Martin E. Héroux

Absence of lateral gastrocnemius activity and differential motor unit behavior in soleus and medial gastrocnemius during standing balance.

In a standing position, the vertical projection of the center of mass passes in front of the ankle, which requires active plantar-flexor torque from the triceps surae to maintain balance. We recorded motor unit (MU) activity in the medial (MG) and lateral (LG) gastrocnemius muscle and the soleus (SOL) in standing balance and voluntary isometric contractions to understand the effect of functional requirements and descending drive from different neural sources on motoneuron behavior. Single MU activity was recorded in seven subjects with wire electrodes in the triceps surae. Two 3-min standing balance trials and several ramp-and-hold contractions were performed. Lateral gastrocnemius MU activity was rarely observed in standing. The lowest thresholds for LG MUs in ramp contractions were 20-35 times higher than SOL and MG MUs (P < 0.001). Compared with MUs from the SOL, MG MUs were intermittently active (P < 0.001), had higher recruitment thresholds (P = 0.022), and greater firing rate variability (P < 0.001); this difference in firing rate variability was present in standing balance and isometric contractions. In SOL and MG MUs, both recruitment of new MUs (R(2) = 0.59-0.79, P < 0.01) and MU firing rates (R(2) = 0.05-0.40, P < 0.05) were associated with anterior-posterior and medio-lateral torque in standing. Our results suggest that the two heads of the gastrocnemius may operate in different ankle ranges with the larger MG being of primary importance when standing, likely due to its fascicle orientation. These differences in MU discharge behavior were independent of the type of descending neural drive, which points to a muscle-specific optimization of triceps surae motoneurons.

Changes in the length and three-dimensional orientation of muscle fascicles and aponeuroses with passive length changes in human gastrocnemius muscles

The human gastrocnemius muscle is slack at its shortest lengths in vivo. We hypothesised that when the muscle is lengthened slack is taken up progressively, first in some muscle fascicles, then in others. A new, quasi‐three‐dimensional ultrasound imaging method was used to test this hypothesis directly. Contrary to our prediction, the slack was taken up nearly simultaneously in muscle fascicles located throughout the gastrocnemius muscle. Dynamic three‐dimensional reconstructions indicated that, over the range of lengths that was investigated, passive changes in the length of gastrocnemius muscle–tendon units are due to changes in the length of both muscle fascicles and tendinous structures, in approximately equal measure. Changes in the alignment of muscle fascicles and the curvature of aponeuroses have a negligible contribution to change in muscle length. These findings give insights into how skeletal muscles change their lengths in vivo.

The effect of inertial loading on wrist postural tremor in essential tremor

OBJECTIVES Determine the effect of inertial loading on the strength of motor unit entrainment and the synergistic/competitive interaction between central and mechanical reflex tremor components in subjects with essential tremor (ET). METHODS Twenty-three subjects with ET and 22 controls held their hand in an outstretched position while supporting sub-maximal loads (no-load, 5%, 15% and 25% 1-repetition maximum). Hand postural tremor and wrist extensor neuromuscular activity were recorded. RESULTS Inertial loading resulted in a reduction in postural tremor in all ET subjects. The largest reduction in tremor amplitude occurred between 5% and 15% loads, which was associated with spectral separation of the mechanical reflex and central tremor components in a large number of ET subjects. Despite an increase in overall neuromuscular activity with inertial loading, EMG tremor spectral power did not increase with loading. CONCLUSIONS The effect of inertial loading on postural tremor amplitude appears to be mediated in large part by its effect on the interaction between mechanical reflex and central tremor components. Also, ET is associated with a constant absolute level of motor unit entrainment. SIGNIFICANCE The amplitude of postural tremor is dependent on both central and peripheral factors, with proportionally greater motor unit entrainment occurring at low contraction intensities.

The Use and Abuse of Transcranial Magnetic Stimulation to Modulate Corticospinal Excitability in Humans

The magnitude and direction of reported physiological effects induced using transcranial magnetic stimulation (TMS) to modulate human motor cortical excitability have proven difficult to replicate routinely. We conducted an online survey on the prevalence and possible causes of these reproducibility issues. A total of 153 researchers were identified via their publications and invited to complete an anonymous internet-based survey that asked about their experience trying to reproduce published findings for various TMS protocols. The prevalence of questionable research practices known to contribute to low reproducibility was also determined. We received 47 completed surveys from researchers with an average of 16.4 published papers (95% CI 10.8–22.0) that used TMS to modulate motor cortical excitability. Respondents also had a mean of 4.0 (2.5–5.7) relevant completed studies that would never be published. Across a range of TMS protocols, 45–60% of respondents found similar results to those in the original publications; the other respondents were able to reproduce the original effects only sometimes or not at all. Only 20% of respondents used formal power calculations to determine study sample sizes. Others relied on previously published studies (25%), personal experience (24%) or flexible post-hoc criteria (41%). Approximately 44% of respondents knew researchers who engaged in questionable research practices (range 32–70%), yet only 18% admitted to engaging in them (range 6–38%). These practices included screening subjects to find those that respond in a desired way to a TMS protocol, selectively reporting results and rejecting data based on a gut feeling. In a sample of 56 published papers that were inspected, not a single questionable research practice was reported. Our survey revealed that approximately 50% of researchers are unable to reproduce published TMS effects. Researchers need to start increasing study sample size and eliminating—or at least reporting—questionable research practices in order to make the outcomes of TMS research reproducible.

