First, let’s address the arm numbness. If your arm only numbs up while you’re running, there are two likely causes: poor posture, and the way you carry your equipment. Here's why, and what to do.
Fix Your Form
“There are little tiny blood vessels that go to the muscles in your arms,” says Dr. Bob Adams, chair of USA Track and Field’s Sports Medicine and Science Committee. “When the muscles in your neck, upper back, and arms are in a tense position, they clamp down on those blood vessels and can give you cramping, pain, and numbness.”
If that’s the case, adjusting your posture can fix the problem. “Ideal running form is pretty much upright, with a very slight lean forward,” Adams says. “The pelvis should be horizontal, maybe tilted 10-degrees forward from the trunk, and the hands and shoulders should be really relaxed.” Your elbow should make a 90 to 100-degree angle between your bicep and forearm, and your forearm should make about a 60-degree angle with your torso.
“A lot of people run with their bodies tensed, and their arms swinging across their bodies,” Adams says. The angles described above should feel more natural and relaxed than a cross-body swing.
Sit Up Straight
Pay attention to your posture at work as well. If you hunch over at a desk all day, Adams warns, your neck, shoulders, and arms could already be tense before you hit the pavement, upping your chances of getting numb.
Before you run, try loosening up your upper body by lying on a foam roller placed along your spine, then moving your arms up and down 30 times or so, like you’re making a snow angel. “This’ll get blood flowing through there, and get your shoulders relaxed,” Adams says.
Go for the Fanny Pack
Carrying anything in your hands, around your arm, or in a pack with shoulder straps can also cause numbness, Adams says. Holding a water bottle, for instance, will make you tense your arm muscles as you run. A band around your arm can impede circulation, as can the straps of a hydration pack. If you do any of these things, try carrying whatever you need around your waist instead. You may find the numbness will clear up with that simple fix alone.
If it persists, however, or you’re worried about it, see your doctor. Numbness can also be a sign of several other issues, including a pinched nerve, arthritis, and carpal tunnel syndrome.
As for cramping in general, recent research suggests it may not have much to do with dehydration or an electrolyte imbalance. Instead, fatigue and muscle damage might be to blame.
Bottom line: A relaxed upper body should not go numb. Avoid carrying things in your hands, or on your arms, back, or shoulders, and work on your form to avoid numbing out.