Does normalization of voluntary EMG amplitude to MMAX account for the influence of electrode location and adiposity?

Determining intracortical, corticospinal and alpha motoneurone excitability in athletes with patellar tendinopathy compared to asymptomatic controls

BACKGROUND

Lower capacity to generate knee extension maximal voluntary force (MVF) has been observed in individuals affected with patellar tendinopathy (PT) compared to asymptomatic controls. This MVF deficit is hypothesized to emanate from alterations in corticospinal excitability (CSE). The modulation of CSE is intricately linked to the excitability levels at multiple sites, encompassing neurones within the corticospinal tract (CST), intracortical neurones within the primary motor cortex (M1), and the alpha motoneurone. The aim of this investigation was to examine the excitability of intracortical neurones, CST neurones, and the alpha motoneurone, and compare these between volleyball and basketball athletes with PT and matched asymptomatic controls.

METHOD

Nineteen athletes with PT and 18 asymptomatic controls participated in this cross-sectional study. Transcranial magnetic stimulation was utilized to assess CST excitability, corticospinal inhibition (silent period, and short-interval cortical inhibition). Peripheral nerve stimulation was used to evaluate lumbar spine and alpha motoneurone excitability, including the evocation of lumbar-evoked potentials and maximal compound muscle action potential (MMAX ), and CSE with central activation ratio (CAR). Knee extension MVF was also assessed.

RESULTS

Athletes with PT exhibited longer silent period duration and greater electrical stimulator output for MMAX , as well as lower MVF, compared to asymptomatic controls (p < 0.05).

CONCLUSION

These findings indicate volleyball and basketball athletes with PT exhibit reduced excitability of the alpha motoneurone or the neuromuscular junction, which may be linked to lower MVF. Subtle alterations at specific sites may represent compensatory changes to excitability aiming to maintain efferent drive to the knee extensors.

Effects of Acute Sleep Deprivation on the Sequential Rate of Torque Development throughout the Force-Time Curve

Objective  The impact of sleep deprivation on the physiological determinants of explosive torque production remains poorly understood. We aimed at determining the acute effects of 24 hours of sleep deprivation on the sequential rate of torque development (RTD) obtained during plantar flexion through maximum voluntary isometric contraction (MVIC). Materials and Methods  The study included 14 healthy-young adults (8 men and 6 women). The participants visited the laboratory on 2 different occasions: without and with 24 hours of sleep deprivation. In each session, the subjects were tested for RTD of the plantar flexors with concomitant recordings of the electromyographic (EMG) amplitude of the soleus over the following time intervals: 0 to 30, 30 to 50, 50 to 100, and 100 to 150 ms. Results  Sleep deprivation did not affect peak RTD (without sleep deprivation: 283.3 ± 111.6 N.m.s -1 versus with sleep deprivation: 294.9 ± 99.2 N.m.s -1 ; p  > 0.05) of plantar flexion. The sequential values of RTD, as well as the normalized amplitude of the soleus EMG, remained similar between both conditions (p > 0.05). Discussion  In conclusion, we found that 24 hours of sleep deprivation do not affect muscle activation, nor explosive torque production throughout the torque-time curve. Thus, exercise performance and daily functionality in tasks involving rapid torque development might remain well preserved after 24 hours of acute sleep deprivation.

Corticospinal excitability across lower limb muscles in humans

Electrophysiological studies in nonhuman primates reported the existence of strong corticospinal output from the primary motor cortex to distal compared with proximal hindlimb muscles. The extent to which corticospinal output differs across muscles in the leg in humans remains poorly understood. Using transcranial magnetic stimulation over the leg representation of the primary motor cortex, we constructed motor evoked potential (MEP) recruitment curves to measure the resting motor threshold (RMT), maximum MEP amplitude (MEP-max), and slope in the biceps femoris, rectus femoris, tibialis anterior, soleus, and a foot muscle (i.e., abductor hallucis) in intact humans. We found that the RMT was lower and the MEP-max and slope were larger in the abductor hallucis compared with most other muscles tested. In contrast, the RMT was higher and the MEP-max and slope were lower in the biceps femoris compared to all other muscles tested. Corticospinal responses in the rectus femoris, tibialis anterior, and soleus were in between those obtained from other leg muscles, with the soleus having a higher RMT and lower MEP-max and slope than the rectus femoris and tibialis anterior. To examine the origin of increases in corticospinal excitability in the abductor hallucis, we compared short-interval intracortical inhibition (SICI) and F-waves between the abductor hallucis and tibialis anterior. SICI was similar across muscles while the F-wave amplitude was larger in the abductor hallucis compared with the tibialis anterior. These results support a nonuniform distribution of corticospinal output to leg muscles, highlighting that increases in corticospinal excitability in a foot muscle could be related to a spinal origin.NEW & NOTEWORTHY We provide evidence on how corticospinal output differs across muscles in the leg in intact humans. We found that corticospinal responses were larger in a distal intrinsic foot muscle and were smaller in the biceps femoris compared to all other muscles in the leg. Increases in corticospinal excitability to an intrinsic foot muscle could have a spinal origin.

