The Relationship Between Neuromuscular Function and the W' in Elite Cyclists.

This study compared neuromechanical characteristics of voluntary (maximum voluntary contraction (MVC) peak torque, rate of torque development (RTD), voluntary activation (VA)) and electrically stimulated contractions (peak torque, RTD) when performed under the same temperature conditions. Twelve physically active males performed two isometric MVCs of the quadriceps muscle group in an isokinetic dynamometer. The MVCs were performed after lower limb submersion for 20 min in hot (40°C) or cold (10°C) water. A control MVC was performed in ambient room temperature (17 ± 0.7°C). Electrical twitches were delivered at rest pre-MVC (Unpotentiated), during the plateau phase of the MVC (Superimposed) and post-MVC (Potentiated). Peak torque for MVC, Unpotentiated and Potentiated was recorded. RTD was calculated for the MVC (at 50, 100, 150, 200 ms and peak torque time points), Unpotentiated and Potentiated twitches, while VA (using the central activation ratio method) was calculated. There was no significant change between conditions in MVC peak torque, MVC RTD, VA and (averaged) twitch peak torque (p > 0.05). Twitch RTD for the hot condition (1025.0 ± 163.0 N·m·s−1) was significantly higher (p = 0.003) than control (872.3 ± 142.9 N·m·s−1). In conclusion, environmental temperature changes, in the range examined, do not affect the ability to generate maximum torque or any of the RTD parameters in maximum voluntary isometric contractions. In contrast, increased heat results in higher RTD in electrically stimulated contractions, most likely induced by reduced contraction time. This has practical implications for the use of electromyostimulation for injury prevention.

Quadriceps function has been demonstrated to predict disability in a variety of pathological patient populations (chronic obstructive pulmonary disorder, knee osteoarthritis, diabetes). Recent evidence suggests that knee extensor power may be more closely associated with disability than typical measures of strength. Unlike isometric strength assessments, evaluating power requires the use of cumbersome and expensive equipment. However, rate of torque development (RTD) during a maximal voluntary isometric contraction (MVIC) may closely associate with power outcomes; if so, RTD may be a surrogate outcome for power that can be assessed more easily in clinical settings. PURPOSE: Determine the association between knee extensor RTD and peak isokinetic power at 60, 180 and 240°/s after accounting for isometric strength. METHODS: The dominant limb of 26 healthy volunteers was secured to an isokinetic dynamometer for testing (21 females, 5 males; Age = 20.1±0.95 yr; Height = 170.4±7.7m; Mass = 66.0±10.44kg). Peak MVIC torque (strength) and peak RTD outcomes were collected from the mean of two MVIC at 90° of knee flexion, where individuals were instructed to generate an MVIC as fast as possible. Peak isokinetic power was extracted during the concentric knee extension phase at each of the contraction speeds (60, 180 and 240°/s). Linear regression models were used to determine the amount of variance in peak power (at each contraction speed) that could be explained by RTD. To determine the change in R2 due to adding RTD in to each of the peak power speeds, separate models were fitted hierarchically with MVIC only, and then with MVIC and RTD jointly. RESULTS: Together, MVIC and RTD explained R2=0.54 (P<0.001), R2=0.48 (P=0.001), R2=0.57 (P<0.001) at 60, 180 and 240°/s, respectively. After accounting for MVIC, RTD accounted for ΔR2= 0.08 (P=0.06), ΔR2= 0.17 (P=0.01), ΔR2= 0.09 (P=0.04), of the variance in peak power at 60, 180 and 240°/s, respectively. DISCUSSION: Together, MVIC and RTD predicted between 48% and 57% of the variance in peak power. RTD significantly predicted between 9% and 17% of the variance in peak power after accounting for MVIC. RTD may not be an adequate surrogate for peak power outcomes. Future studies should determine if RFD, MVIC, and isokinetic power provide unique information regarding muscle dysfunction.

Rate of torque development (RTD) is considered important for improving the performance in sports requiring explosive strength. Although RTD was reported to be increased after explosive-type strength training (Tillin et...

