Human Elbow Flexors Research Articles

Low-Dose of β-Hydroxy-β-Methylbutyrate (HMB) Alleviates Muscle Strength Loss and Limited Joint Flexibility Following Eccentric Contractions

Abstract Objectives The excessive exercise including eccentric contractions (ECCs) cause the reduction of maximal strength, limitation of range of motion (ROM), developed muscle soreness, and muscle swelling for several days from immediately after exercise. Therefore, it is important to promote the recovery of muscle damage and minimize decreases in muscle function and muscle soreness after exercise. We recently examined the effects of 2- and 4-week β-hydroxy-β-methylbutyrate (HMB) supplementation (3 g/day) on ECCs-induced muscle damage. As the result, both the 2- and 4-week treatments decreased muscle strength reduction, ROM, and muscle swelling. However, the optimal dose of HMB required for decreasing muscle damage remains unknown. Therefore, this study investigated the supplemental effect of low-dose HMB on muscle damage after ECCs of human elbow flexor muscles. Methods Twenty untrained men (age, 21.0 ± 1.3 years) were completed the double-blind, placebo-controlled, parallel design study. The subjects were randomly assigned to the ingestion of HMB supplement (HMB, n = 10) or placebo group (PL, n = 10). After the subjects of 1.5 g HMB or placebo pills per day for 2 weeks, they performed six sets of 10 ECCs at 100% maximal voluntary contraction (MVC) using dumbbell. Changes in MVC torque, ROM, upper arm circumference, and muscle soreness were assessed before, immediately after, 1, 2, 3, and 5 days after exercise. Results The MVC torque was significantly higher in the HMB than in the PL group immediately after (HMB, −56.8%; PL, −67.1%) as well as 3 (HMB: −25.5%, PL: −48.7%) and 5 (HMB: −22.5%, PL: −44.0%) days after performing ECCs (p < 0.05). Additionally, the ROM was significantly higher in the HMB than in the PL group immediately after (HMB, −29.8%; PL, −50.5%) and 5 (HMB: −26.1%, PL: −43.3%) days after performing ECCs (p < 0.05). However, no between-group differences were observed in upper arm circumference, and muscle soreness. Conclusions The results demonstrated that 1.5 g/day HMB significantly inhibits the reduction in muscle strength and ROM after ECCs. In addition, 1.5 g/day HMB has no effect on muscle soreness, swelling, or stiffness. We conclude that 1.5 g/day of HMB supplementation plays a role in preventing muscle damage after performing ECCs, even if the effects are limited. Funding Sources The present study was supported by a research funding by Kobayashi Perfumery Co., Ltd.

Activity of Motor Units in Human Elbow Flexor and Extensor Muscles during Task-Dependent Unloading

Surface EMGs and activity of 73 single motor units (MUs) of the m. triceps brachii caput laterale (TBcl) and 25 units of the m. biceps brachii caput longum (BBcl) were recorded in five adults during unloading within the ramp phase of the isotorque ramp-and-hold movements in the elbow joint (“movement” task) and during maintaining of a certain position (“position control” task). Analysis of the surface EMGs showed that patterns of the MU responses to unloading in different motor tasks are dissimilar. Depending on the responses to unloading, the examined MUs were classified into two groups. Motor units of the first group, including 14 TBcl MUs and 22 BBcl ones, were characterized as task-independent units. Their spiking was completely inhibited in response to unloading in both motor tasks. The second group (44 TBcl and 3 BBcl MUs) should be considered as a group of the task-dependent units. These MUs were active during unloading in the “movement” task but were fully inhibited in the “position control” task. This specificity in the reactions of MUs during movements may be related to their different functions. Presumably, the former group of MUs is mainly involved in the force control, while the latter one is associated with the control of the movement per se.