Surface Electromyography-Driven Parameters for Representing Muscle Mass and Strength

The need for developing a simple and effective assessment tool for muscle mass has been increasing in a rapidly aging society. This study aimed to evaluate the feasibility of the surface electromyography (sEMG) parameters for estimating muscle mass. Overall, 212 healthy volunteers participated in this study. Maximal voluntary contraction (MVC) strength and root mean square (RMS) values of motor unit potentials from surface electrodes on each muscle (biceps brachii, triceps brachii, biceps femoris, rectus femoris) during isometric exercises of elbow flexion (EF), elbow extension (EE), knee flexion (KF), knee extension (KE) were acquired. New variables (MeanRMS, MaxRMS, and RatioRMS) were calculated from RMS values according to each exercise. Bioimpedance analysis (BIA) was performed to determine the segmental lean mass (SLM), segmental fat mass (SFM), and appendicular skeletal muscle mass (ASM). Muscle thicknesses were measured using ultrasonography (US). sEMG parameters showed positive correlations with MVC strength, SLM, ASM, and muscle thickness measured by US, but showed negative correlations with SFM. An equation was developed for ASM: ASM = -26.04 + 20.345 × Height + 0.178 × weight - 2.065 × (1, if female; 0, if male) + 0.327 × RatioRMS(KF) + 0.965 × MeanRMS(EE) (SEE = 1.167, adjusted R² = 0.934). sEMG parameters in controlled conditions may represent overall muscle strength and muscle mass in healthy individuals.

A Dynamic Procedure to Detect Maximum Voluntary Contractions in Low Back

Surface electromyography (sEMG) is generally used to measure muscles' activity. The sEMG signal can be affected using several factors and vary among individuals and even measurement trials. Thus, to consistently evaluate data among individuals and trials, the maximum voluntary contraction (MVC) value is usually calculated and used to normalize sEMG signals. However, the sEMG amplitude collected from low back muscles can be frequently larger than that found when conventional MVC measurement procedures are used. To address this limitation, in this study, we proposed a new dynamic MVC measurement procedure for low back muscles. Inspired by weightlifting, we designed a detailed dynamic MVC procedure, and then collected data from 10 able-bodied participants and compared their performances using several conventional MVC procedures by normalizing the sEMG amplitude for the same test. The sEMG amplitude normalized by our dynamic MVC procedure showed a much lower value than those obtained using other procedures (Wilcoxon signed-rank test, with p < 0.05), indicating that the sEMG collected during dynamic MVC procedure had a larger amplitude than those of conventional MVC procedures. Therefore, our proposed dynamic MVC obtained sEMG amplitudes closer to its physiological maximum value and is thus more capable of normalizing the sEMG amplitude for low back muscles.

The Role of Fascial Tissue Layer in Electric Signal Transmission from the Forearm Musculature to the Cutaneous Layer as a Possibility for Increased Signal Strength in Myoelectric Forearm Exoprosthesis Development

Myoelectric exoprostheses serve to aid in the everyday activities of patients with forearm or hand amputations. While electrical signals are known key factors controlling exoprosthesis, little is known about how we can improve their transmission strength from the forearm muscles as to obtain better sEMG. The purpose of this study is to evaluate the role of the forearm fascial layer in transmitting myoelectrical current. We examined the sEMG signals in three individual muscles, each from six healthy forearms (Group 1) and six amputation stumps (Group 2), along with their complete biometric characteristics. Following the tests, one patient underwent a circumferential osteoneuromuscular stump revision surgery (CONM) that also involved partial removal of fascia and subcutaneous fat in the amputation stump, with re-testing after complete healing. In group 1, we obtained a stronger sEMG signal than in Group 2. In the CONM case, after surgery, the patient’s data suggest that the removal of fascia, alongside the fibrotic and subcutaneous fat tissue, generates a stronger sEMG signal. Therefore, a reduction in the fascial layer, especially if accompanied by a reduction of the subcutaneous fat layer may prove significant for improving the strength of sEMG signals used in the control of modern exoprosthetics.

Eccentric exercise-induced muscle weakness abolishes sex differences in fatigability during sustained submaximal isometric contractions

BACKGROUND

Females are typically less fatigable than males during sustained isometric contractions at lower isometric contraction intensities. This sex difference in fatigability becomes more variable during higher intensity isometric and dynamic contractions. While less fatiguing than isometric or concentric contractions, eccentric contractions induce greater and longer lasting impairments in force production. However, it is not clear how muscle weakness influences fatigability in males and females during sustained isometric contractions.

