MOTOR UNIT DISCHARGE BEHAVIOR DURING 10% MVC ISOMETRIC CONTRACTIONS FOLLOWING RESISTANCE EXERCISE TRAINING IN YOUNG AND OLDER ADULTS

Abstract

We sought to determine whether motor unit firing rates, recorded during low force isometric contractions in young and older adults, would be altered by a resistance training program. Eight young (mean = 21.4 yrs) and seven older (mean = 77.0 yrs) adults attended the laboratory for assessment of maximal knee extensor torque and discharge rate on each of four separate occasions. The first two test sessions were spaced one week apart. Progressive resistance exercise training using both isometric and dynamic 10-RM contractions began immediately following the second test session. Subjects re-visited the laboratory for follow-up testing after two- and six weeks of resistance exercise training. A four-wire needle electrode inserted in the vastus lateralis, was used to record motor unit activity while the subject maintained a 10% MVC isometric contraction. The signals were amplified, digitized (51,200 samples/s) and stored for later identification of individual motor unit action potential trains from the resultant multi-channel recordings. The exercise training produced similar, significant (p < .001) increases in knee extensor torque in both subject groups, with an average torque gain of 34%. Little difference in motor unit discharge rate at 10% MVC was noted between young (7.1 imp/s) and older (6.7 imp/s) subject groups during 90-s contractions. The peak discharge rate during this interval was 9.4 imp/s for young, 9.0 imp/s for older adults. Motor unit discharge rates were stable during exercise training, with little change evident in either group. These data are in contrast to the results presented earlier for maximal voluntary force contractions, and suggest that motor unit discharge rate adaptations are specific to tasks requiring high levels of muscular force.