Imagined Contractions Research Articles

Task-dependent activation of distinct fast and slow(er) motor pathways during motor imagery

BackgroundMotor imagery and actual movements share overlapping activation of brain areas but little is known about task-specific activation of distinct motor pathways during mental simulation of movements. For real contractions, it was demonstrated that the slow(er) motor pathways are activated differently in ballistic compared to tonic contractions but it is unknown if this also holds true for imagined contractions. ObjectiveThe aim of the present study was to assess the activity of fast and slow(er) motor pathways during mentally simulated movements of ballistic and tonic contractions. MethodsH-reflexes were conditioned with transcranial magnetic stimulation at different interstimulus intervals to assess the excitability of fast and slow(er) motor pathways during a) the execution of tonic and ballistic contractions, b) motor imagery of these contraction types, and c) at rest. ResultsIn contrast to the fast motor pathways, the slow(er) pathways displayed a task-specific activation: for imagined ballistic as well as real ballistic contractions, the activation was reduced compared to rest whereas enhanced activation was found for imagined tonic and real tonic contractions. ConclusionsThis study provides evidence that the excitability of fast and slow(er) motor pathways during motor imagery resembles the activation pattern observed during real contractions. The findings indicate that motor imagery results in task- and pathway-specific subliminal activation of distinct subsets of neurons in the primary motor cortex.

Does a Mental Training Session Induce Neuromuscular Fatigue?

Mental training, as physical training, enhances muscle strength. Whereas the repetition of maximal voluntary contractions (MVC) induces neuromuscular fatigue, the effect of maximal imagined contractions (MIC) on neuromuscular fatigue remains unknown. Here, we investigated neuromuscular alterations after a mental training session including MIC, a physical training session including MVC, and a combined training session including both MIC and MVC of the elbow flexor muscles. Ten participants performed 80 MIC (duty cycle, 5-s MIC and 10-s rest), 80 MVC (identical duty cycle), or 80 MVC and 80 MIC (5-s MVC, 2-s rest, 5-s MIC, and 3-s rest) in three separate sessions. MVC torque was assessed five times over the course of the training and 10 min after the end of the training in the three protocols. Central activation ratio (CARc), reflecting central fatigue, and corticospinal excitability, at rest and during MIC, were estimated using transcranial magnetic stimulation. Both the physical training and the combined training induced an approximately 40% drop of MVC torque, accompanied with an approximately 10% decrease of CARc without significant difference between the two sessions. On the contrary, the repetition of MIC did not reduce maximal force production capacity and did not alter CARc. Corticospinal excitability was always facilitated during MIC compared with that during rest, ensuring that the participants imagined the desired movement. These results suggested that one session of mental training alone or combined with physical training do not induce (additional) neuromuscular fatigue despite the repetitive activation of the corticospinal track. Motor imagery may be added to physical practice to increase the total workload without exacerbating neuromuscular fatigue.

Kinesthetic Motor Imagery Acutely Increases Spinal Excitability

INTRODUCTION & PURPOSE: Functional brain imaging studies indicate motor imagery (MI) of a task or movement utilizes similar neural substrates as actual performance. Data on whether MI results in a descending volley of neural/spinal activity is equivocal. Therefore, the purpose of this study was to determine if imagined plantar flexion (PF) muscle contractions alter spinal excitability. METHODS: 17 healthy volunteers (30 Â ± 8 years) were asked to perform imagined (kinesthetic imagery approach) contractions of their left PF at 25% and 100% of MVC, after performing several practice trials of actual muscle contraction at those intensities. Upon a verbal command and in randomized fashion each subject performed ten imagined contractions at both intensities (∼ 15-sec duration). Randomly between the 8th and 15th second during a contraction the soleus H-reflex was evoked to assess spinal excitability. Additionally, the H-reflex was assessed 15 seconds before and after each imagined contraction. The H-reflexes were elicited with an intensity equal to 20 + 2.5% of Mmax and expressed relative to this measure (H20:Mmax). A repeated measures ANOVA was performed to assess the effect of learning (trials 2–4, 5–7, 8–10), time (at rest, during MI and recovery) and intensity (25% vs. 100% MVC). RESULTS: During MI the H-reflex was facilitated, and immediately returned to baseline values after cessation (see Figure; p < 0.01; Î-=0.40). There was a trend for the MVC intensity level to result in a slightly heightened response than the submaximal MI (see Figure; p=0.06; Î-=0.19).FigureCONCLUSION: MI acutely results in a heightened H-reflex excitability, which tended to be more exaggerated during higher intensity contractions. This finding suggests MI results in brain activation, and sends a descending volley increasing excitatory input at the level of the spinal cord. ACKNOWLEDGEMENTS: This work was supported in part by a NASA Training Grant (NGT5-50446).