METHODS

We investigated the effects of eccentric exercise-induced muscle weakness on time to task failure (TTF) during a sustained submaximal isometric contraction in young (18-30 years) healthy males (n = 9) and females (n = 10). Participants performed a sustained isometric contraction of the dorsiflexors at 35° plantar flexion by matching a 30% maximal voluntary contraction (MVC) torque target until task failure (i.e., falling below 5% of their target torque for ≥2 s). The same sustained isometric contraction was repeated 30 min after 150 maximal eccentric contractions. Agonist and antagonist activation were assessed using surface electromyography over the tibialis anterior and soleus muscles, respectively.

RESULTS

Males were ∼41% stronger than females. Following eccentric exercise both males and females experienced an ∼20% decline in maximal voluntary contraction torque. TTF was ∼34% longer in females than males prior to eccentric exercise-induced muscle weakness. However, following eccentric exercise-induced muscle weakness, this sex-related difference was abolished, with both groups having an ∼45% shorter TTF. Notably, there was ∼100% greater antagonist activation in the female group during the sustained isometric contraction following exercise-induced weakness as compared to the males.

CONCLUSION

This increase in antagonist activation disadvantaged females by decreasing their TTF, resulting in a blunting of their typical fatigability advantage over males.

Increases in muscle temperature by hot water improve muscle contractile function and reduce motor unit discharge rates

PURPOSE

Examine the effects of 42°C hot-water immersion on muscle contraction function and motor unit discharge rates. Voluntary and evoked contraction assessments were examined first with a concomitant increase in the core and muscle temperature, and thereafter with increased muscle temperature but cooled core temperature.

METHODS

Fifteen participants (24.9 ± 5.6 years) performed neuromuscular assessments before, after, and ~15-min after either 90-min of 42°C (hot) or 36°C (control) water immersion. Maximal voluntary contraction (MVC) assessment of knee extension was performed along with surface electromyography (sEMG) (vastus lateralis and medialis [VL, VM]) and voluntary activation level (VAL). Resting evoked twitch was elicited for peak torque and time to peak torque analysis. In addition, the VL and VM motor unit discharge rates (MUDR) were measured.

RESULTS

After hot-water immersion (core temperature ↑1°C; muscle temperature ↑2.4°C), MVC torque and VAL decreased (p < 0.05). The sEMG (VL and VM) and peak twitch torque did not change (p > 0.05), while time to peak torque decreased (p = 0.007). The VL and VM MUDR decreased, showing a time effect, after both water immersion conditions (36 and 42°C) (p > 0.001). Fifteen minutes after the hot-water immersion (core temperature at baseline; muscle temperature ↑1.4°C), MVC torque returned to baseline, but VAL remained lower. The sEMG (VL and VM) remained unchanged. Peak twitch torque increased (p < 0.002) and time to peak torque remained lower (p = 0.028). The MUDR remained lower after both water immersion conditions (p < 0.05).

CONCLUSION

Increased core temperature evoked by 42°C hot-water immersion decreases MVC torque and VAL. However, a passive increase in muscle temperature improved evoked muscle contractile function (i.e., time to peak torque [after] and peak twitch torque [~15 min after]). Moreover, a passive increase in muscle temperature reduced the required MUDR to attain the same torque.

Ultrasound measures of muscle thickness and subcutaneous tissue from the hip abductors: Inter- and intra-rater reliability

BACKGROUND

Ultrasound imaging is important in many fields such as medicine, sports, and health sciences to assess parts of muscle structure (e.g., muscle thickness [MT]) or composition (subcutaneous tissue [Sub_T]).

OBJECTIVE

The aim of the present study was to investigate the intra- and inter-rater reliability of MT and Sub_T measurements of the hip abductor muscles gluteus medius (GM) and tensor fascia latae (TFL).

DESIGN

Cross-sectional study.

METHOD

Twenty young adults participated in the study. Intra-rater reliability was established by measuring the same two images twice by the same rater, while inter-rater reliability was assessed between two raters by measuring the same two images for each muscle. For both intra- and inter-rater reliability, the reliability of the TFL and GM outcomes (MT- Sub_T) were determined by the intraclass correlation coefficient (ICC), coefficient of variation (CV), standard error of the measurement (SEM), and Bland-Altman plots.

RESULTS

For intra-rater reliability, variables of both muscles showed an excellent ICC (≥0.90), lower CV and SEM, and bias near zero. Inter-rater reliability also showed an excellent ICC for both variables and muscles (≥0.81) with lower CV, SEM, and bias.

CONCLUSION

Therefore, these results provide strong evidence of a reliable measure of MT and Sub_T from GM and TFL. The present study provides health care professionals and researchers increased confidence in using 2D ultrasound to assess the hip abductors muscles reliably.

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.

