Repeatability of corticospinal and spinal measures during lengthening and shortening contractions in the human tibialis anterior muscle

Elements of the human central nervous system (CNS) constantly oscillate. In addition, there are also methodological factors and changes in muscle mechanics during dynamic muscle contractions that threaten the stability and consistency of transcranial magnetic stimulation (TMS) and perpherial nerve stimulation (PNS) measures.

Purpose

To determine the repeatability of TMS and PNS measures during lengthening and shortening muscle actions in the intact human tibialis anterior.

Methods

On three consecutive days, 20 males performed lengthening and shortening muscle actions at 15, 25, 50 and 80% of maximal voluntary contraction (MVC). The amplitude of the Motor Evoked Potentials (MEPs) produced by TMS was measured at rest and during muscle contraction at 90° of ankle joint position. MEPs were normalised to Mmax determined with PNS. The corticospinal silent period was recorded at 80% MVC. Hoffman reflex (H-reflex) at 10% isometric and 25% shortening and lengthening MVCs, and V-waves during MVCs were also evoked on each of the three days.

Results

With the exception of MEPs evoked at 80% shortening MVC, all TMS-derived measures showed good reliability (ICC = 0.81–0.94) from days 2 to 3. Confidence intervals (CI, 95%) were lower between days 2 and 3 when compared to days 1 and 2. MEPs significantly increased at rest from days 1 to 2 (P = 0.016) and days 1 to 3 (P = 0.046). The H-reflex during dynamic muscle contraction was reliable across the three days (ICC = 0.76–0.84). V-waves (shortening, ICC = 0.77, lengthening ICC = 0.54) and the H-reflex at 10% isometric MVC (ICC = 0.66) was generally less reliable over the three days.

Conclusion

Although it is well known that measures of the intact human CNS exhibit moment-to-moment fluctuations, careful experimental arrangements make it possible to obtain consistent and repeatable measurements of corticospinal and spinal excitability in the actively lengthening and shortening human TA muscle.     

Increased Residual Force Enhancement in Older Adults Is Associated with a Maintenance of Eccentric Strength

Despite an age-related loss of voluntary isometric and concentric strength, muscle strength is well maintained during lengthening muscle actions (i.e., eccentric strength) in old age. Additionally, in younger adults during lengthening of an activated skeletal muscle, the force level observed following the stretch is greater than the isometric force at the same muscle length. This feature is termed residual force enhancement (RFE) and is believed to be a combination of active and passive components of the contractile apparatus. The purpose of this study was to provide an initial assessment of RFE in older adults and utilize aging as a muscle model to explore RFE in a system in which isometric force production is compromised, but structural mechanisms of eccentric strength are well-maintained. Therefore, we hypothesised that older adults will experience greater RFE compared with young adults. Following a reference maximal voluntary isometric contraction (MVC) of the dorsiflexors in 10 young (26.1±2.7y) and 10 old (76.0±6.5y) men, an active stretch was performed at 15°/s over a 30° ankle joint excursion ending at the same muscle length as the reference MVCs (40° of plantar flexion). Any additional torque compared with the reference MVC therefore represented RFE. In older men RFE was ∼2.5 times greater compared to young. The passive component of force enhancement contributed ∼37% and ∼20% to total force enhancement, in old and young respectively. The positive association (R ² = 0.57) between maintained eccentric strength in old age and RFE indicates age-related mechanisms responsible for the maintenance of eccentric strength likely contributed to the observed elevated RFE. Additionally, as indicated by the greater passive force enhancement, these mechanisms may be related to increased muscle series elastic stiffness in old age.     

