Active Voice: Fight Between Your Muscles – Beat Common Drive for Steady Cocontraction
By Minoru Shinohara, Ph.D., FACSM
Viewpoints presented in SMB commentaries reflect opinions of the authors and do not necessarily reflect positions or policies of ACSM.
Minoru “Shino” Shinohara, Ph.D., FACSM, is an associate professor in biological sciences and director of the Human Neuromuscular Physiology Lab at the Georgia Institute of Technology in Atlanta, Georgia. He studies neurophysiological mechanisms underlying motor control by analyzing electromyograms (EMG), including the analysis of motor unit discharges, surface EMG and evoked EMG. He has been a member of ACSM since 1989, was a recipient of the ACSM Visiting Scholar Award in 2009 and has been an associate editor for Medicine & Science in Sports & Exercise® (MSSE) since 2010.
This commentary presents Dr. Shinohara’s views on the topic of a research article he and a colleague jointly authored. Their article appeared in last month’s issue of MSSE.
Why do some people have better motor skills than others – even in tasks that simply involve contraction of antagonistic muscles to achieve steady motor control? With aging, you not only lose muscle volume and strength, but also steadiness of movement. For instance, as an older person, you would find it more difficult to stabilize posture in activities such as yoga and tai chi. Athletes train in stabilizing themselves in gymnastics, against their opponent in judo, and with the equipment in archery. Various clinical populations suffer from instability. Unfortunately, we scientists do not know what neurophysiological mechanisms determine steady control of muscle contraction for stabilization and its improvement.
For gaining stability, you would contract both agonist and antagonist muscles at the same time (i.e., cocontraction). You may think you can stabilize more effectively if you better synchronize the contractions between antagonistic muscles. Note that your brain naturally sends associated motor drive to multiple muscles via slow in-phase oscillations called “common drive.” As presented in our September 2017 MSSE research report, we aimed to find out if this common drive phenomenon helps steady the control of muscle cocontraction.
We recruited 60 healthy young adults and tested their performance in a difficult steady cocontraction task involving selected elbow flexor and extensor muscles. All subjects watched real-time recordings (from surface signals) of their muscle EMG activity and an unbalanced LOW/HIGH or HIGH/LOW target pair (in these trials, 4 percent/12 percent or 12 percent/4 percent of maximal EMG) for the biceps/triceps pair. Subjects had their right arm fixed with the elbow joint angle at 90 degrees and with surface EMG electrodes attached to the biceps and triceps. EMG signals moved up and down on a monitor in response to the instantaneous activity of the corresponding muscle. They tried to maintain their muscle activities at the respective unbalanced targets — as close and steady as possible. Subjects struggled in producing unbalanced cocontraction while naturally sending common drive.
We quantified common drive by measuring the associated slow oscillations in rectified EMG (less than three cycles per second) between the muscles. Interestingly, subjects with greater common drive performed worse in steady cocontraction – they exhibited greater error and variability than subjects with lower common drive. You would be better at steady cocontraction if you could suppress common drive and dissociate muscles. If you have difficulty in stabilizing yourself, your opponent or your equipment in the right position, your brain may be producing too much common drive without your intention.
We also examined whether one bout of “antagonist dissociation” practice can attenuate common drive and improve steady cocontraction. We split the subjects into cocontraction, contraction and control groups. After the first steady cocontraction test, the cocontraction group practiced out-of-phase cocontraction by alternating between LOW/HIGH and HIGH/LOW target pairs, back and forth, repeatedly. The contraction group practiced only one muscle at a time without cocontraction. The control group just rested. After an hour of practice or resting, all subjects performed the steady cocontraction test again. Surprisingly, we saw comparable improvements across groups – just due to test repetition. Against our expectation, we saw no specific effect of the dissociation practice on common drive or steady cocontraction performance.
Common drive appears to be strongly embedded in our nervous system, and we may not be able to modulate it with just a bout of practice. Well, that may be why we are supposed to keep training for a longer term to achieve better stability in yoga, gymnastics and rehabilitation – until we invent a better strategy.