Correlation between subcutaneous adipose tissue of the head and body mass index: Implications for functional neuroimaging

High body mass index (BMI) is generally assumed to represent overall amounts of body adipose tissue (fat). Increased adipose tissue amounts in persons with increased BMI has been cited as a barrier to assessment of body tissues such as muscle. Significant increases in the amount of adipose tissue between the dermal layer and the skull may result in high electrical impedance and/or increased light diffusion causing a lower signal to noise ratio during use of neuroimaging tools such as electroencepholography (EEG), transcranial direct current stimulation (tDCS), and functional near infrared spectroscopy (fNIRS). Investigating how subcutaneous adipose tissue in the head region increases with respect to total body fat percentage and BMI is an important step in developing mathematical corrections in neuroimaging measurements as BMI increases, as recommended in other measurement modalities such as electromyography (EMG). We hypothesized that percentage of subcutaneous adipose tissue in the head region would increase with respect to both total body fat percentage and BMI. A statistically significant increase in subcutaneous head fat percentage occurred with increased BMI and total body fat percentage. The data investigated in this study indicate that participant age, sex, and BMI are important features to consider in model corrections during data signal processing and analyses for subcutaneous head fat in neuroimaging approaches. The data in this project serve to provide physiological justification for this practice along with regression analyses to be considered for physiologically-based signal to noise correction algorithms.

Effect of long-term maximum strength training on explosive strength, neural, and contractile properties

The purpose of this cross‐sectional study was to compare explosive strength and underpinning contractile, hypertrophic, and neuromuscular activation characteristics of long‐term maximum strength‐trained (LT‐MST; ie, ≥3 years of consistent, regular knee extensor training) and untrained individuals. Sixty‐three healthy young men (untrained [UNT] n = 49, and LT‐MST n = 14) performed isometric maximum and explosive voluntary, as well as evoked octet knee extension contractions. Torque, quadriceps, and hamstring surface EMG were recorded during all tasks. Quadriceps anatomical cross‐sectional area (QACSA_MAX; via MRI) was also assessed. Maximum voluntary torque (MVT; +66%) and QACSA_MAX (+54%) were greater for LT‐MST than UNT ([both] p < 0.001). Absolute explosive voluntary torque (25–150 ms after torque onset; +41 to +64%; [all] p < 0.001; 1.15≤ effect size [ES]≤2.36) and absolute evoked octet torque (50 ms after torque onset; +43, p < 0.001; ES = 3.07) were greater for LT‐MST than UNT. However, relative (to MVT) explosive voluntary torque was lower for LT‐MST than UNT from 100 to 150 ms after contraction onset (−11% to −16%; 0.001 ≤ p ≤ 0.002; 0.98 ≤ ES ≤ 1.11). Relative evoked octet torque 50 ms after onset was lower (−10%; p < 0.001; ES = 1.14) and octet time to peak torque longer (+8%; p = 0.001; ES = 1.18) for LT‐MST than UNT indicating slower contractile properties, independent from any differences in torque amplitude. The greater absolute explosive strength of the LT‐MST group was attributable to higher evoked explosive strength, that in turn appeared to be due to larger quadriceps muscle size, rather than any differences in neuromuscular activation. In contrast, the inferior relative explosive strength of LT‐MST appeared to be underpinned by slower intrinsic/evoked contractile properties.

The Influence of Motor Unit Number and Muscle Activation on Early Phase Rate of Torque Development in Younger and Older Men

This study examined the influence of muscle activation and motor unit number estimation (MUNE) on early phase voluntary rate of torque development (RTD) in younger (YM) and older (OM) men. Thirty-two YM (n = 17; Age = 22 yrs) and OM (n = 15; Age = 74 yrs) volunteered to participate in this study. Early phase RTD (first 50 ms of a rapid isometric contraction; RTD₅₀) and normalized surface electromyography (first 50 ms of muscle excitation; nEMG₅₀) were recorded from the right quadricep muscle group. MUNE was examined from the right vastus lateralis. Multiple linear regression analyses revealed that nEMG₅₀ had a significant effect on RTD₅₀ independent of age group (p ≤ 0.001). nEMG₅₀ had a significant effect on RTD₅₀ in the OM group (p = 0.037). MUNE had no effect on RTD₅₀ independent of age. Older adults may depend more on muscle activation at contraction onset of early phase RTD compared to younger adults.

Gender response to 10 weeks acupuncture-TENS application on patients who presented with post-injection sciatic pain

Background

Studies to determine gender response to transcutaneous electrical nerve stimulation (TENS) application on individuals who presented with post-injection sciatic pain (PISP) following gluteal injection is not common. A total of 40 subjects comprising 20 males and 20 females who were purposively recruited and conveniently assigned to group A (male) and group B (female) completed the study. Acupuncture-like TENS (AL-TENS) was applied on the 20 male and 20 female subjects, 1 h per session, 3 times per week for the 10 weeks the study lasted.