Residual force enhancement following eccentric induced muscle damage

During lengthening of an activated skeletal muscle, the force maintained following the stretch is greater than the isometric force at the same muscle length. This is termed residual force enhancement (RFE), but it is unknown how muscle damage following repeated eccentric contractions affects RFE. Using the dorsiflexors, we hypothesised muscle damage will impair the force generating sarcomeric structures leading to a reduction in RFE. Following reference maximal voluntary isometric contractions (MVC) in 8 young men (26.5±2.8y) a stretch was performed at 30°/s over a 30° ankle excursion ending at the same muscle length as the reference MVCs (30° plantar flexion). Surface electromyography (EMG) of the tibialis anterior and soleus muscles was recorded during all tasks. The damage protocol involved 4 sets of 25 isokinetic (30°/s) lengthening contractions. The same measures were collected at baseline and immediately post lengthening contractions, and for up to 10min recovery. Following the lengthening contraction task, there was a 30.3±6.4% decrease in eccentric torque (P<0.05) and 36.2±9.7% decrease in MVC (P<0.05) compared to baseline. Voluntary activation using twitch interpolation and RMS EMG amplitude of the tibialis anterior remained near maximal without increased coactivation for MVC. Contrary to our hypothesis, RFE increased (～100-250%) following muscle damage (P<0.05). It appears stretch provided a mechanical strategy for enhanced muscle function compared to isometric actions succeeding damage. Thus, active force of cross-bridges is decreased because of impaired excitation-contraction coupling but force generated during stretch remains intact because force contribution from stretched sarcomeric structures is less impaired.     

Effect of Hypohydration on Peripheral and Corticospinal Excitability and Voluntary Activation

We investigated whether altered peripheral and/or corticospinal excitatory output and voluntary activation are implicated in hypohydration-induced reductions in muscle isometric and isokinetic (90°.s⁻¹) strength. Nine male athletes completed two trials (hypohydrated, euhydrated) comprising 90 min cycling at 40°C, with body weight losses replaced in euhydrated trial. Peripheral nerve and transcranial magnetic stimulations were applied during voluntary contractions pre- and 40 min post-exercise to quantify voluntary activation and peripheral (M-wave) and corticospinal (motor evoked potential) evoked responses in m. vastus medialis. Both maximum isometric (−15.3±3.1 vs −5.4±3.5%) and isokinetic eccentric (−24.8±4.6 vs −7.3±7.2%) torque decreased to a greater extent in hypohydrated than euhydrated trials (p<0.05). Half relaxation time of the twitch evoked by peripheral nerve stimulation during maximal contractions increased after exercise in the hypohydrated (21.8±9.3%) but stayed constant in the euhydrated (1.6±10.7%; p = 0.017) condition. M-wave amplitude during maximum voluntary contraction increased after exercise in the heat in hypohydrated (10.7±18.0%) but decreased in euhydrated condition (−17.4±16.9%; p = 0.067). Neither peripheral nor cortical voluntary activation were significantly different between conditions. Motor evoked potential amplitude increased similarly in both conditions (hypohydrated: 25.7±28.5%; euhydrated: 52.9±33.5%) and was accompanied by lengthening of the cortical silent period in euhydrated but not hypohydrated condition (p = 0.019). Different neural strategies seem to be adopted to regulate neural drive in the two conditions, with increases in inhibitory input of either intracortical or corticospinal origin during the euhydrated trial. Such changes were absent in the hypohydrated condition, yet voluntary activation was similar to the euhydrated condition, perhaps due to smaller increases in excitatory drive rather than increased inhibition. Despite this maximal isometric and eccentric strength were impaired in the hypohydrated condition. The increase in peripheral muscle excitability evident in the hypohydrated condition was not sufficient to preserve performance in the face of reduced muscle contractility or impaired excitation-contraction coupling.     

