Effect of exercise training on neuromuscular function of elbow flexors and knee extensors of type 2 diabetic patients

Mechanisms underlying altered neuromuscular function in people with DPN

Diabetes alters numerous physiological functions and can lead to disastrous consequences in the long term. Neuromuscular function is particularly affected and is impacted early, offering an opportunity to detect the onset of diabetes-related dysfunctions and follow the advancement of the disease. The role of physical training for counteracting the deleterious effects of diabetes is well accepted but at the same time, it appears difficult to reliably assess the effects of exercise on functional capacity in patients with diabetic peripheral neuropathy (DPN). In this paper, we will review the specific characteristics of various neuromuscular dysfunctions associated with diabetes according to the DPN presence or not, and their changes over time. We present several propositions regarding the onset of neuromuscular alterations in people with diabetes compared to people with DPN. It appears that motor unit loss and neuromuscular transmission impairment are among the main mechanisms explaining the considerable degradation of neuromuscular function in the transition from a diabetic to neuropathic state. Rate of force development and contractile properties could start to decrease with the onset of preferential type II fiber atrophy, commonly reported in people with DPN. Finally, M_max amplitude could decrease with neuromuscular fatigue only in people with DPN, reflecting the fatigue-related neuromuscular transmission impairment reported in people with DPN. In this review, we show that the different neuromuscular parameters are altered at different stages of diabetes, according to the presence of DPN or not. The precise evaluation of these parameters might participate in adapting the physical training prescription.

Walking Attenuates Postprandial Glycemic Response: What Else Can We Do without Leaving Home or the Office?

We evaluated the effects of different exercise types suitable for a home/work setting on the postprandial glucose response. Twenty-three healthy, active, young individuals performed one of two studies (12 in Study 1 and 11 in Study 2), with four randomized protocols each. After a meal high in carbohydrate content (1 g of carbohydrate per kg of body weight), in Study 1, participants performed 30 min of either walking (WALK), bench stepping exercise (STEP) or isometric wall squat (SQUAT); in Study 2, participants performed 30 min of either walking (WALK), neuromuscular electrical stimulation alone (P_NMES) or superimposed on voluntary muscle contraction (VC_NMES). In both studies, participants performed a prolonged sitting condition (CON) that was compared to the exercise sessions. In Study 1, WALK and STEP significantly reduced the glucose peak compared to CON (p < 0.011). In Study 2, the peak was significantly reduced in WALK compared to CON, P_NMES and VC_NMES (p < 0.011) and in VC_NMES compared to CON and P_NMES (p < 0.011). A significant reduction of 3 h glucose iAUC was found for WALK and VC_NMES compared to CON and P_NMES (p < 0.033). In conclusion, WALK is the most effective strategy for improving the postprandial glycemic response. However, STEP and VC_NMES can also be used for reducing postprandial glycemia.

Greater fatigability and motor unit discharge variability in human type 2 diabetes

This study determined the discharge characteristics of motor units from two lower limb muscles before and after fatiguing exercise in people with type 2 diabetes (T2D) with no symptoms of polyneuropathy and activity‐matched controls. Seventeen people with T2D (65.0 ± 5.6 years; 8 women) and 17 controls (63.6 ± 4.5 years; 8 women) performed: (a) intermittent, isometric contractions at 50% maximal voluntary isometric contraction (MVIC) sustained to failure with the ankle dorsiflexors, and (b) a dynamic fatiguing task (30% MVIC load) for 6 min with the knee extensors. Before and after the fatiguing tasks, motor unit characteristics (including coefficient of variation (CV) of interspike intervals (ISI)) were quantified from high‐density electromyography and muscle contractile properties were assessed via electrical stimulation. Fatigability was ~50% greater for people with T2D than controls for the dorsiflexors (time‐to‐failure: 7.3 ± 4.1 vs. 14.3 ± 9.1 min, p = .010) and knee extensors (power reduction: 56.7 ± 11.9 vs. 31.5 ± 25.5%, p < .001). The CV of ISI was greater for the T2D than control group for the tibialis anterior (23.1 ± 11.0 vs. 21.3 ± 10.7%, p < .001) and vastus lateralis (27.8 ± 20.2 vs. 24.5 ± 16.1%, p = .011), but these differences did not change after the fatiguing exercises. People with T2D had greater reductions in the electrically evoked twitch amplitude of the dorsiflexors (8.5 ± 5.1 vs. 4.0 ± 3.4%·min^‐1, p = .013) and knee extensors (49.1 ± 10.0 vs. 31.8 ± 15.9%, p = .004) than controls. Although motor unit activity was more variable in people with T2D than controls, the greater fatigability of the T2D group for lower limb muscles was due to mechanisms involving disruption of contractile function of the exercising muscles rather than motor unit behavior.

