Temporal modulations of agonist and antagonist muscle activities accompanying improved performance of ballistic movements.

Although many studies have examined performance improvements of ballistic movement through practice, it is still unclear how performance advances while maintaining maximum velocity, and how the accompanying triphasic electromyographic (EMG) activity is modified. The present study focused on the changes in triphasic EMG activity, i.e., the first agonist burst (AG1), the second agonist burst (AG2), and the antagonist burst (ANT), that accompanied decreases in movement time and error. Twelve healthy volunteers performed 100 ballistic wrist flexion movements in ten 10-trial sessions under the instruction to "maintain maximum velocity throughout the experiment and to stop the limb at the target as fast and accurately as possible". Kinematic parameters (position and velocity) and triphasic EMG activities from the agonist (flexor carpi radialis) and antagonist (extensor carpi radialis) muscles were recorded. Comparison of the results obtained from the first and the last 10 trials, revealed that movement time, movement error, and variability of amplitudes reduced with practice, and that maximum velocity and time to maximum velocity remained constant. EMG activities showed that AG1 and AG2 durations were reduced, whereas ANT duration did not change. Additionally, ANT and AG2 latencies were reduced. Integrated EMG of AG1 was significantly reduced as well. Analysis of the alpha angle (an index of the rate of recruitment of the motoneurons) showed that there was no change in either AG1 or AG2. Correlation analysis of alpha angles between these two bursts further revealed that the close relationship of AG1 and AG2 was kept constant through practice. These findings led to the conclusion that performance improvement in ballistic movement is mainly due to the temporal modulations of agonist and antagonist muscle activities when maximum velocity is kept constant. Presumably, a specific strategy is consistently applied during practice.

Motor strategies and excitability changes of human hand motor area are dependent on different voluntary drives.

The present study examined whether there were different voluntary drives between intended and non-intended muscle contractions. In experiment 1, during intended and non-intended muscle contractions, electromyograms (EMGs) were recorded from the first dorsal interosseous (FDI) and extensor carpi radialis (ECR) muscles when force levels were varied from 10% to 50% maximal voluntary contraction (MVC) in 10% MVC steps. In experiment 2, using transcranial magnetic stimulation, motor-evoked potentials (MEPs) were recorded from the FDI muscle when EMGs were varied from 10% to 40% EMGmax (EMG activities during MVC) in 10% EMGmax steps during intended and non-intended muscle contractions. In experiment 3, at 10% MVC force level MEPs were recorded before and after practice. The results showed that, in the FDI muscle, EMGs during intended muscle contractions were larger than those during non-intended ones at higher force levels (30-50% MVC). In the ECR muscle, reverse results were observed. At comparable EMG levels of the FDI muscle MEPs were the same during intended and non-intended muscle contractions. After practice, MEPs during intended muscle contraction became larger than those during non-intended at 10% MVC force level, while EMGs were the same between two muscle contractions. It is concluded that motor strategies and excitability changes of hand motor area are different during intended and non-intended muscle contractions, and these differences are due to the different voluntary drives of intended and non-intended. The present findings may contribute to the understanding of rehabilitation for patients suffering from damages of the central motor system.

Excitability changes in human hand motor area induced by voluntary teeth clenching are dependent on muscle properties.

To investigate whether the early effects of voluntary teeth clenching (VTC) among the first dorsal interosseous (FDI), abductor digiti minimi (ADM), and abductor pollicis brevis (APB) muscles are differently modulated depending on their muscle properties, we examined the responses of motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation with selected current directions and by brainstem magnetic stimulation (BMS). Although MEP responses with anterior-medially current direction (preferentially elicited I1-waves) were facilitated in all three muscles, those responses with posterior-laterally current direction (preferentially elicited I3-waves) were different among FDI, ADM, and APB muscles. That is, MEP responses in FDI and APB muscles were significantly reduced, whereas those responses in ADM muscle were not significantly reduced. Further, inhibitory effects of VTC in FDI muscle were more potent than those in ADM or APB muscles. On the other hand, the responses to BMS were unchanged by VTC in all three muscles, suggesting that the modulations of MEP were attributed to the cortical origin. On the basis of our previous findings that the inhibitory connections in FDI muscle are more potent than those in ADM muscle (Takahashi et al. in Clin Neurophysiol 116:2757-2764, 2005), the cortical effects of VTC among three hand muscles are differently modulated, depending on muscle properties, presumably the extents of inhibitory connections to corticospinal tract neurons. Considering that the functional capacity in FDI muscle is higher than that in ADM or APB muscles, the cortical inhibitory effect of VTC might contribute to the sophisticated regulation of the motor outputs even during VTC.

