The purpose of the study was to determine the effect of motor-unit synchronization on the surface electromyogram (EMG) and isometric force using a computer model of muscle contraction. The EMG and force were simulated by generating muscle fiber action potentials, defining motor-unit mechanical characteristics and territories, estimating motor-unit action potentials, specifying motor-unit discharge times, and imposing various levels of motor-unit synchronization. The output (EMG and force) was simulated at 11 levels of excitation, ranging from 5 to 100% of maximum. To synchronize motor-unit activity, selected motor-unit discharge times were adjusted; however, the number of motor units recruited and the average discharge rate of each unit was constant across synchronization conditions for a given level of excitation. Two levels of synchronization were imposed on the discharge times: a moderate and a high level, which approximated the experimentally observed range of motor-unit synchronization. The moderate level of synchrony caused the average EMG to increase by approximately 65%, whereas the high level caused a 130% increase in the EMG with respect to the no-synchrony condition. Neither synchrony condition influenced the magnitude of the average force. However, motor-unit synchronization did increase the amplitude of the fluctuations in the simulated force, especially at intermediate levels of excitation. In conclusion, motor-unit synchronization increased the amplitude of the average rectified EMG and decreased the steadiness of the force exerted by the muscle in simulated contractions.

† These authors have contributed equally to this work.The purpose of this study was to compare the effect of training using internal imagery (IMI; also known as kinesthetic imagery or first person imagery) with that of external imagery (EMI; also known as third-person visual imagery) of strong muscle contractions on voluntary muscle strengthening. Eighteen young, healthy subjects were randomly assigned to one of three groups (6 in each group): internal motor imagery (IMI), external motor imagery (EMI), or a no-practice control (CTRL) group. Training lasted for 6 weeks (∼15 min/day, 5 days/week). The participants' right arm elbow-flexion strength, muscle electrical activity, and movement-related cortical potential (MRCP) were evaluated before and after training. Only the IMI group showed significant strength gained (10.8%) while the EMI (4.8%) and CTRL (−3.3%) groups did not. Only the IMI group showed a significant elevation in MRCP on scalp locations over both the primary motor (M1) and supplementary motor cortices (EMI group over M1 only) and this increase was significantly greater than that of EMI and CTRL groups. These results suggest that training by IMI of forceful muscle contractions was effective in improving voluntary muscle strength without physical exercise. We suggest that the IMI training likely strengthened brain-to-muscle (BTM) command that may have improved motor unit recruitment and activation, and led to greater muscle output. Training by IMI of forceful muscle contractions may change the activity level of cortical motor control network, which may translate into greater descending command to the target muscle and increase its strength.

Recent research on bilateral transfer suggests that imagery training can facilitate the transfer of motor skill from a trained limb to that of an untrained limb above and beyond that of physical practice. To further explore this effect, the present study examined the influence of practice duration and task difficulty on the extent to which imagery training and physical training influences bilateral transfer of a sequential key pressing task. In experiment 1, participants trained on the key pressing task using their non-dominant arm under one of three conditions (physical practice, imagery practice, and no practice). In a subsequent bilateral transfer test, participants performed the sequential task using their untrained dominant arm in either an original order or mirror-ordered sequence. In experiment 2, the same procedures were followed as in experiment 1 except that participants trained with their dominant arm and performed the bilateral transfer task with their non-dominant arm. Results indicated that with extended practice beyond what has been employed in previous studies, physical practice is more effective at facilitating bilateral transfer compared to training with imagery. Interestingly, significant bilateral transfer was only observed for transfer from the non-dominant to the dominant arm with no differences observed between performing the task in an original or mirror ordered sequence. Overall, these findings suggest that imagery training may benefit bilateral transfer primarily at the initial stages of learning, but with extended training, physical practice leads to larger influences on transfer.

No abstract

We investigated the effect of exercise timing on attenuation of postprandial hyper-triglyceridemia (PHTG) in individuals with hypertriglyceridemia (HTG). Subjects were 10 males (TG = 290.1 +/- 28.5 mg/dl). Each subject performed a control trial (Ctr), 12-hr premeal exercise trial (12-hr Pre), and 24-hr premeal exercise trial (24-hr Pre). In each trial, subjects had a fat-rich meal. In the exercise trials they jogged on a treadmill at 60% of their VO2max for 1 hr at a designated time. Blood samples were taken at 0 (immediately before the fat meal), and at 2, 4, 6, 8, and 24 hrs after the meal. The results indicated that plasma TG concentrations in 12-hr Pre were lower than in Ctr and 24-hr Pre (p < 0.03). The area score under the TG concentration curve (TG AUC score) in 12-hr Pre was 37% and 33% lower than in 24-hr Pre and Ctr (p < 0.02), respectively. Insulin concentrations in 12-hr Pre were lower than Ctr and 24-hr Pre (p < 0.001). The plasma nonesterified fatty acid (NEFA) concentration was higher in 12-hr Pre than in both 24-hr Pre and Ctr (p < 0.003). There were no trial differences in both HDLtot-Ch and HDL2-Ch. These results suggest that exercising 12 hrs prior to a fat-meal intake significantly reduces PHTG response whereas exercising 24 hrs prior to the meal does not attenuate PHTG in hypertriglyceridemic men. The effect of an acute exercise bout on PHTG lowering may be short-lived and diminished by 24 hrs.