The Relation between Aerobic Fitness, Muscular Fitness, and Obesity in Children from Three Countries at Different Stages of the Physical Activity Transition.

Background. The physical activity transition is contributing to an increase in childhood obesity and a decrease in fitness worldwide. This study compared body composition and fitness measures in children from three countries and examined intercountry differences in the relationship between these variables. Methods. Participants consisted of 736 Canadian, 193 Mexican, and 179 Kenyan children aged 9–13 years. Body mass index (BMI), waist circumference, triceps skinfolds, aerobic fitness, and muscular fitness were measured. Linear regression was used to examine associations between variables. Results. The prevalence of obesity was the highest in Mexican children (9.2% boys, 8.4% girls) and the lowest in Kenyan children (0.9% boys, 2.8% girls). Aerobic fitness (VO2max in mL/kg/min) was the highest in Kenyan children (50.2 boys, 46.7 girls) and the lowest in Canadian children (41.3 boys, 38.3 girls). Aerobic fitness was negatively associated with body composition measures irrespective of country and sex. Mexican children with low aerobic fitness had higher body composition measures than Canadian and Kenyan children. Muscular fitness was not associated with the body composition measures in Kenyan children but was a weak positive correlate of BMI and waist circumference in Canadian and Mexican children. Conclusion. The current study provides some evidence to support the physical activity transition hypothesis.

Human muscle fatigue, eccentric damage and coherence in the EMG.

Eccentric exercise involves contraction of a muscle when it is being lengthened. An untrained person performing eccentric exercise will experience stiffness, swelling, soreness and weakness due to muscle fibre damage that persists for several days (Proske & Morgan 2001). The physiological mechanisms surrounding this muscle damage have received much attention (e.g. Clarkson & Hubal 2002). Much less is known about the effect of muscle damage on the neural drive to muscles after eccentric exercise. In this issue, Semmler et al. (2013) confirm the effects of eccentric exercise on maximal force, muscle endurance, force fluctuations and electromyography (EMG) amplitude. They also show that coherence between EMG recordings of agonist elbow flexor muscles increases during a fatiguing contraction 2 h after eccentric exercise, but not before or 48 h after. While oscillatory drive appears to increase during a fatiguing contraction after eccentric exercise, certain factors need to be considered before the reported changes in EMG–EMG coherence can be attributed to eccentric muscle damage. Corticomuscular and EMG–EMG coherence are measures commonly used to investigate descending cortical oscillatory drive to a muscle or common oscillatory inputs to different muscles (Farmer 1998). Assessment of the neural processes involved in EMG– EMG coherence, and its functional significance is an active research area. Currently, coherence in various frequency bands is thought to reflect different neural processes: 0–5 Hz ‘common drive’, 8–12 Hz physiological tremor involving subcortical and spinal mechanisms and 15–30 Hz beta activity from descending corticospinal drive (Farmer 1998). Synchrony in the beta range increases force production (Baker et al. 1999), which would be beneficial during a fatiguing contraction. However, synchrony and coherence in other bands can increase force fluctuations and EMG amplitude, but not necessarily force production (Halliday et al. 1999, Yao et al. 2000). Thus, a global increase in EMG–EMG coherence across a wide band, such as the 0–60 Hz reported by Semmler et al. (2013), is difficult to interpret beyond providing evidence for a widespread increase in oscillatory drive during a fatiguing contraction. Within the first 2 h following eccentric exercise, there is acute inflammation, reduced maximal force production, impaired proprioception, altered muscle length–tension relationship, inhibition or disfacilitation of the motor cortex or motor neurones, increased muscle stiffness and focal disruption in about onethird of muscle fibres on biopsy (e.g. Clarkson & Hubal 2002, Prasartwuth et al. 2005, 2006, Hoang et al. 2007, Torres et al. 2010). By 48 h, intramuscular oedema and muscle soreness peak, plasma creatine kinase levels begin to rise, and about half of the muscle fibres show focal disruption (reviewed in Clarkson & Hubal 2002). Although increased EMG–EMG coherence during a fatiguing contraction has been seen by others in the absence of eccentric exercise (see Semmler et al. 2013 for references), Semmler et al. (2013) observed no rise in coherence either before or 48 h after eccentric exercise. This raises the question of what aspect of eccentric exercise, present at 2 h but not before or 48 h after exercise, generated the observed effect. In the absence of a control condition such as concentric exercise or a rise in EMG–EMG coherence at 48 h, it is probably premature to attribute the observed changes in EMG–EMG coherence to muscle damage caused by the eccentric exercise. Although coherence measures can provide key insights into neuromuscular coupling, there is vigorous debate about whether the EMG should be rectified prior to coherence analysis (see introduction in Farina et al. 2013 for a recent review). The argument in support of this pre-processing step is that it facilitates the detection of oscillatory inputs by increasing the spectral power associated with these inputs, while the argument against rectification is that it is a nonlinear process that distorts the frequency content of the original signal. Farina et al. (2013) have recently shown that the decision to use rectification should be based on the amount of EMG signal cancellation, which occurs when overlapping positive and negative phases of different motor unit potentials cancel. Specifically, when contraction levels are low, rectification is a desirable pre-processing step to identify common oscillatory inputs to motor neurones, but not at higher contraction intensities when amplitude cancellation is greater. They emphasized that rectification, a pre-processing step used by Semmler et al. (2013), should be avoided when coherence is compared across conditions in which the amount of amplitude cancellation varies because it changes the estimates of linear transmission of oscillatory inputs to motor neurones (i.e. coherence). Importantly, amplitude cancellation increases during fatiguing submaximal contractions

