Fig 1.
The experimental protocol was repeated on 4 separate days. Two days included a central fatigue task that involved a series of 10 isometric contractions of the right FDI at 40% MVC (each set of 10 contractions is denoted by one box labeled D). On two days, the fatigue task was replaced with a rest period to provide a “no-fatigue” control. On one fatigue day, and on one control day, 5-Hz stimulation was omitted to provide “no-stimulation” controls. These days are not shown in the figure, but are identical aside from omission of the dashed boxed arrow denoting 5-Hz rTMS. During the baseline and recovery periods, the following measures were made: voluntary activation assessed during maximal contractions of the right FDI (A), cortical excitability and inhibition assessed with TMS (B) and sense of effort (C). During the baseline period the order of these measures were set to prevent fatigue. During the recovery period, the order of these measures was set to minimize recovery in the interval between assessing voluntary activation and cortical excitability. During the fatigue task (and control day rest periods), single measures of cortical excitability, inhibition and maximal voluntary activation were assessed every minute (E).
Fig 2.
Paradigm for Transcranial Magnetic Stimulation.
Cortical excitability, intracortical inhibition (SICI) and intracortical facilitation (ICF) were assessed within the left (active) motor cortex and interhemispheric inhibition (IHI) was assessed from the right (resting) to the left (active) 500ms prior to a low level contraction of the right FDI (A.). Measures of cortical excitability and inhibition were made 500ms prior to rhythmic low-level isometric contractions (5% MVC) of the right FDI. Contractions were maintained for 2 seconds with 3 seconds rest between contractions to prevent neuromuscular fatigue. The frame-based software allowed static cursors to be placed at 500ms and 2.5s to be used as cues to contract and relax the right FDI. This allowed for contractions to be rhythmic and predictable in nature Magnetic stimuli are represented as downward arrows and were applied at the start of each frame. (B).
Fig 3.
Constant Force Sensation Contractions.
Sense of force was measured using a constant sense of effort contraction of the right FDI. Participants had 5 seconds to view the target force and prepare to contract and 3 seconds to reach and hold force at the 50%-MVC target. Visual feedback of force and the target were then removed and participants were instructed to continue the contraction for 30 seconds but to adjust their force as required to “make the force feel the same”. The resultant decline in force is fitted to a double exponential function (black dashed line and inset equation) yielding two rate constants (a1 and a3) that are used as a measure of sense of effort. Percent decline from the start to finish of the contraction is also used to index the amount of effort required to sustain the contraction.
Table 1.
Motor threshold was assessed at the beginning of each experiment on each day with the muscle at rest (RMT) and at rest but 500ms prior to a contraction (Pre-contraction motor threshold (PCMT)) using a single monophasic magnetic pulse from the Bistim2 stimulator over the left motor cortex.
Resting motor threshold was also assessed in the left and right primary motor cortex using the Rapid2 stimulator and are presented as average between the two hemispheres*. Motor thresholds were consistent at baseline on all 4 days. Bolded averages across the four days indicate that motor threshold was lower when assessed prior to a contraction (PCMT) compared to at rest† and higher using a biphasic magnetic stimulus from the Rapid2 stimulator††. Data are presented as mean±sd.
Table 2.
Stimulator output was adjusted at the start of each experiment to elicit unconditioned MEPs of 1mV in the left and right FDI with the muscle at rest but 500ms prior to a low-level unimanual contraction of the right FDI.
Unconditioned test MEPs of 1mV were elicited in the right FDI by applying single monophasic magnetic stimuli to the left motor cortex (M1) using a Bistim module (Test Pulse). Conditioning MEPs of 1mV were elicited in the left FDI by applying single biphasic magnetic stimuli to the right M1 to activate the interhemispheric inhibitory pathway using a Rapid2 stimulator (Conditioning Pulse). Stimulator intensities and elicited MEPs were matched across days however the stimulator output required to elicit 1mV conditioning stimuli on the Fatigue+No stim day was slightly elevated indicating reduced excitability of the ipsilateral M1 at baseline. Data are presented as mean±sd.
Table 3.
Changes in maximal force, EMG, voluntary activation, M wave, muscle twitch characteristics, and cortically evoked potentials from baseline to post-fatigue (at Tlim) prior to the application of rTMS to the supplementary motor area or no-stimulation control period.
F and p values from 2 factor repeated measures ANOVA are displayed for interaction between day (fatigue, control) and time (baseline, Tlim). In this analysis, the days with and without rTMS stimulation were collapsed (as all measures shown in this table were assessed prior to rTMS stimulation).
Fig 4.
Changes in Cortical Circuitry During Fatigue.
Voluntary activation (A) and normalized test MEP (B) declined over the course of the fatigue protocol and were significant at task failure. The ipsilateral MEP (C) did not change and intracortical facilitation (D) was signicantly reduced at task failure on the fatigue days. Intracortical (E) and interhemispheric inhibition (F) did not significantly change over the course of the contraction protocols on the fatigue and no fatigue control days. The days with and without rTMS were pooled in this analysis because rTMS was applied after these measures were made (at task failure). Data are represented at mean±S.E asterisks denote a significant difference from baseline measures when with a Fisher LSD when RM ANOVA was significant. p<0.05, **, p<0.01, ***p<0.001.
Fig 5.
Effect of 5Hz rTMS over the SMA on Recovery from Fatigue.
Maximal torque (A), maximal voluntary activation (B) and test MEP amplitude (C) were significantly reduced at task failure however rTMS to the supplementary motor area (SMA) increased the rate of recovery of maximal torque but not maximal voluntary activation or MEP amplitude. Data are presented as mean±S.E on a fatigue day where stimulation was applied to the supplementary motor area (circles) or no stimulation control day (triangles). ** indicate significant differences from baseline on both days. † indicate significant difference between days (Fishers LSD post hoc, p<0.05).
Fig 6.
Effect of 5Hz rTMS over the SMA on the Relationship between ICF and Sense of Effort Following Fatigue.
Following fatigue, and in the absence of rTMS, there was no relationship between intracortical facilitation and sense of force using the first (a1;C) or second(a3; A) rate constants of the sense of effort contraction however a strong relationship developed following high frequency rTMS of the supplementary motor area (B&D). Sense of force is displayed on the y axes using either the inverse of the first (a1) or second (a3) rate constants of the sense of effort contraction where a lower value represents increased rate of decline and greater sense of force. Intracortical facilitation is displayed on the x axis as the ratio between conditioned MEP and unconditioned MEP.