Inge Zijdewind

Motor fatigue and cognitive task performance in humans

During fatiguing submaximal contractions a constant force production can be obtained at the cost of an increasing central command intensity. Little is known about the interaction between the underlying central mechanisms driving motor behaviour and cognitive functions. To address this issue, subjects performed four tasks: an auditory choice reaction task (CRT), a CRT simultaneously with a fatiguing or a non‐fatiguing submaximal muscle contraction task, and a fatiguing submaximal contraction task alone. Results showed that performance in the single‐CRT condition was relatively stable. However, in the fatiguing dual‐task condition, performance levels in the cognitive CRT deteriorated drastically with time‐on‐task. Moreover, in the fatiguing dual‐task condition the rise in force variability was significantly larger than during the fatiguing submaximal contraction alone. Thus, our results indicate a mutual interaction between cognitive functions and the central mechanisms driving motor behaviour during fatigue. The precise nature of this interference, and at what level this interaction takes place is still unknown.

Corticospinal excitability during observation and imagery of simple and complex hand tasks: Implications for motor rehabilitation

Movement observation and imagery are increasingly propagandized for motor rehabilitation. Both observation and imagery are thought to improve motor function through repeated activation of mental motor representations. However, it is unknown what stimulation parameters or imagery conditions are optimal for rehabilitation purposes. A better understanding of the mechanisms underlying movement observation and imagery is essential for the optimization of functional outcome using these training conditions. This study systematically assessed the corticospinal excitability during rest, observation, imagery and execution of a simple and a complex finger-tapping sequence in healthy controls using transcranial magnetic stimulation (TMS). Observation was conducted passively (without prior instructions) as well as actively (in order to imitate). Imagery was performed visually and kinesthetically. A larger increase in corticospinal excitability was found during active observation in comparison with passive observation and visual or kinesthetic imagery. No significant difference between kinesthetic and visual imagery was found. Overall, the complex task led to a higher corticospinal excitability in comparison with the simple task. In conclusion, the corticospinal excitability was modulated during both movement observation and imagery. Specifically, active observation of a complex motor task resulted in increased corticospinal excitability. Active observation may be more effective than imagery for motor rehabilitation purposes. In addition, the activation of mental motor representations may be optimized by varying task-complexity.

Effects of imagery motor training on torque production of ankle plantar flexor muscles

The aim of this study was to investigate in control subjects the effect of imagery training on the torque of plantar‐flexor muscles of the ankle. Twenty‐nine subjects were allocated to one of three groups that performed either imagery training, low‐intensity strength training, or no training (only measurements). The low‐intensity training served as an attention control group. Plantar‐flexor torques were measured before, during, directly after, and 4 weeks after the training period. At the end of a 7‐week training program, significant differences were observed between the maximal voluntary torque production of the imagery training group (136.3 ± 21.8% of pretraining torque) vs. the low‐intensity training group (112.9 ± 29.0%; P < 0.02) and the control group (113.6 ± 19.2%; P < 0.02). The results of this study show that imagery training of lower leg muscles significantly increased voluntary torque production of the ankle plantar‐flexor muscles and that the force increase was not due to nonspecific motivational effects. Such muscle strengthening effects might be beneficial in rehabilitation for improving or maintaining muscle torque after immobilization. Muscle Nerve 28: 168–173, 2003

Muscle fatigue induced by stimulation with and without doublets.

Muscles are usually stimulated by shocks delivered at some constant rate. However, human thenar motor units generate optimum force per pulse when excited by impulse trains that begin with one or two short interpulse intervals (“doublets”), followed by longer intervals. Our aim was to determine whether the rate of force and force–time integral reduction during fatigue of thenar muscles is influenced by an initial doublet, and/or the number of pulses per train. We first matched thenar force–time integral using two different pulse patterns, one of which began with a doublet. Fatigue induced by trains that contained a doublet resulted in slower rates of force and force–time integral reduction and smaller increases in half‐relaxation time than that evoked by bursts of 40‐HZ stimulation. When the force was measured in each protocol after equal numbers of pulses had been delivered, the force loss was still significantly less for pulse trains containing a doublet. These results have useful implications when designing stimulation to strengthen weak muscles or to drive paralyzed muscles.

