The effect of abdominal functional electrical stimulation on blood pressure in people with high level spinal cord injury.

STUDY DESIGN: Single centre training study. OBJECTIVES: To investigate, in a group of people with spinal cord injury (SCI), the effect of transcutaneous functional electrical stimulation of the abdominal muscles (abdominal FES) during cough training on blood pressure (BP), and how it is affected by injury characteristics and alters over time. SETTING: Laboratory and community. METHODS: Sixteen participants with SCI (C4-T5) underwent 25 of abdominal FES cough training (5 sets of 10 stimulated coughs) over 6 weeks as part of a previously published study on the effect of abdominal FES training on cough. Systolic BP (SBP), diastolic BP (DBP) and calculated mean arterial pressure (MAP) were measured at the completion of each set. RESULTS: Abdominal FES coughing resulted in an average ~30% acute increase in BP from initial resting BP across all sessions in almost all participants (p < 0.001). However, the increase in BP during abdominal FES coughs from rest reduced over the 25 sessions of training by ~35% for SBP, MAP and DBP (p = 0.024, p = 0.013 and p = 0.042, respectively). There was no meaningful change in resting BP over time (p = 0.935, p = 0.705 and p = 0.988, respectively). Overall, increases in BP during abdominal FES coughs were greatest for those with chronic injuries and cervical injuries. CONCLUSIONS: Transcutaneous abdominal FES during cough training acutely increases BP. However, the magnitude of the increase is reduced after 25 sessions of training. Abdominal FES may offer a solution to combat orthostatic hypotension, but its effectiveness may diminish over time.

Pain tolerance and the thresholds of human sensory and motor axons to single and repetitive bursts of kilohertz-frequency stimulation.

Transcutaneous electrical stimulation with repetitive bursts of a kilohertz carrier frequency is thought to be less painful than conventional pulsed currents by reducing the sensitivity of pain receptors. However, no purported benefit has been shown unequivocally. We compared the effects of carrier-frequency stimulation and conventional stimulation on pain tolerance and the thresholds for sensory and motor axons in twelve participants. The ulnar nerve was stimulated transcutaneously with a conventional single pulse and 5 and 10 kHz carrier-frequency waveforms that had 5 and 10 pulses, respectively, when delivered in bursts of ∼1 ms duration. Phase durations were adjusted across waveform types to match the total charge for a given current amplitude. Single bursts of stimulation were delivered from 1 mA up until no longer tolerable. This was repeated with repetitive bursts of stimulation at 20 Hz for 1 s. Participants tolerated higher current amplitudes with both carrier-frequency waveforms than conventional stimulation, with repetitive bursts more painful than single bursts. However, compared to conventional stimulation, carrier-frequency waveforms required more current to produce sensory and motor-threshold responses and to obtain a maximal motor response (Mmax). When the current at pain tolerance was normalised to the current at Mmax, participants tolerated lower stimulus intensities with carrier-frequency waveforms than conventional stimulation. These findings indicate that there is little to no benefit in using carrier-frequency waveforms to minimise the discomfort from electrical stimulation as the increase in stimulus intensity at pain tolerance is more than offset by reduced effectiveness in the activation of sensory and motor axons. KEY POINTS: Transcutaneous electrical stimulation with repetitive bursts of a kilohertz carrier-frequency waveform is thought to be less painful than conventional pulsed currents. For ulnar nerve stimulation, when stimulus waveforms were matched for total phase charge, participants tolerated higher current amplitudes with carrier-frequency stimulation than conventional stimulation. However, compared to conventional stimulation, carrier-frequency waveforms required more current to produce a threshold response in both sensory and motor axons and to produce a maximal motor response (Mmax). When current at pain tolerance was normalised to current at Mmax, participants tolerated lower stimulus intensities with carrier-frequency waveforms than conventional stimulation. Carrier-frequency waveforms provide little to no benefit in minimising the discomfort from transcutaneous electrical stimulation as the increase in stimulus intensity at pain tolerance is more than offset by reduced effectiveness in activating sensory and motor axons.

Motor-evoked potentials in the human upper and lower limb do not increase after single 30-min sessions of acute intermittent hypoxia.

