Factors influencing the relation between corticospinal output and muscle force during voluntary contractions.

The mechanisms by which voluntary forces of different strengths are produced in human muscles are not clear. We studied the relation between force and surface electromyography (sEMG) variables over a wide range of voluntary contraction strengths of biceps brachii (BIC) and abductor digiti minimi (ADM). The relation between force and motor evoked potentials (MEPs) to transcranial magnetic stimulation of motor cortex was also assessed. The root mean square of sEMG and median frequency (Mf) of the sEMG power spectrum as well as the MEP area of ADM and BIC were calculated up to the maximum voluntary contraction (MVC). The root mean square of ADM and BIC increased with increasing force levels up to the MVC. The Mf of BIC increased with force levels up to 70% MVC after which it rapidly declined. The Mf of ADM peaked at 40% MVC to slowly decline thereafter. The MEP changes with force were similar to Mf changes. Thus, corticospinal output, as tested by the Mf and MEPs, does not parallel force increments across the contraction range. This decline, which is contingent on the relative contribution of motor unit recruitment and rate coding to force production in each muscle, may depend on reduced motoneurone responsiveness at high firing rates. We suggest that, under controlled conditions, the frequency content of the sEMG signal may be taken to indicate motor unit recruitment range. This information may improve the utility of the Mf to enable evaluation of voluntary activation under different conditions.

Recurrence quantification analysis of surface EMG detects changes in motor unit synchronization induced by recurrent inhibition.

The systemic injection of L-Acetylcarnitine (L-Ac) induces a reversible increase in recurrent inhibition. In addition, L-Ac potentiation of recurrent inhibition has been found to increase the synchronous activity of single motor units, as detected by traditional linear analysis in the time domain. This result has been recently confirmed using a nonlinear method based on the analysis of embedded determinism (%DET) extracted from the surface EMG. The present study aimed at testing the general applicability of RQA methodology, as a viable tool for assessing motor unit synchronization, by extending the analysis of surface EMG, as revealed by changes in %DET induced by L-Ac, to many upper and lower limb muscles and to muscles that are not easily studied by needle electrodes, such as the orbicularis oculi. Subjects performed brief periods of tonic contractions, alternated to periods of rests to avoid muscle fatigue. Pharmacological enhancement of recurrent inhibition was obtained by a short-lasting intravenous injection of L-Ac. Control experiments were performed replacing L-Ac injection with saline injection. The average %DET showed a significant increase during L-Ac injection in the deltoid, biceps brachii, extensor carpi radialis, while no effect was observed in the opponens pollicis and abductor digiti minimi for the upper limb muscles. Similarly, the average %DET showed a significant increase during L-Ac injection in the quadriceps, soleus, and tibialis anterior, while no effect was observed in the abductor hallucis for the lower limb muscles. RQA of orbicularis oculi muscle activity showed no increase in %DET during L-Ac injection in analogy to what found in the intrinsic muscles of the hand and foot, known to be devoid of recurrent inhibition. The presence or absence of drug-induced increase in motor unit synchronization agrees with the known distribution of recurrent inhibition in the various motor nuclei. The overall significance of these findings is the potential application of RQA methodology as a reliable and independent tool for generally assessing motor unit synchronization from surface EMG under strictly controlled experimental condition.

Motor unit synchronous firing as revealed by determinism of surface myoelectric signal.

Information on motor strategies can be extracted from the surface electromyogram (EMG) by non-linear methods. The percentage of determinism (%DET) obtained from recurrence quantification analysis (RQA) may be a sensitive variable to detect synchronous motor unit behaviour. The purpose of the present study was to validate this methodology by comparing it with an established technique estimating the degree of synchronization of pairs of voluntary activated motor units from the correlation of their firing in the time-domain. Single motor unit activity was recorded in extensor carpi radialis (ECR) muscle by pairs of tungsten microelectrodes inserted into the muscle belly. Cross-correlation analysis was performed in order to determine synchronization peak area by computing synchronous impulse probability. Surface EMG activity was recorded in parallel by electrodes placed over the skin of the same muscle and %DET was used as a measure of synchronous activity. The %DET appeared to be a valid measure of synchronization yielding results comparable to those obtained with cross-correlation analysis. Increases in %DET (t = 64.59, P < 0.0001) highly correlated (r2 = 0.70, P = 0.0013) with pharmacologically induced increases in the synchronization activity of pairs of ECR motor units (t = 8.71, P < 0.0001). RQA may be used as an alternative methodology for testing synchronous motor unit behaviour from surface EMG under physiological and pathological conditions.