Voluntary strength has been shown to be influenced by the stiffness of the muscle-tendon unit. Measures of stiffness have also been reported to be linearly related to muscle cross-sectional area (CSA). However, no previous studies have examined the relationship between passive stiffness and evoked twitch properties when accounting for the influence of muscle CSA. PURPOSE: To examine the relationship between passive stiffness and evoked twitch properties prior to and following the normalization of passive stiffness to muscle CSA. METHODS: Ten healthy participants (age = 23 ± 3 yr) performed a passive range of motion and evoked twitch assessment of their right plantar flexors on a calibrated Biodex System 3 dynamometer. Passive stiffness was determined from the slope of the final 5° of the angle-torque curve. One doublet was administered via transcutaneous electrical stimulation (Digitimer DS7AH) to determine peak twitch torque (PTT) and the maximal rate of torque development (RTD). PTT was defined as the highest average torque value achieved for any 20 consecutive data points, while RTD was determined as the highest slope value (torque/time) calculated with the same epoch from the onset of torque production. Muscle CSA was determined using a peripheral quantitative computed tomography (pQCT) scanner (XCT 3000) and manually analyzed using DicomWorks v1.3.5 software. Pearson product moment correlation coefficients (r) were used to quantify the relationship between passive stiffness and PTT and RTD and normalized passive stiffness (passive stiffness · muscle CSA-1) and PTT and RTD. An alpha of P≤0.05 was used to determine statistical significance. RESULTS: There were significant positive relationships between passive stiffness and PTT (P = 0.003, r = 0.828) and passive stiffness and RTD (P = 0.003, r = 0.825). When stiffness was normalized to muscle CSA, there were no observed significant relationships between passive stiffness and PTT (P = 0.290, r = 0.372) or RTD (P = 0.353, r = 0.329). CONCLUSIONS: These findings indicated a significant linear relationship between passive stiffness and evoked twitch properties, however when passive stiffness was normalized to muscle CSA the relationship was no longer significant and accounted for only 11 - 14% of the variance in twitch properties.

Previously published studies on the effect of short-term endurance training on neuromuscular function of the plantar flexors have shown that the H-reflex elicited at rest and during weak voluntary contractions was increased following the training regime. However, these studies did not test H-reflex modulation during isometric maximum voluntary contraction (iMVC) and did not incorporate a control group in their study design to compare the results of the endurance training group to individuals without the endurance training stimulus. Therefore, this randomized controlled study was directed to investigate the neuromuscular function of the plantar flexors at rest and during iMVC before and after 8 weeks of cycling endurance training. Twenty-two young adults were randomly assigned to an intervention group and a control group. During neuromuscular testing, rate of torque development, isometric maximum voluntary torque and muscle activation were measured. Triceps surae muscle activation and tibialis anterior muscle co-activation were assessed by normalized root mean square of the EMG signal during the initial phase of contraction (0–100, 100–200 ms) and iMVC of the plantar flexors. Furthermore, evoked spinal reflex responses of the soleus muscle (H-reflex evoked at rest and during iMVC, V-wave), peak twitch torques induced by electrical stimulation of the posterior tibial nerve at rest and fatigue resistance were evaluated. The results indicate that cycling endurance training did not lead to a significant change in any variable of interest. Data of the present study conflict with the outcome of previously published studies that have found an increase in H-reflex excitability after endurance training. However, these studies had not included a control group in their study design as was the case here. It is concluded that short-term cycling endurance training does not necessarily enhance H-reflex responses and fatigue resistance.

