Assessment of voluntary muscle activation using magnetic stimulation

Abstract

The present study investigated the applicability of magnetic stimulation (MS) for estimating activation capacity. Ten men performed isometric knee extensions at 95% of maximum voluntary contraction level on two testing sessions. MS and electrical stimulation (ES) were applied by placing the coil and stimulating electrodes, respectively, on the quadriceps muscle group. In session 1, MS and ES were applied to allow a comparison between the two stimulating devices. During session 2, MS was applied again to assess the reproducibility of MS measurements. Activation level was estimated with the interpolated twitch technique (ITT) and the central activation ratio (CAR). Activation level determined with MS was highly repeatable, but was only comparable to ES when using ITT (standard error < 1.45%, mean difference < 0.63%, for all). MS with CAR overestimated activation by 3% compared to ES (standard error = 1.13%, mean difference = 2.76%) because the power of the magnetic field was limited. These results indicate that MS is a promising tool for reliable and valid quantification of activation levels when using the ITT approach and potentially suitable for studies involving children or other subjects where the pain of conventional ES is a problem.

Effect of stimulation intensity on assessment of voluntary activation

“…First, tendon slack can reduce resting doublet torque to a greater extent with submaximal muscle stimulation compared with maximal nerve stimulation and lead to a relatively high superimposed torque during submaximal stimulation. 30 However, the absolute torque increments upon MVT were high during submaximal muscle stimulation (5.4 Nm) and were not statistically different from those obtained with maximal nerve stimulation (6.3 Nm). Moreover, for some subjects the absolute torque increments were even systematically larger for submaximal muscle stimulation upon MVT compared with maximal activation (e.g., Fig.…”

The effects of agonist and antagonist muscle activation on the knee extension moment–angle relationship in adults and children

“…Ninety degrees was chosen as the reference joint angle as it was the most flexed joint angle examined, and therefore, the one that had the least muscletendon slack that would affect the estimation of activation using the ITT method. The level of voluntary activation was quantified from superimposed and resting doublets evoked using magnetic stimulation applied percutaneously (pulse duration 1 ms, rise and fall durations 1 ls, inter-stimulus gap 10 ms and a magnetic field of 2.0 T, produced by a 70-mm diameter ''Double coil''; MagStim, Bi-Stim2, Carmarthenshire, Wales), as previously described (O'Brien et al 2008). The strength of the doublet evoked by magnetic stimulation depends on the placement of the coil relative to the muscle.…”

Relationship between ventilatory constraint and muscle fatigue during exercise in COPD

“…Based on pilot work, the current authors found that they were able to best demonstrate a plateau in isometric torque and M-wave amplitude at the motor point rather than the femoral triangle, and that this technique was more comfortable than femoral triangle stimulation [29]. A plateau in M-wave amplitude during progressive intensity motor stimulation is a critical criterion for fatigue studies, to ensure that stimulation is supramaximal [4,25].…”

“…A plateau in M-wave amplitude during progressive intensity motor stimulation is a critical criterion for fatigue studies, to ensure that stimulation is supramaximal [4,25]. Supramaximal magnetic motor point stimulation has been used previously in other populations and has been shown to be valid and reliable [26][27][28][29]. The current authors were able to demonstrate an M-wave plateau in all of the study subjects during magnetic stimulation, and the TwVL measurements demonstrated excellent reliability; therefore, the technique appears to be appropriate.…”

“…Supramaximal muscle stimulation [26,27] was applied over the right vastus lateralis motor point using a magnetic stimulator (Magpro R30; Medtronic Inc., Minneapolis, MN, USA) and parabolic coil (MMC 140; Medtronic Inc.) [28,29]. The coil was placed over the approximate location of the vastus lateralis motor point then repeatedly stimulated the muscle at 50% of the stimulator output.…”