Harri Piitulainen

Innervation zone shift at different levels of isometric contraction in the biceps brachii muscle

Experiments were carried out to examine whether innervation zone (IZ) location remains stable at different levels of isometric contraction in the biceps brachii muscle (BB), and to determine how the proximity of the IZ affects common surface electromyography (sEMG) parameters. Twelve subjects performed maximal (MVC) and submaximal voluntary isometric contractions at 10%, 20%, 30%, 40%, 50% and 75% of MVC. sEMG signals were recorded with a 13 rows x 5 columns grid of electrodes from the short head of BB. The IZ shifted in the proximal direction by up to 2.4 cm, depending upon the subject and electrode column. The mean shift of all the columns was 0.6+/-0.4 cm (10% vs. 100% MVC, P<0.001). This shift biased the average values of mean frequency (+21.8+/-9.9 Hz, P<0.001), root mean square (-0.16+/-0.15 mV, P<0.05) and conduction velocity (-1.15+/-0.93 m/s, P<0.01) in the channels immediately proximal to the IZ. The shift in IZ could be explained by shortening of the muscle fibers, and thus lengthening of the (distal) tendon due to increasing force. These results underline the importance of individual investigation of IZ locations before the placement of sEMG electrodes, even in isometric contractions.

Corticokinematic coherence mainly reflects movement-induced proprioceptive feedback

Corticokinematic coherence (CKC) reflects coupling between magnetoencephalographic (MEG) signals and hand kinematics, mainly occurring at hand movement frequency (F0) and its first harmonic (F1). Since CKC can be obtained for both active and passive movements, it has been suggested to mainly reflect proprioceptive feedback to the primary sensorimotor (SM1) cortex. However, the directionality of the brain–kinematics coupling has not been previously assessed and was thus quantified in the present study by means of renormalized partial directed coherence (rPDC). MEG data were obtained from 15 subjects who performed right index-finger movements and whose finger was, in another session, passively moved, with or without tactile input. Four additional subjects underwent the same task with slowly varying movement pace, spanning the 1–5 Hz frequency range. The coupling between SM1 activity recorded with MEG and finger kinematics was assessed with coherence and rPDC. In all conditions, the afferent rPDC spectrum, which resembled the coherence spectrum, displayed higher values than the efferent rPDC spectrum. The afferent rPDC was 37% higher when tactile input was present, and it was at highest at F1 of the passive conditions; the efferent rPDC level did not differ between conditions. The apparent latency for the afferent input, estimated within the framework of the rPDC analysis, was 50–100 ms. The higher directional coupling between hand kinematics and SM1 activity in afferent than efferent direction strongly supports the view that CKC mainly reflects movement-related somatosensory proprioceptive afferent input to the contralateral SM1 cortex.

Corticokinematic coherence during active and passive finger movements

Corticokinematic coherence (CKC) refers to coupling between magnetoencephalographic (MEG) brain activity and hand kinematics. For voluntary hand movements, CKC originates mainly from the primary sensorimotor (SM1) cortex. To learn about the relative motor and sensory contributions to CKC, we recorded CKC from 15 healthy subjects during active and passive right index-finger movements. The fingertip was either touching or not touching table, resulting in active-touch, active-no-touch, passive-touch, and passive-no-touch conditions. The kinematics of the index-finger was measured with a 3-axis accelerometer. Beamformer analysis was used to locate brain activations for the movements; somatosensory-evoked fields (SEFs) elicited by pneumatic tactile stimulation of the index finger served as a functional landmark for cutaneous input. All active and passive movements resulted in statistically significant CKC at the movement frequency (F0) and its first harmonic (F1). The main CKC sources at F0 and F1 were in the contralateral SM1 cortex with no spatial differences between conditions, and distinct from the SEF sources. At F1, the coherence was by two thirds stronger for passive than active movements, with no difference between touch vs. no-touch conditions. Our results suggest that the CKC occurring during repetitive finger movements is mainly driven by somatosensory, primarily proprioceptive, afferent input to the SM1 cortex, with negligible effect of cutaneous input.

