We read with interest the study by Krishnan and Williams (2) on sex differences in quadriceps and hamstrings EMG-moment relationships. They observed that females exhibited higher antagonist muscle activity than males during open kinetic chain quadriceps and hamstrings muscle activation. We are concerned that these findings were made using surface EMG electrodes and run contrary to what is normally seen during needle electromyography.
The performance of diagnostic needle electromyography starts with the evaluation of a muscle completely at rest to assess for abnormal spontaneous activity. As often as not, one or more motor units can be seen firing spontaneously. To completely relax the muscle and eliminate all spontaneously firing motor units, a commonly used technique is activation of the antagonist muscle (1,3). Thus, for example, if the needle is in the biceps brachii, the patient is asked to gently activate the triceps by extending the elbow and this eliminates all activity in the biceps. This technique also applies to the hamstrings and quadriceps for open kinetic chain muscle activation.
We suspect that the EMG activity Krishnan and Williams thought arose from underlying antagonist muscles actually came from agonist muscles farther away, a phenomenon known as cross talk.
Furthermore, because females are generally smaller than males (as was presumably the case in their study), then the distance between recording electrodes located over the antagonist muscles and the actual source EMG signal in agonist muscles would be less, resulting in higher amplitudes and the impression that females had greater antagonist muscle activity.
We are unaware of any studies using needle or wire electrodes that show that the hamstrings are at all active during open kinetic chain knee extension in normal individuals. However, in individuals with upper motor neuron disorders such as cerebral palsy, agonists and antagonists cocontract during open kinetic chain activities resulting in spastic movement patterns.
As for females having higher early vastus medialis recruitment in response to increasing moment, we think this is an important finding that is related to but not the cause of their greater anterior cruciate ligament injury risk. Because of their higher Q-angles, females likely recruit their vastus medialis earlier and more to counteract the greater lateral component of their quadriceps vector, which might otherwise produce excessive lateral patellar tilt at high and subluxation at low flexion angles.
MARKO BODOR, MD
CHRISTOPHER R. MARTIN, MD
Synergy Medical Fitness Program
Queen of the Valley Medical Center
1. Daube JR, Rubin DI. Needle electromyography. Muscle Nerve
2. Krishnan C, Williams GN. Sex differences in quadriceps and hamstrings EMG-moment relationships. Med Sci Sports Exerc
3. Terebuh BM, Johnson EW. The electrodiagnostic consultation including EMG examination. In: Johnson EW, Pease WS, editors. Practical Electromyography
. 3rd ed. Baltimore (MD): Williams & Wilkins; 1997.