Functional demanded excitability changes of human hand motor area.

The present study was performed to examine if there are functional differences between the first dorsal interosseous (FDI) and the abductor digit minimi (ADM) muscles during different muscle contractions, namely dynamic and static contractions of the index and little finger abductions. It was also examined whether these functional differences occur at the cortical level. The motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) and force curves, during the muscle contractions, were simultaneously recorded. Rest motor thresholds (RMTs) and active motor thresholds (AMTs), during dynamic and static contractions, were determined in the two muscles. In all trials, the background EMGs (B.EMGs) were kept at the same level in each muscle. Results showed that the target matching errors of dynamic contractions were statistically smaller in the FDI muscle than those in the ADM. In the FDI muscle, the AMT during dynamic contractions was significantly lower than during static ones and the MEPs elicited by TMS were larger during dynamic contractions than those during static ones. However, such results were not found in the ADM muscle. In order to investigate whether the differences were caused by the excitability changes that occurred in the cortical level, the responses elicited by subcortical stimulations were recorded using the same procedures as the experiment of TMS. Responses to subcortical stimulations during dynamic contractions were similar to those during static ones in either muscle. It is concluded that there are differences in the task-dependent MEP facilitations between the FDI and ADM muscles. And the differences are due to the functional demanded excitability changes accompanied by the cortical activation.

Differential modulations of intracortical neural circuits between two intrinsic hand muscles.

OBJECTIVE: To investigate whether the intracortical inhibitory (ICI) and facilitatory (ICF) circuits in the primary motor cortex between the first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles are modulated differently. METHODS: We conducted paired-pulse transcranial magnetic stimulation in combination with different current directions (anterior-medially: AM, and posterior-laterally: PL) under relaxed and active muscle conditions with interstimulus intervals (ISIs) between 2 and 16 ms. RESULTS: In both muscle conditions, the conditioned motor-evoked potential (MEP) responses obtained with the AM current direction (preferentially eliciting early I-waves) were similar between the two muscles at all ISIs, but the MEP responses obtained with the PL current direction (preferentially eliciting late I-waves) were different between FDI and ADM muscles, in that the conditioned MEP responses in FDI muscle were inhibited at all ISIs under both muscle conditions, whereas those in ADM muscle were suppressed at only short ISIs (2-4 ms). CONCLUSIONS: These results indicate that the inhibitory connections operating for the corticospinal tract neurons in FDI muscle are more potent, and, conversely, that those in ADM muscle are weaker. SIGNIFICANCE: The different modulations of ICI circuits between FDI and ADM muscles is an important neural mechanism which may contribute to different functional demands (finger dexterity).

Role of human SII cortices in sensorimotor integration.

OBJECTIVES: To elucidate the functional properties of neurons in the human primary (SI) and ipsilateral and contralateral secondary (iSII or cSII) cortices in response to stimuli during finger movement. METHODS: We measured somatosensory evoked fields (SEFs) produced by electric stimuli delivered to the median nerve at 0.2 Hz in 6 healthy subjects. RESULTS: The amplitudes of evoked fields from both iSII and cSII were gradually attenuated with time. Consecutive blocks of trials were obtained to assess the habituation of each evoked field. Complex finger movements with attention (gating session) increased the amplitude of evoked fields from the iSII cortices but reduced the amplitudes of evoked fields from the cSII cortices (P<0.01). In contrast, the amplitude of P30 m from the SI did not show habituation effects but decreased significantly in the gating session (P<0.01). CONCLUSIONS: The enhanced iSII as well as suppressed cSII cortices during complex finger movements with attention are not only considered to be result of gating effect but also attention.