Coherence between magnetoencephalography and hand-action-related acceleration, force, pressure, and electromyogram

Hand velocity and acceleration are coherent with magnetoencephalographic (MEG) signals recorded from the contralateral primary sensorimotor (SM1) cortex. To learn more of this interaction, we compared the coupling of MEG signals with four hand-action-related peripheral signals: acceleration, pressure, force, and electromyogram (EMG). Fifteen subjects performed self-paced repetitive hand-action tasks for 3.5min at a rate of about 3Hz. Either acceleration, pressure or force signal was acquired with MEG and EMG signals during (1) flexions-extensions of right-hand fingers, with thumb touching the other fingers (acceleration; free), (2) dynamic index-thumb pinches against an elastic rubber ball attached to a pressure sensor (pressure and acceleration; squeeze), and (3) brief fixed-finger-position index-thumb pinches against a rigid load cell (force; fixed-pinch). Significant coherence occurred between MEG and all the four peripheral measures at the fundamental frequency of the hand action (F0) and its first harmonic (F1). In all tasks, the cortical sources contributing to the cross-correlograms were located at the contralateral hand SM1 cortex, with average inter-source distance (mean±SEM) of 9.5±0.3mm. The coherence was stronger with respect to pressure (0.40±0.03 in squeeze) and force (0.38±0.04 in fixed-pinch) than acceleration (0.24±0.03 in free) and EMG (0.25±0.02 in free, and 0.29±0.04 in fixed-pinch). The results imply that the SM1 cortex is strongly coherent at F0 and F1 with hand-action-related pressure and force, in addition to the previously demonstrated EMG, velocity, and acceleration. All these measures, especially force and pressure, are potential tools for functional mapping of the SM1 cortex.

Human primary motor cortex is both activated and stabilized during observation of other person's phasic motor actions

When your favourite athlete flops over the high-jump bar, you may twist your body in front of the TV screen. Such automatic motor facilitation, ‘mirroring’ or even overt imitation is not always appropriate. Here, we show, by monitoring motor-cortex brain rhythms with magnetoencephalography (MEG) in healthy adults, that viewing intermittent hand actions of another person, in addition to activation, phasically stabilizes the viewers primary motor cortex, with the maximum of half a second after the onset of the seen movement. Such a stabilization was evident as enhanced cortex–muscle coherence at 16–20 Hz, despite signs of almost simultaneous suppression of rolandic rhythms of approximately 7 and 15 Hz as a sign of activation of the sensorimotor cortex. These findings suggest that inhibition suppresses motor output during viewing another persons actions, thereby withholding unintentional imitation.

Impaired action potential conduction at high force levels after eccentric exercise

High-density surface electromyography was used to examine whether gross sarcolemmal function is impaired in m. biceps brachii after intensive eccentric elbow flexor exercise, when measured at wide range of isometric contraction levels. Root mean square (RMS), mean power frequency (MNF) and mean muscle fibre conduction velocity (CV) were calculated before and up to four days post-exercise. Maximal isometric voluntary (MVC) force decreased by 21.3+/-5.6% two hours after exercise, and by 12.6+/-11.1% two days post-exercise. CV and MNF decreased both during MVC (CV from 4.1+/-0.3m/s to 3.8+/-0.4m/s and MNF from 92.6+/-10 Hz to 85.2+/-11 Hz) and during electrically evoked maximal M-wave (CV from 4.1+/-0.3m/s to 3.0+/-0.5m/s and MNF from 97.1+/-27.2 Hz to 78.0+/-24.4 Hz) two hours post-exercise. Furthermore, at submaximal isometric force levels, CV and MNF decreased only at higher contraction levels (40%, 50% and 75% of MVC) two hour post-exercise. It can be concluded that intensive exercise can temporarily impair gross sarcolemmal function. In addition, since this only occurred at high force levels, based on Hennemans size principle, it seems that higher threshold motor units were predominantly affected.

Changes in motor unit characteristics after eccentric elbow flexor exercise.

Morphological evidence suggests that fast‐twitch fibers are prone to disruption of their membrane structures by eccentric exercise. However, it is unclear how this is reflected in the discharge rate and action potential propagation of individual motor units, especially at high contraction levels. High‐density surface electromyograms were recorded from biceps brachii muscle and decomposed to individual motor unit action potentials at isometric contraction levels between 10% and 75% of maximal voluntary contraction (MVC) before intermittent maximal elbow flexor eccentric exercise, and two hours (2H), two days (2D) and four days (4D) post‐exercise. Maximal voluntary force decreased by 21.3±5.6% 2H and by 12.6±11.1% 2D post‐exercise. Motor unit discharge rate increased and mean muscle fiber conduction velocity decreased, at the highest isometric contraction levels only (50% and 75% of MVC) at 2H post‐exercise. These results indicate that eccentric exercise can disturb the function of motor units active at high contraction levels in the early stages after exercise, which seems to be compensated by the central nervous system with an increase in neural drive during submaximal isometric contractions.

