Transcranial magnetic stimulation intensity affects exercise-induced changes in corticomotoneuronal excitability and inhibition and voluntary activation.

Transcranial magnetic stimulation (TMS) of the motor cortex during voluntary contractions elicits electrophysiological and mechanical responses in the target muscle. The effect of different TMS intensities on exercise-induced changes in TMS-elicited variables is unknown, impairing data interpretation. This study aimed to investigate TMS intensity effects on maximal voluntary activation (VATMS), motor-evoked potentials (MEPs), and silent periods (SPs) in the quadriceps muscles before, during, and after exhaustive isometric exercise. Eleven subjects performed sets of ten 5-s submaximal isometric quadriceps contractions at 40% of maximal voluntary contraction (MVC) strength until task failure. Three different TMS intensities (I100, I75, I50) eliciting MEPs of 53 ± 6%, 38 ± 5% and 25 ± 3% of maximal compound action potential (Mmax) at 20% MVC were used. MEPs and SPs were assessed at both absolute (40% baseline MVC) and relative (50%, 75%, and 100% MVC) force levels. VATMS was assessed with I100 and I75. When measured at absolute force level, MEP/Mmax increased during exercise at I50, decreased at I100 and remained unchanged at I75. No TMS intensity effect was observed at relative force levels. At both absolute and relative force levels, SPs increased at I100 and remained stable at I75 and I50. VATMS assessed at I75 tended to be lower than at I100. TMS intensity affects exercise-induced changes in MEP/Mmax (only when measured at absolute force level), SPs, and VATMS. These results indicate a single TMS intensity assessing maximal voluntary activation and exercise-induced changes in corticomotoneuronal excitability/inhibition may be inappropriate.

Assessement of quadriceps strength, endurance and fatigue in FSHD and CMT: benefits and limits of femoral nerve magnetic stimulation.

OBJECTIVES: To (i) evaluate the feasibility and the reliability of a test assessing quadriceps strength, endurance and fatigue in patients with fascioscapulohumeral dystrophy (FSHD) and Charcot-Marie-Tooth disease (CMT), (ii) compare quadriceps function between patients and healthy controls. METHODS: Controls performed the test once and patients twice on two separate visits. It involved progressive sets of 10 isometric contractions each followed by neuromuscular assessments with FNMS. RESULTS: Volitional assessment of muscle strength, endurance and fatigue appeared to be reliable in FSHD and CMT patients. Supramaximal FNMS was achieved in ∼70% of FSHD patients and in no CMT patients. In FSHD patients, Femoral nerve magnetic stimulation (FNMS) provided reliable assessment of central (typical error as a coefficient of variation (CVTE)<8% for voluntary activation) and peripheral (CVTE<10% and intraclass coefficient correlation >0.85 for evoked responses) function. Patients and controls had similar reductions in evoked quadriceps responses, voluntary activation and similar endurance. CONCLUSIONS: This test provides reliable evaluation but FNMS exhibits limitations due to insufficient stimulation intensity particularly in neurogenic conditions. It showed similar central and peripheral quadriceps fatigability in patients and controls. SIGNIFICANCE: This test may be a valuable tool for patient follow-up although further development of magnetic stimulation devices is needed to extend its applicability.

Potential interests and limits of magnetic and electrical stimulation techniques to assess neuromuscular fatigue.

Neuromuscular function can change under different conditions such as ageing, training/detraining, long-term spaceflight, environmental conditions (e.g. hypoxia, hyperthermia), disease, therapy/retraining programs and also with the appearance of fatigue. Neuromuscular fatigue can be defined as any decrease in maximal voluntary strength or power. There is no standardized method to induce fatigue and various protocols involving different contraction patterns (such as sustained or intermittent submaximal isometric or dynamic contractions on isokinetic or custom chairs) have been used. Probably due to lack of motivation/cooperation, results of fatigue resistance protocols are more variable in patients than in healthy subjects. Magnetic and electrical stimulation techniques allow non-invasive assessment of central and peripheral origins of fatigue. They also allow investigation of different types of muscle fatigue when combining various types of stimulation with force/surface EMG measurements. Since maximal electrical stimuli may be uncomfortable or even sometimes painful, several alternative methods have been recently proposed: submaximal muscle stimulation, low/high-frequency paired pulses instead of tetanic stimuli and the use of magnetic stimulation at the peripheral level.