Tom Osler, a top ultrarunner in the 60s who later became a math professor and author of The Serious Runner, conducted a comprehensive study of runners and found that for every extra pound you carry, you will be 2.5 seconds per mile slower. Dr. Steve Gangemi aka “Sock Doc,” examines the biomechanical relationship between excess body weight and gait.
Let’s face it, if you’re a big person, you’re going to be slower than someone smaller, provided your fitness levels are identical. But studies don’t take into account where the weight is and how it’s actually affecting your body. For example, huge thighs on a runner is most likely going to slow him or her down more than someone with  big upper arms. Excess fat is going to slow one down more than more muscle; it’s towing some extra weight and not giving anything back in return. Excess weight in certain areas can definitely affect gait.

Let’s take the common large belly found on many people, even some runners. (Usually due to eating too many unhealthy carbs.) That excess weight on the front of the body is going to cause the body to compensate so the runner is more sway-back, putting more pressure on the lumbar spine. That, in turn, will put more stress on the back of the knees, hamstrings, calves, and create more of a heel-strike landing. So the gut can result in symptoms that are ultimately diagnosed as plantar fasciitis, heel spurs,and tendonitis. Further compensations can develop after that. In this example. the extra stress on the calf muscles in the back of the lower leg (the gastrocnemius and soleus) will then result in more of a workload being distributed to the front of the lower leg (the tibialis anterior primarily) and next thing ya’ know, shin splints appear. Take out the Dixie cups and ice them down! – That was the primitive treatment we used during high school cross country and it’s still being used today. Maybe in this case, the belly causing the gait imbalance should be getting the paper-cup service.

You actually don’t need much to change gait. Gait is a continuous pattern of facilitation and inhibition. You take a step forward with your right leg, the muscles in the front of your thigh (the rectus femoris) should neurologically facilitate – that means they should “fire” or “turn-on” more than their antagonist, or opposing muscles. In this case, that would be the muscles in the back of the thigh (the hamstrings); they should normally turn themselves down somewhat, or become neurologically inhibited. When muscles don’t work in harmony, problems arise.
So which runner is more susceptible to an injury? – the one who is overfat? Overweight? Too skinny? Too muscular? There really is no specific answer because it all comes down to individuality and what is exactly going on with the health and fitness of that person in regards to their weight – whether it is fat or muscle, or lack thereof.

Runners are going to be handling their weight throughout their entire run differently than most other sports or activities. Take swimming for example. Fat is buoyant in water, so a great swimmer doesn’t need to be as lean and muscular as a runner. Sure the swimmer doesn’t want to have too much body fat, except for, say,  crossing the English Channel, since the fat will act as insulation in the cold water. Or, a heavier cyclist will have an advantage on descents, but he will get smoked on ascents by smaller and lighter riders.

The gait is also going to be significantly affected by the type of fuel a runner is burning, even more than by additional weight. Anaerobic running (done at a high heart rate using primarily sugar (glucose) for fuel) will result in many more muscle imbalances long before the runner who is aerobic (lower heart rate burning more fat for fuel). The muscles throughout the entire body have such as strong connection to body chemistry and changes in glucose metabolism that they will soon fatigue when their glycogen levels are used up as a result of overtraining and in racing. Then, the gait changes. The feet land differently – they kick out or land with a harder strike. The center of gravity shifts. The legs get heavier. Body weight, no matter fat or muscle, or whether in the hips, glutes, or belly now takes its toll on the tiring runner.

This article originally appeared on Zero Drop.