Result

The pre-intervention baseline scores for the two groups were 8.80 + 1.05 (Female) and 8.60 + 1.27 (Male). The result revealed that after 10 weeks of intervention the VAS scores were 2.60+ 3.28 (p < 0.001) and 2.40 + 3.28 (p < 0.001) for the female and male subjects, respectively. The mean comparison of the female mean VAS scores (2.60 + 3.28) and male VAS scores 2.40 + 3.25 after 10 weeks of AL-TENS intervention shows no statistically significant difference (p > 0.85) in pain intensity (pain perception).

Conclusions

There was no gender variation in pain perception in subjects with post-injection sciatic pain (PISP) following gluteal muscle injection after 10 weeks of AL-TENS application. Therefore, gender-based variation should not be considered when applying AL-TENS as an instrument of intervention in subjects with PISP.

Trial registration

PACTR2018050034082

Time to Save Time: Beneficial Effects of Blood Flow Restriction Training and the Need to Quantify the Time Potentially Saved by Its Application During Musculoskeletal Rehabilitation

The main goal of musculoskeletal rehabilitation is to achieve the pre-injury and/or pre-surgery physical function level with a low risk of re-injury. Blood flow restriction (BFR) training is a promising alternative to conventional therapy approaches during musculoskeletal rehabilitation because various studies support its beneficial effects on muscle mass, strength, aerobic capacity, and pain perception. In this perspective article, we used an evidence-based progressive model of a rehabilitative program that integrated BFR in 4 rehabilitation phases: (1) passive BFR, (2) BFR combined with aerobic training, (3) BFR combined with low-load resistance training, and (4) BFR combined with low-load resistance training and traditional high-load resistance training. Considering the current research, we propose that a BFR-assisted rehabilitation has the potential to shorten the time course of therapy to reach the stage where the patient is able to tolerate resistance training with high loads. The information and arguments presented are intended to stimulate future research, which compares the time to achieve rehabilitative milestones and their physiological bases in each stage of the musculoskeletal rehabilitation process. This requires the quantification of BFR training-induced adaptations (eg, muscle mass, strength, capillary-to-muscle-area ratio, hypoalgesia, molecular changes) and the associated changes in performance with a high measurement frequency (≤1 week) to test our hypothesis. This information will help to quantify the time saved by BFR-assisted musculoskeletal rehabilitation. This is of particular importance for patients, because the potentially accelerated recovery of physical functioning would allow them to return to their work and/or social life earlier. Furthermore, other stakeholders in the health care system (eg, physicians, nurses, physical therapists, insurance companies) might benefit from that with regard to work and financial burden.

The Strength of the Corticospinal Tract Not the Reticulospinal Tract Determines Upper-Limb Impairment Level and Capacity for Skill-Acquisition in the Sub-Acute Post-Stroke Period

Background. Upper-limb impairment in patients with chronic stroke appears to be partly attributable to an upregulated reticulospinal tract (RST). Here, we assessed whether the impact of corticospinal (CST) and RST connectivity on motor impairment and skill-acquisition differs in sub-acute stroke, using transcranial magnetic stimulation (TMS)–based proxy measures. Methods. Thirty-eight stroke survivors were randomized to either reach training 3-6 weeks post-stroke (plus usual care) or usual care only. At 3, 6 and 12 weeks post-stroke, we measured ipsilesional and contralesional cortical connectivity (surrogates for CST and RST connectivity, respectively) to weak pre-activated triceps and deltoid muscles with single pulse TMS, accuracy of planar reaching movements, muscle strength (Motricity Index) and synergies (Fugl-Meyer upper-limb score). Results. Strength and presence of synergies were associated with ipsilesional (CST) connectivity to the paretic upper-limb at 3 and 12 weeks. Training led to planar reaching skill beyond that expected from spontaneous recovery and occurred for both weak and strong ipsilesional tract integrity. Reaching ability, presence of synergies, skill-acquisition and strength were not affected by either the presence or absence of contralesional (RST) connectivity. Conclusion. The degree of ipsilesional CST connectivity is the main determinant of proximal dexterity, upper-limb strength and synergy expression in sub-acute stroke. In contrast, there is no evidence for enhanced contralesional RST connectivity contributing to any of these components of impairment. In the sub-acute post-stroke period, the balance of activity between CST and RST may matter more for the paretic phenotype than RST upregulation per se.