Acute intermittent hypoxia (AIH) can induce sustained facilitation of motor output in people with spinal cord injury (SCI). Most studies of corticospinal tract excitability in humans have used 9% fraction inspired oxygen ([Formula: see text]) AIH (AIH-9%), with inconsistent outcomes. We investigated the effect of single sessions of 9% [Formula: see text] and 12% [Formula: see text] AIH (AIH-12%) on corticospinal excitability of a hand and leg muscle in able-bodied adults. Ten naïve participants completed three sessions on separate days comprising 15 epochs of 1 min of AIH-9%, AIH-12%, or sham (SHAM-21%) followed by 1 min of room air (21% [Formula: see text]) in a randomized crossover design. Motor-evoked potentials (MEPs; n = 30, ∼1 mV) elicited at rest by transcranial magnetic stimulation and maximal M-waves (Mmax) evoked by peripheral nerve stimulation were measured from the first dorsal interosseous (FDI) and tibialis anterior (TA) muscles at baseline and at ∼0, 20, 40, and 60 min post intervention. AIH-9% induced the greatest reduction in peripheral oxygen saturation (to 85% vs. 93% and 100% in AIH-12% and SHAM-21%, respectively; P < 0.001) and the greatest increase in ventilation [by 22% vs. 12% and -3% in AIH-9%, AIH-12%, and SHAM-21%, respectively (P < 0.001)]. There was no difference in MEP amplitudes (%Mmax) after any of the three conditions (AIH-9%, AIH-12%, SHAM-21%) for both the FDI (P = 0.399) and TA (P = 0.582). Despite greater cardiorespiratory changes during AIH-9%, there was no evidence of corticospinal facilitation (tested with MEPs) in this study. Further studies could explore variability in response to AIH between individuals and other methods to measure motor facilitation in people with and without spinal cord injuries.NEW & NOTEWORTHY This is the first study that tests whether acute intermittent hypoxia (AIH) induces motor output facilitation in humans after two different doses of AIH (9% and 12% [Formula: see text]) and the reproducibility of participant responses after a repeat AIH intervention at 9% AIH. There was no motor output facilitation in response to either dose of AIH. The results question the effectiveness of a single 30-min session of AIH in inducing motor output facilitation, tested in this way.

Graded onset of parasternal intercostal inspiratory activity detected with surface electromyography in healthy young females and males.

Intramuscular recordings of single motor unit activity from parasternal intercostal muscles show a rostrocaudal gradient in timing and amplitude of inspiratory activity. This study determined the feasibility of surface electromyographic activity (EMG) to measure graded parasternal intercostal activity in young females and males during quiet breathing and breathing with inspiratory resistive loads. Surface EMGs were recorded from the 1st-to-5th parasternal intercostal muscles during 10 min of breathing. EMGs were processed to remove 50 Hz and electrocardiogram artifacts and integrated. Amplitude and onset time of inspiratory activity were measured from waveform averages triggered at the onset of inspiratory flow. Onset times were measured independently by two assessors, blinded to interspace and EMG scale, with excellent agreement (ICC3,k = 0.86). The onset of inspiratory activity in the 1st-to-3rd interspaces was at or within ∼400 ms of the start of inspiratory airflow, but activity in the caudal (4th and 5th) spaces was delayed by up to ∼1,000 ms (P < 0.001). There was no main effect of sex on onset time (P = 0.07), but an interaction with interspace (P < 0.001) revealed that inspiratory activity in the caudal interspaces was delayed by 15% of inspiratory time in female participants compared with 30% of inspiratory time in male participants. Inspiratory loads did not affect EMG onset time (P = 0.31). Thus, surface EMG is feasible to assess the onset time of inspiratory activity as a marker of inspiratory neural drive and pattern of activation across spaces, in both females and males.NEW & NOTEWORTHY We demonstrated that surface EMG is a valid method to measure graded inspiratory EMG in the parasternal intercostal muscles in healthy young male and female participants during quiet breathing and loaded breathing. Across the 1st-to-5th interspaces, there was more homogenous activation in women and more graded activity in men across parasternal intercostal muscles during breathing. By recording surface EMG from both male and female participants, we have revealed sex differences in inspiratory activity across intercostal muscles.

Regional associations between inspiratory tongue dilatory movement and genioglossus activity during wakefulness in people with obstructive sleep apnoea.

