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Cross-education enhances the performance of muscles not directly involved in the chronic conditioning of the muscles in a remote limb. Substantial cross-education occurs after training with eccentric contractions or with contractions evoked by electromyostimulation (EMS). Since during EMS and eccentric contractions, skin and muscle afferents are activated that have excitatory effects on contralateral homologous muscles, it was hypothesized that exercise training with stimulated vs. voluntary eccentric contractions would lead to greater cross-education. Thirty-two women were randomly assigned to a voluntary (Vol), an EMS, or a remote EMS (rEMS) exercise group and performed 840 voluntary or stimulated eccentric contractions over 6 weeks. All subjects, including nonexercising controls (Con), were tested pre- and posttraining for maximal voluntary and stimulated isometric and eccentric quadriceps strength. Ipsilateral voluntary and stimulated forces increased in all groups. Changes in EMG activity paralleled those in voluntary force in each limb. No changes occurred in grip strength. The greater contra- and ipsilateral strength gains after EMS training were most likely related to an additive effect of EMS and muscle lengthening.

In this study we used a biofeedback system to evaluate the joint movements of 3-month-old infants in real-time. The computer was set to discriminate a specific leg position as the motor task infants had to discover to receive the reinforcement from a mobile. Two groups of infants were given two different tasks: (1) to cross 85° knee flexion in the flexion group, and (2) to cross 35° knee extension in extension group. The results from this experiment suggest that infants in both groups learned the task; however, they used different motor solutions. Infants in the flexion group demonstrated two distinctly different motor solutions to make the mobile move. One was movement-based and was characterized by an increase in kicking frequency, while the other was posture-based. In contrast, most infants in the extension group only used the movement-based solution to gain the reinforcements. Controlling the knee at the 35° extension positions seems to be more difficult than at the 85° flexion positions for 3-month-old infants. These results indicate that infants are capable of discovering narrowly defined leg motor solutions and that, depending on their individual characteristics and the task demands, they select different motor solutions.

The purpose of this study was to simulate the control of an external force using different strategies of muscle coordination and to compare the predicted patterns of muscle forces with those of electromyographic activity reported in the literature for the same task. We simulated a motor task in which a person sitting on a chair exerts an external force by pushing on the ground (or pulling a strap) in five different directions with two different force magnitudes. The results of this study suggest that during the control of an external force in pushing directions, more force is allocated to muscles with long moment arms and a large physiological cross-sectional area, and the number of simultaneously active muscles is increased. This strategy of muscle coordination corresponds to the strategy of minimizing muscle fatigue, and it is characterized by features of muscle coordination that agree with those reported in experimental studies of walking, running, jumping, and cycling.

Differences in the directions of torque produced by motor units might be used by the nervous system to coordinate posture and movement. Here we report plantar flexion and abduction isometric torques exerted at the ankle by 158 motor units in the cat medial gastrocnemius (MG) muscle. In five cats, motor unit torque direction differed by an average of 10°. Torque direction was weakly correlated with unit contraction time in 3 of 5 experiments, with tetanic force in 3 of 5 experiments, and with conduction velocity in 1 of 5 experiments. The direction of whole muscle torque, however, was constant at all levels of MG activation produced in sural and crossed extension reflexes. Thus, although there is a range in the direction of torque produced by motor units in the cat MG, we find no evidence for the ordering of motor units according to torque direction during MG activation.

This cross-sectional study aimed at developing a biomechanical method to objectify voluntary and unpredictable movements, using an automated three-dimensional motion capture system and surface electromyography. Fourteen experienced theater performers were tested while executing the old man exercise, wherein they have to walk like an old man, building up a sustained high intensive muscular activity and tremor. Less experienced performed showed a different kinematics of movement, a slower speed of progression and more variable EMG signals at higher intensity. Female performers also differed from males in movement kinematics and muscular activity. The number of the trial only influenced the speed of progression. The performers showed results which could be well placed within the stages of learning and the degrees of freedom problem.

Cutaneous reflexes have been described primarily according to their actions in the flexion/extension plane. It is shown here, by measuring electromyography and isometric force in decerebrate cats, that ankle muscles are activated in relation to their actions in the abduction/adduction plane during sural nerve (SNR) and crossed-extension (XER) reflexes. Ankle adductors (tibialis posterior, extensor digitorum longus, and flexors digitorum and hallucis longus) were active in XER, but not in SNR. Muscles producing ankle abduction (medial and lateral gastrocnemii and peroneus longus and brevis) were often activated in both reflexes, and medial gastrocnemius and peroneus longus were consistently more strongly activated in SNR than in XER. This differential pattern of muscle activation results in greater abduction torque at the ankle in SNR than in XER. These data demonstrate reflex organization in relation to the multidirectional torque generated by individual muscles.

It has been shown that crossing the midline affects the performance of fine motor skills but the underlying mechanisms are not well understood. This issue is particularly important with respect to the development of motor activities such as writing or pointing in children. Forty-eight right-handed children performed goal-directed movements toward targets positioned either at the midline, or in the left (contralateral side), or right (ipsilateral) hemispace. Findings revealed that movements were more accurate in ipsilateral than in contralateral space and their overall accuracy increased by 42% between 6 and 10 years of age. Differences in movement time among hemispaces depended on the joints predominantly involved in producing the movements (wrist versus fingers). Lower accuracy of movements in contralateral workspace is also present when participants do not have to cross the midline but only move within this workspace. In motor proficient children, no developmental trends were found for these hemispace effects.