Real-Time Changes in Corticospinal Excitability during Voluntary Contraction with Concurrent Electrical Stimulation

While previous studies have assessed changes in corticospinal excitability following voluntary contraction coupled with electrical stimulation (ES), we sought to examine, for the first time in the field, real-time changes in corticospinal excitability. We monitored motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation and recorded the MEPs using a mechanomyogram, which is less susceptible to electrical artifacts. We assessed the MEPs at each level of muscle contraction of wrist flexion (0%, 5%, or 20% of maximum voluntary contraction) during voluntary wrist flexion (flexor carpi radialis (FCR) voluntary contraction), either with or without simultaneous low-frequency (10 Hz) ES of the median nerve that innervates the FCR. The stimulus intensity corresponded to 1.2× perception threshold. In the FCR, voluntary contraction with median nerve stimulation significantly increased corticospinal excitability compared with FCR voluntary contraction without median nerve stimulation (p<0.01). In addition, corticospinal excitability was significantly modulated by the level of FCR voluntary contraction. In contrast, in the extensor carpi radialis (ECR), FCR voluntary contraction with median nerve stimulation significantly decreased corticospinal excitability compared with FCR voluntary contraction without median nerve stimulation (p<0.05). Thus, median nerve stimulation during FCR voluntary contraction induces reciprocal changes in cortical excitability in agonist and antagonist muscles. Finally we also showed that even mental imagery of FCR voluntary contraction with median nerve stimulation induced the same reciprocal changes in cortical excitability in agonist and antagonist muscles. Our results support the use of voluntary contraction coupled with ES in neurorehabilitation therapy for patients.     

Residual force enhancement after lengthening is present during submaximal plantar flexion and dorsiflexion actions in humans.

Stretch of an activated muscle causes a transient increase in force during the stretch and a sustained, residual force enhancement (RFE) after the stretch. The purpose of this study was to determine whether RFE is present in human muscles under physiologically relevant conditions (i.e., when stretches were applied within the working range of large postural leg muscles and under submaximal voluntary activation). Submaximal voluntary plantar flexion (PF(v)) and dorsiflexion (DF(v)) activation was maintained by providing direct visual feedback of the EMG from soleus or tibialis anterior, respectively. RFE was also examined during electrical stimulation of the plantar flexion muscles (PF(s)). Constant-velocity stretches (15 degrees /s) were applied through a range of motion of 15 degrees using a custom-built ankle torque motor. The muscles remained active throughout the stretch and for at least 10 s after the stretch. In all three activation conditions, the stable joint torque measured 9-10 s after the stretch was greater than the isometric joint torque at the final joint angle. When expressed as a percentage of the isometric torque, RFE values were 7, 13, and 12% for PF(v), PF(s), DF(v), respectively. These findings indicate that RFE is a characteristic of human skeletal muscle and can be observed during submaximal (25%) voluntary activation when stretches are applied on the ascending limb of the force-length curve. Although the underlying mechanisms are unclear, it appears that sarcomere popping and passive force enhancement are insufficient to explain the presence of RFE in these experiments.     

Stimulation at the cervicomedullary junction in human subjects.

In awake human subjects, corticospinal axons can be activated at the level of the cervicomedullary junction by electrical or magnetic stimulation. Such stimuli evoke single descending volleys which activate motoneurones and elicit responses in muscles of the upper limb. These responses (cervicomedullary motor evoked potentials, CMEPs) have a large monosynaptic component and can be used to test motoneurone excitability in a variety of tasks. CMEPs can be elicited in resting muscle and during all strengths of voluntary contraction. Examination of CMEPs during and after voluntary contractions reveals changes in motoneurone excitability but also suggests activity-dependent changes in the efficacy of the corticospinal pathway. Because they test the same subcortical pathway as transcranial magnetic stimulation, but are unaffected by altered excitability at a cortical level, CMEPs often offer the most appropriate comparison to allow interpretation of changes in motor evoked potentials. The advantages and disadvantages of stimulation at the cervicomedullary junction as a test of motoneurone excitability are reviewed.     

Central and peripheral contributions to fatigue in relation to level of activation during repeated maximal voluntary isometric plantar flexions.