Motor unit activation order during electrically evoked contractions of paralyzed or partially paralyzed muscles

The activation order of motor units during electrically evoked contractions of paralyzed or partially paralyzed thenar muscles was determined in seven subjects with chronic cervical spinal cord injury. The median nerve was stimulated percutaneously with pulses of graded intensity to produce increments in the compound electromyogram (EMG) and force. Each increment corresponded to the activation of another unit. The evoked unit EMG and force was obtained by digital subtraction. The thenar muscles had between 15 and 83 units (26 ± 19) that produced 114.3 ± 127.1 mN force (n = 290). In six subjects, a significant positive correlation was found between activation order and unit force indicating that weaker units were excited before stronger units. These data are contrary to the notion that a reversal of unit activation order occurs during evoked versus voluntary contractions.

Fatigue of muscles weakened by death of motoneurons

Weakness is a characteristic of muscles influenced by the postpolio syndrome (PPS), amyotrophic lateral sclerosis (ALS), and spinal cord injury (SCI). The strength deficits relate to changes in muscle use and to the chronic denervation that can follow the spinal motoneuron death common to these disorders. PPS, ALS, and SCI also involve variable amounts of supraspinal neuron death, the effects of which on muscle weakness remains unclear. Nevertheless, weakness of muscle itself defines the functional consequences of these disorders. A weaker muscle requires an individual to work that muscle at higher than usual intensities relative to its maximal capacity, inducing progressive fatigue and an increased sense of effort. Little evidence is available to suggest that the fatigue commonly experienced by individuals with these disorders relates to an increase in the intrinsic fatigability of the muscle fibers. The only exception is when SCI induces chronic muscle paralysis. To reduce long‐term functional deficits in these disorders, studies must identify the signaling pathways that influence neuron survival and determine the factors that encourage and limit sprouting of motor axons. This may ensure that a greater proportion of the fibers in each muscle remain innervated and available for use. Muscle Nerve, 2005

Relation between muscle and brain activity during isometric contractions of the first dorsal interosseus muscle

We studied the relationship between muscle activity (electromyography, EMG), force, and brain activity during isometric contractions of the index finger, on a group and individual level. Ten subjects contracted their right or left index finger at 5, 15, 30, 50, and 70% of their maximal force. Subjects received visual feedback of the produced force. We focused our analysis on brain activation that correlated with EMG. Brain activity of specific anatomical areas (region‐of‐interest analysis, ROI) was quantified and correlated with EMG activity. Furthermore, we tried to distinguish between brain areas in which activity was modulated by the amount of EMG and areas that were active during the task but in which the activity was not modulated. Therefore, we used two regressors simultaneously: (1) the produced EMG and (2) the task (a categorical regressor). As expected, activity in the motor areas (contralateral sensorimotor cortex, premotor areas, and ipsilateral cerebellum) strongly correlated with the amount of EMG. In contrast, activity in frontal and parietal areas (inferior part of the right precentral sulcus, ipsilateral supramarginal gyrus, bilateral inferior parietal lobule, bilateral putamen, and insular cortex) correlated with activation per se, independently of the amount of EMG. Activity in these areas was equal during contractions of the right or left index finger. We suppose that these areas are more involved in higher order motor processes during the preparatory phase or monitoring feedback mechanisms. Furthermore, our ROI analysis showed that muscle and brain activity strongly correlate in traditional motor areas, both at group and at subject level. Hum Brain Mapp, 2008.

Influence of a voluntary fatigue test on the contralateral homologous muscle in humans

Influences of a submaximal endurance test in the right first dorsal interosseus on force and fatigue-related parameters of activating the contralateral muscle were studied. The test consisted of a 30% maximum voluntary contraction (MVC), regularly interrupted by maximal contractions and brief rest periods. Despite the induced central fatigue, as tested with the MVC-superimposed twitch technique, and substantial peripheral fatigue, only minor effects of the previous fatigue test were seen for the contralateral hand. No significant influence was found on endurance time, the perceived effort for maintaining 30% MVC force or the MVC-superimposed twitch. Thus, our fatigue protocol induced both central and peripheral fatigue but only minor cross-over effects of fatigue were found for the homologous contralateral muscle.

