A muscle imbalance can be a lot of things. But a bilateral deficiency is just one thing, and you can test yourself to see if you have one, says Red Bull's Sports Performance Manager.
Andy Walshe, who designs workouts for the world's best surfers, BMXers, skydivers, skateboarders and other elite adventure athletes, says that a bilateral deficiency is different from a muscle imbalance.
"A muscle imbalance can be a weakness around the joint, a weakness in the antagonist muscles (opposing muscles, example: antagonist for quadriceps is hamstrings; for biceps, it's the triceps), it could be a lot of things. But a bilateral deficiency is a difference between two sides of the body in terms of strength," Walshe says.
In most cases, one arm or one leg will be much stronger than the other, as much as 60-70 percent stronger. Walshe warns, "If the problem is significant and isn't corrected, it can lead to biomechanical imbalance and overuse injuries and muscle strains."
That means that the repeated hamstring pulls, shoulder strains or knee problems suffered by many elite and pro athletes---and recreational or Masters athletes as well--- can stem from a long term, undetected bilateral deficiency. "It's common in tennis players. You see it in the very best athletes," Walshe says.
You can, and should, test yourself for the condition by doing exercises first with one limb, then the other. For example, do biceps curls to exhaustion with one arm, counting each rep. Then test the other arm. Walsh explains that there will always be some difference. "Most humans have a general deficiency when you compare left to right side, depending on whether they have a right or left hand dominance. But it's only about ten percent of a difference. When it's more than ten percent, that's a bilateral deficiency," he says.
There are numerous testing exercises you can use. A one-legged squat will test your glutes and quadriceps. To do it, use a cable machine and a portable step or platform. Put the step between the cables. Put the cable down at the lowest setting. Use half your regular resistance weight. Put one foot close to the edge of the step, with the other foot hanging off the side. Pulling the cable, start doing squats on the one leg, being careful not to let the knee flex in front of the toes. Turn and repeat with the other leg. Work in front of a mirror when testing to be sure you aren't leaning to one side or hunching instead of holding your torso straight.
If you find a bilateral deficiency, Walshe says to work on correcting it. "You can work the weaker muscle with exercises so that you train it to be within a ten percent strength difference of the stronger side, since that amount is not significant," he advises.
But, overcoming the muscle memory of a bilateral deficiency will be hard. Once you have corrected it, you will need to keep testing and training the weaker side, probably for many years.
The equalization of your muscle strength will help prevent nagging injuries that keep you away from the activities you enjoy. Even better, you will be a balanced---and thus better---athlete.