Effect of innervation zones in estimating biceps brachii force–EMG relationship during isometric contraction

Measuring muscle forces in vivo is invasive and consequently indirect methods e.g., electromyography (EMG) are used in estimating muscular force production. The aim of the present paper was to examine what kind of effect the disruption of the physiological signal caused by the innervation zone has in predicting the force/torque output from surface EMG. Twelve men (age 26 (SD ±3)years; height 179 (±6)cm; body mass 73 (±6)kg) volunteered as subjects. They were asked to perform maximal voluntary isometric contraction (MVC) in elbow flexion, and submaximal contractions at 10%, 20%, 30%, 40%, 50% and 75% of the recorded MVC. EMG was measured from biceps brachii muscle with an electrode grid of 5 columns×13 rows. Force-EMG relationships were determined from individual channels and as the global mean value. The relationship was deemed inconsistent if EMG value did not increase in successive force levels. Root mean squared errors were calculated for 3rd order polynomial fits. All subjects had at least one (4-52) inconsistent channel. Two subjects had inconsistent relationship calculated from the global mean. The mean root mean squared error calculated using leave one out method for the fits of the individual channels (0.33±0.17) was higher (P<0.001) than the error for the global mean fit (0.16±0.08). It seems that the disruption of the physiological signal caused by the innervation zone affects the consistency of the force-EMG relationship on single bipolar channel level. Multichannel EMG recordings used for predicting force overcame this disruption.

MEG-compatible pneumatic stimulator to elicit passive finger and toe movements

Magnetoencephalographic (MEG) signals recorded from the primary sensorimotor (SM1) cortex are coherent with kinematics of both active and passive finger movements. The coherence mainly reflects movement-related proprioceptive afference to the cortex. Here we describe a novel MEG-compatible stimulator to generate computer-controlled passive finger and toe movements that can be used as stimuli in functional brain-imaging experiments. The movements are produced by pneumatic artificial muscle (PAM), elastic actuator that shortens with increasing air pressure. To test the applicability of the stimulator to functional brain-imaging, 4-min trains of passive repetitive 5-mm flexion-extension movements of the right and left index finger and the right hallux were produced at 3Hz while the subjects brain activity was measured with whole-scalp MEG and finger or toe kinematics with an accelerometer. In all ten subjects studied, statistically significant coherence (up to 0.78) occurred between the accelerometer and MEG signals at the movement frequency or its first harmonic. Sources of coherent activity were in the contralateral hand or foot SM1 cortices. Movement-evoked fields elicited with intermittent movements of the right index finger (once every 3.2-4.0s; mean±SD peak response latency 88±25ms) were co-located with the respective coherent sources. We further moved the right index finger at 3, 6, and 12Hz (movement ranges 5, 3, and 2mm, respectively), and analyzed the first 1, 2, and 4-min epochs of data. One minute of data was sufficient to locate the left hand area of the SM1 cortex at all movement frequencies. Sound-induced spurious coherence was reliably ruled out in a control experiment. Our novel movement stimulator thus provides a robust and reliable tool to track proprioceptive afference to the cortex and to locate the SM1 cortex.

Effect of electrode location on surface electromyography changes due to eccentric elbow flexor exercise

Experiments were carried out to determine whether the location of electrodes has an effect on eccentric exercise–induced changes in surface electromyography (sEMG) variables in the biceps brachii muscle. sEMG signals were recorded with a grid of 64 electrodes before and up to 4 days post‐exercise. Root mean square (RMS) and mean power frequency (MNF) were calculated for: (1) each channel; (2) as an average of all channels; and (3) as an average of individual channel rows and columns. Mean muscle‐fiber conduction velocity (CV) was estimated similarly but was based on double‐differential channels. Maximal isometric voluntary torque decreased 21.3 ± 5.6% post‐exercise. The average sEMG variables decreased after the exercise and recovered 2 days (RMS and CV) or 4 days (MNF) post‐exercise. Site‐dependent changes were observed in sEMG variables. We conclude that site‐dependent changes in sEMG variables after eccentric exercise can be detected and are influenced in part by anatomical factors. Muscle Nerve, 2009