This study aimed to investigate central and peripheral contributions to fatigue during repeated maximal voluntary isometric plantar flexions (MVCs). Changes in joint torque, level of activation (LOA), resting twitch amplitude (RT), electromyographic signals (EMG), and presynaptic inhibition of Ia afferents were investigated during 9 bouts of 10 MVCs. MVCs lasted for 2 s and were separated by 1 s. The interval between bouts was 10 s. Electrical stimulation was applied to the tibial nerve; at rest to evoke RTs, M waves, and two (1.5-s interval) H reflexes; with the soleus EMG at 30% of that during MVC to evoke M waves and two H reflexes; and during MVCs to measure LOA. Over the nine bouts, LOA decreased by 12.6% and RT by 16.2%. EMG root mean square during MVCs remained unchanged for the soleus and tibialis anterior muscles, but it decreased for medial gastrocnemius. Peripheral fatigue (decrease in RT) was positively correlated to LOA, whereas central fatigue (decrease in LOA) was not. Depression of both H reflexes suggests that presynaptic inhibition after the first bout was partly induced by homosynaptic postactivation depression of the Ia terminal. The H-reflex-to-M-wave ratio increased with fatigue in both passive and active states, with no change in the ratio of the second H reflex to the first, thereby indicating a decrease of presynaptic inhibition during fatigue. The results indicate that both central and peripheral mechanisms contributed to the fatigue observed during repeated MVCs and that the development of peripheral fatigue was influenced by the level of voluntary activation and initial plantar flexor torque.     

Effects of fatiguing unilateral plantar flexions on corticospinal and transcallosal inhibition in the primary motor hand area

Background

Corticospinal excitability of the primary motor cortex (M1) representing the hand muscle is depressed by bilateral lower limb muscle fatigue. The effects of fatiguing unilateral lower limb contraction on corticospinal excitability and transcallosal inhibition in the M1 hand areas remain unclear. The purpose of this study was to determine the effects of fatiguing unilateral plantar flexions on corticospinal excitability in the M1 hand areas and transcallosal inhibition originated from the M1 hand area contralateral to the fatigued ankle.

Methods

Ten healthy volunteers (26.2 ± 3.8 years) participated in the study. Using transcranial magnetic stimulation, we examined motor evoked potentials (MEPs) and interhemispheric inhibition (IHI) recorded from resting first dorsal interosseous (FDI) muscles before, immediately after, and 10 min after fatiguing unilateral lower limb muscle contraction, which was consisted of 40 unilateral maximal isometric plantar flexions intermittently with a 2-s contraction followed by 1 s of rest.

Results

We demonstrated no significant changes in MEPs in the FDI muscle ipsilateral to the fatigued ankle and decrease in IHI from the M1 hand area contralateral to the fatigued ankle to the ipsilateral M1 hand area after the fatiguing contraction. MEPs in the FDI muscle contralateral to the fatigued ankle were increased after the fatiguing contraction.

Conclusions

These results suggest that fatiguing unilateral lower limb muscle contraction differently influences corticospinal excitability of the contralateral M1 hand area and IHI from the contralateral M1 hand area to the ipsilateral M1 hand area. Although fatiguing unilateral lower limb muscle contraction increases corticospinal excitability of the ipsilateral M1 hand area, the increased corticospinal excitability is not associated with the decreased IHI.     

Supraspinal fatigue during intermittent maximal voluntary contractions of the human elbow flexors.

Responses to transcranial magnetic stimulation in human subjects (n = 9) were studied during series of intermittent isometric maximal voluntary contractions (MVCs) of the elbow. Stimuli were given during MVCs in four fatigue protocols with different duty cycles. As maximal voluntary torque fell during each protocol, the torque increment evoked by cortical stimulation increased from approximately 1.5 to 7% of ongoing torque. Thus "supraspinal" fatigue developed in each protocol. The motor evoked potential (MEP) and silent period in the elbow flexor muscles also changed. The silent period lengthened by 20-75 ms (lowest to highest duty cycle protocol) and recovered significantly with a 5-s rest. The MEP increased in area by >50% in all protocols and recovered significantly with 10 s, but not 5 s, of rest. These changes are similar to those during sustained MVC. The central fatigue demonstrated by the torque increments evoked by the stimuli did not parallel the changes in the electromyogram responses. This suggests that part of the fatigue developed during intermittent exercise is "upstream" of the motor cortex.     