Force estimation processing as a function of the input--output relationship of the corticospinal pathway in humans.

Static changes of the shoulder joint from 30 degrees adduction (ANT) to 30 degrees abduction (POST) in the horizontal plane reduce the gain of the input--output relationship of the corticospinal pathway to the abductor digiti minimi (ADM) muscle [F. Ginanneschi et al. (2005)Exp. Brain Res., 161, 374--382]. The present study examined force estimation under conditions in which the input--output relationship of the corticospinal innervation to ADM was modified by changing shoulder position as above. The input--output relationship was studied using transcranial magnetic stimulation. Estimates of force were assessed using a matching procedure; subjects first matched a target level (10--40% of maximum) on a screen by applying a reference (Ref) isometric contraction of ADM and then they reproduced the same level of force without visual feedback by a test contraction (Test). When Ref and Test contractions were performed at either ANT or POST (i.e. the same input--output), the respective force levels were closely matched. In contrast, when the Test and Ref were performed in POST and ANT, respectively (i.e. different input--output), subjects exerted more force than required. Errors were in the opposite direction when the Test and Ref were in ANT and POST, respectively. The present results suggest that the process of force estimation is based on the effort : tension ratio which is a direct function of the corticomotoneuronal input--output relationship. This notion may contribute to explaining the pathophysiology of central fatigue.

Changes in corticomotor excitability of hand muscles in relation to static shoulder positions.

We examined whether the recruitment properties of the corticospinal pathway to intrinsic hand muscles are influenced by variations of the shoulder joint angle. Abductor digiti minimi (ADM) motor evoked potentials (MEPs) in response to transcranial magnetic stimulation were examined during different static positions of the shoulder joint in the horizontal plane from 30 degrees adduction to 30 degrees abduction with respect to the neutral position at 0 degrees, while elbow and wrist joints were constrained statically at 90 degrees and 180 degrees respectively. We found that 30 degrees abduction of the shoulder significantly depressed MEP size and prolonged MEP latency in comparison with 30 degrees shoulder adduction. The neutral shoulder angle position (at 0 degrees ) significantly reduced MEP size but had no effect on MEP latency in comparison with 30 degrees shoulder abduction. The input-output relationship between MEP size and stimulus intensity was sigmoidal. The plateau value and maximum slope were significantly lower at 30 degrees abduction than at 30 degrees adduction of the shoulder. However, the threshold value did not differ significantly between the two positions. To differentiate excitability changes at cortical versus subcortical sites, intracortical inhibition (ICI) and intracortical facilitation (ICF) were assessed using a paired-magnetic pulse paradigm. A significant decrease in ICF was observed after changing shoulder position from 30 degrees adduction to 30 degrees abduction. In contrast, no variation in the amount of ICI occurred in relation to the same changes in shoulder position. ADM F-waves elicited by electrical stimulation of the ulnar nerve at the wrist were significantly decreased at 30 degrees shoulder abduction in comparison with 30 degrees adduction. A similar pattern was observed in one subject in whom the H-reflex could be exceptionally elicited in ADM. We conclude that shoulder position influences the recruitment efficiency (gain) of the corticospinal volleys to motoneurons of intrinsic hand muscles. It is proposed that activity of peripheral receptors signalling static shoulder position influences corticomotor excitability of hand muscles both at the cortical and at the spinal level. This modulation may be functionally relevant when reaching to grasp objects.