Supplementation of eicosapentaenoic acid-rich fish oil attenuates muscle stiffness after eccentric contractions of human elbow flexors

BackgroundThis study aimed to investigate the effect of supplementation of fish oil rich in eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on the damage of the biceps brachii after eccentric contractions (ECCs) of the elbow flexors, particularly focusing on muscle stiffness.MethodsSixteen men were included in this double-blind, placebo-controlled, parallel design study and the participants were randomly assigned to the EPA and DHA supplement group (EPA, n = 8) and placebo group (PL, n = 8). They consumed either EPA 600 mg and DHA 260 mg per day or placebo supplement for 8 weeks prior to exercise. Moreover, they performed six sets of 10 ECCs at 100% maximal voluntary contraction (MVC) using a dumbbell. Changes in MVC torque, range of motion (ROM), upper arm circumference, muscle soreness, muscle echo intensity, and muscle stiffness were assessed before exercise; immediately after exercise; and 1, 2, and 5 days after exercise.ResultsMVC torque and ROM were significantly higher in the EPA group than in the PL group after ECCs (p < 0.05). Muscle soreness, upper arm circumference, and muscle echo intensity were significantly higher in the PL group than in the EPA group after ECCs (p < 0.05). In addition, muscle stiffness at 150° was significantly higher in the PL group than in the EPA group immediately after ECCs (p < 0.05).ConclusionThe present study showed that EPA and DHA supplementation has a positive role in inhibiting muscle stiffness after ECCs.Trial registrationThis trial (UMIN000028165) was registered on 10th/July/2017.

Szubmaximális izometrikus kontrakció alatt bekövetkező fáradás hatása a motoros egységek aktivációjára a könyökhajlító izmokban

Szubmaximális izometrikus kontrakció alatt bekövetkező fáradás hatása a motoros egységek aktivációjára a könyökhajlító izmokban

Muscular recruitment is associated with muscular function and swelling following eccentric contractions of human elbow flexors.

This study investigated the relationship between muscle fiber recruitment and the magnitude of muscle damage by isotonic eccentric contractions (ECCs). Ten healthy men (age: 27.2±6.0 y, height: 174. 0±5.3 cm, body mass 70.1±2.1 kg) were recruited in this study. Subjects performed 30 ECCs of the elbow flexors using a dumbbell adjusted to 40% and 80% one repetition maximum of each individual. The dependent variables maximal voluntary isometric contraction (MVC) torque, Range of Motion (ROM), delayed onset muscle soreness (DOMS), cross-sectional area (CSA) and transverse relaxation time (T2) of magnetic resonance imaging (MRI) were measured immediately before, immediately after and 1, 3, and 5 days after each exercise. The decreased MVC and limited ROM were significantly greater for 80% than that 40% (P<0.05) at immediately after ECCs. However, no significant difference between 40% and 80% was found for DOMS. CSA at 5 days after ECCs was significantly higher 80% (P<0.05) than 40% (P<0.05). No significant changes in post T2 (acute T2) was found for 40%, but an increased acute T2 was observed in 80% (P<0.05). We found a significant correlation between the change in T2 at immediately after and MVC at immediately after (r=0.77, P<0.05). In addition, a significant correlation between the change in T2 at immediately after and change in CSA at 3 days after (r=-0.83, P<0.05) was found. We conclude that the muscle strength loss and swelling following ECCs are related to the muscle fiber recruitment.

Impact of Loading and Rest Intervals on Muscle Microtrauma

The purpose of this study is to determine whether different rest intervals and load-repetition combinations with different frequencies and durations would have an impact on the microtrauma response of the muscle and the development of musculoskeletal disorders (MSDs). Serum Creatine Kinase (CK) level, the physical response measured in this study, is collected before exercise and at days 1, 2, 4, 8 post- eccentric exercise of the human elbow flexor muscles through phlebotomy and serum analysis. A two-factor (loading and rest interval) factorial regression model is established for muscle microtrauma response. The findings of the study would assist in understanding the effect of different rest intervals and load-repetition combinations on muscle microtrauma response and healing process to eccentric exercise, foster injury-prevention recommendations to manufacturing job design in terms of promoting rest intervals with suitable frequency, as well as force and repetition balance. The regression model could provide guidance to future ergonomic assessment tool design. The results of the study show that higher force with a low repetition frequency leads to greater response than lower force with high repetition frequency, when given a predetermined total work time, total rest time and total work volume. Though not greatly significant, experiment data show that longer rest intervals, although given less frequently, discourages the peak response of muscle microtrauma compared to shorter rest intervals given more frequently, when total work time, rest time and workload are predetermined.

Intrinsic excitability of human motoneurons in biceps brachii versus triceps brachii.