The Magnitude of Peripheral Muscle Fatigue Induced by High and Low Intensity Single-Joint Exercise Does Not Lead to Central Motor Output Reductions in Resistance Trained Men

Purpose

To examine quadriceps muscle fatigue and central motor output during fatiguing single joint exercise at 40% and 80% maximal torque output in resistance trained men.

Method

Ten resistance trained men performed fatiguing isometric knee extensor exercise at 40% and 80% of maximal torque output. Maximal torque, rate of torque development, and measures of central motor output and peripheral muscle fatigue were recorded at two matched volumes of exercise, and after a final contraction performed to exhaustion. Central motor output was quantified from changes in voluntary activation, normalized surface electromyograms (EMG), and V-waves. Quadriceps muscle fatigue was assessed from changes in the size and shape of the resting potentiated twitch (Q._pot.tw). Central motor output during the exercise protocols was estimated from EMG and interpolated twitches applied during the task (VA_sub).

Results

Greater reductions in maximal torque and rate of torque development were observed during the 40% protocol (p<0.05). Maximal central motor output did not change for either protocol. For the 40% protocol reductions from pre-exercise in rate and amplitude variables calculated from the Q._pot.tw between 66.2 to 70.8% (p<0.001) exceeded those observed during the 80% protocol (p<0.01). V-waves only declined during the 80% protocol between 56.8 ± 35.8% to 53.6 ± 37.4% (p<0.05). At the end of the final 80% contraction VA_sub had increased from 91.2 ± 6.2% to 94.9 ± 4.7% (p = 0.005), but a greater increase was observed during the 40% contraction where VA_sub had increased from 67.1 ± 6.1% to 88.9 ± 9.6% (p<0.001).

Conclusion

Maximal central motor output in resistance trained men is well preserved despite varying levels of peripheral muscle fatigue. Upregulated central motor output during the 40% contraction protocol appeared to elicit greater peripheral fatigue. V-waves declines during the 80% protocol suggest intensity dependent modulation of the Ia afferent pathway.     

Central excitability does not limit postfatigue voluntary activation of quadriceps femoris.

After fatigue, motor evoked potentials (MEP) elicited by transcranial magnetic stimulation and cervicomedullary evoked potentials elicited by stimulation of the corticospinal tract are depressed. These reductions in corticomotor excitability and corticospinal transmission are accompanied by voluntary activation failure, but this may not reflect a causal relationship. Our purpose was to determine whether a decline in central excitability contributes to central fatigue. We hypothesized that, if central excitability limits voluntary activation, then a caffeine-induced increase in central excitability should offset voluntary activation failure. In this repeated-measures study, eight men each attended two sessions. Baseline measures of knee extension torque, maximal voluntary activation, peripheral transmission, contractile properties, and central excitability were made before administration of caffeine (6 mg/kg) or placebo. The amplitude of vastus lateralis MEPs elicited during minimal muscle activation provided a measure of central excitability. After a 1-h rest, baseline measures were repeated before, during, and after a fatigue protocol that ended when maximal voluntary torque declined by 35% (Tlim). Increased prefatigue MEP amplitude (P=0.055) and cortically evoked twitch (P<0.05) in the caffeine trial indicate that the drug increased central excitability. In the caffeine trial, increased MEP amplitude was correlated with time to task failure (r=0.74, P<0.05). Caffeine potentiated the MEP early in the fatigue protocol (P<0.05) and offset the 40% decline in placebo MEP (P<0.05) at Tlim. However, this was not associated with enhanced maximal voluntary activation during fatigue or recovery, demonstrating that voluntary activation is not limited by central excitability.     

