Greater movement-related cortical potential during human eccentric versus concentric muscle contractions

Distinct brain activation patterns for human maximal voluntary eccentric and concentric muscle actions

Eccentric muscle contractions generate greater force at a lower level of activation and subject muscles to more severe damage than do concentric actions. A recent investigation has revealed that electroencephalogram (EEG)-derived movement-related cortical potential (MRCP) is greater and occurs earlier for controlling human eccentric than concentric submaximal muscle contractions. However, whether the central nervous system (CNS) control signals for high-intensity or maximal-effort eccentric movements differ from those for concentric actions is unknown. The purpose of this study was to determine whether the MRCP signals differ between the two types of maximal-effort contractions. Eight volunteers performed 40 maximal voluntary eccentric and 40 maximal voluntary concentric elbow flexor contractions on a Kin-Com isokinetic dynamometer. Scalp EEG signals (62 channels) were measured along with force, joint angle, and electromyographic (EMG) signals of the performing muscles. MRCP-based two-dimensional brain maps were created to illustrate spatial and temporal distributions of the MRCP signals. Although the level of elbow flexor muscle activity was lower during eccentric than concentric movements, MRCP-indicated cortical activation was greater both in amplitude and area dimension for the eccentric task. Detailed comparisons of individual electrode signals suggested that eccentric movements needed a significantly longer time for early preparation and a significantly greater magnitude of cortical activity for later movement execution. The extra preparation time and higher amplitude of activation may reflect CNS activities that account for the higher risk of injury, higher degree of movement difficulty, and unique motor unit activation pattern associated with maximal-level eccentric muscle actions.

From mental power to muscle power--gaining strength by using the mind

The purposes of this project were to determine mental training-induced strength gains (without performing physical exercises) in the little finger abductor as well as in the elbow flexor muscles, which are frequently used during daily living, and to quantify cortical signals that mediate maximal voluntary contractions (MVCs) of the two muscle groups. Thirty young, healthy volunteers participated in the study. The first group (N = 8) was trained to perform "mental contractions" of little finger abduction (ABD); the second group (N = 8) performed mental contractions of elbow (ELB) flexion; and the third group (N = 8) was not trained but participated in all measurements and served as a control group. Finally, six volunteers performed training of physical maximal finger abductions. Training lasted for 12 weeks (15 min per day, 5 days per week). At the end of training, we found that the ABD group had increased their finger abduction strength by 35% (P < 0.005) and the ELB group augmented their elbow flexion strength by 13.5% (P < 0.001). The physical training group increased the finger abduction strength by 53% (P < 0.01). The control group showed no significant changes in strength for either finger abduction or elbow flexion tasks. The improvement in muscle strength for trained groups was accompanied by significant increases in electroencephalogram-derived cortical potential, a measure previously shown to be directly related to control of voluntary muscle contractions. We conclude that the mental training employed by this study enhances the cortical output signal, which drives the muscles to a higher activation level and increases strength.

Differences in recruitment properties of the corticospinal pathway between lengthening and shortening contractions in human soleus muscle

The purpose of this study was to investigate how the recruitment properties of the corticospinal pathway are modulated in the soleus muscle of the lower limb during lengthening (LEN) and shortening (SHO) contractions by comparing the shape of the input-output (I/O) relation of the corticospinal pathway. To this end, we investigated the relationship between various stimulus intensities applied via transcranial magnetic stimulation and the size of motor-evoked potentials in 14 healthy subjects during voluntary plantarflexion and dorsiflexion (active lengthening) with a similar background activity (BGA) level. The shape of this relationship was sigmoidal and was characterized by a plateau value, maximum slope and threshold. The plateau value was clearly lower during LEN contractions than during SHO contractions. Likewise, the maximum slope was lower during LEN contractions. However, the threshold did not differ significantly between the two tasks. Since the plateau value and the maximum slope clearly differed between LEN and SHO contractions despite the similarity of their BGA levels, the central nervous system appears to have a different activation strategy for each of these tasks. Namely, the relative balance between excitatory and inhibitory components of the corticospinal volleys, as well as the subliminal fringe of the corticospinal pathway, were reduced during LEN contractions compared with SHO contractions. These strategies may help to avoid reflexive contractions brought about by higher discharge of muscle spindles and enable fine motor actions in voluntary lengthening contractions.

Intent-Related Differences in Surface EMG of Maximum Eccentric and Concentric Contractions

For this study it was hypothesized that when participants intended to perform a maximum voluntary concentric (or eccentric) contraction but had an eccentric (or concentric) contraction imposed upon them, the initial EMG measured during the isometric phase preceding the onset of the dynamometer motion would reflect the intended contraction condition. The surface EMG of the vastus lateralis muscle was measured in 24 participants performing isokinetic concentric and eccentric maximum voluntary knee extensor contractions. The contractions were initiated from rest and from the same knee flexion angle and required the same level of external force to trigger the onset of dynamometer motion. Vastus lateralis EMG were quantified during the isometric phase preceding the onset of the dynamometer motion. When participants intended to perform a concentric contraction but had an eccentric contraction imposed upon them, the initial EMG resembled that of a concentric contraction. When they intended to perform an eccentric contraction but had a concentric contraction imposed upon them, the initial EMG resembled that of an eccentric contraction. Overall, the difference between concentric and eccentric contractions observed during the period of theinitialmuscle activation implies that descending signals include information that distinguishes between eccentric and concentric contractions.

Motor output is more variable during eccentric compared with concentric contractions

PURPOSE

This study examined the ability of 10 young (25.3 +/- 2.8 yr) healthy individuals to control knee-extension force during several discrete concentric and eccentric contractions.

METHODS

Subjects performed maximal and submaximal tasks on a KIN-COM isokinetic dynamometer. The submaximal tasks were to match a force-time parabola with a time to peak force of 200 ms at five target-forces (50, 100, 150, 200, and 250 N).

RESULTS

Mean peak force produced by the subjects at each target-force during concentric and eccentric contractions was similar. Mean time to peak force, however, was much shorter for eccentric contractions and was not influenced by increases in the level of force. The standard deviation (SD) and coefficient of variation (CV) of peak force was greater during eccentric compared with concentric contractions. The SD of time to peak force was greater for concentric contractions; however, when normalized to the mean time to peak force produced (CV), eccentric contractions were more variable.

CONCLUSIONS

Results provide evidence that even in young adults the control of motor output is different for eccentric compared with concentric contractions.