Hold a plank long enough, and your muscles will start to quiver. (Suprijono Suharjoto/Stocksy)
If you’ve ever tried to hold a plank as long as you can, you know that something strange happens as you push past your comfort zone: Your muscles start to tremble like a dog afraid of a lightning storm. But shakes are just one in a long list of bizarre and puzzling bodily reactions that can happen during physical activity. It seems there’s a whole host of problems that only creep up when you start to sweat, from numb feet, to blurry vision, to itchy skin.
Curious what’s going on with those random itches, twitches, and more? Yahoo Health consulted with top experts to discover the reasons behind these body oddities.
One disclaimer: Most of these problems are nothing to worry about, our sources stress. But there are some instances where you should seek medical attention, which we’ve outlined below. When in doubt, check with your doctor or a physical therapist.
Now, let’s scratch our curiosity, shall we?
What happens: Your muscles shake during planks and barre classes.
Why it happens: If you’ve ever had a barre instructor say that shaking muscles tell you the move is working, that’s not that far off: The shuddering sensation is a normal result of muscle fatigue, says exercise scientist Len Kravitz, PhD, of the University of New Mexico.
But why does your body quiver instead of, say, collapsing all at once, when your muscles are tired? It has to do with the relationship between your nerve cells and muscle fibers. Challenging exercise depletes the chemical messengers that carry the signals between nerves and muscle cells. This causes some of the nerves and their corresponding fibers to drop out of service, Kravitz tells Yahoo Health. And since your cells don’t all fire at once — some are contracting as others are relaxing — your body shakes like a car sputtering on a low gas tank.
Exercises such as planks are especially likely to trigger the trembles because your muscles must generate a lot of force to hold your body in one position, explains Alice Holland, DPT, director of Stride Strong Physical Therapy in Portland, Oregon. Holding a plank, squat, or a dumbbell at the midway point of a bicep curl — common elements of barre workouts — are especially difficult because you’re fighting gravity in addition to maintaining one position, Holland says.
Like with any type of physical activity, your body will adapt to the challenge if you perform these exercises consistently, so your muscles will shake less as you become more fit, says Linda S. Pescatello, PhD, a kinesiology professor with the University of Connecticut. Just don’t overdo it, she cautions. “Muscle shakes are benign unless you continue to subject yourself to pretty intense resistance exercise without taking rest days.”
Dehydration can also contribute to shaking, Kravitz adds. “Muscle proteins contract in a fluid environment, and sometimes if a person is dehydrated it will disrupt the muscle contraction signals,” he tells Yahoo Health. “So make sure you are hydrated when you exercise.”
What happens: Your feet go numb when you run or use the elliptical.
Why it happens: There’s a nerve that runs along the top of your foot, Holland tells Yahoo Health, which can get pinched when you exercise with too-tight shoelaces.
The fix: Skip a loop or two in the middle when lacing your shoes to give the top of your feet some extra room. And before you exercise, stretch your feet so that the nerve has a little more room to travel without impingement.
Holland suggests these two stretches:
• Calf stretch: Stand facing a wall within arm’s reach. Step back with your right foot and pin your heel to the ground. Drive your left knee toward the wall until you feel a stretch in your right calf. Hold for 20 to 30 seconds, and repeat on the other side.
• Foot stretch: Sit in a chair in socks or bare feet. Bend your foot so that the tops of your toes touch the floor. Gently press into the floor enough to feel tension in the top of your foot. Stretch each foot for 20 to 30 seconds.
Related: 5 Expert Tips for Winter Running
See a physical therapist, however, if your feet are numb even when you’re standing, walking, or sitting, Holland cautions; these symptoms might signal a herniated disc in your back.
What happens: Your skin itches when you exercise.
Why it happens: You’re probably familiar with histamine, the body chemicals that make you sneezy, itchy, and generally miserable during hay fever season. These same compounds play a role in exercise, too, helping the body exchange oxygen and carbon dioxide. When your blood starts pumping, your body releases histamine, and you start to itch. “Then we sweat, or worse, we scratch, and the whole thing gets worse because our body just releases more histamine,” says dermatologist Ruth Tedaldi, founder of Dermatology Partners, Inc. Wear loose clothing, and resist the urge to scratch, she tells Yahoo Health.
What happens: You get headaches when you work out.
Why they happen: A grueling interval workout or a long cardio session can give you sore muscles, but a sore head? It’s more common than you might think. In a study of 4,000 cyclists published in the journal Headache, 37 percent of people surveyed reported having at least one exercise-induced headache per month. Exercise dilates the blood vessels in your head, which may trigger headaches by exciting branches of the trigeminal nerve — the nerve in the brain that carries painful sensations from within the skull outward, explains neurologist Lawrence C. Newman, MD, president of the American Headache Society and director of the Headache Institute at Mount Sinai St. Luke’s Roosevelt.
Most exercise-induced headaches are innocuous, but some are symptoms of more serious problems. How can you tell the difference? The harmless kind typically affects both sides of the head and lasts between five minutes and one day, Newman tells Yahoo Health. “The benign form is more common when exercising in the heat and humidity, in high-altitude locations, and in people with a history of migraines,” he says. But if the pain lasts for several days or is accompanied by vomiting, numbness, double vision, or loss of consciousness, stop your workout and see a doctor.
If your pounding head only happens when you’re pumping iron, your posture might be the culprit. “Rounding the shoulders forward can cause excessive muscle contractions in the neck, which can cause spasms and trigger a headache,” Holland says. Be sure to keep your shoulders back and your posture tall.
What happens: Your hands swell up during or after exercise.
Why it happens: Most of the time, swelling in your hands or feet is simply a result of increased blood flow. “It’s not uncommon for feet to swell after a day of shopping, for instance, and the hands kind of do the same thing,” says Holland. If your veins aren’t effectively returning the blood in your body to your heart and lungs, the blood will stay in your hands and cause more prolonged swelling.
To know if it might be cause for concern, try this test: Push into the swollen area with your thumb. If the indentation lasts for more than one second, it’s a sign that the flow of blood and fluids beneath your skin is poor. It’s not an emergency, Holland says, but mention it to your doctor.
Less commonly, extreme hand swelling and redness during exercise in cold temperatures is caused by Raynaud’s disease. “It’s not just like ‘Oh, my fingertips feel pinkish and cold,’ it’s severe, tomato red-colored fingers,” Holland explains. Fortunately, the condition is benign, and most people can easily avoid the symptoms by wearing gloves during chilly workouts, she says.
What happens: Your vision goes blurry, starry, black, or white when you exert yourself.
Why it happens: It depends on your specific symptoms. Here’s what’s normal: Seeing spots during heavy full-body weightlifting moves, such as deadlifts or cleans. Going from a low to a high body position causes blood pressure changes and forces your heart to pump more blood to the brain, Holland explains. You might see stars for a second while that shift occurs. It’s the same thing that happens when you stand after sitting for a long time, and is generally harmless.
What’s not normal: If your vision goes completely white or black, especially if it’s accompanied by other symptoms. “If you have vision issues, dizziness, or fainting, that’s one of the signs that you need to go see your doctor,” Pescatello tells Yahoo Health. “I wouldn’t advise keeping up with your exercise program until you do so.”