Transpinal and Transcortical Stimulation Alter Corticospinal Excitability and Increase Spinal Output

The objective of this study was to assess changes in corticospinal excitability and spinal output following noninvasive transpinal and transcortical stimulation in humans. The size of the motor evoked potentials (MEPs), induced by transcranial magnetic stimulation (TMS) and recorded from the right plantar flexor and extensor muscles, was assessed following transcutaneous electric stimulation of the spine (tsESS) over the thoracolumbar region at conditioning-test (C-T) intervals that ranged from negative 50 to positive 50 ms. The size of the transpinal evoked potentials (TEPs), induced by tsESS and recorded from the right and left plantar flexor and extensor muscles, was assessed following TMS over the left primary motor cortex at 0.7 and at 1.1× MEP resting threshold at C-T intervals that ranged from negative 50 to positive 50 ms. The recruitment curves of MEPs and TEPs had a similar shape, and statistically significant differences between the sigmoid function parameters of MEPs and TEPs were not found. Anodal tsESS resulted in early MEP depression followed by long-latency MEP facilitation of both ankle plantar flexors and extensors. TEPs of ankle plantar flexors and extensors were increased regardless TMS intensity level. Subthreshold and suprathreshold TMS induced short-latency TEP facilitation that was larger in the TEPs ipsilateral to TMS. Noninvasive transpinal stimulation affected ipsilateral and contralateral actions of corticospinal neurons, while corticocortical and corticospinal descending volleys increased TEPs in both limbs. Transpinal and transcortical stimulation is a noninvasive neuromodulation method that alters corticospinal excitability and increases motor output of multiple spinal segments in humans.     

The Relationship between Central Visual Field Damage and Motor Vehicle Collisions in Primary Open-Angle Glaucoma Patients

Purpose

To investigate the relationship between visual field (VF) damage and history of motor vehicle collisions (MVCs) in subjects with primary open-angle glaucoma (POAG).

Methods

MVC history and driving habits were recorded using patient questionnaires in 247 POAG patients. Patients'' driving attitudes (carefulness) were estimated using Rasch analysis. The relationship between MVC outcomes and 52 total deviation (TD) values of integrated binocular VF (IVF), better and worse visual acuities (VAs), age and gender was analyzed using principal component analysis and logistic regression.

Results

51 patients had the history of MVCs. Significant difference was observed between patients with and without history of MVCs only for: better VA, a single TD value in the superior-right VF, and the typical distance driven in a week (unpaired t-test, p = 0.002, 0.015 and 0.006, respectively). There was not a significant relationship between MVCs and mean deviation (MD) of IVF (p = 0.41, logistic regression). None of the principal components were significantly correlated with MVC outcome (p>0.05, polynomial logistic regression analysis). There was a significant relationship between IVF MD and Rasch derived Person parameter (R² = 0.023, p = 0.0095). There was also a significant positive relationship between MVCs and the distance driven in a week (p = 0.005, logistic regression).

Conclusions

In this study of POAG patients, MVCs were not related to central binocular VF damage. These results suggest the relationship between visual function and driving is not straightforward, and careful consideration should be given when predicting patients'' driving ability using their VF.     

Motor cortex excitability does not increase during sustained cycling exercise to volitional exhaustion

The excitability of the motor cortex increases as fatigue develops during sustained single-joint contractions, but there are no previous reports on how corticospinal excitability is affected by sustained locomotor exercise. Here we addressed this issue by measuring spinal and cortical excitability changes during sustained cycling exercise. Vastus lateralis (VL) and rectus femoris (RF) muscle responses to transcranial magnetic stimulation of the motor cortex (motor evoked potentials, MEPs) and electrical stimulation of the descending tracts (cervicomedullary evoked potentials, CMEPs) were recorded every 3 min from nine subjects during 30 min of cycling at 75% of maximum workload (W(max)), and every minute during subsequent exercise at 105% of W(max) until subjective task failure. Responses were also measured during nonfatiguing control bouts at 80% and 110% of W(max) prior to sustained exercise. There were no significant changes in MEPs or CMEPs (P > 0.05) during the sustained cycling exercise. These results suggest that, in contrast to sustained single-joint contractions, sustained cycling exercise does not increase the excitability of motor cortical neurons. The contrasting corticospinal responses to the two modes of exercise may be due to differences in their associated systemic physiological consequences.     