Inspiratory tongue dilatory movement is believed to be mediated via changes in neural drive to genioglossus. However, this has not been studied during quiet breathing in humans. Therefore, this study investigated this relationship and its potential role in obstructive sleep apnoea (OSA). During awake supine quiet nasal breathing, inspiratory tongue dilatory movement, quantified with tagged magnetic resonance imaging, and inspiratory phasic genioglossus EMG normalised to maximum EMG were measured in nine controls [apnoea-hypopnea index (AHI) ≤5 events/h] and 37 people with untreated OSA (AHI >5 events/h). Measurements were obtained for 156 neuromuscular compartments (85%). Analysis was adjusted for nadir epiglottic pressure during inspiration. Only for 106 compartments (68%) was a larger anterior (dilatory) movement associated with a higher phasic EMG [mixed linear regression, beta = 0.089, 95% CI [0.000, 0.178], t(99) = 1.995, P = 0.049, hereafter EMG↗/mvt↗]. For the remaining 50 (32%) compartments, a larger dilatory movement was associated with a lower phasic EMG [mixed linear regression, beta = -0.123, 95% CI [-0.224, -0.022], t(43) = -2.458, P = 0.018, hereafter EMG↘/mvt↗]. OSA participants had a higher odds of having at least one decoupled EMG↘/mvt↗ compartment (binary logistic regression, odds ratio [95% CI]: 7.53 [1.19, 47.47] (P = 0.032). Dilatory tongue movement was minimal (>1 mm) in nearly all participants with only EMG↗/mvt↗ compartments (86%, 18/21). These results demonstrate that upper airway dilatory mechanics cannot be predicted from genioglossus EMG, particularly in people with OSA. Tongue movement associated with minimal genioglossus activity suggests co-activation of other airway dilator muscles. KEY POINTS: Inspiratory tongue movement is thought to be mediated through changes in genioglossus activity. However, it is unknown if this relationship is altered by obstructive sleep apnoea (OSA). During awake supine quiet nasal breathing, inspiratory tongue movement, quantified with tagged magnetic resonance imaging (MRI), and inspiratory phasic genioglossus EMG normalised to maximum EMG were measured in four tongue compartments of people with and without OSA. Larger tongue anterior (dilatory) movement was associated with higher phasic genioglossus EMG for 68% of compartments. OSA participants had an ∼7-times higher odds of having at least one compartment for which a larger anterior tongue movement was not associated with a higher phasic EMG than controls. Therefore, higher genioglossus phasic EMG does not consistently translate into tongue dilatory movement, particularly in people with OSA. Large dilatory tongue movements can occur despite minimal genioglossus inspiratory activity, suggesting co-activation of other pharyngeal muscles.

Comparison of two inspiratory muscle training protocols in people with spinal cord injury: a secondary analysis.

STUDY DESIGN/SETTING: Secondary analysis. OBJECTIVES: To compare the change in maximal inspiratory pressure (PImax) over the first 4 weeks of two different inspiratory muscle training (IMT) protocols and explore if either method is more effective for people with spinal cord injury. METHODS: Data originated from two published studies. Participants completed flow-resistive IMT (F-IMT) at 80% daily PImax, 7 days/week (supervised weekly), or threshold IMT (T-IMT) at 30-80% weekly PImax, twice-daily, 5 days/week (supervised every session). Seven participants from each trial were matched by training adherence, level of spinal cord injury, impairment grade (A-C), and height. Differences between F-IMT and T-IMT groups in training intensity, breaths taken, inspiratory work, and the change in the PImax from baseline at the end of week four were analysed. RESULTS: Over 4 weeks, there was no difference in the change in PImax between groups (Absolute change in PImax (cmH2O): p = 0.456, Percent change in PImax relative to baseline: p = 0.128). F-IMT participants trained at a higher intensity (median: 77 vs 22 cmH2O, p = 0.001 and 80% baseline vs 61% baseline, p = 0.038) but took fewer breaths (840 vs 1404 breaths, p = 0.017) than T-IMT participants. Inspiratory work was similar between groups (64,789 vs 65,910 (% PImax × number of breaths), p = 0.535). CONCLUSIONS: Our findings support both methods of IMT as the change in PImax and inspiratory work were similar between groups. However, daily high-intensity F-IMT with intermittent supervision, required fewer breaths and less participant and therapist time. Future studies should examine optimal dosage and supervision required to achieve increased PImax.

Effect of respiratory muscle training on load sensations in people with chronic tetraplegia: a secondary analysis of a randomised controlled trial.

