J. Greig Inglis

Reliability of the biceps brachii M-wave

BackgroundThe peak-to-peak (P-P) amplitude of the maximum M-wave and the area of the negative phase of the curve are important measures that serve as methodological controls in H-reflex studies, motor unit number estimation (MUNE) procedures, and normalization factors for voluntary electromyographic (EMG) activity. These methodologies assume, with little evidence, that M-wave variability is minimal. This study therefore examined the intraclass reliability of these measures for the biceps brachii.MethodsTwenty-two healthy adults (4 males and 18 females) participated in 5 separate days of electrical stimulation of the musculocutaneous nerve supplying the biceps brachii muscle. A total of 10 stimulations were recorded on each of the 5 test sessions: a total of fifty trials were used for analysis. A two-factor repeated measures analysis of variance (ANOVA) evaluated the stability of the group means across test sessions. The consistency of scores within individuals was determined by calculating the intraclass correlation coefficient (ICC). The variance ratio (VR) was then used to assess the reproducibility of the shape of the maximum M-wave within individual subjects.ResultsThe P-P amplitude means ranged from 12.62 ± 4.33 mV to 13.45 ± 4.07 mV across test sessions. The group means were highly stable. ICC analysis also revealed that the scores were very consistent (ICC = 0.98). The group means for the area of the negative phase of the maximum M-wave were also stable (117 to 126 mV·ms). The ICC analysis also indicated a high degree of consistency (ICC = 0.96). The VR for the sample was 0.244 ± 0.169, which suggests that the biceps brachii maximum M-wave shape was in general very reproducible for each subject.ConclusionThe results support the use of P-P amplitude of the maximum M-wave as a methodological control in H-reflex studies, and as a normalization factor for voluntary EMG. The area of the negative phase of the maximum M-wave is both stable and consistent, and the shape of the entire waveform is highly reproducible and may be used for MUNE procedures.

Skeletal muscle type comparison of pyruvate dehydrogenase phosphatase activity and isoform expression: effects of obesity and endurance training

Pyruvate dehydrogenase (PDH) plays an important role in regulating carbohydrate metabolism in skeletal muscle. PDH is activated by PDH phosphatase (PDP) and deactivated by PDH kinase (PDK). Obesity has a large negative impact on skeletal muscle carbohydrate metabolism, whereas endurance training has been shown to improve regulatory control of skeletal muscle carbohydrate metabolism, more so when coupled with obesity. A majority of this literature has focused on PDK, with little information available on PDP. To determine the relative role of PDP in regulating skeletal muscle PDH activity with obesity and endurance training, obese and lean Zucker rats remained sedentary or were endurance trained (1 h/day, 5 days/wk) for a period of 8 wk. Soleus, red gastrocnemius, (RG), and white gastrocnemius (WG) muscles were sampled after the training period. The main findings were 1) obesity resulted in a 46% decrease in PDP activity expressed per milligram extracted mitochondrial protein only in RG, while PDP isoform content was unchanged; 2) 8 wk of endurance training led to a significant 1.4-2.2-fold increase in PDP activity of all muscle examined from obese rats, and the concomitant increase in PDP1 protein was only seen in soleus and RG; 3) 8 wk of endurance training led to a trending 1.4-2.2-fold increase in PDP activity of all muscle examined from obese rats, and the concomitant increase in PDP1 protein was only seen in soleus and RG; and 4) PDP2 protein content was not affected by obesity or training. These results suggest that decreased PDP activity in oxidative skeletal muscles may play a role in the impairment of carbohydrate metabolism in obese rats, which is reversible with endurance training.

PDH activation during in vitro muscle contractions in PDH kinase 2 knockout mice: effect of PDH kinase 1 compensation.

Pyruvate dehydrogenase (PDH) plays an important role in regulating carbohydrate oxidation in skeletal muscle. PDH is deactivated by a set of PDH kinases (PDK1, PDK2, PDK3, PDK4), with PDK2 and PDK4 being the most predominant isoforms in skeletal muscle. Although PDK2 is the most abundant isoform, few studies have examined its physiological role. The role of PDK2 on PDH activation (PDHa) at rest and during muscle stimulation at 10 and 40 Hz (eliciting low- and moderate-intensity muscle contractions, respectively) in isolated extensor digitorum longus muscles was studied in PDK2 knockout (PDK2KO) and wild-type (WT) mice (n = 5 per group). PDHa activity was unexpectedly 35 and 77% lower in PDK2KO than WT muscle (P = 0.043), while total PDK activity was nearly fourfold lower in PDK2KO muscle (P = 0.006). During 40-Hz contractions, initial force was lower in PDK2KO than WT muscle (P < 0.001) but fatigued similarly to ∼75% of initial force by 3 min. There were no differences in initial force or rate of fatigue during 10-Hz contractions. PDK1 compensated for the lack of PDK2 and was 1.8-fold higher in PDK2KO than WT muscle (P = 0.019). This likely contributed to ensuring that resting PDHa activity was similar between the groups and accounts for the lower PDH activation during muscle contraction, as PDK1 is a very potent inhibitor of the PDH complex. Increased PDK1 expression appears to be regulated by hypoxia inducible factor-1α, which was 3.5-fold higher in PDK2KO muscle. It is clear that PDK2 activity is essential, even at rest, in regulation of carbohydrate oxidation and production of reducing equivalents for the electron transport chain. In addition, these results underscore the importance of the overall kinetics of the PDK isoform population, rather than total PDK activity, in determining transformation of the PDH complex and PDHa activity during muscle contraction.