Spatial differences in fatigue‐associated electromyographic behaviour of the human first dorsal interosseus muscle.

1. Fatigue‐associated electromyographic (EMG) reactions of intrinsic hand muscles were studied during maintained isometric voluntary contractions of normal subjects. Most measurements concerned actions of the first dorsal interosseus (FDI). In a smaller number of subjects, complementary measurements were obtained for adductor pollicis (AP). 2. Measurements were made of isometric force (thumb adduction, index finger abduction and flexion) and of surface EMG amplitudes (AP and FDI) after rectification and smoothing (rsEMG). 3. In the analysis of fatigue, the subjects were required to maintain a steady isometric force (index finger abduction or thumb adduction) of half their maximum voluntary contraction (1/2MVC test) for as long as possible. Average endurance times were 88 +/‐ 19 s (mean +/‐ S.D.) for FDI and 119 +/‐ 29 s for AP (Students t test, P < 0.02). 4. Pronounced differences in fatigue‐associated EMG behaviour were observed between AP and FDI. In AP the reaction was as expected: a rise of EMG during maintained force (mean rsEMG at end of fatigue test/mean rsEMG at start of test (rsEMG‐FI): 181 +/‐ 64%). In FDI this reaction was seen in half of the recorded cases, the remainder displaying bidirectional changes or a more or less marked decrease of EMG during the endurance task (mean for all cases together: rsEMG‐FI, 103 +/‐ 15%; difference between AP vs. FDI significant, P < 0.01). 5. The unexpected EMG variability of the FDI reactions was further analysed with multiple bipolar recordings of surface EMG. For all the four thoroughly studied subjects, recordings were obtained which showed simultaneously occurring EMG changes in opposite directions (decrease and increase) at different sites of FDI while force was kept constant at 50% of the maximum voluntary contraction (MVC). 6. Further observations on FDI showed that EMGs simultaneously obtained from different recording sites could show dramatic differences in their responses depending on ‘synergistic context’ (e.g. in relation to changes in index finger extension force during maintained abduction at 50% MVC). Evidence for ‘task switching’ (shift in rsEMG distribution, shift in hand muscle synergy) was frequently observed during the performance of the 1/2MVC test. 7. The results indicate that FDI is not handled in a topographically homogeneous manner during the execution of an isometric constant force endurance test. Furthermore, the results suggest that this seemingly simple motor performance can be executed in several alternative manners associated with the activation of different muscle synergies and with different distributions of activity within the FDI.

Voluntary activation and cortical activity during a sustained maximal contraction: An fMRI study

Motor fatigue is an exercise‐induced reduction in the force‐generating capacity. The underlying mechanisms can be separated into factors residing in the periphery or in the central nervous system. We designed an experiment in which we investigated central processes underlying motor fatigue by means of magnetic resonance imaging in combination with the twitch interpolation technique. Subjects performed a sustained maximal abduction (2 min) with the right index finger. Brain activation was recorded with an MR scanner, together with index finger abduction force, EMG of several hand muscles and interpolated twitches. Mean activity per volume was calculated for the primary motor cortex and the secondary motor areas (supplementary motor, premotor, and cingulate areas) as well as mean force and mean rectified EMG amplitude. Results showed a progressive decline in maximal index finger abduction force and EMG of the target muscles combined with an increase in brain activity in the contralateral primary motor cortex and secondary motor areas. Analysis of the twitches superimposed on the sustained contraction revealed that during the contraction the voluntary drive decreased significantly. In conclusion, our data showed that despite an increase in brain activity the voluntary activation decreased. This suggests that, although the CNS increased its input to the relevant motor areas, this increase was insufficient to overcome fatigue‐related changes in the voluntary drive. Hum Brain Mapp, 2009.