STUDY DESIGN: Secondary analysis of a randomised controlled trial. OBJECTIVES: Our primary study showed that increasing inspiratory muscle strength with training in people with chronic (>1 year) tetraplegia corresponded with reduced sensations of breathlessness when inspiration was loaded. This study investigated whether respiratory muscle training also affected the respiratory sensations for load detection and magnitude perception. SETTING: Independent research institute in Sydney, Australia. METHODS: Thirty-two adults with chronic tetraplegia participated in a 6-week, supervised training protocol. The active group trained the inspiratory muscles through progressive threshold loading. The sham group performed the same protocol with a fixed threshold load (3.6 cmH2O). Primary measures were load detection threshold and perceived magnitudes of six suprathreshold loads reported using the modified Borg scale. RESULTS: Maximal inspiratory pressure (PImax) increased by 32% (95% CI, 18-45) in the active group with no change in the sham group (p =  0.51). The training intervention did not affect detection thresholds in the active (p =  0.24) or sham (p =  0.77) group, with similar overall decreases in Borg rating of 0.83 (95% CI, 0.49-1.17) in active and 0.72 (95% CI, 0.32-1.12) in sham group. Increased inspiratory muscle strength reduced slope magnitude between Borg rating and peak inspiratory pressure (p =  0.003), but not when pressure was divided by PImax to reflect contraction intensity (p =  0.92). CONCLUSIONS: Training reduces the sensitivity of load sensations for a given change in pressure but not for a given change in contraction intensity.

Transcutaneous spinal stimulation in people with and without spinal cord injury: Effect of electrode placement and trains of stimulation on threshold intensity.

Transcutaneous spinal cord stimulation (TSS) is purported to improve motor function in people after spinal cord injury (SCI). However, several methodology aspects are yet to be explored. We investigated whether stimulation configuration affected the intensity needed to elicit spinally evoked motor responses (sEMR) in four lower limb muscles bilaterally. Also, since stimulation intensity for therapeutic TSS (i.e., trains of stimulation, typically delivered at 15-50 Hz) is sometimes based on the single-pulse threshold intensity, we compared these two stimulation types. In non-SCI participants (n = 9) and participants with a SCI (n = 9), three different electrode configurations (cathode-anode); L1-midline (below the umbilicus), T11-midline and L1-ASIS (anterior superior iliac spine; non-SCI only) were compared for the sEMR threshold intensity using single pulses or trains of stimulation which were recorded in the vastus medialis, medial hamstring, tibialis anterior, medial gastrocnemius muscles. In non-SCI participants, the L1-midline configuration showed lower sEMR thresholds compared to T11-midline (p = 0.002) and L1-ASIS (p < 0.001). There was no difference between T11-midline and L1-midline for participants with SCI (p = 0.245). Spinally evoked motor response thresholds were ~13% lower during trains of stimulation compared to single pulses in non-SCI participants (p < 0.001), but not in participants with SCI (p = 0.101). With trains of stimulation, threshold intensities were slightly lower and the incidence of sEMR was considerably lower. Overall, stimulation threshold intensities were generally lower with the L1-midline electrode configuration and is therefore preferred. While single-pulse threshold intensities may overestimate threshold intensities for therapeutic TSS, tolerance to trains of stimulation will be the limiting factor in most cases.

Exploration of the contribution of physiotherapy students to the delivery of health services: a qualitative study.

OBJECTIVE: To understand the perspectives of physiotherapists on the contribution of students to the delivery of health services during clinical placements. METHODS: Focus groups with a semi-structured interview guide were completed separately with new graduate physiotherapists reflecting on their student experience and experienced physiotherapists from five Queensland public health-sector hospitals. Interviews were transcribed verbatim in preparation for thematic analysis. Interview manuscripts were read independently and initially coding completed. Codes were compared and further refinement of themes occurred. Themes were reviewed by two investigators. RESULTS: There were 38 new graduate participants across nine focus groups and 35 experienced physiotherapists across six focus groups who participated in this study. Students participate in a range of activities during clinical placements some of which contribute to delivery of health services and others which support student learning. Three major themes were identified: 1) tangible student contribution; 2) non-tangible student contribution; and 3) factors that influence the student contribution. CONCLUSIONS: Overwhelmingly, both new graduate and experienced physiotherapists felt that students do contribute to the delivery of health services however careful consideration of a variety of factors is necessary to maximize the student contribution.

Early lateral root formation in response to calcium and nickel shows variation within disjunct populations of Arabidopsis lyrata spp. lyrata.