Experimental and modelling investigation of surface EMG spike analysis

A pattern classification method based on five measures extracted from the surface electromyographic (sEMG) signal is used to provide a unique characterization of the interference pattern for different motor unit behaviours. This study investigated the sensitivity of the five sEMG measures during the force gradation process. Tissue and electrode filtering effects were further evaluated using a sEMG model. Subjects (N=8) performed isometric elbow flexion contractions from 0 to 100% MVC. The sEMG signals from the biceps brachii were recorded simultaneously with force. The basic building block of the sEMG model was the detection of single fibre action potentials (SFAPs) through a homogeneous, equivalent isotropic, infinite volume conduction medium. The SFAPs were summed to generate single motor unit action potentials. The physiologic properties from a well-known muscle model and motor unit recruitment and firing rate schemes were combined to generate synthetic sEMG signals. The following pattern classification measures were calculated: mean spike amplitude, mean spike frequency, mean spike slope, mean spike duration, and the mean number of peaks per spike. Root-mean-square amplitude and mean power frequency were also calculated. Taken together, the experimental data and modelling analysis showed that below 50% MVC, the pattern classification measures were more sensitive to changes in force than traditional time and frequency measures. However, there are additional limitations associated with electrode distance from the source that must be explored further. Future experimental work should ensure that the inter-electrode distance is no greater than 1cm to mitigate the effects of tissue filtering.

A Comparison of Statistical Models for Calculating Reliability of the Hoffmann Reflex.

The Hoffmann reflex is obtained through surface electromyographic recordings, and it is one of the most common neurophysiological techniques in exercise science. Measurement and evaluation of the peak-to-peak amplitude of the Hoffmann reflex has been guided by the observation that it is a variable response that requires multiple trials to obtain a stable mean. As a result, reliability assessment of the Hoffmann reflex has thus far been limited to trial-to-trial variability or the mean of multiple trials across test days to evaluate day-to-day variability, without considering how the two dimensions of measurement interact to affect test–retest reliability. This article compares two analysis of variance models for reliability assessment of the peak-to-peak amplitude of the Hoffmann reflex: one model incorporated both days and trials, and the other used the mean of multiple trials on each test day. The intraclass correlation coefficient calculated from an analysis of variance model that incorporated both trial-to-trial and day-to-day error variances was R = .93. Using the mean of the trials for each test day as a single score resulted in a decrease in the intraclass correlation coefficient (R 2,1 = .76). This study demonstrated that an analysis of variance model that accounts for both trial-to-trial and day-to-day error variances provided a higher estimate of reliability. Further, reliability estimation based on the interaction between both trials and days revealed that a schedule with additional test days had a disproportionate impact upon securing a reliable measure of Hoffmann reflex amplitude.

Carbohydrate refeeding after a high-fat diet rapidly reverses the adaptive increase in human skeletal muscle PDH kinase activity