Different motor learning effects on excitability changes of motor cortex in muscle contraction state

Abstract

We aimed to investigate whether motor learning induces different excitability changes in the human motor cortex (M1) between two different muscle contraction states (before voluntary contraction [static] or during voluntary contraction [dynamic]). For the same, using motor evoked potentials (MEPs) obtained by transcranial magnetic stimulation (TMS), we compared excitability changes during these two states after pinch-grip motor skill learning. The participants performed a force output tracking task by pinch grip on a computer screen. TMS was applied prior to the pinch grip (static) and after initiation of voluntary contraction (dynamic). MEPs of the following muscles were recorded: first dorsal interosseous (FDI), thenar muscle (Thenar), flexor carpi radialis (FCR), and extensor carpi radialis (ECR) muscles. During both the states, motor skill training led to significant improvement of motor performance. During the static state, MEPs of the FDI muscle were significantly facilitated after motor learning; however, during the dynamic state, MEPs of the FDI, Thenar, and FCR muscles were significantly decreased. Based on the results of this study, we concluded that excitability changes in the human M1 are differentially influenced during different voluntary contraction states (static and dynamic) after motor learning.     

Sustained contraction at very low forces produces prominent supraspinal fatigue in human elbow flexor muscles.

During sustained maximal voluntary contractions (MVCs), most fatigue occurs within the muscle, but some occurs because voluntary activation of the muscle declines (central fatigue), and some of this reflects suboptimal output from the motor cortex (supraspinal fatigue). This study examines whether supraspinal fatigue occurs during a sustained submaximal contraction of 5% MVC. Eight subjects sustained an isometric elbow flexion of 5% MVC for 70 min. Brief MVCs were performed every 3 min, with stimulation of the motor point, motor cortex, and brachial plexus. Perceived effort and pain, elbow flexion torque, and surface EMGs from biceps and brachioradialis were recorded. During the sustained 5% contraction, perceived effort increased from 0.5 to 3.9 (out of 10), and elbow flexor EMG increased steadily by approximately 60-80%. Torque during brief MVCs fell to 72% of control values, while both the resting twitch and EMG declined progressively. Thus the sustained weak contraction caused fatigue, some of which was due to peripheral mechanisms. Voluntary activation measured by motor point and motor cortex stimulation methods fell to 90% and 80%, respectively. Thus some of the fatigue was central. Calculations based on the fall in voluntary activation measured with cortical stimulation indicate that about two-thirds of the fatigue was due to supraspinal mechanisms. Therefore, sustained performance of a very low-force contraction produces a progressive inability to drive the motor cortex optimally during brief MVCs. The effect of central fatigue on performance of the weak contraction is less clear, but it may contribute to the increase in perceived effort.     

Residual force enhancement during submaximal and maximal effort contractions of the plantar flexors across knee angle

Following active muscle lengthening, steady-state isometric force is elevated compared with an isometric contraction without prior lengthening for the same muscle length and activation level. This property of muscle contraction is known as residual force enhancement (RFE). Here, we aimed to determine whether neural factors may mask some of the mechanical benefits of RFE on plantar flexion torque production. Inherent to lengthening contractions is an increase in cortical and spinal-mediated inhibition, while knee flexion places the medial gastrocnemius at a neuromechanical disadvantage. Neuromuscular properties of the plantar flexors were investigated with a Humac Norm dynamometer in 10 males (∼27 years) with a flexed (90°) and extended (180°) knee and with or without calcaneal tendon vibration (frequency range: 80–110 Hz). There was no effect for vibration (p > 0.05), but there was an effect for knee angle (p < 0.05) such that there was a 2 fold increase in RFE with the knee flexed compared with extended. During submaximal torque matching, following active lengthening there was an activation reduction (electromyography; EMG) of 7.2 and 4.7% with the knee flexed and extended, respectively for soleus as compared with the reference isometric contraction, but no difference for the medial gastrocnemius. Despite attempting to excite Ia input onto the plantar flexor motor neuron pool, vibration had no influence on RFE. Surprisingly, RFE was elevated more for the knee flexed than extended, which was possibly owing to the activation differences across the disparate muscles of the triceps surae during the plantar flexion task.     