Root architecture is important in nutrient uptake and avoidance of toxic compounds within the soil. Arabidopsis lyrata spp. lyrata has widespread distribution in disjunct environments that encounter unique stressors starting at germination. Five populations of A. lyrata spp. lyrata show local adaptation to Nickel (Ni) but cross-tolerance to variations in Calcium (Ca) concentration within the soil. Differentiation among the populations begins early in development and appears to impact timing of lateral root formation; therefore the purpose of the study was to understand changes in root architecture and root exploration in response to Ca and Ni within the first three weeks of growth. Lateral root formation was first characterized under one concentration of Ca and Ni. Lateral root formation and tap root length were reduced in all five populations in response to Ni compared to Ca, with the least reduction in the three serpentine populations. When the populations were exposed to a gradient (either Ca or Ni) there were differences in population response based on the nature of the gradient. Start side was the greatest determinant of root exploration and lateral root formation under a Ca gradient, while population was the greatest determinant of root exploration and lateral root formation under a Ni gradient. All populations exhibited about the same frequency of root exploration under a Ca gradient, while the serpentine populations exhibited much higher levels of root exploration under a Ni gradient compared to the two non-serpentine populations. Differences among populations in response to Ca and Ni demonstrate the importance of stress responses early in development, particularly in species that have widespread distribution among disparate habitats.

A collaborative clinical placement model for physiotherapy students results in equivalent (or greater) direct patient care activity than that delivered by physiotherapists alone: an observational study.

BACKGROUND: The demand for physiotherapy clinical placements is rising which requires innovative approaches and an understanding of clinical placement models. OBJECTIVE: To determine physiotherapy student contribution to direct patient care activity during a collaborative clinical placement model. Secondary aims determined the impact of clinical area and clinical educator to student (CE:student) ratio and if a group of students could reach equivalent direct patient care activity of a junior or senior physiotherapist. METHOD: Physiotherapy student, and junior and senior physiotherapist occasions of service (OOS) were collected from five Queensland Public Health Sector hospital information management systems from four physiotherapy clinical areas (i.e. cardiorespiratory, musculoskeletal, neurorehabilitation, and orthopedics). Number of days of clinical activity was recorded to provide average OOS/day. RESULTS: Across a 5-week clinical placement a group of physiotherapy students in a collaborative clinical placement model provided on average 10.6 OOS/day (95%CI 10.1-11.2). In three (75%) clinical areas, a group of students participating in higher CE:student ratios produced more OOS/day. Clinical area and CE:student ratio predicted 39% of the variance in student average OOS/day. On average a group of students reached the equivalent direct patient care activity of a junior and senior physiotherapist by week two of a 5-week clinical placement. CONCLUSION: Physiotherapy students in a collaborative clinical placement model met or exceeded the direct patient care activity of a physiotherapist, irrespective of clinical area and CE:student ratio.

Motor cortical excitability and pre-supplementary motor area neurochemistry in healthy adults with substantia nigra hyperechogenicity.

Substantia nigra (SN) hyperechogenicity, viewed with transcranial ultrasound, is a risk marker for Parkinson's disease. We hypothesized that SN hyperechogenicity in healthy adults aged 50-70 years is associated with reduced short-interval intracortical inhibition in primary motor cortex, and that the reduced intracortical inhibition is associated with neurochemical markers of activity in the pre-supplementary motor area (pre-SMA). Short-interval intracortical inhibition and intracortical facilitation in primary motor cortex was assessed with paired-pulse transcranial magnetic stimulation in 23 healthy adults with normal (n = 14; 61 ± 7 yrs) or abnormally enlarged (hyperechogenic; n = 9; 60 ± 6 yrs) area of SN echogenicity. Thirteen of these participants (7 SN- and 6 SN+) also underwent brain magnetic resonance spectroscopy to investigate pre-SMA neurochemistry. There was no relationship between area of SN echogenicity and short-interval intracortical inhibition in the ipsilateral primary motor cortex. There was a significant positive relationship, however, between area of echogenicity in the right SN and the magnitude of intracortical facilitation in the right (ipsilateral) primary motor cortex (p = .005; multivariate regression), evidenced by the amplitude of the conditioned motor evoked potential (MEP) at the 10-12 ms interstimulus interval. This relationship was not present on the left side. Pre-SMA glutamate did not predict primary motor cortex inhibition or facilitation. The results suggest that SN hyperechogenicity in healthy older adults may be associated with changes in excitability of motor cortical circuitry. The results advance understanding of brain changes in healthy older adults at risk of Parkinson's disease.

