The feel of one’s heart pounding in their chest and their shortness of breath at the top of a hill attest to what hills provide the cardiorespiratory system. But they also provide a great workout for skeletal muscles. Hill training increases leg muscle power, can be used as a transition into more formal speed work, improves the performance of the heart since heart rate can easily climb up to its maximum when running up a hill, and uses the muscles of the legs, arms, and trunk in ways that are different from running over flat terrain. Since hill running uncouples the effort from the speed (i.e., the speed is relatively slow even though it’s hard work), the exact pace is not as important as the effort. When running hills, the runner should aim for a specific effort rather than a specific speed.

Even though running uphill seems harder because the heart must do more work to send blood and oxygen to the muscle mass that is working against gravity, downhill running causes the biggest problems. If someone has ever run a race with long or steep downhills, he knows what running downhill can do to his legs. The reason downhills are so tough is because of all the gravity-induced eccentric muscle contractions, during which the muscle fibers are forced to lengthen, causing them to tear. Eccentric contractions are also unique in that fewer muscle fibers are active compared to concentric or isometric muscle contractions, causing the force generated to be distributed over a smaller area of muscle. A greater force over a smaller area equals greater tension, which causes even more damage.

The forces of impact and braking are also greater during downhill running compared to uphill and flat running. Therefore, running downhill carries a greater risk of overuse injury compared to uphill or flat running. The muscle damage decreases the muscles’ ability to produce force, which slows the pace on the flat and uphill portions of the race and leads to delayed-onset muscle soreness, which includes an inflammatory response and lasts for a few days following the race as the muscle fibers heal. The good news is that damaging muscle fibers with eccentric contractions makes them heal back stronger, protecting them from future damage. While a runner can expect their muscles to be sore after the first-time running downhill, subsequent downhill workouts will cause less soreness since running downhill has a prophylactic effect on muscle damage and soreness.