A Scientific Analysis of the Quads During Leg Extensions
It’s easy to become imbalanced in quadriceps development, which can result in injury or even just an odd-looking quad muscle. In a recent Journal of Strength and Conditioning Research study, researchers examined EMG data to find out how we can address this issue.
In the study, researchers analyzed the EMG data of the three superficial quadriceps muscles on a leg extension machine:
- The rectus femoris: This is the muscle on top, right in the center.
- The vastus medialis: This is the “teardrop” on the inside.
- The vastus lateralis: The vastus lateralis is on the outside part of the front of your leg.
The researchers measured the data for each rep out of a set of eight and for each third (bottom, middle, and top) of the range of motion (ROM). They also looked at what happens when you change toe angle. They compared turning the toe in, turning it out, and maintaining a neutral foot position.
Range of Motion
All three superficial muscles of the quadriceps responded differently to the ROM used.
- For the vastus lateralis, the activity increased towards the middle and end of the ROM (the top two-thirds of the leg extension).
- For the vastus medialis, the same was true, but only as the set progressed. By rep eight, the most activity was seen in the latter half of the movement.
- For the rectus femoris, there was no difference in activation at any part of the ROM.
- The rectus femoris steadily increased in activity as the set progressed from rep one to eight.
- The vastus medialis saw a decline in activity with each rep, but as noted above, this only occurred in the lower part of the ROM.
- For the vastus lateralis, there wasn’t much difference as the set progressed.
Each muscle responded differently to changes in toe position. For the vastus lateralis, the toes-in position generally outperformed the others throughout the course of the rep, but the neutral position was a little better at the top of the ROM. Toe position didn't seem to affect the vastus medialis.
The rectus femoris benefited more from the toes-out position. It was at its strongest at any part of the ROM when the toes were turned out. It makes sense that the rectus femoris would be most affected by toe position, since it crosses the joint to the hip and would be stretched or relaxed by changes in rotation.
Clinically, these results are of important use for physical therapists and doctors who can prescribe different maneuvers to preferentially select one of the heads of the quadriceps muscle. Differences in strength of each head might cause further stress in the long term. An imbalance may also cause the patella (kneecap) to get off track.
Furthermore, because the angle of the leg was measured throughout the movement, it allowed the researchers to study the shearing forces of the limbs. The most shearing occured in the final third of a full range of motion during the concentric phase, at the very top of the leg extension. For this reason, the researchers recommended leaving the top part of the ROM out during healing.
For non-clinical purposes, this information could be used to preferentially target muscles for strength and hypertrophy development. It seems the old bodybuilding myths of targeting the teardrop aren’t totally accurate, but some preferential recruitment is possible.
1. Oseph Signorile, et. al., “Range of Motion and Leg Rotation Affect Electromyography Activation Levels of the Superficial Quadriceps Muscles During Leg Extension,” Journal of Strength and Conditioning Research 2014, 28:9
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