Respiratory-related evoked potentials in chronic obstructive pulmonary disease and healthy aging.

Altered neural processing and increased respiratory sensations have been reported in chronic obstructive pulmonary disease (COPD) as larger respiratory-related evoked potentials (RREPs), but the effect of healthy-aging has not been considered adequately. We tested RREPs evoked by brief airway occlusions in 10 participants with moderate-to-severe COPD, 11 age-matched controls (AMC) and 14 young controls (YC), with similar airway occlusion pressure stimuli across groups. Mean age was 76 years for COPD and AMC groups, and 30 years for the YC group. Occlusion intensity and unpleasantness was rated using the modified Borg scale, and anxiety rated using the Hospital Anxiety and Depression Scale. There was no difference in RREP peak amplitudes across groups, except for the N1 peak, which was significantly greater in the YC group than the COPD and AMC groups (p = 0.011). The latencies of P1, P2 and P3 occurred later in COPD versus YC (p < 0.05). P3 latency occurred later in AMC than YC (p = 0.024). COPD and AMC groups had similar Borg ratings for occlusion intensity (3.0 (0.5, 3.5) [Median (IQR)] and 3.0 (3.0, 3.0), respectively; p = 0.476) and occlusion unpleasantness (1.3 (0.1, 3.4) and 1.0 (0.75, 2.0), respectively; p = 0.702). The COPD group had a higher anxiety score than AMC group (p = 0.013). A higher N1 amplitude suggests the YC group had higher cognitive processing of respiratory inputs than the COPD and AMC groups. Both COPD and AMC groups showed delayed neural responses to the airway occlusion, which may indicate impaired processing of respiratory sensory inputs in COPD and healthy aging.

Effectiveness of Abdominal Functional Electrical Stimulation for Improving Bowel Function in People With a Spinal Cord Injury: A Study Protocol for a Double-Blinded Randomized Placebo-Controlled Clinical Trial.

Background: People with a spinal cord injury (SCI) have a high rate of bowel-related morbidity, even compared with people with other neurological disorders. These complications lower quality of life and place a financial burden on the health system. A noninvasive intervention that improves the bowel function of people with an SCI should reduce morbidity, improve quality of life, and lead to cost savings for health care providers. Objectives: To investigate the effectiveness of noninvasive abdominal functional electrical stimulation (FES) for improving bowel function in people with a chronic SCI. Methods: A prospective, double-blinded, 1:1 randomized, placebo-controlled intervention trial will be conducted with 80 adults with chronic SCI (>12 months since injury) above T8 single neurological level. The intervention will be a 45-minute abdominal FES (or placebo) session, 3 days per week, for 6 weeks. Main Study Parameters/Endpoints: Primary endpoint is whole gut transit time before and after 6 weeks of abdominal FES. Secondary endpoints measured before and after 6 weeks of abdominal FES are (1) colonic transit time; (2) quality of life (EQ-5D-5L); (3) participant-reported bowel function (International SCI Bowel Function Basic Data Set Questionnaire and visual analogue scale); (4) respiratory function (forced vital capacity, forced expiratory volume in 1 second, peak expiratory flow, maximal inspiratory pressure, and maximal expiratory pressure); (5) bladder symptoms (Neurogenic Bladder Symptom Score); (6) daily bowel management diary; and (7) unplanned hospital visits. Conclusion: Safety data will be collected, and a cost utility analysis using quality of life scores will be performed. Trial registration: Australian New Zealand Clinical Trials Registry (ANZCTR): ACTRN12621000386831.

Muscle electromyographic activity normalized to maximal muscle activity, not to Mmax, better represents voluntary activation.

In human applied physiology studies, the amplitude of recorded muscle electromyographic activity (EMG) is often normalized to maximal EMG recorded during a maximal voluntary contraction. When maximal contractions cannot be reliably obtained (e.g. in people with muscle paralysis, anterior cruciate ligament injury, or arthritis), EMG is sometimes normalized to the maximal compound muscle action potiential evoked by stimulation, the Mmax. However, it is not known how these two methods of normalization affect the conclusions and comparability of studies. To address this limitation, we investigated the relationship between voluntary muscle activation and EMG normalized either to maximal EMG or to Mmax. Twenty-five able-bodied adults performed voluntary isometric ankle plantarflexion contractions to a range of percentages of maximal voluntary torque. Ankle torque, plantarflexor muscle EMG, and voluntary muscle activation measured by twitch interpolation were recorded. EMG recorded at each contraction intensity was normalized to maximal EMG or to Mmax for each plantarflexor muscle, and the relationship between the two normalization approaches quantified. A slope >1 indicated EMG amplitude normalized to maximal EMG (vertical axis) was greater than EMG normalized to Mmax (horizontal axis). Mean estimates of the slopes were large and had moderate precision: soleus 8.7 (95% CI 6.9 to 11.0), medial gastrocnemius 13.4 (10.5 to 17.0), lateral gastrocnemius 11.4 (9.4 to 14.0). This indicates EMG normalized to Mmax is approximately eleven times smaller than EMG normalized to maximal EMG. Normalization to maximal EMG gave closer approximations to the level of voluntary muscle activation assessed by twitch interpolation.

