Tendon (Sinew)

What is a tendon (sinew)?

A tendon, or sinew, is a cord of strong, flexible tissue, similar to a rope. Tendons connect your muscles to your bones. Tendons let you move your limbs. They also help prevent muscle injury by absorbing some of the impact your muscles take when you run, jump or do other movements.

Your body contains thousands of tendons. You can find tendons from your head all the way down to your toes. Your Achilles tendon, which connects your calf muscle to your heel bone, is the largest tendon in your body.

Tendons are highly resistant to tearing but aren’t stretchy. This means they can be easily injured when strained (stretched to the point of partial tearing) and may take a long time to heal.

What does a tendon do?

When you contract (squeeze) your muscle, your tendon pulls the attached bone, causing it to move. Tendons essentially work as levers to move your bones as your muscles contract and relax.

Tendons are stiffer than muscles and have great strength. For instance, the flexor tendons in your foot can handle more than eight times your body weight.

Where are your tendons?

Tendons are located all over your body. For example, tendons connect your muscles to your bones in your elbow, heel, knee, shoulder and wrist.

What are the parts of a tendon?

A tendon consists of:

• Epitenon, a thin layer of connective tissue that surrounds the entire tendon.
• Paratenon, a loose layer of connective tissue that lets the tendon move against the epitenon and other tissues that the tendon touches. Located outside the epitenon.
• Sharpey fibers, collagen fibers that attach the tendon to the bone.
• Endotenon, connective tissue that surrounds the primary, secondary and tertiary fiber bundles. Helps the bundles glide against each other inside the tendon.
• Sheath (synovium), some tendons in your hands and feet have a sheath (synovium). This is the protective outer covering of the tendon. It produces a lubricating fluid called synovial fluid, which helps the tendon slide smoothly where it meets muscle and bone. Not all tendons have a sheath, however.

What do tendons look like?

Tendons are mostly collagen, one of the most abundant proteins in your body. Tendons also contain blood vessels and nerves.

Collagen fibers are flexible, strong and resistant to damage. A tendon’s structure is similar to a fiber-optic cable or a rope, with small collagen fibers arranged in bundles. This bundling reinforces the tendon and makes it stronger.

The collagen fibers in a tendon group are separated into:

• Primary fiber bundles (subfascicles), the smallest bundle
• Secondary fiber bundles (fascicles) made of groups of subfascicles
• Tertiary (third) fiber bundles containing groups of fascicles that form the tendon itself

How do tendons connect muscles to bones?

Tendons have two areas of transitions or junctions:

• Musculotendinous junction (MTJ). The point where the tendon attaches to your muscle. This is a frequent site of injury.
• Osteotendinous junction (OTJ). The point where the tendon attaches to your bone.

The Sharpey fibers that are part of the tendon extend into the bone. Tendons of the hand or foot commonly slide through a connection called a reflection pulley that helps hold them in place. Small, fluid-filled pads called tendon bursae (plural of bursa) cushion tendons where they meet the bone.

What conditions and disorders can affect tendons?

Because tendons connect every muscle in your body, a wide range of injuries and disorders can affect them. Tendon issues are more common with age. As people get older, tendons become thinner, have less blood flow and accumulate microscopic damage to fibers that weaken the tendon.

Common disorders that affect your tendons include:

• Enthesopathy and enthesitis
• Strains
• Tendinosis
• Tendonitis
• Tenosynovitis

How can I keep my tendons healthy?

To help reduce your risk of sinew conditions:

• Balance cardio exercise, strength training and flexibility. Keeping your body moving in a variety of ways can prevent you from overtaxing your tendons.
• Listen to your body. Taking it easy when you’re tired or stressed can lessen your risk of injuries.
• Stop an activity if pain occurs. If you experience any pain during an activity, stop doing it and try again later to see if the pain reoccurs.
• Stretch after exercise. Stretching your muscles when they’re more pliable after exercise, and never to the point of pain, can help prevent tendon injury.
• Warm up before you exercise. Doing some light aerobic activities or running in place before more intense exercise increases blood flow rates and loosens up tendons.
• Wear proper athletic shoes. Making sure your shoes fit well and are designed for the sport you’re playing can help keep your body in alignment.
• Work in rest days. Scheduling regular days off can lessen the chances of overstressing your tendons.