Effect of a 6-week strength-training program on neuromuscular efficiency in type 2 diabetes mellitus patients

Background

The neuromuscular system generates human movement. The functional capacity of the neuromuscular system in patients with type 2 diabetes mellitus (T2DM) is decreased and this affects the generation of muscle force. Exercise is recommended as an effective treatment in such cases. Short-duration strength training causes neural adaptations in healthy participants, but the effects of strength training on T2DM are unclear. The present study aimed to evaluate the effect of strength training on neuromuscular efficiency of lower limb muscles in T2DM.

Methods

Surface electromyograms (SEMG) of the knee flexors and extensors were recorded during isometric contractions. The ratio of peak torque to SEMG amplitude was calculated as neuromuscular efficiency. Measurements were taken before the intervention after 6 weeks of non-training, and after 6 weeks of strength training.

Results

SEMG amplitudes did not differ among the subsequent measurement sessions. Flexor and extensor peak torque increased after the strength-training program. The neuromuscular efficiency of all muscles increased after the 6 weeks of strength training.

Conclusion

A 6-week strength-training program increased the neuromuscular efficiency and peak torque in patients with T2DM; however, the electrical properties of the muscles did not change. These results may be related to increased neural adaptations and motor learning in the early stages of strength training.

Greater fatigue resistance of dorsiflexor muscles in people with prediabetes than type 2 diabetes

Although exercise can prevent progression to T2D among people with prediabetes, little is known about fatigue during exercise in people with prediabetes compared to T2D. The purpose of the study was to compare the magnitude and mechanisms of fatigability of the ankle dorsiflexor muscles between people with prediabetes and T2D. Ten people with prediabetes (6 females, 51.7 ± 6.9 years) and fourteen with T2D (6 females, 52.6 ± 6.2 years) who were matched for age, body mass index and physical activity performed an intermittent (6 s contraction: 4 s relaxation) fatiguing task at 75% maximal voluntary contraction (MVC) with the dorsiflexors. Electrical stimulation was used to assess contractile properties of the dorsiflexor muscles before and after the fatiguing task. People with prediabetes had a longer time-to-task failure, i.e. greater fatigue resistance (7.9 ± 5.1 vs. 4.9 ± 2.5 min, P = 0.04), and slower rate of decline of the (potentiated) twitch amplitude (6.5 ± 3.1 vs. 16.5 ± 11.7%·min^-1, P = 0.03) than people with T2D. Shorter time-to-task failure (i.e. greater fatigability) was associated with greater baseline MVC torque (r² = 0.21, P = 0.02) and faster rate of decline of twitch amplitude (r² = 0.39, P = 0.04). The ankle dorsiflexor muscles of males and females with prediabetes were more fatigue resistant than people with T2D, and fatigability was associated with contractile mechanisms.

Reproducibility of muscle fibre conduction velocity during linearly increasing force contractions

Muscle fibre conduction velocity (MFCV) is a basic physiological parameter biophysically related to the diameter of muscle fibres and properties of the sarcolemma. The aim of this study was to assess the intersession reproducibility of the relation between voluntary force and estimates of average muscle fibre conduction velocity (MFCV) from multichannel high-density surface electromyographic recordings (HDsEMG). Ten healthy men performed six linearly increasing isometric ankle dorsiflexions on two separate experimental sessions, 4 weeks apart. Each session involved the recordings of voluntary force during maximal isometric (MViF) and submaximal ramp contractions at 35-50-70% of MViF. Concurrently, the HDsEMG activity was detected from the tibialis anterior muscle and MFCV estimates were derived in 250-ms epochs. Absolute and relative reproducibility of MFCV initial value (intercept) and rate of change (regression slope) as a function of force were assessed by within-subject coefficient of correlation (CV_w) and with intraclass correlation coefficient (ICC). MFCV was positively correlated with voluntary force (R² = 0.75 ± 0.12) in all individuals and test conditions (P < 0.001). Average CV_w for MFCV intercept and slope were of 2.6 ± 2.0% and 11.9 ± 3.2% and ICC values of 0.96 and 0.94, respectively. Overall, MFCV regression coefficients showed a high degree of intersession reproducibility in both absolute and relative terms. These results may have important practical implications in the tracking of training-induced neuromuscular changes and/or in the monitoring of the progress of neuromuscular disorders when a full sEMG signal decomposition is problematic or not possible.