The intrinsic excitability of spinal motoneurons is mediated in part by the presence of persistent inward currents (PICs), which amplify synaptic input and promote self-sustained firing. Studies using animal models have shown that PICs are greater in extensor motoneurons over flexor motoneurons, but this difference has not yet been demonstrated in humans. The primary objective of this study was to determine whether a similar difference exists in humans by recording from motor units in biceps and triceps brachii during isometric contractions. We compared firing rate profiles of pairs of motor units, in which the firing rate of the lower-threshold "control" unit was used as an indicator of common drive to the higher-threshold "test" unit. The estimated contribution of the PIC was calculated as the difference in firing rate of the control unit at recruitment versus derecruitment of the test unit, a value known as the delta-F (ΔF). We found that ΔF values were significantly higher in triceps brachii (5.4 ± 0.9 imp/s) compared with biceps brachii (3.0 ± 1.4 imp/s; P < 0.001). This difference was still present even after controlling for saturation in firing rate of the control unit, rate modulation of the control unit, and differences in recruitment time between test and control units, which are known to contribute to ΔF variability. We conclude that human elbow flexor and extensor motor units exhibit differences in intrinsic excitability, contributing to different neural motor control strategies between muscle groups.

Impact of Loading and Rest Intervals on Muscle Inflammation

The efficacy of many existing musculoskeletal disorders (MSDs) treatment and preventive approaches remains unclear in current literature. Instead of applying additional measures that generate additional costs, redesigning the job in a MSD preventive way in the first place could be a cost effective and health promoting alternative. Manipulating rest intervals and load-repetition combinations within the work time and workload capacity constraints could qualify one of the beneficial alternatives. The purpose of this study is to determine whether different load-repetition combinations and rest intervals with different frequencies and durations would have an impact on the inflammatory response of the muscle and the development of MSDs within certain constraints – predetermined total work time, total rest time and total work volume. In this study, a total of 24 healthy college men with homogenous anthropometry measures will be randomly assigned to a 19-minute bicep eccentric exercise task including a total of 4-minute rest time. The total workload for all eccentric exercise treatment combinations is 27 times the maximum voluntary isometric contraction (MIVC). Within the shift, the subjects will be randomly assigned to 90% MIVC or 30% MIVC workloads; each subject will also be assigned with 2 minutes or 0.5 minute rest intervals. This pilot study uses the recoverable short-term delayed onset muscle soreness (DOMS) analogous to the long-term MSDs to imitate muscle inflammation and healing processes in MSD cases. Three physical responses: serum CK level (a quantitative marker for skeletal muscle microtrauma), MIVC (a good measure of the degree of muscle recovery), and muscle soreness are measured before exercise and at days 1, 2, 4, 8 post-exercise of the human elbow flexor muscles. A two-factor (loading and rest interval) factorial regression model will be established for muscle inflammatory response. The findings of the study would assist in understanding the effect of loading and rest on muscle inflammatory response and healing process as well as designing jobs with suitable rest intervals and load-repetition balance to prevent MSDs at manufacturing settings. The statistical regression model could provide guidance to implement ergonomic assessment tools.

Reduced short-interval intracortical inhibition after eccentric muscle damage in human elbow flexor muscles

The purpose of this study was to use paired-pulse transcranial magnetic stimulation (TMS) to examine the effect of eccentric exercise on short-interval intracortical inhibition (SICI) after damage to elbow flexor muscles. Nine young (22.5 ± 0.6 yr; mean ± SD) male subjects performed maximal eccentric exercise of the elbow flexor muscles until maximal voluntary contraction (MVC) force was reduced by ∼40%. TMS was performed before, 2 h after, and 2 days after exercise under Rest and Active (5% MVC) conditions with motor-evoked potentials (MEPs) recorded from the biceps brachii (BB) muscle. Peripheral electrical stimulation of the brachial plexus was used to assess maximal M-waves, and paired-pulse TMS with a 3-ms interstimulus interval was used to assess changes in SICI at each time point. The eccentric exercise resulted in a 34% decline in strength (P < 0.001), a 41% decline in resting M-wave (P = 0.01), changes in resting elbow joint angle (10°, P < 0.001), and a shift in the optimal elbow joint angle for force production (18°, P < 0.05) 2 h after exercise. This was accompanied by impaired muscle strength (27%, P < 0.001) and increased muscle soreness (P < 0.001) 2 days after exercise, which is indicative of muscle damage. When the test MEP amplitudes were matched between sessions, we found that SICI was reduced by 27% in resting and 23% in active BB muscle 2 h after exercise. SICI recovered 2 days after exercise when muscle pain and soreness were present, suggesting that delayed onset muscle soreness from eccentric exercise does not influence SICI. The change in SICI observed 2 h after exercise suggests that eccentric muscle damage has widespread effects throughout the motor system that likely includes changes in motor cortex.

