscles have a poor “moment arm,” which is the force used to produce a motion multiplied by the distance of that force from the axis of rotation. In essence, the closer a muscle is to the joint’s axis of rotation, the less capable it is of producing significant force. The lumbar extensor muscles lie directly along the spine (very close to their axis of rotation) and are in a poor mechanical position for lifting activities, which can lead to muscle fatigue and injury (Image 1).

The strong lumbodorsal fascia helps the lumbar muscles by lending tensile power from the other muscles connected to it, including the latissimus dorsi, gluteus maximus, transverse abdominis, and abdominal obliques. Their mechanical role is to produce tension on the lumbodorsal fascia, which then supports the lumbar muscles in lifting actions. Consequently, it is very important to address these other muscles when treating lumbar pain.

Poor sitting postures and improper lifting techniques can produce radiating back pain. Long periods of isometric muscle contractions from sitting lead to chronic tightness, pain, and myofascial trigger points in the lumbar erector spinae muscle group. These chronic contractions, exacerbated by heavy lifting, can lead to facet joint dysfunction and disc pathology.

Facet Joint Dysfunction

The facet joints of the spine (zygapophysial joints), located on the posterior aspect of the vertebral arch, are the only bone-to-bone contact surface of adjacent vertebra (Image 2). Facet joint dysfunctions occur as a result of compression forces on the spine. When the disc loses height from compression, the facet joints compress together, irritating the adjacent contact surfaces of each vertebra.

Even slight overloading of the back with the spine in extension can put excessive loads on the facet joints. Those with exaggerated lumbar lordosis have increased pressure on these joints.

Disc Pathology

Most disc herniations occur as a result of moderate compressive loads endured over time. Disc herniations develop in this worker population because of chronic compressive forces (sitting) and heavy lifting that places significant compressive loads on the discs, producing disc pathology.

The disc is thicker on the anterior portion than the posterior in order to maintain the normal lordotic curvature of the spine (Image 3). When the spine is flexed, as it is during lifting activities, there is greater compression on the anterior aspect of the annulus fibrosis, while the posterior annulus is overstretched. The tensile forces on the posterior annulus, where it is overstretched, lead to weakening and eventual disc degeneration and herniation.

Knee Biomechanics and Pathology

Delivery drivers also put an enormous demand on their knees. Constant bending of the leg with weight places adverse wear on the patella and its tendon, the anterior cruciate ligament (ACL), as well as the meniscus. Repetitive loads on the knee extensor structures make them susceptible to injury.

The patella functions to increase the power of the quadriceps, acting as a fulcrum during extension, and is embedded within the tendon of the quadriceps muscles (Image 4). Consequently, there is a great deal of tensile (pulling) force on the patellar tendon with knee extension during lifting.

As the knee extends, the patella moves in a superior direction. Sometimes the forces on the tendon are not equal; most commonly there is a greater pull on the lateral side compared to the medial side of the patella. The imbalanced pull can produce a patellar tracking disorder, a primary cause of anterior knee pain.

Delivery workers also sustain chronic overuse tendon pathologies such as patellar tendinosis, which is chronic overload on the tendon but not necessarily a tracking disorder. Patellar tendinosis involves chronic degeneration of the collagen matrix within the patellar tendon. Unfortunately, tendinosis heals slowly, and people often go back to activities long before they are at optimum function, thereby aggravating and prolonging the existing injury.

ACL and meniscus pathology are also common. Powerful contractions of the quadriceps, as during lifting and bending, produce a high tensile force on the ACL, which can, with repetition, cause gradual breakdown and sprain injury to the ACL.

The ACL can be injured with simultaneous bending and twisting movements, which are common for delivery drivers. Rotational stress to the knee puts the ACL in a compromised position, a common mechanism of injury, and can seriously stress the medial collateral ligament (MCL) and the medial meniscus. In fact, these three structures are so frequently injured together that they are known as the “terrible triad.”

The ACL and medial meniscus are structures deep within the knee and inaccessible to massage treatment. Conditions involving these two structures should be treated by an orthopedist. The MCL, however, is accessible for massage treatment and responds well to deep-friction massage, along with other movement methods used in physical therapy.