SIGNIFICANCE OF ELECTROMYOGRAPHY IN THE ASSESSMENT OF DIABETIC NEUROPATHY

Diabetic neuropathy is one of the physical complications of diabetes mellitus (DM) patients with a long history of diabetes. An electromyography (EMG)-based assessment may be very useful for the management of diabetic neuropathy. In the present study, we aimed to summarize all of the findings and recommendations obtained from previous studies that investigated the application of EMG to the assessment of diabetic neuropathy. An extensive search of the prominent electronic databases PubMed, Google Scholar and Scopus was performed to evaluate the following areas: (i) what are the muscles to be evaluated by EMG for neuropathy assessment, (ii) what type of EMG methodologies have been used and (iii) what recommendation can be made for neuropathy detection. The major findings are summarized as follows: (i) very few studies have analyzed the correlation of the EMG signals acquired from peripheral muscles affected in neuropathy with those obtained with non-neuropathic complications, such as ankle sprain; (ii) EMG has been applied for the detection of diabetic neuropathy more than diabetes treatment; and (iii) neuropathy detection using an EMG-based assessment were mainly performed for type 2 DM patients aged at least 50 years.

The Effects of Quercetin Supplementation on Eccentric Exercise-Induced Muscle Damage

The aim of the present investigation was to test the hypothesis that quercetin (Q) may prevent the strength loss and neuromuscular impairment associated with eccentric exercise-induced muscle damage (EEIMD). Twelve young men (26.1 ± 3.1 years) ingested either Q (1000 mg/day) or placebo (PLA) for 14 days using a randomized, double-blind, crossover study design. Participants completed a comprehensive neuromuscular (NM) evaluation before, during and after an eccentric protocol able to induce a severe muscle damage (10 sets of 10 maximal lengthening contractions). The NM evaluation comprised maximal voluntary isometric contraction (MVIC) and force–velocity relationship assessments with simultaneous recording of electromyographic signals (EMG) from the elbow flexor muscles. Soreness, resting arm angle, arm circumference, plasma creatine kinase (CK) and lactate dehydrogenase (LDH) were also assessed. Q supplementation significantly increased the isometric strength recorded during MVIC compared to baseline (+4.7%, p < 0.05). Moreover, the torque and muscle fiber conduction velocity (MFCV) decay recorded during the eccentric exercise was significant lower in Q compared to PLA. Immediately after the EEIMD, isometric strength, the force–velocity relationship and MFCV were significantly lower when participants were given PLA rather than Q. Fourteen days of Q supplementation seems able to attenuate the severity of muscle weakness caused by eccentric-induced myofibrillar disruption and sarcolemmal action potential propagation impairment.

Mechanisms for the increased fatigability of the lower limb in people with type 2 diabetes