Changes in kinematic variables at various muscle lengths of human elbow flexors following eccentric exercise

Exercise-induced muscle damage causes a disproportionally larger drop in maximal force when measured at short versus optimal or long muscle lengths, resulting in a shift of the length (angle)-force relationship towards longer lengths. However, little attention has been given to the potential effect of this shift on the rate of force development (RFD) and isotonic function at different muscle lengths. This study examined RFD at various elbow angles and kinematic variables at two different ranges of elbow flexion, so as to include mainly the ascending (S condition) or the descending limb (L condition) of the angle-force curve, following eccentric exercise. Seven male volunteers performed an eccentric exercise protocol with the elbow flexors, which caused significant changes in indicators of muscle damage (P<0.05-0.001). Optimum angle for force generation was significantly shifted towards longer elbow flexors lengths post exercise (P<0.05-0.01). RFD was significantly decreased at all the angles tested but no differences were revealed between angles (P<0.05-0.001). The kinematic variables measured were also significantly impaired following eccentric damage (P<0.05-0.001). Maximal isotonic force showed a greater impairment in the S condition, however no significant differences between the S and L condition were found in maximal angular velocity (MAV) and time, angle and isotonic force needed to achieve MAV. These results suggest that impairment of RFD following muscle damage is not muscle-length dependent and the rightward shift of the angle-force curve is not the determinant of the decline in either RFD or the isotonic performance at the different ranges of the elbow flexion movement.

Effects of aging and sex on voluntary activation and peak relaxation rate of human elbow flexors studied with motor cortical stimulation

Data are equivocal on whether voluntary activation is preserved or decreased in old compared to young adults. Further, data are scant on the effect of age on the rate of muscle relaxation when the muscle is contracting voluntarily. Assessment of both measures with transcranial magnetic stimulation (TMS) yields information which cannot be obtained with traditional peripheral nerve stimulation. Hence, voluntary activation and peak relaxation rate of the elbow flexors were assessed with TMS during repeated maximal efforts in 30 men and 28 women between the ages of 22-84 years. Voluntary activation was similar for the two sexes (P = 0.154) and was not affected by age in men (96.2 ± 2.7 %; P = 0.887) or women (95.1 ± 3.0 %; P = 0.546). Men had a significantly faster peak rate of relaxation than women in absolute units (-880.0 ± 223.2 vs. -360.2 ± 78.5 Nm/ s, respectively; P < 0.001) and when normalized to subject strength (-12.5 ± 2.1 vs. -8.7 ± 1.0 s(-1), respectively; P < 0.001). Absolute and normalized relaxation rates slowed with age in men (P = 0.002 and P = 0.006, respectively), but not women (P = 0.142 and P = 0.950, respectively). Across the age range studied, all subjects, regardless of age or sex, were able to achieve high voluntary activation scores for the elbow flexors (~95 %). In contrast, peak relaxation rate was markedly faster in men than women and slowed with age in men but not women. Normalization of relaxation rates to strength did not affect the influence of age or sex.