Strength-trained individuals (ST) develop greater levels of force compared with untrained subjects. These differences are partly of neural origin and can be explained by training-induced changes in the neural drive to the muscles. In the present study we hypothesize a greater rate of torque development (RTD) and faster recruitment of motor units with greater muscle fiber conduction velocity (MFCV) in ST compared with a control cohort. MFCV was assessed during maximal voluntary isometric explosive contractions of the elbow flexors in eight ST and eight control individuals. MFCV was estimated from high-density surface electromyogram recordings (128 electrodes) in intervals of 50 ms starting from the onset of the electromyogram. RTD and MFCV were computed and normalized to their maximal voluntary torque (MVT) values. The explosive torque of the ST was greater than in the control group in all time intervals analyzed (P < 0.001). The absolute MFCV values were also greater for the ST than for controls at all time intervals (P < 0.001). ST also achieved greater normalized RTD in the first 50 ms of contraction [887.6 (152) vs. 568.5 (148.66)%MVT/s, mean (SD), P < 0.001] and normalized MFCV before the rise in force compared with controls. We have shown for the first time that ST can recruit motor units with greater MFCV in a shorter amount of time compared with untrained subjects during maximal voluntary isometric explosive contractions.NEW & NOTEWORTHY Strength-trained individuals show neuromuscular adaptations. These adaptations have been partly related to changes in the neural drive to the muscles. Here, we show for the first time that during the initial phase of a maximal isometric explosive contraction, strength-trained individuals achieve higher levels of force and recruit motor units with greater conduction velocities.

Measuring rate of torque development (RTD) and peak torque (PT) for hip muscle performance presents challenges in clinical practice. This study investigated the construct validity of a handheld dynamometer (HHD) versus an isokinetic dynamometer (IKD), and intra-rater repeated reliability for RTD and PT and their relationship in hip joint movements. Thirty healthy individuals (mean age = 30 ± 8 years, 13 males) underwent two test sessions in a single day. RTD (0-50, 0-100, 0-150, 0-200ms) and PT normalised to body mass in maximal voluntary isometric contractions were measured using a HHD and an IKD in hip flexion, extension, abduction, adduction, internal and external rotation. For validity between the devices, RTD0-50 exhibited the largest significant systematic bias in all hip movements (3.41-11.99 Nm·s-1 kg-1) and widest limits-of-agreement, while RTD0-200 had the lowest bias (-1.33-3.99 Nm·s-1 kg-1) and narrowest limits-of-agreement. For PT, agreement between dynamometers was observed for hip flexion (0.08 Nm·kg-1), abduction (-0.09 Nm·kg-1), internal (-0.01 Nm·kg-1), and external rotation (0.05 Nm·kg-1). For reliability, intra-rater intraclass correlation coefficient (ICC2,1) ranged from moderate to good in RTD0-50 and RTD0-100 (0.5-0.88), and good to excellent in RTD0-150 and RTD0-200 (0.87-0.95) in all movements. The HHD displayed excellent intra-rater, relative reliability values (ICC2,1) in all movements (0.85-0.95). Pearson's correlation revealed good linear correlation between PT and RTD0-150 and RTD0-200 in all movements (r = .7 to .87, p = < .001). Validity analysis demonstrated significant systematic bias and lack of agreement in RTD measures between the HHD and IKD. However, the HHD displays excellent to moderate intra-rater, relative reliability for RTD and PT measures in hip movements. Clinicians may use the HHD for hip muscle PT assessment but note, late phase RTD measures are more reliable, valid, and relate to PT than early phase RTD. Additionally, the correlation between RTD and PT at various time epochs was examined to better understand the relationship between these measures.

Neural and muscular alterations of the plantar flexors in middle-aged women.

Time-dependent neuromuscular parameters in the plantar flexors support greater fatigability of old compared with younger males

The objective of this study was to test the hypothesis that neuromuscular fatigue influences the rate of torque development (RTD) in a similar manner to isometric torque. Nine men participated in this study and performed 5-min all-out isometric tests for knee extensors (KE) and plantar flexors (PF) muscles, to determine the end-test torque (ET) and the critical rate of torque development (critical RTD). Additionally, participants performed submaximal constant-torque tests to task failure for KE and PF muscles. Both maximal voluntary contraction and RTD exhibited hyperbolic behavior and reached an asymptote at the end of the 5-min all out isometric test with similar relative values (KE 29.5 ± 5.6% MVC and PF 50.9 ± 2.9% MVC and KE 25.1 ± 3.6 to 28.5 ± 4.4% RTD and PF 48.4 ± 6.5 to 52.4 ± 5.8% RTD). However, both % MVC and % RTD were statistically different between muscle groups (P < 0.05), even when normalized by muscle volume (P < 0.05). Torque and RTD after the constant-torque test were similar to the values of ET and critical RTD (P > 0.05), respectively. In this study, it was observed that neuromuscular fatigue affects RTD and torque similarly, with the magnitude of this effect varying according to the muscle size.