Fatiguing exercise is the basis of exercise training and a cornerstone of management of type 2 diabetes mellitus (T2D); however, little is known about the fatigability of limb muscles and the involved mechanisms in people with T2D. The purpose of this study was to compare fatigability of knee extensor muscles between people with T2D and controls without diabetes and determine the neural and muscular mechanisms for a dynamic fatiguing task. Seventeen people with T2D [ten men and seven women: 59.6 (9.0) yr] and twenty-one age-, body mass index-, and physical activity-matched controls [eleven men and ten women: 59.5 (9.6) yr] performed one hundred twenty high-velocity concentric contractions (one contraction/3 s) with a load equivalent to 20% maximal voluntary isometric contraction (MVIC) torque with the knee extensors. Transcranial magnetic stimulation (TMS) and electrical stimulation of the quadriceps were used to assess voluntary activation and contractile properties. People with T2D had larger reductions than controls in power during the fatiguing task [42.8 (24.2) vs. 26.4 (15.0)%; P < 0.001] and MVIC torque after the fatiguing task [37.6 (18.2) vs. 26.4 (12.1)%; P = 0.04]. People with T2D had greater reductions than controls in the electrically evoked twitch amplitude after the fatiguing task [44.0 (20.4) vs. 35.4 (12.1)%, respectively; P = 0.01]. However, the decrease in voluntary activation was similar between groups when assessed with electrical stimulation [12.1 (2.6) vs. 12.4 (4.4)% decrease; P = 0.84] and TMS ( P = 0.995). A greater decline in MVIC torque was associated with larger reductions of twitch amplitude ( r2 = 0.364, P = 0.002). Although neural mechanisms contributed to fatigability, contractile mechanisms were responsible for the greater knee extensor fatigability in men and women with T2D compared with healthy controls.

NEW & NOTEWORTHY Transcranial magnetic stimulation and percutaneous muscle stimulation were used to determine the contributions of neural and contractile mechanisms of fatigability of the knee extensor muscles after a dynamic fatiguing task in men and women with type 2 diabetes (T2D) and healthy age-, body mass index-, and physical activity-matched controls. Although neural and contractile mechanisms contributed to greater fatigability of people with T2D, fatigability was primarily associated with impaired contractile mechanisms and glycemic control.

Distribution of muscle fibre conduction velocity for representative samples of motor units in the full recruitment range of the tibialis anterior muscle

AIM

Motor units are recruited in an orderly manner according to the size of motor neurones. Moreover, because larger motor neurones innervate fibres with larger diameters than smaller motor neurones, motor units should be recruited orderly according to their conduction velocity (MUCV). Because of technical limitations, these relations have been previously tested either indirectly or in small motor unit samples that revealed weak associations between motor unit recruitment threshold (RT) and MUCV. Here, we analyse the relation between MUCV and RT for large samples of motor units.

METHODS

Ten healthy volunteers completed a series of isometric ankle dorsiflexions at forces up to 70% of the maximum. Multi-channel surface electromyographic signals recorded from the tibialis anterior muscle were decomposed into single motor unit action potentials, from which the corresponding motor unit RT, MUCV and action potential amplitude were estimated. Established relations between muscle fibre diameter and CV were used to estimate the fibre size.

RESULTS

Within individual subjects, the distributions of MUCV and fibre diameters were unimodal and did not show distinct populations. MUCV was strongly correlated with RT (mean (SD) R²  = 0.7 (0.09), P < 0.001; 406 motor units), which supported the hypothesis that fibre diameter is associated with RT.

CONCLUSION

The results provide further evidence for the relations between motor neurone and muscle fibre properties for large samples of motor units. The proposed methodology for motor unit analysis has also the potential to open new perspectives in the study of chronic and acute neuromuscular adaptations to ageing, training and pathology.

Associations between motor unit action potential parameters and surface EMG features

The surface interference EMG signal provides some information on the neural drive to muscles. However, the association between neural drive to muscle and muscle activation has long been debated with controversial indications due to the unavailability of motor unit population data. In this study, we clarify the potential and limitations of interference EMG analysis to infer motor unit recruitment strategies with an experimental investigation of several concurrently active motor units and of the associated features of the surface EMG. For this purpose, we recorded high-density surface EMG signals during linearly increasing force contractions of the tibialis anterior muscle, up to 70% of maximal force. The recruitment threshold (RT), conduction velocity (MUCV), median frequency (MDF_MU), and amplitude (RMS_MU) of action potentials of 587 motor units from 13 individuals were assessed and associated with features of the interference EMG. MUCV was positively associated with RT (R² = 0.64 ± 0.14), whereas MDF_MU and RMS_MU showed a weaker relation with RT (R² = 0.11 ± 0.11 and 0.39 ± 0.24, respectively). Moreover, the changes in average conduction velocity estimated from the interference EMG predicted well the changes in MUCV (R² = 0.71), with a strong association to ankle dorsiflexion force (R² = 0.81 ± 0.12). Conversely, both the average EMG MDF and RMS were poorly associated with motor unit recruitment. These results clarify the limitations of EMG spectral and amplitude analysis in inferring the neural strategies of muscle control and indicate that, conversely, the average conduction velocity could provide relevant information on these strategies.NEW & NOTEWORTHY The surface EMG provides information on the neural drive to muscles. However, the associations between EMG features and neural drive have been long debated due to unavailability of motor unit population data. Here, by using novel highly accurate decomposition of the EMG, we related motor unit population behavior to a wide range of voluntary forces. The results fully clarify the potential and limitation of the surface EMG to provide estimates of the neural drive to muscles.