The effects of short-term hypoxia on motor cortex excitability and neuromuscular activation.

The effects of acute hypoxia on motor cortex excitability, force production, and voluntary activation were studied using single- and double-pulse transcranial magnetic stimulation techniques in 14 healthy male subjects. Electrical supramaximal stimulations of the right ulnar nerve were performed, and transcranial magnetic stimulations were delivered to the first dorsal interosseus motor cortex area during short-term hypoxic (HX) and normoxic (NX) condition. M waves, voluntary activation, F waves, resting motor threshold (rMT), recruitment curves (100-140% of rMT), and short-interval intracortical inhibition and intracortical facilitation were measured. Moreover, motor-evoked potentials (MEPs) and cortical silent periods were determined during brief isometric maximum right index finger abductions. Hypoxia was induced by breathing a fraction of inspired oxygen of 12% via a face mask. M waves, voluntary activation, and F waves did not differ between NX and HX. The rMT was significantly lower in HX (55.79 +/- 9.40%) than in NX (57.50 +/- 10.48%) (P < 0.01), whereas MEP recruitment curve, short-interval intracortical inhibition, intracortical facilitation, maximum right index finger abduction, and MEPs were unaffected by HX. In contrast, the cortical silent periods in HX (158.21 +/- 33.96 ms) was significantly shortened compared with NX (169.42 +/- 39.69 ms) (P < 0.05). These data demonstrate that acute hypoxia results in increased cortical excitability and suggest that acute hypoxia alters motor cortical ion-channel function and GABAergic transmission.     

Voluntary activation level and muscle fiber recruitment of human quadriceps during lengthening contractions.

Voluntary activation levels during lengthening, isometric, and shortening contractions (angular velocity 60 degrees/s) were investigated by using electrical stimulation of the femoral nerve (triplet, 300 Hz) superimposed on maximal efforts. Recruitment of fiber populations was investigated by using the phosphocreatine-to-creatine ratio (PCr/Cr) of single characterized muscle fibers obtained from needle biopsies at rest and immediately after a series of 10 lengthening, isometric, and shortening contractions (1 s on/1 s off). Maximal voluntary torque was significantly higher during lengthening (270 +/- 55 N.m) compared with shortening contractions (199 +/- 47 N.m, P < 0.05) but was not different from isometric contractions (252 +/- 47 N.m). Isometric torque was higher than torque during shortening (P < 0.05). Voluntary activation level during maximal attempted lengthening contractions (79 +/- 8%) was significantly lower compared with isometric (93 +/- 5%) and shortening contractions (92 +/- 3%, P < 0.05). Mean PCr/Cr values of all fibers from all subjects at rest were 2.5 +/- 0.6, 2.0 +/- 0.7, and 2.0 +/- 0.7, respectively, for type I, IIa, and IIax fibers. After 10 contractions, the mean PCr/Cr values for grouped fiber populations (regardless of fiber type) were all significantly different from rest (1.3 +/- 0.2, 0.7 +/- 0.3, and 0.8 +/- 0.6 for lengthening, isometric, and shortening contractions, respectively; P < 0.05). The cumulative distributions of individual fiber populations after either contraction mode were significantly different from rest (P < 0.05). Curves after lengthening contractions were less shifted compared with curves from isometric and shortening contractions (P < 0.05), with a smaller shift for the type IIax compared with type I fibers in the lengthening contractions. The results indicate a reduced voluntary drive during lengthening contractions. PCr/Cr values of single fibers indicated a hierarchical order of recruitment of all fiber populations during maximal attempted lengthening contractions.