Inspiratory muscle reflex control after incomplete cervical spinal cord injury.

In healthy individuals, loading inspiratory muscles by brief inspiratory occlusion produces a short-latency inhibitory reflex (IR) in the electromyographic (EMG) activity of scalene and diaphragm muscles. This IR may play a protective role to prevent aspiration and airway collapse during sleep. In people with motor and sensory complete cervical spinal cord injury (cSCI), who were able to breathe independently, this IR was predominantly absent. Here, we investigated the reflex response to brief airway occlusion in 16 participants with sensory incomplete cSCI [American spinal injury association impairment scale (AIS) score B or C]. Surface EMG was recorded from scalene muscles and the lateral chest wall (overlying diaphragm). The airway occlusion evoked a small change in mouth pressure resembling a physiological occlusion. The short-latency IR was present in 10 (63%) sensory incomplete cSCI participants; significantly higher than the IR incidence observed in complete cSCI participants in our previous study (14%; P = 0.003). When present, mean IR latency across all muscles was 58 ms (range 29-79 ms), and mean rectified EMG amplitude decreased to 37% preocclusion levels. Participants without an IR had untreated severe obstructive sleep apnea (OSA), in contrast to those with an IR, who had either had no, mild, or treated OSA (P = 0.002). Insufficient power did not allow statistical comparison between IR presence or absence and participant clinical characteristics. In conclusion, spared sensory connections or intersegmental connections may be necessary to generate the IR. Future studies to establish whether IR presence is related to respiratory morbidity in the tetraplegic population are required.NEW & NOTEWORTHY Individuals with incomplete cSCI were tested for the presence of a short latency reflex inhibition of inspiratory muscles, by brief airway occlusion. The reflex was 4.5 times more prevalent in this group compared with those with complete cSCI and is similar to the incidence in able-bodied people. Participants without this reflex all had untreated severe OSA, in contrast to those with an IR, who either had no, mild, or treated OSA. This work reveals novel differences in the reflex control of inspiratory muscles across the cSCI population.

The detection and sensory perception of inspiratory resistive loads in people with chronic tetraplegia.

This study investigated sensations of breathing following tetraplegia. Fifteen people with chronic tetraplegia and fifteen healthy able-bodied controls matched for age, sex, height, and weight participated. Sensations of breathing were quantified by determining the threshold for detecting an added resistance during inspiration. In a separate task, the perceived magnitudes of six suprathreshold resistive loads were determined with a modified Borg scale. The detection threshold of 0.34 cmH2O/L/s [standard deviation (SD) 0.14] in the tetraplegia group was higher than the 0.23 cmH2O/L/s (SD 0.10) threshold for able-bodied controls (P = 0.004). Both participant groups perceived larger loads to be more effortful, with the Borg effort rating increasing linearly with the peak inspiratory pressure generated at each load. The relationship between Borg effort rating and peak inspiratory pressure was steeper in participants with tetraplegia than in able-bodied controls (P = 0.001), but there was no difference when pressure was divided by maximal inspiratory pressure (P = 0.95). Despite a higher detection threshold, the findings suggest that the perceived magnitude of a suprathreshold inspiratory load is not impaired in chronic tetraplegia and that load magnitude perception is related to the maximal, and not absolute, inspiratory muscle force.NEW & NOTEWORTHY Sensations of breathing are thought to be impaired following chronic tetraplegia. The detection threshold for an added resistive load during inspiration was higher in people with tetraplegia than in healthy able-bodied participants. However, for inspiratory loads above the detection threshold, the perceived magnitude of a resistive load as a function of the peak inspiratory pressure was greater in tetraplegia. Load magnitude perception was comparable between participant groups when peak pressure was divided by maximal inspiratory pressure.