A new optical flow model for motor unit conduction velocity estimation in multichannel surface EMG

Many studies have demonstrated the feasibility and benefits of Conduction Velocity (CV) estimation from surface electromyograms (EMGs) in various experimental conditions. Among them, a method based on optical flow was proposed recently, demonstrating relatively accurate CV estimation for EMG signals acquired in monopolar mode. We extended this method by a new data model that compensates more realistically for the spatial Motor Unit Action Potential (MUAP) shape variability and enables accurate CV estimation also in single-differential acquisition mode. The proposed modification was validated on 5000 synthetic Motor Units (MUs) with known CV and direction of fibres. It was shown that, in the noiseless case, the mean CV estimation error was significantly lower for our proposed modification compared to the original CV estimation procedure by up to 2% in the case of monopolar EMG signals and by up to 18.6% for single-differential EMG signals. When estimating fibre directions, the mean error was lower by up to 2.4° (for monopolar EMG signals) and 9.6° (for single-differential EMG signals). The results of tests with 10dB and 20dB noise further demonstrated the robustness of the proposed algorithm to noise in MUAP estimation.

Decrease of muscle fiber conduction velocity correlates with strength loss after an endurance run

Monitoring surface electromyographic (EMG) signals can provide useful insights for characterizing muscle fatigue, which is defined as an exercise-induced strength loss. This experiment investigated the muscle fiber conduction velocity (CV) changes induced by an endurance run. The day before and immediately after a half-marathon run (21.097 km) 11 amateur runners performed maximum voluntary contractions (MVCs) of knee extensor muscles. During the MVC, multichannel EMG was recorded from the vastus lateralis and EMG amplitude and CV were calculated. After the run, knee extensors showed a decreased strength (-13  ±  9%, p  =  0.001) together with a reduction in EMG amplitude (-13  ±  10%, p  =  0.003) and in CV (-6  ±  8%, p  =  0.032). Knee extensor strength loss positively correlated with vastus lateralis CV differences (r  =  0.76, p  =  0.006). Thus, the exercises-induced muscle fatigue was associated not only with a decrease in EMG amplitude, but also with a reduction in CV. This finding suggests that muscle fibers with higher CV (i.e. those with greater fiber size) were the most impaired during strength production after an endurance run.

Muscle fatigability in type 2 diabetes

BACKGROUND

Patients with type 2 diabetes (T2D) may be subject to premature muscle fatigue. However, the effect of diabetes on muscle fatigability has not yet been thoroughly examined. The purpose of this study was to investigate the effect of T2D on muscle fatigability at the upper and lower body.

METHODS

Thirty-three T2D patients (18 men and 15 women; mean age, 59.3 ± 5.3 years) and 34 matched healthy control participants (17 men and 17 women; mean age, 60.1 ± 6.1 years) were recruited. Clinical characteristics of diabetic patients were assessed by considering a wide range of vascular and neurological parameters in order to exclude the presence of micro- and macro-vascular complications. Gender-specific muscle function was evaluated measuring the maximal voluntary isometric contraction (MVIC), and the endurance time at 50% of the MVIC at the shoulder and at the knee extensor muscles.

RESULTS

Muscle strength in the upper body was similar among groups, whereas in the lower body, it was significantly reduced in T2D men (-16%) and women (-22%) compared with the controls. Additionally, the endurance time in both upper and lower body was significantly lower in T2D men (-18% and -29%) and women (-19% and -25%, respectively) than controls.

CONCLUSIONS

Besides the reduction in strength, muscle dysfunction in T2D is characterized by a higher fatigability that affects both upper and lower body muscles. This effect is independent to the presence of diabetic complications and may represent a more sensitive marker of muscular dysfunction than muscle strength. Copyright © 2016 John Wiley & Sons, Ltd.