Written by: Matt Fitzgerald
The most famous feature of the Boston Marathon racecourse is Heartbreak Hill, a climb of 88 feet over 0.6 miles that begins just past the 20-mile mark of the race. But the most challenging feature of the course is probably its extensive downhill sections, including a 150-foot plummet in the first mile.
While running downhill generally feels easier, and is less taxing on the metabolic and cardiovascular systems then level or uphill running, it subjects the tissues of the lower extremities to significantly more strain. This strain causes muscle damage, soreness, and neuromuscular fatigue that can cripple runners later in the race. Runners who enter the Boston Marathon, or another marathon that has extensive downhill sections, without specific preparation for this challenge are often shocked by the toll exacted by the descents, and seldom run as well as they hope or expect to run.
The specific nature of the strain imposed by downhill running is called eccentric loading. An eccentric load occurs when an outside force tries to stretch a muscle as the muscle itself tries to resist that stretch by contracting. Eccentric loading takes place every time your foot makes contact with the ground when you run. The quadriceps muscles on the front of the thigh are subjected to the greatest eccentric loading. When your foot strikes the ground, impact forces try to make your knee buckle. Unconsciously, you contract your quadriceps to stabilize your knee and remain upright. But your knee does flex and your quads do stretch a bit when you land, so those muscles are essentially pulled in two directions simultaneously. This strain causes microscopic trauma to the muscle fibers. While eccentric loading occurs on all gradients, it is much greater when you are running downhill.
There are two consequences of the strain of running downhill. First, it limits performance and causes fatigue to occur more quickly in runners (and triathletes) who are unaccustomed to downhill running. The second consequence of the strain associated with running downhill is delayed-onset muscle soreness (DOMS). Indeed, when exercise physiologists wish to study exercise-induced muscle damage and DOMS, they almost always use downhill running to cause damage and soreness, because it does so more effectively than most other kinds of exercise.
Fortunately, practicing downhill running greatly increases fatigue resistance and eccentric loading tolerance in subsequent downhill runs. In fact, a single downhill run that is extreme enough to cause significant soreness provides a protective effect that lasts up to two months. However, it takes more than that to get the benefit that triathletes training for events featuring run courses with significant downhill sections want most, which is greater fatigue resistance on descents. This is probably because the body needs more exposure to downhill running to improve downhill running efficiency and to overcome the neuromuscular inhibition that limits downhill running performance.
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