Neural adaptations to long-term resistance training: evidence for the confounding effect of muscle size on the interpretation of surface electromyography

This study compared elbow flexor (EF; experiment 1) and knee extensor (KE; experiment 2) maximal compound action potential (M_max) amplitude between long-term resistance trained (LTRT; n = 15 and n = 14, 6 ± 3 and 4 ± 1 yr of training) and untrained (UT; n = 14 and n = 49) men, and examined the effect of normalizing electromyography (EMG) during maximal voluntary torque (MVT) production to M_max amplitude on differences between LTRT and UT. EMG was recorded from multiple sites and muscles of EF and KE, M_max was evoked with percutaneous nerve stimulation, and muscle size was assessed with ultrasonography (thickness, EF) and magnetic resonance imaging (cross-sectional area, KE). Muscle-electrode distance (MED) was measured to account for the effect of adipose tissue on EMG and M_max. LTRT displayed greater MVT (+66%-71%, P < 0.001), muscle size (+54%-56%, P < 0.001), and M_max amplitudes (+29%-60%, P ≤ 0.010) even when corrected for MED (P ≤ 0.045). M_max was associated with the size of both muscle groups (r ≥ 0.466, P ≤ 0.011). Compared with UT, LTRT had higher absolute voluntary EMG amplitude for the KE (P < 0.001), but not the EF (P = 0.195), and these differences/similarities were maintained after correction for MED; however, M_max normalization resulted in no differences between LTRT and UT for any muscle and/or muscle group (P ≥ 0.652). The positive association between M_max and muscle size, and no differences when accounting for peripheral electrophysiological properties (EMG/M_max), indicates the greater absolute voluntary EMG amplitude of LTRT might be confounded by muscle morphology, rather than providing a discrete measure of central neural activity. This study therefore suggests limited agonist neural adaptation after LTRT.NEW & NOTEWORTHY In a large sample of long-term resistance-trained individuals, we showed greater maximal M-wave amplitude of the elbow flexors and knee extensors compared with untrained individuals, which appears to be at least partially mediated by differences in muscle size. The lack of group differences in voluntary EMG amplitude when normalized to maximal M-wave suggests that differences in muscle morphology might impair interpretation of voluntary EMG as an index of central neural activity.

Reliability of corticospinal excitability estimates for the vastus lateralis: Practical considerations for lower limb TMS task selection

Transcranial magnetic stimulation (TMS) is increasingly used to examine lower extremity corticospinal excitability (CSE) in clinical and sports research. Because CSE is task-specific, there is growing emphasis on the use of ecological tasks. Nevertheless, the comparative reliability of CSE measurements during established (e.g. knee extensions; KE) and more recent ecological (e.g. squats; SQT) lower extremity tasks has received less attention. The aim of this study was to compare the test-retest reliability of CSE, force, and muscle activity (EMG) during isometric SQT and KE. 19 right-footed men (age: 25 ± 5 yrs) with similar fitness and body composition performed SQT (N = 7) or KE (N = 12) on two consecutive days. Force and EMG were recorded during maximum voluntary isometric contractions (MVC). Corticospinal excitability was determined in the dominant leg during light (15% MVC) contractions based on motor evoked potential (MEP) stimulus-response-curves (SRC). Test-retest reliability, absolute agreement, and consistency were determined for force, EMG, and SRC MEP maximum (MEPMAX) and rising phase midpoint (V50). As a secondary analysis, all outcomes were compared between groups with mixed-methods ANCOVAs (Task × Time, covariate: body-fat-percentage). Compared with SQT, KE displayed better test-retest reliability and agreement for MEPMAX whereas V50, force, and EMG were similarly reliable. Force (p = 0.01) and MEPMAX (p = 0.02) were also greater during KE despite a similar V50 (p = 0.11). Differences in test-retest reliability, absolute agreement, and between-group comparisons highlight the need to carefully select lower limb TMS assessment tasks and encourage future efforts to balance ecological validity with statistical sensitivity.

Intramuscular Fat Influences Neuromuscular Activation of the Gluteus Medius in Older Adults

The amount of tissue between the muscle and surface electromyography (sEMG) electrode influences the sEMG signals. Increased intramuscular adipose tissue (IMAT) of the hip abductor muscles negatively impacts balance in older individuals, but it is unknown if this is related to the ability to activate the muscles. The aim of this preliminary study was to investigate the influence of gluteus medius (GM) IMAT on sEMG amplitude during maximal voluntary isometric contractions (MVIC) of the hip abductors in older adults. We recruited 12 healthy community-dwelling older adults that underwent a spiral computerized tomography scan. High density lean (HDL), IMAT, and subcutaneous adipose tissue (SUBFAT) cross-sectional area of the GM were assessed. sEMG signal from the GM was recorded while participants performed an MVIC of the hip abductors. There was a negative correlation between GM activation and IMAT (r = -0.58, P = 0.046), and also SUBFAT (r = -0.78, P = 0.002) and a positive correlation with HDL (r = 0.73, P = 0.006). When controlling for SUBFAT, the partial correlations demonstrated a consistent negative correlation between GM activation and IMAT (r = -0.60, P = 0.050) but no relationship with HDL. The current results are important for helping to interpret the results from sEMG by accounting for IMAT. In conclusion, the neuromuscular activation of the GM may be reduced by the quantity of IMAT.

