Impact of Doublet Discharges on Force Production in Young and Older Humans

Abstract

Doublet discharges, in which a motor unit fires two action potentials close together in time (inter-spike interval of < 10 ms), have been demonstrated in both animals and humans. However, the functional role of doublets is not yet fully understood in humans. PURPOSE: To determine the functional conditions favorable for the production of doublets. The impact of doublet discharges on the rate of force production was also investigated, and compared between young and older individuals. METHODS: Participants included eightyoung (21.9 ± 3.5 years) and eight older (74.1 ± 8.7 years) individuals. Motor unit activity was recorded from the tibialis anterior during isometric dorsiflexion using a four-wire needle electrode. Subjects performed isometric ramp contractions up to 50% MVC and the occurrence of doublets was identified. A subset of subjects (n=4) also performed a constant force contraction at 50% MVC. The force level and the rate of change in force were assessed 100 ms prior to the first firing of the doublet and 100 ms following the second firing of the doublet. The force and rate of change in force were compared between age groups and pre- vs post-doublet discharge. Similar comparisons were made between constant force and changing force contractions in a subset of individuals. RESULTS: During the ramp contractions the force level prior to the doublet was similar to the force level following the doublet (p=.38). The rate of change of the force was significantly higher following the firing of doublets than prior to the doublets in the older group (p=.008), but not in the young subjects (p=.74). There was a tendency for a greater number of doublets per unit time during ramp contractions (mean=0.66/s) than during constant force contractions (mean=0.14/s). The changes in the level of force and in the rate of change in force from pre-post doublet were also greater during ramp contractions than during constant force contractions, in both young and older subjects. CONCLUSION: These results suggest a role for doublets in increasing both the level and rate of force production in humans, particularly in older individuals. Although data comparing constant force and ramp contractions is limited, the evidence suggests that doublets are more likely to occur, and have a greater impact on force production during changing force conditions.