Pyruvate dehydrogenase (PDH) regulates oxidative carbohydrate disposal in skeletal muscle and is downregulated by reversible phosphorylation catalyzed by PDH kinase (PDK). Previous work has demonstrated increased PDK activity and PDK4 expression in human skeletal muscle following a high-fat low-carbohydrate (HF) diet, which leads to decreased PDH in the active form (PDHa activity) and carbohydrate oxidation. The purpose of this study was to examine the time course of changes in PDK and PDHa activities with refeeding of carbohydrates after an HF diet in human skeletal muscle. Healthy male volunteers (n = 8) consumed a standardized 3-day Pre-diet with the same energy content as their habitual diet, followed by a eucaloric 6-day HF diet (Pre-diet: 50:30:20%; HF diet: 5:75:20%; carbohydrate/fat/protein). Muscle biopsies were taken before and after the HF diet and at 45 min and 3 h after carbohydrate refeeding with a single high-glycemic index carbohydrate meal (88:5:7% carbohydrate/fat/protein) representing approximately one third of the individual subjects habitual energy intake. PDK activity increased from 0.08 +/- 0.01 Pre- to 0.25 +/- 0.02 min (P < 0.001) Post-HF diet, and decreased with carbohydrate refeeding to 0.17 +/- 0.05 (P = 0.014) and 0.11 +/- 0.01 min (P = 0.006) at 45 min and 3 h, respectively. PDHa decreased from 0.89 +/- 0.20 to 0.32 +/- 0.05 (P = 0.007) mmol x min(-1) x kg wet wt(-1) following the HF diet, and was increased transiently with refeeding at 45 min, but returned to lower values by 3 h (P = 0.025 compared with Pre). The potential mechanism(s) for this attenuation of PDHa activity remains unclear. These data demonstrate that in human skeletal muscle, the adaptive increase in PDK activity following an HF diet is rapidly reversed to Pre-diet activity levels within 45 min to 3 h, and this is accompanied by a short-term increase in PDHa activity.

The relationship between human skeletal muscle pyruvate dehydrogenase phosphatase activity and muscle aerobic capacity

Pyruvate dehydrogenase (PDH) is a mitochondrial enzyme responsible for regulating the conversion of pyruvate to acetyl-CoA for use in the tricarboxylic acid cycle. PDH is regulated through phosphorylation and inactivation by PDH kinase (PDK) and dephosphorylation and activation by PDH phosphatase (PDP). The effect of endurance training on PDK in humans has been investigated; however, to date no study has examined the effect of endurance training on PDP in humans. Therefore, the purpose of this study was to examine differences in PDP activity and PDP1 protein content in human skeletal muscle across a range of muscle aerobic capacities. This association is important as higher PDP activity and protein content will allow for increased activation of PDH, and carbohydrate oxidation. The main findings of this study were that 1) PDP activity (r(2) = 0.399, P = 0.001) and PDP1 protein expression (r(2) = 0.153, P = 0.039) were positively correlated with citrate synthase (CS) activity as a marker for muscle aerobic capacity; 2) E1α (r(2) = 0.310, P = 0.002) and PDK2 protein (r(2) = 0.229, P =0.012) are positively correlated with muscle CS activity; and 3) although it is the most abundant isoform, PDP1 protein content only explained ∼ 18% of the variance in PDP activity (r(2) = 0.184, P = 0.033). In addition, PDP1 in combination with E1α explained ∼ 38% of the variance in PDP activity (r(2) = 0.383, P = 0.005), suggesting that there may be alternative regulatory mechanisms of this enzyme other than protein content. These data suggest that with higher muscle aerobic capacity (CS activity) there is a greater capacity for carbohydrate oxidation (E1α), in concert with higher potential for PDH activation (PDP activity).

Elevated concentrations of circulating cytokines and correlations with nerve conduction velocity in human peripheral nerves

The purpose of the current study was to quantify the potential relationship between various cytokines and peripheral nerve function in humans, in-vivo. Measures of nerve conduction velocity (NCV) were examined prior to and following the induction of a cytokine spike. A significant negative correlation was found between the change in IL-1ra and the change in NCV at 24h post-exercise (r=-0.65, p=0.02) while a significant positive correlation was found between the change in IL-6 and the change in NCV at 2h post-exercise (r=0.61, p=0.048). It may be possible that different cytokines induce a unique neural influence at elevated concentrations.

Practice Evoking the Flexor Carpi Radialis H-Reflex: A Guideline for Proficiency

The purpose of this research was to identify the number of sessions required for a new investigator to become proficient at evoking an H-reflex in the flexor carpi radialis (FCR), in comparison to an experienced investigator. 31 students from Brock University in the greater Niagara region (16 women M age = 32.2, SD = 8.9 yr.; 15 men M age = 27.8, SD = 7.8 yr.) with no known neurological disorders volunteered and completed two test sessions performed by either an experienced or a novice investigator. In randomized order, both investigators stimulated each subjects median nerve 10 times, once every 15 sec. Each session included the measurement of the subjects flexor carpi radialis maximal M-wave amplitude and H-reflex amplitude and latency with surface electromyographic electrodes. The intraclass correlation coefficients (ICC) indicated an adequate correlation between investigators for both M-wave maximal amplitude and H-reflex at 5% of the M-wave maximal amplitude (.84 and .70, respectively). However, there was a low correlation (.38) between the latency values obtained by the two investigators. The peak-to-peak amplitudes of the H-reflex and M-wave do not appear to be influenced by experience of the tester. The latency of the response, however, appears to have an associated learning curve, improving in consistency with increasing practice of tester.