BackgroundNeuromuscular deficits are common following knee injuries and may contribute to early-onset post-traumatic osteoarthritis, likely mediated through quadriceps dysfunction.ObjectiveTo identify how peri-articular neuromuscular function changes over time after knee injury and surgery.DesignSystematic review with meta-analyses.Data SourcesPubMed, Web of Science, Embase, Scopus, CENTRAL (Trials).Eligibility Criteria for Selecting StudiesModerate and high-quality studies comparing neuromuscular function of muscles crossing the knee joint between a knee-injured population (ligamentous, meniscal, osteochondral lesions) and healthy controls. Outcomes included normalized isokinetic strength, muscle size, voluntary activation, cortical and spinal-reflex excitability, and other torque related outcomes.ResultsA total of 46 studies of anterior cruciate ligament (ACL) and five of meniscal injury were included. For ACL injury, strength and voluntary activation deficits were evident (moderate to strong evidence). Cortical excitability was not affected at < 6 months (moderate evidence) but decreased at 24+ months (moderate evidence). Spinal-reflex excitability did not change at < 6 months (moderate evidence) but increased at 24+ months (strong evidence). We also found deficits in torque variability, rate of torque development, and electromechanical delay (very limited to moderate evidence). For meniscus injury, strength deficits were evident only in the short-term. No studies reported gastrocnemius, soleus or popliteus muscle outcomes for either injury. No studies were found for other ligamentous or chondral injuries.ConclusionsNeuromuscular deficits persist for years post-injury/surgery, though the majority of evidence is from ACL injured populations. Muscle strength deficits are accompanied by neural alterations and changes in control and timing of muscle force, but more studies are needed to fill the evidence gaps we have identified. Better characterisation and therapeutic strategies addressing these deficits could improve rehabilitation outcomes, and potentially prevent PTOA.Trial Registration NumberPROSPERO CRD42019141850.

ObjectivesTo assess the time course of changes in rapid muscle force/torque production capacity and neuromuscular activity of lower limb muscles in response to prolonged (∼2 h) match-play tennis under heat...

Low-dose hip abductor-adductor power training improves neuromechanical weight-transfer control during lateral balance recovery in older adults

ACL Research Retreat V

The aim of this study was to investigate the effects of the ageing process in the electromechanical delay (EMD), rate of torque development (RTD) and peak torque (PT) of the knee extensor muscles. The volunteers were assigned to three groups: young group (YG - 23·44 ± 4·74 years, 78·14 ± 15·11 kg, 1·72 ± 0·05 m), middle-aged group (MAG - 49·56 ± 6·06 years, 72·01 ± 14·07 kg, 1·67 ± 0·06 m) and elderly group (EG - 68·67 ± 9·06 years, 67·96 ± 7·60 kg, 1·64 ± 0·07 m). The PT and RTD were assessed during maximal voluntary ballistic isometric contractions (MVBIC) in the isokinetic dynamometer. Muscle electrical activity was recorded (EMG) during MVBIC in the vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) muscles. The EMD was calculated during the MVBIC, through the time interval between the EMG onset and torque onset. The PT and RTD were higher in the YG than in the MAG (P = 0·02; P = 0·01, respectively) and in the EG (P = 0·002; P = 0·0004, respectively). There were no significant differences in EMD among the three age groups for the VL, VM and RF (P>0·05) muscles. We conclude that age affects the PT and RTD, but not EMD of the VL, VM and RF muscles.