Repetitive transcranial magnetic stimulation attenuates the perception of force output production in non-exercised hand muscles after unilateral exercise

Abstract

We examined whether unilateral exercise creates perception bias in the non-exercised limb and ascertained whether rTMS applied to the primary motor cortex (M1) interferes with this perception. All participants completed 4 interventions: 1) 15-min learning period of intermittent isometric contractions at 35% MVC with the trained hand (EX), 2) 15-min learning period of intermittent isometric contractions at 35% MVC with the trained hand whilst receiving rTMS over the contralateral M1 (rTMS+EX); 3) 15-min of rTMS over the 'trained' M1 (rTMS) and 4) 15-min rest (Rest). Pre and post-interventions, the error of force output production, the perception of effort (RPE), motor evoked potentials (MEPs) and compound muscle action potentials (CMAPs) were measured in both hands. EX did not alter the error of force output production in the trained hand (Δ3%; P>0.05); however, the error of force output production was reduced in the untrained hand (Δ12%; P<0.05). rTMS+EX and rTMS alone did not show an attenuation in the error of force output production in either hand. EX increased RPE in the trained hand (9.1±0.5 vs. 11.3±0.7; P<0.01) but not the untrained hand (8.8±0.6 vs. 9.2±0.6; P>0.05). RPE was significantly higher after rTMS+EX in the trained hand (9.2±0.5 vs. 10.7±0.7; P<0.01) but ratings were unchanged in the untrained hand (8.5±0.6 vs. 9.2±0.5; P>0.05). The novel finding was that exercise alone reduced the error in force output production by over a third in the untrained hand. Further, when exercise was combined with rTMS the transfer of force perception was attenuated. These data suggest that the contralateral M1 of the trained hand might, in part, play an essential role for the transfer of force perception to the untrained hand.

Figure 1. The experimental protocol.

Motor evoked potentials (MEP), F-waves, maximal M-waves (M_max), perceived force error and ratings of perceived exertion (RPE) were recorded before and after each intervention in the trained and untrained hands. The order of interventions was randomised and performed on separate days. Each of the interventions, performed with the ‘trained hand’, lasted for 15 mins and during this time the ‘untrained hand’ remained at rest. Within 5 minutes after each intervention post responses were acquired. EX –15 min of force training with the trained hand; rTMS+EX –15 min of force training and rTMS over M1 region of the trained hand performed concurrently; rTMS –15 min of rTMS over M1 region corresponding to the trained hand; Rest – no intervention.

Figure 2. The error in force output production after the fifteen minute interventions (EX –15 min of force training with the trained hand; rTMS+EX –15 min of force training and rTMS over M1 region of the trained hand performed concurrently; rTMS –15 min of rTMS over M1 region corresponding to the trained hand; Rest – no intervention) in both the trained (open bars) and untrained (closed bars) hands.

Data are means ± SE for 13 participants. * - P<0.05 trained vs. untrained hand.

Figure 3. Changes in ratings of perceived exertion (RPE) in the trained (A) and untrained (B) hand before and after the 15 min interventions (EX –15 min of force training with the trained hand; rTMS+Ex –15 min of force training and rTMS over M1 region of the trained hand performed concurrently; rTMS –15 min of rTMS over M1 region corresponding to the trained hand; Rest – no intervention).

Data are means ± SE for 13 participants. * - P<0.05 pre vs. post; $ - P<0.05 vs. rTMS post; # - P<0.01 vs. Rest post; ^∧ - P<0.05 vs. trained hand at the same time point.

Figure 4. Raw force and EMG traces while a performed the EX intervention.

EMG activity shown was recorded from the FPB muscle. Note, during the 35% contractions with the trained hand, no EMG activity was recorded in the untrained hand.

Figure 5. Motor evoked potentials (MEP) evoked in the flexor pollicis brevis (FPB; A and B) and flexor digitorum superficialis (FDS; C and D) muscles of the trained (A and C) and untrained hand (B and D) before and after the 15 min interventions.

Data are means ± SE.

Figure 6. Maximal M-waves (M_max) evoked in the flexor pollicis brevis (FPB) of the trained (A) and untrained (B) hands before and after the 15 min interventions.

Data are means ± SE.