Post-exercise depression following submaximal and maximal isometric voluntary contraction

Abstract

It is well known that corticomotor excitability is altered during the post-exercise depression following fatigue within the primary motor cortex (M1). However, it is currently unknown whether corticomotor reorganization following muscle fatigue differs between magnitudes of force and whether corticomotor reorganization occurs measured with transcranial magnetic stimulation (TMS). Fifteen young healthy adults (age 23.8±1.4, 8 females) participated in a within-subjects, repeated measures design study, where they underwent three testing sessions separated by one-week each. Subjects performed separate sessions of each: low-force isometric contraction (30% maximal voluntary contraction [MVC]), high-force isometric contraction (95% MVC) of the first dorsal interosseous (FDI) muscle until self-perceived exhaustion, as well as one session of a 30-min rest as a control. We examined changes in corticomotor map area, excitability and location of the FDI representation in and around M1 using TMS. The main finding was that following low-force, but not high-force fatigue (HFF) corticomotor map area and excitability reduced [by 3cm(2) (t(14)=-2.94, p=0.01) and 56% respectively t(14)=-4.01, p<0.001)]. Additionally, the region of corticomotor excitability shifted posteriorly (6.4±2.5mm) (t(14)=-6.33, p=.019). Corticomotor output became less excitable particularly in regions adjoining M1. Overall, post-exercise depression is present in low-force, but not for HFF. Further, low-force fatigue (LFF) results in a posterior shift in corticomotor output. These changes may be indicative of increased sensory feedback from the somatosensory cortex during the recovery phase of fatigue.

Keywords: fatigue; healthy; isometric contraction; post-exercise depression; primary motor cortex; transcranial magnetic stimulation.

Figure 1

Experimental design. For each TMS session, subjects first underwent TMS mapping of the FDI. This mapping was followed by a thirty minute rest period. Following the rest period, subjects underwent either protocol 1 (low-force fatigue) or protocol 2 (high-force fatigue). For the low-force protocol, subjects maintained contraction of the FDI at 30% maximal force of contraction. For the high-force protocol, subjects maintained contraction of the FDI at 95% maximal force of contraction. After the fatigue protocol, subjects immediately underwent a second TMS motor mapping of the FDI. For the no fatigue protocol, no contraction was performed in order to ensure same day reliability of TMS outcomes. *Only 30 minutes of rest was given for protocol 3, where motor mapping was immediately followed.

Figure 2

a) Subjects accurately maintained 30% and 95% of their MVC throughout the duration of the fatiguing protocol. b) Subjects were able to maintain low force fatigue (LFF) for a significantly longer time than high force fatigue (HFF). c) Example EMG during LFF indicating the 1^st and last 10^th, where the RMS was taken in order to analyze increase in EMG activation during fatigue. d) There was a significant increase in EMG amplitude throughout the low-force fatigue protocol but not the high-force fatigue protocol. Initially there was no difference between LFF and HFF; however, 12 out of 15 subjects only increased their EMG by 8.4±26.7% during the high force condition, where the remaining 3 increased their EMG by 127±18% %. Interestingly, the 3 outliers did not demonstrate the longest duration of high-fatigue out of the 15 subjects. The figure includes all 15 subjects. e) Example median power frequency (MDF) analysis for one subject during the LFF. f) There was a significant difference pre to post EMG in the decrease of MDF for both LFF and HFF; however there was no difference between the low-force fatigue and the high-force fatigue protocol. ‡ indicates a significant difference pre-to-post fatigue. * indicated significant difference between task conditions. Error bars indicated SEs. p < .05

Figure 3

a) Map area count significantly decreased after the low force fatigue (LFF), but not after high force (HFF) or control no fatigue (NF). Map area count decreased in 12 out of 15 patients during low force fatigue and 7 out of 15 patients after high force fatigue. b) Representative map area count between the low force fatigue day and the high force fatigue day. A – Anterior, P = Posterior, L = Left, R = Right. Error bars indicated SEs. *P < .05

Figure 4

a) Total map volume decreased after the low force fatigue day but not after high force. b) There was no decrease in map volume in the least excitable regions of the map after low force fatigue (<50% normalized motor evoked potential) c), but there was a significant decrease in map volume from the most excitable regions (>50% normalized motor evoked potentials). d) Representation of the total map before and after LFF in a representative subject. Hotter colors represent more excitable points where colder colors represent least excitable points. Error bars indicate SEs. *p < .05.

Figure 5

Location of MEP_maxima. There was a significant posterior shift of the MEP_maxima following low-force fatigue but not following no fatigue and high-force fatigue. Error bars indicate SEs. * p < .05.