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Many runners look forward to a downhill break after a tough climb during a run. While the aerobic cost of running downhill is lower than running uphill, the strong eccentric contractions associated with downhill running take a toll on your body that you may not feel immediately.

An eccentric contraction occurs when the muscle lengthens as it contracts, while a concentric contraction is when the muscle shortens as it contracts. For example, when doing biceps curls in the weight room, bicep muscles are contracting concentrically as you lift the weight up and eccentrically as you lower the weight. A runner’s quadriceps and calf muscles contract eccentrically to help the runner catch his body weight each time his foot strikes the ground before a concentric contraction propels him forward again. Downhill running exaggerates the eccentric contractions. Though they require less oxygen and adenosine triphosphate (ATP) than an equivalent concentric contraction, they also cause more muscle damage than concentric contractions because they recruit fewer fibers for a given amount of force.

Delayed Onset Muscle Soreness (DOMS) is a phenomenon almost every athlete has encountered at some point during his or her training. DOMS usually sets in 24-48 hours after an event or training session, which is why you wake up with sore legs the next morning despite not feeling sore immediately after the race. The exact science behind DOMS is unknown. However, muscle damage is a likely cause, and the repeated eccentric contractions a racer’s body endures on a predominantly downhill course can increase the amount of damage muscles sustain. If you plan on participating in a race with a lot of downhill, you can help your body prepare by practicing downhill running during your training. Performing one to eight intervals of two to four minutes once a week can help your body adapt to the stress of downhill running, and icing after these workouts can minimize the soreness you may have from them.