Runners know how to rank their body parts. Most vulnerable? It has to be the knees. Most tortured? Feet. Most powerful? When it comes to speed, endurance and holding the strength that gets us uphill, downhill and everywhere in between, most of us would find it hard to credit anything other than our thighs. And for good reason. The muscles that make up our upper legs drive our running - whether we're sprinting for 100 yards or battling our way through 26.2 miles.

Run enough hills and you'll develop an appreciation for the demands on your hamstrings (which power you up) and quadriceps (which steady you down). Three muscles make up the hamstring, which runs down the back of the thigh, bends the knee and extends the leg at the hip. The quadriceps, on the front of the thigh, comprises four muscles that extend and stabilise the knee and decelerate the forces of impact when the foot lands.

Unlike most muscles, the hamstrings and the quadriceps cross two joints - the hip and knee - which means that they assist with the function of both, making an injury to either muscle group extremely disruptive to running. Both can be strained (or pulled) if they are overextended to the point that they rip slightly. A complete tear of the muscle is called a rupture. These injuries usually come after a burst of speed, and with a popping sound or sensation.

Overtraining, forceful stretching, excessive speedwork or speedwork without a proper warm-up can strain a muscle. Strength imbalances also pose a threat. If the fronts of your legs are stronger than the backs (this is especially common among triathletes and runners who cycle), the quadriceps can put extra strain on the hamstrings.

But even if you take great care of your hamstrings and quadriceps by regularly stretching, strengthening and massaging them, they still won't necessarily be complaint-free. That's because the source of thigh issues is often not in the legs at all. You have to go higher. "Many thigh injuries can be traced back to weak hip muscles, such as the gluteals," says Dr Brian Krabak, a sports medicine and rehabilitation specialist at Johns Hopkins University in Baltimore, USA.

The gluteus maximus and gluteus medius are the two large muscles in the buttocks that propel you forward and support the hip joint and pelvis while running. "As soon as your foot hits the ground, your glutes should fire first, followed by hamstrings and then quadriceps," says Nancy Cummings, a certified strength and conditioning specialist. "If the glutes aren't strong enough to activate, the quads and hamstrings will have to pick up the slack. This throws off the alignment and mechanics of the entire leg and can lead to knee and foot problems."

If you've ever had iliotibial band syndrome (ITBS) you've experienced this trickle-down effect - probably without realising. Runners who develop ITBS curse their knees because that's where they feel the pain. But weak glutes are often responsible. The iliotibial band runs from the pelvis down the side of the thigh to the knee, so when it's stressed, pain can radiate anywhere along its path - the glute, hip, thigh or knee.

Piriformis syndrome is what Krabak calls "a catchy phrase for a pain in the backside". The piriformis muscle lies deep in your buttocks - so hidden that many runners don't even know it's there. But if you overtrain and understretch, the piriformis can become so tight that it'll restrict leg movement and irritate the sciatic nerve (the largest nerve in the body, responsible for motor and sensory function in the legs).

Some older runners may experience hip pain due to osteoarthritis, the loss of cartilage in the joint. Others (especially women with low bone density) can be prone to stress fractures in the thigh or hip bones.

The majority of upper-leg issues in runners result from lack of strength and flexibility - perhaps helped along by a dose of overuse and/or sudden changes in training. Fortunately, most of these problems are preventable, says Krabak, by following smart training tactics, namely, listening to your body and backing off when necessary. As extra insurance, work to strengthen your quads, hamstrings and glutes.

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