Have you ever noticed how running gives you a certain type of soreness in your muscles that activities like swimming and bicycling don’t? I sure did and I always wondered why. Through our research, I’ve come to learn that the reason is not what many people think.
It’s not the “impact” that occurs when our foot hits the ground, but rather, just a difference in how our muscles naturally work when we run (and also walk) compared to when we swim or bike.
When we think of a muscle contracting we typically picture it shortening in length. But a muscle doesn’t always have to shorten as it works – it can also lengthen. When you pick up a weight, your biceps (the muscle that bends your elbow) shortens as it works to bend your elbow. But when you put the weight back down, your biceps muscle continues to work as it lengthens, preventing the weight from crashing back down with the force of gravity.
When a muscle shortens as it works, we say that it is concentrically contracting. When a muscle lengthens as it works, we say that it is eccentrically contracting. (When the overall length of the muscle stays the same while it is active…say if you were to just hold the weight in one position…we say that the muscle is isometrically contracting.)
Back to running. In a modern-day 3-D human motion laboratory that can generate an image such as the one below, it is possible to study the position and motion of the joints in the body, along with the gravitational forces applied about the joints, simultaneously with the electrical activity emitted from the muscles (called electromyographic activity). Analyzing that data all together, and using a 3-D robotic muscle-modeling software that can quantify muscle lengths throughout a stride, it is possible to see when a muscle is concentrically contracting (shortening) versus when it’s eccentrically contracting (lengthening).
It was our ability to do this type of comprehensive study which led to a number of discoveries with respect to walking and running and the effects of different types of footwear that had never before been appreciated. Those same studies also could tell us why running and also walking makes us sore.
During both walking and running, most of the muscle activity that occurs isn’t concentric, but rather is eccentric. That is, most of the work done by each of the muscles in the leg and the foot is used to resist the weight of gravity as the muscle gradually lengthens. Only at the very end of a muscle contraction does any particular muscle in the foot or leg actually shorten. This implies that most of the muscle work in walking and running comes not from propelling ourselves forward but rather from controlling ourselves from falling, or more specifically, controlling ourselves from collapsing at each joint when our weight is borne on each foot. This is the case whether we run or walk on flat ground or on an incline or on a decline (with slightly more eccentric activity on a decline and slightly less eccentric activity on an incline).
In comparison, during swimming or bicycling, there is essentially no eccentric activity; the entire time that any particular muscle is working, it is concentrically contracting.
At a microscopic level, each muscle fiber is enveloped by a membrane or lining, called the endomysium. A family of muscle fibers, each enveloped by endomysium, are further enveloped by another larger and thicker membrane or lining, called the perimysium. Finally, a group of muscle families are further enveloped by an even larger and thicker membrane or lining called the ectromysium. The endomysium, perimysium and ectomysium together form what’s called the muscle “fascia.”
When a muscle is concentrically contracting, the muscle fibers actively shorten and all those layers of fascia surrounding the muscle fibers slacken. In contrast, when a muscle is eccentrically contracting, as the muscle fibers lengthen, so do all the fascia surrounding the muscle fibers. During walking and running both the fascia and the muscles are working together to resist the outside forces that come from our body weight. The fascial structures don’t really do any active work like the muscles do. Rather, they are just being passively stretched by the body weight force.
It’s the stretching of the fascia that makes our muscles feel sore. There are nine to ten times more sensory nerve cell endings in the fascia than there are in the muscle fibers themselves. As the muscle is being stretched (eccentric activity), the stretch in the fascia triggers the nerve cell endings, telling the brain, “I’m sore.” In contrast, in swimming or bicycling, the fascia doesn’t stretch but rather just slackens (concentric activity) such that those same nerve endings aren’t being stretched and triggered.
When you run, recognize that it’s not “impact” that makes you sore. Rather, soreness comes from the fascia in and around your muscles being stretched.
Think about how that stretching is helping you achieve and maintain flexibility in and around your muscles, which optimizes your biomechanics and posture, and ultimately helps protect your body from injury. It is important to also recognize that too much muscle soreness, i.e., soreness that you still feel 24 hours after a run, is a warning you’ve overworked your muscles and fascial structures.
That “good” sore is unique to running and walking–actions that lengthen your muscles. Understand that the soreness you get with running and walking essentially just means you’re stretching as you run or walk. Which is a very good thing.