What causes musculoskeletal pain? Pain can come from several different structures in the body including joints, ligaments, muscles, tendons and nerves. You will often have more than one type of structure acting as a pain generator. Pain can be generated in any body structure that has nociceptive (pain-generating) nerve endings. Pain nerve endings can be triggered by chemicals released by inflamed tissue or by mechanical means such as direct compression. As pain becomes more chronic, scarring and adhesions may develop in the body tissues which can cause further stimulation of the pain nerve endings.
Let’s take a step back and look at the human body as a whole. How does it move? Your skeleton is covered in layers of muscle. These muscles pull (but never push) on the bones of the skeleton, allowing you to move your body. In order to move the skeleton, a muscle typically spans one or more joints. Tendons, made of collagen, connect muscles to bones. Ligaments, also made of collagen, connect bone to bone. A different type of connective tissue, articular cartilage, lubricates bony surfaces of weight bearing joints such as knee.
A common cause of pain is an overuse injury. Overuse injuries are due to repetitive microtrauma on the body. These injuries are often seen in muscles, tendons and bursae (the lubricating sacs positioned between tendon and bone). Overuse injuries are often related to muscle weakness, lack of flexibility and malalignments in the body such as flat feet, high arches of the feet and inward rotation of the knees. Poor posture and repetitive movements throughout the day increase risk for certain overuse injuries. For example, a common postural abnormality is a forward head posture and slouched shoulders. This improper alignment of the body imposes stress on muscles in the neck and upper back, causing them to become tight and tense. This chronic muscle tension can cause neck pain and headache.
Let’s focus on the most common types of overuse injuries involving tendons and bursae. A tendon is like a bungee cord attaching muscle to bone. A bursa is a small sac that provides lubrication. It may be positioned between bone and tendon or between muscles around a joint. A bursa reduces friction and helps tendons glide smoothly over bone. Sometimes a bursa can become inflamed and cause localized pain. Likewise, the strong material of a tendon can deform and sometimes tear if subjected to enough tension.
Tendons are made of tightly compacted collagen bundles which transmit the force generated by muscle to the attached bone. Tendons also store and release mechanical energy much like a spring. Tendons are able to withstand strong tensile forces. However, a tendon can only stretch up to a certain point before it begins to deform and tear. The stress-strain curve for the tendon shows how a tendon can tolerate a certain amount of stress (defined as the amount of force applied over an area) before the strain (defined as the % change in tendon length) on the tendon becomes too great. Some fibers of the tendon will rupture if the tendon is stretched to its breaking point. Complete rupture of the tendon occurs at 8% to 10% strain.1
A tendinopathy (tendon injury) can be understood as a gradual accumulation of microscopic damage to the collagen bundles of the tendon. With repetitive loading, the collagen bundles are no longer tightly bound in an orderly way. Instead, the collagen bundles fall into disarray and tear apart. This damaged tendon is not as strong and will not be able to transmit as much muscle tension to the attached bone.
The behavior of a tendon is influenced by its size (larger tendons can tolerate more force), length (longer tendons can stretch more), amount of collagen (the more collagen, the stronger the tendon) and the amount of collagen cross-linking (increased crosslinking results in a stiffer tendon that deforms less under the same load). A tendon often heals very slowly due to poor blood supply. In fact, ligaments and tendons, which have a similar collagen structure, do not heal as well as bone does. It is usually easier for your body to completely repair a bone fracture than a tendon tear.
There are many common overuse injuries involving tendons throughout the body. A tendon moving over bones in the hand or foot is typically covered with a protective sheath. This tendon sheath can also cause pain when it becomes inflamed. A good example is a condition called deQuervain’s tenosynovitis which causes pain at the base of the thumb.
The largest tendon in the body is the Achilles tendon which is located at the back of the ankle just underneath the skin. Achilles tendinopathy is common in runners. The injured Achilles tendon often feels swollen and lumpy when you run your finger along the back of your ankle. Other common tendon injuries include patellar tendinopathy in the knee, rotator cuff tendinopathy in the shoulder and extensor tendinopathy in the elbow (known as lateral epicondylitis).
Another common type of overuse injury is bursitis. In this condition, a small sac filled with synovial fluid becomes inflamed. Recall that bursa are often situated between tendon and bone to facilitate smooth movement. In bursitis, there is inflammation in the sac itself. In contrast, tendinopathy does not usually involve inflammation. Instead it is due to breakdown of the collagen fibers that make up the tendon. Bursitis often presents as a small area of tenderness directly over the bursa. Common examples of bursitis include trochanteric bursitis in the lateral hip area, anserine bursitis in the knee and subacromial bursitis in the shoulder.
How are tendinopathy and bursitis treated? First, it is important to identify the underlying cause of the injury whether it be poor posture, repetitive movements on the job, muscle imbalances or a rapid increase in running mileage.2 It is important to ensure that you have an ergonomically designed workspace. Many individuals may benefit from regular stretching of tight muscle groups such as the hamstrings and calf muscles. Keep in mind that the muscle is attached to the tendon as a unit. A relaxed muscle that is able to stretch will help the tendon work better.
Almost all musculoskeletal tissues become stronger with increased load. Much like muscle, tendon responds to increased loads by becoming larger. With repetitive loading, a damaged tendon also becomes stronger by increasing the amount of cross-linking in its collagen fibers.1
Thus, it is important to develop an appropriate rehabilitation program based on the phase of tendon healing. Most patients with chronic tendon injury will respond well to an appropriate tensile loading program of the tendon. This should initially be done under the supervision of a physical therapist. For example, the Achilles tendon can be loaded by performing eccentric heel raises on a step. Injured wrist extensor tendons can be loaded with wrist flexion/ extension exercises and the patellar tendon of the knee can be loaded by performing squat-type exercises.1 Imposing appropriate tensile loads on the injured tendon help generate stronger collagen tissue in the tendon.
The way your body works (or doesn’t work) is a direct result of your movements over a lifetime. Over the decades, poor posture, impaired flexibility and a sedentary lifestyle can take a toll on the body. The human body is a kinetic chain and all components of the chain- including bone, muscle and tendon- should be working properly for you to feel your best.
References
- Maffulli, N., Renström, P., & Leadbetter, W. B. (2010). Tendon injuries: Basic science and clinical medicine. London: Springer.
- Brukner, P. (2010). Clinical sports medicine. Sydney: McGraw-Hill.