The effect of contraction intensity on force fluctuations and motor unit entrainment in individuals with essential tremor

OBJECTIVES Quantify the effect of increasing contraction intensity on the amplitude of force fluctuations and neuromuscular and force tremor spectral power. METHODS Twenty-one subjects with essential tremor (ET) and 22 healthy controls applied isometric wrist extension contractions. Various sub-maximal contraction intensities were evaluated (5%-, 10%-, 20%- and 30%-MVC). Force fluctuations and wrist extensor neuromuscular activity were recorded using a load cell and electromyography (EMG). RESULTS Higher contraction intensities were associated with larger amplitude force fluctuations and greater neuromuscular activation. However, spectral power associated with tremor peaks remained relatively constant (EMG) or decreased (force) with increasing contraction intensity. CONCLUSIONS Motor unit entrainment associated with centrally generated oscillatory inputs does not increase with greater levels of muscle activation. SIGNIFICANCE Rather than influencing a constant proportion of active motor units, abnormal oscillatory drive influences a relative constant number of total motor units. When combined with the findings from our previous study on postural tremor, the present results provide preliminary evidence that abnormal stretch reflex activity may contribute to this motor unit entrainment.

Motor units in the human medial gastrocnemius muscle are not spatially localized or functionally grouped

Human medial gastrocnemius (MG) motor units (MUs) are thought to occupy small muscle territories or regions, with low‐threshold units preferentially located distally. We used intramuscular recordings to measure the territory of muscle fibres from MG MUs and determine whether these MUs are grouped by recruitment threshold or joint action (ankle plantar flexion and knee flexion). The territory of MUs from the MG muscle varied from somewhat localized to highly distributed, with approximately half the MUs spanning at least half the length and width of the muscle. There was also no evidence of regional muscle activity based on MU recruitment thresholds or joint action. The CNS does not have the means to selectively activate regions of the MG muscle based on task requirements.

Is this my finger? Proprioceptive illusions of body ownership and representation

•  The brain keeps a representation of which things are part of our body. This sense of ownership is easily manipulated using brushing of the skin or movement of a limb to create an illusion of ownership over an inanimate object, such as a rubber hand. •  We induced a sense of ownership of an artificial finger using movement of the index finger without vision of the hands. As cutaneous receptors had been anaesthetised, this illusion depended on proprioceptive signals from muscle receptors. •  In addition, we found a new grasp illusion in which perceived distance between the index fingers decreases when subjects hold an artificial finger. •  These results increase understanding of how the brain generates our body representation and may help in understanding diseases in which the sense of ownership is disrupted.

Questionable science and reproducibility in electrical brain stimulation research

Electrical brain stimulation (EBS) is a trendy new technique used to change brain function and treat neurological, psychiatric and psychological disorders. We were curious whether the published literature, which is dominated by positive results, reflects the experience of researchers using EBS. Specifically, we wanted to know whether researchers are able to reproduce published EBS effects and whether they engage in, but fail to report, questionable research practices. We invited 976 researchers to complete an online survey. We also audited 100 randomly-selected published EBS papers. A total of 154 researchers completed the survey. Survey respondents had a median of 3 [1 to 6, IQR] published EBS papers (1180 total) and 2 [1 to 3] unpublished ones (380 total). With anodal and cathodal EBS, the two most widely used techniques, 45–50% of researchers reported being able to routinely reproduce published results. When asked about how study sample size was determined, 69% of respondents reported using the sample size of published studies, while 61% had used power calculations, and 32% had based their decision on pilot data. In contrast, our audit found only 6 papers where power calculations were used and a single paper in which pilot data was used. When asked about questionable research practices, survey respondents were aware of other researchers who selectively reported study outcomes (41%) and experimental conditions (36%), adjusted statistical analysis to optimise results (43%), and engaged in other shady practices (20%). Fewer respondents admitted to engaging in these practices themselves, although 25% admitted to adjusting statistical analysis to optimize results. There was strong agreement that such practices should be reported in research papers; however, our audit found only two such admissions. The present survey confirms that questionable research practices and poor reproducibility are present in EBS studies. The belief that EBS is effective needs to be replaced by a more rigorous approach so that reproducible brain stimulation methods can be devised and applied.