Neural Adaptations to Resistive Exercise

It is generally accepted that neural factors play an important role in muscle strength gains. This article reviews the neural adaptations in strength, with the goal of laying the foundations for practical applications in sports medicine and rehabilitation. An increase in muscular strength without noticeable hypertrophy is the first line of evidence for neural involvement in acquisition of muscular strength. The use of surface electromyographic (SEMG) techniques reveal that strength gains in the early phase of a training regimen are associated with an increase in the amplitude of SEMG activity. This has been interpreted as an increase in neural drive, which denotes the magnitude of efferent neural output from the CNS to active muscle fibres. However, SEMG activity is a global measure of muscle activity. Underlying alterations in SEMG activity are changes in motor unit firing patterns as measured by indwelling (wire or needle) electrodes. Some studies have reported a transient increase in motor unit firing rate. Training-related increases in the rate of tension development have also been linked with an increased probability of doublet firing in individual motor units. A doublet is a very short interspike interval in a motor unit train, and usually occurs at the onset of a muscular contraction. Motor unit synchronisation is another possible mechanism for increases in muscle strength, but has yet to be definitely demonstrated. There are several lines of evidence for central control of training-related adaptation to resistive exercise. Mental practice using imagined contractions has been shown to increase the excitability of the cortical areas involved in movement and motion planning. However, training using imagined contractions is unlikely to be as effective as physical training, and it may be more applicable to rehabilitation. Retention of strength gains after dissipation of physiological effects demonstrates a strong practice effect. Bilateral contractions are associated with lower SEMG and strength compared with unilateral contractions of the same muscle group. SEMG magnitude is lower for eccentric contractions than for concentric contractions. However, resistive training can reverse these trends. The last line of evidence presented involves the notion that unilateral resistive exercise of a specific limb will also result in training effects in the unexercised contralateral limb (cross-transfer or cross-education). Peripheral involvement in training-related strength increases is much more uncertain. Changes in the sensory receptors (i.e. Golgi tendon organs) may lead to disinhibition and an increased expression of muscular force. Agonist muscle activity results in limb movement in the desired direction, while antagonist activity opposes that motion. Both decreases and increases in co-activation of the antagonist have been demonstrated. A reduction in antagonist co-activation would allow increased expression of agonist muscle force, while an increase in antagonist co-activation is important for maintaining the integrity of the joint. Thus far, it is not clear what the CNS will optimise: force production or joint integrity. The following recommendations are made by the authors based on the existing literature. Motor learning theory and imagined contractions should be incorporated into strength-training practice. Static contractions at greater muscle lengths will transfer across more joint angles. Submaximal eccentric contractions should be used when there are issues of muscle pain, detraining or limb immobilisation. The reversal of antagonists (antagonist-to-agonist) proprioceptive neuromuscular facilitation contraction pattern would be useful to increase the rate of tension development in older adults, thus serving as an important prophylactic in preventing falls. When evaluating the neural changes induced by strength training using EMG recording, antagonist EMG activity should always be measured and evaluated.

Task-involvement and ego-involvement goals during actual and imagined movements: Their effects on cognitions and vegetative responses

It has been experimentally proven many times that the mental rehearsal of an activity not only improves motor performance but also has vegetative effects whose magnitude is correlated with the amount of imagined effort. These beneficial effects of mental imagery have been explained in terms of central programming structures capable of anticipating the metabolic demands of the task. Twenty-four subjects were asked to actually perform and also imagine an isometric contraction of the forearm under various goal conditions: a task-involving goal (8 subjects), an ego-involving goal (8 subjects), and no goal (8 subjects). During the contractions, electromyographic potential and heart rate were measured. Afterwards, the subjects were asked to indicate the amount of effort expended under different feedback conditions. The results showed no trace of electromyographic activity during the imagined contractions when the lack of movement was controlled using a force sensor. On the other hand, a significantly faster in heart rate was observed with a task- or ego-involving goal than with no goal, during both actual and imagined contraction. Similarly, as predicted, subjects said they applied less effort in the positive feedback condition, and more effort in the negative feedback condition with an ego-involving goal. The results are discussed in the light of goal theories, while regarding goals not only as serving to anticipate metabolic expenditures but also as promoting a self-image of competence, particularly in threatening, ego-involving situations.