Sex differences in fatigability following exercise normalised to the power-duration relationship

KEY POINTS

Knee-extensors demonstrate greater fatigue resistance in females compared to males during single-limb and whole-body exercise. For single-limb exercise, the intensity-duration relationship is different between sexes, with females sustaining a greater relative intensity of exercise. This study established the power-duration relationship during cycling, then assessed fatigability during critical power-matched exercise within the heavy and severe intensity domains. When critical power and the curvature constant were expressed relative to maximal ramp test power, no sex difference was observed. No sex difference in time to task failure was observed in either trial. During heavy and severe intensity cycling, females experienced lesser muscle de-oxygenation. Following both trials, females experienced lesser reductions in knee-extensor contractile function, and following heavy intensity exercise, females experienced less reduction in voluntary activation. These data demonstrate that whilst the relative power-duration relationship is not different between males and females, the mechanisms of fatigability during critical power-matched exercise are mediated by sex.

ABSTRACT

Due to morphological differences, females demonstrate greater fatigue resistance of locomotor muscle during single-limb and whole-body exercise modalities. Whilst females sustain a greater relative intensity of single-limb, isometric exercise than males, limited investigation has been performed during whole-body exercise. Accordingly, this study established the power-duration relationship during cycling in 18 trained participants (eight females). Subsequently, constant-load exercise was performed at critical power (CP)-matched intensities within the heavy and severe domains, with the mechanisms of fatigability assessed via non-invasive neurostimulation, near-infrared spectroscopy and pulmonary gas exchange during and following exercise. Relative CP (72 ± 5 vs. 74 ± 2% P_max , P = 0.210) and curvature constant (51 ± 11 vs. 52 ± 10 J P_max ^-1 , P = 0.733) of the power-duration relationship were similar between males and females. Subsequent heavy (P = 0.758) and severe intensity (P = 0.645) exercise time to task failures were not different between sexes. However, females experienced lesser reductions in contractile function at task failure (P ≤ 0.020), and greater vastus lateralis oxygenation (P ≤ 0.039) during both trials. Reductions in voluntary activation occurred following both trials (P < 0.001), but were less in females following the heavy trial (P = 0.036). Furthermore, during the heavy intensity trial only, corticospinal excitability was reduced at the cortical (P = 0.020) and spinal (P = 0.036) levels, but these reductions were not sex-dependent. Other than a lower respiratory exchange ratio in the heavy trial for females (P = 0.039), no gas exchange variables differed between sexes (P ≥ 0.052). Collectively, these data demonstrate that whilst the relative power-duration relationship is not different between males and females, the mechanisms of fatigability during CP-matched exercise above and below CP are mediated by sex.

Attenuated activation of knee extensor muscles during fast contractions in older men and women

AIM

Reduced physical function and increased risk of falls in older adults are accompanied by age-related reductions in torque development of leg muscles, although the mechanisms and potential sex differences are not understood.

PURPOSE

To determine the mechanistic origins (neural vs. muscular) for the age-related reduction in torque development, we compared the peak rates of torque development (RTD) during electrically-evoked and fast voluntary contractions of the knee extensors between young and older men and women.

METHODS

Sets of single- and double-pulse electrical stimulations evoked contractions of the knee extensor muscles in 20 young (23.0 ± 0.8 years; 10 women) and 20 older adults (78.2 ± 1.5 years; 10 women), followed by voluntary isometric knee extension contractions with torque development as fast as possible that matched the torque during electrically-evoked contraction (10-40% maximal torque).

RESULTS

Peak RTD during fast-voluntary contractions was 41% less than electrically-evoked contractions (p < 0.001), but more so for older adults (44%) than young (38%, p = 0.04), with no sex differences. Peak RTD during fast-voluntary contractions was more variable between contractions for the older than young adults (77%MVC s^-1 vs. 47%MVC s^-1, p < 0.001). Additionally, older women exhibited greater variability than older men (81%MVC s^-1 vs. 72%MVC s^-1, p = 0.04) with no sex-related differences within the young adults.

CONCLUSION

Older adults had slower and more variable RTD during voluntary contractions than young adults, particularly older women. The limited age-related differences in electrically-evoked RTD suggest the primary mechanism for the slower torque development of the knee extensor muscles in older men and women involve reduced neural activation.