Regional genioglossus reflex responses to negative pressure pulses in people with obstructive sleep apnea.

Tongue and upper airway dilator muscle movement patterns during quiet breathing vary in people with obstructive sleep apnea (OSA). Many patients have inadequate or counterproductive responses to inspiratory negative airway pressure that likely contributes to their OSA. This may be due, at least in part, to inadequate or nonhomogeneous reflex drive to different regions of the largest upper airway dilator, genioglossus. To investigate potential regional heterogeneity of genioglossus reflex responses in OSA, brief suction pulses were applied via a nasal breathing mask and an electromyogram (EMG) was recorded in four regions (anterior oblique, anterior horizontal, posterior oblique, and posterior horizontal) using intramuscular fine wire electrodes in 15 people with OSA. Genioglossus short-latency reflex excitation amplitude had regional heterogeneity (horizontal vs. oblique regions) when expressed in absolute units but homogeneity when normalized as a percentage of the immediate (100 ms) prestimulus EMG. Regional variability in reflex morphology (excitation and inhibition) was present in one-third of the participants. The minimum cross-sectional area (CSA) of the pharyngeal airway was quantified using MRI and may be related to the amplitude of the short-latency reflex response to negative pressure as we found that people with a smaller CSA tended to have a greater reflex amplitude (e.g., horizontal region r2 = 0.41, P = 0.01). These findings highlight the complexity of genioglossus reflex control, the potential for regional heterogeneity, and the functional importance of upper airway anatomy in mediating genioglossus reflex responses to rapid changes in negative pressure in OSA.NEW & NOTEWORTHY Our findings indicate that 30% of participants had regional heterogeneity in reflex morphology (excitation/inhibition) to brief pulses of negative upper-airway pressure across anterior oblique, anterior horizontal, posterior oblique, and posterior horizontal regions of the genioglossus muscle. Reflex excitation amplitude was proportional to prestimulus drive, with increased activation in oblique compared with horizontal regions of the posterior tongue. People with narrower upper-airway anatomy tended to have increased genioglossus reflex amplitude to negative pressure pulses during wakefulness.

The effect of acute intermittent hypoxia on human limb motoneurone output.

NEW FINDINGS: What is the central question of this study? Does a single session of repeated bouts of acute intermittent hypoxic breathing enhance the motoneuronal output of the limb muscles of healthy able-bodied participants? What is the main finding and its importance? Compared to breathing room air, there were some increases in motoneuronal output following acute intermittent hypoxia, but the increases were variable across participants and in time after the intervention and depended on which neurophysiological measure was checked. ABSTRACT: Acute intermittent hypoxia (AIH) induces persistent increases in output from rat phrenic motoneurones. Studies in people with spinal cord injury (SCI) suggest that AIH improves limb performance, perhaps via postsynaptic changes at cortico-motoneuronal synapses. We assessed whether limb motoneurone output in response to reflex and descending synaptic activation is facilitated after one session of AIH in healthy able-bodied volunteers. Fourteen participants completed two experimental days, with either AIH or a sham intervention (randomised crossover design). We measured H-reflex recruitment curves and homosynaptic post-activation depression (HPAD) of the H-reflex in soleus, and motor evoked potentials (MEPs) evoked by transcranial magnetic stimulation (TMS) and their recruitment curves in first dorsal interosseous. All measurements were performed at rest and occurred at baseline, 0, 20, 40 and 60 min post-intervention. The intervention was 30 min of either normoxia (sham, F i O 2 ${F_{{\rm{i}}{{\rm{O}}_{\rm{2}}}}}$  ≈ 0.21) or AIH (alternate 1-min hypoxia [ F i O 2 ${F_{{\rm{i}}{{\rm{O}}_{\rm{2}}}}}$  ≈ 0.09], 1-min normoxia). After AIH, the H-reflex recruitment curve shifted leftward. Lower stimulation intensities were needed to evoke 5%, 50% and 99% of the maximal H-reflex at 40 and 60 min after AIH (P < 0.04). The maximal H-reflex, recruitment slope and HPAD were unchanged after AIH. MEPs evoked by constant intensity TMS were larger 40 min after AIH (P = 0.027). There was no change in MEP recruitment or the maximal MEP. In conclusion, some measures of the evoked responses from limb motoneurones increased after a single AIH session, but only at discrete time points. It is unclear to what extent these changes alter functional performance.