Many people ARE lopsided in terms of muscle length or strength. You can have muscles on one side that are shorter, and/or weaker than the same muscles on the other side.
Although one may think primarily of the knees being bent, or the movement of the ankles during a frog kick, there's another important action: internal and external rotation at the hip. In plain english, if you're sitting down as you read this, first straighten your right knee. Then turn your right foot so that your toes point in: this action is really coming from your hip, and is called internal rotation. Now turn your foot so your toes point out: this is external rotation at the hip. Both movements are needed in the frog kick.
<anatomy nerd alert on>
Tensor fascia lata and the anterior part of gluteus medius (these are muscles at the front of the hip) cause internal hip rotation. Piriformis and the posterior part of gluteus medius (at the back of the hip) are responsible for external rotation. Interestingly, the sciatic nerve runs underneath piriformis muscle, and piriformis can compress the nerve leading to one cause of "sciatica".
Part of the power in the frog kick comes from snapping the feet together, and this is due to external hip rotation. Piriformis is a small muscle, and one that's not used a lot if you're sitting at a desk during the day, for example. People often have one piriformis shorter than the other, or one piriformis weaker that the other. In either case, this can cause a lopsided feeling when frog kicking.
<anatomy nerd alert off>
If you want to know if one of your piriformis muscles is shorter than the other, you'll need someone to observe the following. Lie face down on the floor, legs together. Bend your knees to 90 degrees, keeping your ankles touching. Now let your ankles move apart as far as they'll go (but keep your knees together, and bent to 90 degrees). Ask the observer (standing at your head) to see how far your feet move from the starting point. If, for example, your right foot moves farther from the upright starting point (in other words, comes closer to the floor) than the left foot, the right piriformis is longer than the left.
So, you'd stretch the left piriformis to correct this. One way to stretch left piriformis is to lie on your back, right foot on the floor, right hip and knee bent. Now bend your left hip and knee, and place your left ankle on your right thigh. Reach both hands behind your right knee, and bring your right thigh towards your torso. This will pull your left ankle and leg towards your torso also, and stretch your left piriformis.
If anyone's interested, I can continue with how to assess for strength of piriformis, and how to strenghten it at a later time.