Mechanical and morphological determinants of peak power output in elite cyclists

Mechanical peak power output (PPO) is a determinant of performance in sprint cycling. The purpose of this study was to examine the relationship between PPO and putative physiological determinants of PPO in elite cyclists, and to compare sprint performance between elite sprint and endurance cyclists. Thirty-five elite cyclists (18 endurance; 17 sprint) performed duplicate sprint cycling laboratory tests to establish PPO and its mechanical components. Quadriceps femoris (Q_VOL ) and hamstring muscle volume (HAM_VOL ) were assessed with MRI, vastus lateralis pennation angle (Pθ_VL ) and fascicle length (FL_VL ) were determined with ultrasound imaging, and neuromuscular activation of three muscles was assessed using EMG at PPO during sprint cycling. For the whole cohort, there was a wide variability in PPO (range 775-2025 W) with very large, positive, bivariate relationships between PPO and Q_VOL (r = .87), HAM_VOL (r = .71), and Pθ_VL (r = .81). Step-wise multiple regression analysis revealed that 87% of the variability in PPO between cyclists was explained by two variables Q_VOL (76%) and Pθ_VL (11%). The sprint cyclists had greater PPO (+61%; P < .001 vs endurance), larger Q_VOL (P < .001), and BF_VOL (P < .001) as well as more pennate vastus lateralis muscles (P < .001). These findings emphasize the importance of quadriceps muscle morphology for sprint cycling events.

Explosive strength: effect of knee-joint angle on functional, neural, and intrinsic contractile properties

Purpose

The present study compared knee extension explosive isometric torque, neuromuscular activation, and intrinsic contractile properties at five different knee-joint angles (35°, 50°, 65°, 80°, and 95°; 0° = full knee extension).

Methods

Twenty-eight young healthy males performed two experimental sessions each involving: 2 maximum, and 6–8 explosive voluntary contractions at each angle; to measure maximum voluntary torque (MVT), explosive voluntary torque (EVT; 50–150 ms after contraction onset) and quadriceps surface EMG (QEMG, 0–50, 0–100, and 0–150 ms after EMG onset during the explosive contractions). Maximum twitch and M-wave (M_MAX) responses as well as octet contractions were evoked with femoral nerve stimulation at each angle.

Results

Absolute MVT and EVT showed an inverted ‘U’ relationship with higher torque at intermediate angles. There were no differences between knee-joint angles for relative EVT (%MVT) during the early phase (≤ 75 ms) of contraction and only subtle differences during the late phase (≥ 75 ms) of contraction (≤ 11%). Neuromuscular activation during explosive contractions was greater at more flexed than extended positions, and this was also the case during MVT. Whilst relative twitch torque (%MVT) was higher at knee flexed positions (P ≤ 0.001), relative octet torque (%MVT) was higher at knee extended positions (P ≤ 0.001).

Conclusion

Relative EVT was broadly similar between joint angles, likely because neuromuscular activation during both explosive and plateau (maximum) phases of contraction changed proportionally, and due to the opposing changes in twitch and octet evoked responses with joint angle.

Reduced corticospinal responses in older compared with younger adults during submaximal isometric, shortening, and lengthening contractions

The aim of this study was to assess differences in motor performance, as well as corticospinal and spinal responses to transcranial magnetic and percutaneous nerve stimulation, respectively, during submaximal isometric, shortening, and lengthening contractions between younger and older adults. Fifteen younger [26 yr (SD 4); 7 women, 8 men] and 14 older [64 yr (SD 3); 5 women, 9 men] adults performed isometric and shortening and lengthening dorsiflexion on an isokinetic dynamometer (5°/s) at 25% and 50% of contraction type-specific maximums. Motor evoked potentials (MEPs) and H reflexes were recorded at anatomical zero. Maximal dorsiflexor torque was greater during lengthening compared with shortening and isometric contractions ( P < 0.001) but was not age dependent ( P = 0.158). However, torque variability was greater in older compared with young adults ( P < 0.001). Background electromyographic (EMG) activity was greater in older compared with younger adults ( P < 0.005) and was contraction type dependent ( P < 0.001). As evoked responses are influenced by both the maximal level of excitation and background EMG activity, the responses were additionally normalized {[MEP/maximum M wave (M_max)]/root-mean-square EMG activity (RMS) and [H reflex (H)/M_max]/RMS}. (MEP/M_max)/RMS and (H/M_max)/RMS were similar across contraction types but were greater in young compared with older adults ( P < 0.001). Peripheral motor conduction times were prolonged in older adults ( P = 0.003), whereas peripheral sensory conduction times and central motor conduction times were not age dependent ( P ≥ 0.356). These data suggest that age-related changes throughout the central nervous system serve to accommodate contraction type-specific motor control. Moreover, a reduction in corticospinal responses and increased torque variability seem to occur without a significant reduction in maximal torque-producing capacity during older age. NEW & NOTEWORTHY This is the first study to have explored corticospinal and spinal responses with aging during submaximal contractions of different types (isometric, shortening, and lengthening) in lower limb musculature. It is demonstrated that despite preserved maximal torque production capacity corticospinal responses are reduced in older compared with younger adults across contraction types along with increased torque variability during dynamic contractions. This suggests that the age-related corticospinal changes serve to accommodate contraction type-specific motor control.