Many people have noticed at some point in their lives that one of their arms or legs is larger than the other. In fact, this phenomenon is probably more common than we realize. In a recent study in the Journal of Strength and Conditioning Research, investigators looked at how this inequity in muscle affects strength and power.


There are a few reasons why an athlete may experience asymmetrical lean mass. The first and most obvious reason is laterality. Laterality is another way of saying handedness or footedness, which is the preference that most people have to perform physical tasks with one side of their body. For the average right-handed person, the right side of their body tends to be both stronger and larger, and the opposite for left-handed people.



Breaking Muscle Shop

It’s also not uncommon for people to have limbs of different lengths. For example, someone could have one femur (the large bone of the upper leg) that was longer than the other. Also, if one foot were larger than the other, it may turn out and flatten more, and could effectively make that same leg shorter. Like laterality, differently sized limbs can create preferences and imbalances that lead to a muscle being larger than its counterpart on the other side of your body.


In the study, researchers measured the muscle masses in various parts of the leg and pelvis of 167 Division I athletes. They then came up with a percentage of asymmetry to compare to the force and power profiles of the same athletes when they jumped. The researchers used what’s called a force plate, which measures the energy an athlete puts into the floor during a jump. Finally, they also measured the jumping height of each athlete.


The researchers learned that asymmetry of muscle development correlated to asymmetry in force and power. Asymmetries of the thigh and shank (the part of your leg between knee and ankle) could explain about twenty percent of force asymmetries. Similarly, imbalance in the pelvis, thigh, and shank together explained 25% of power asymmetry. As a result of muscle assymetry, jump height was reduced by a power asymmetry of ten percent or greater - an average of about 3.5 inches.


You might think these results don’t apply to you, but they certainly may. About five percent of the studied athletes fell into a range of power asymmetry that affected performance, so it wasn’t uncommon. Muscular asymmetry might also be masked by athletic ability. One of the best jumpers in the study had a power asymmetry of fifteen percent. Although he was one of the best, the imbalance may prevent that athlete from becoming even better.


Ensuring that power levels are similar on each side of your body is important for maximum performance in athletics. Maintaining strength on both sides goes a long way in staying healthy and strong.  



1. David Bell, et. al., “Lean Mass Asymmetry Influences Force and Power Asymmetry During Jumping in Collegiate Athletes,Journal of Strength and Conditioning Research, 28(4), 2014


Photo courtesy of the Journal of Strength and Conditioning Research.

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