A spinal pathway between synergists can modulate activity in human elbow flexor muscles

Electrical stimulation of the brachioradialis branch of the radial nerve has been shown to inhibit the discharge of voluntarily activated motor units in biceps brachii during weak contractions with the elbow flexor muscles. The purpose of the present study was to characterise the inhibitory reflex by comparing its strength in the short and long heads of the biceps brachii and examining the influence of forearm position on the strength of the reflex. Spike-triggered stimulation was used to assess the influence of radial nerve stimulation on the discharge of single motor units in the biceps brachii of 15 subjects. Stimulation of the radial nerve prolonged the interspike interval (P < 0.001) of motor units in the long (n = 31, 4.8 +/- 5.6 ms) and short heads (n = 26, 8.1 +/- 12.3 ms) of biceps brachii with no difference between the two heads (P = 0.11). The strength of inhibition varied with forearm position for motor units in both heads (n = 18, P < 0.05). The amount of inhibition was greatest in pronation (7.9 +/- 8.9 ms), intermediate in neutral (5.8 +/- 7.1 ms), and least in supination (2.8 +/- 3.4 ms). These findings indicate that the inhibition evoked by afferent feedback from brachioradialis to low-threshold motor units (mean force 3-5% MVC) in biceps brachii varied with forearm posture yet was similar for the two heads of biceps brachii. This reflex pathway provides a mechanism to adjust the activation of biceps brachii with changes in forearm position, and represents a spinal basis for a muscle synergy in humans.

Electromyographic measures of muscle activation and changes in muscle architecture of human elbow flexors during fatiguing contractions

The study compared changes in intramuscular and surface recordings of EMG amplitude with ultrasound measures of muscle architecture of the elbow flexors during a submaximal isometric contraction. Ten subjects performed a fatiguing contraction to task failure at 20% of maximal voluntary contraction force. EMG activity was recorded in biceps brachii, brachialis, and brachioradialis muscles using intramuscular and surface electrodes. The rates of increase in the amplitude of the surface EMG for the long and short heads of biceps brachii and brachioradialis were greater than those for the intramuscular recordings measured at different depths. The amplitude of the intramuscular recordings from three muscles increased at a similar rate (P = 0.13), as did the amplitude of the three surface recordings from two muscles (P = 0.83). The increases in brachialis thickness (27.7 +/- 5.7 to 30.9 +/- 3.5 mm; P < 0.05) and pennation angle (10.9 +/- 3.5 to 16.5 +/- 4.8 degrees ; P = 0.003) were not associated with the increase in intramuscular EMG amplitude (P > 0.58). The increase in brachioradialis thickness (22.8 +/- 4.8 to 25.5 +/- 3.4 mm; P = 0.0075) was associated with the increase in the amplitude for one of two intramuscular EMG signals (P = 0.007, r = 0.79). The time to failure was more strongly associated with the rate of increase in the amplitude of the surface EMG than that for the intramuscular EMG, which suggests that the surface measurement provides a more appropriate measure of the change in muscle activation during a fatiguing contraction.

Cortical voluntary activation can be reliably measured in human wrist extensors using transcranial magnetic stimulation

Objective A twitch interpolation technique using transcranial magnetic stimulation (TMS) was recently developed to measure motor cortical drive to human elbow flexors. Here, we described studies designed to test the applicability and reliability of the technique for the human wrist extensors and to provide new evidence regarding the sensitivity of the technique to inadvertent antagonist activation. Methods Study 1: we measured amplitudes of superimposed twitches (SITs) produced by TMS during wrist extension at intensities from rest to maximal voluntary contraction on two occasions ( n = 9). Study 2: we assessed the impact of inadvertent antagonist activation by TMS on measurement of voluntary activation using a muscle potentiation technique to increase mechanical efficiency of the wrist flexors ( n = 6). Results The SITs decreased linearly between 25% and 100% MVC and voluntary activation could be reliably estimated across days (ICC 2,1 = 0.963, p < 0.001). Prior potentiation of the wrist flexors had little impact on extension SITs and voluntary activation. Conclusions TMS allows valid and reliable measurement of voluntary activation of the wrist extensors. Significance TMS can be used to assess effects of supraspinal fatigue, pathology and rehabilitation interventions on cortical activation in upper limb muscle groups.

Effect of elbow joint angle on force–EMG relationships in human elbow flexor and extensor muscles

The purpose of this study was to examine the effect of joint angle on the relationship between force and electromyogram (EMG) amplitude and median frequency, in the biceps, brachioradialis and triceps muscles. Surface EMG were measured at eight elbow angles, during isometric flexion and extension at force levels from 10% to 100% of maximum voluntary contraction (MVC). Joint angle had a significant effect on MVC force, but not on MVC EMG amplitude in all of the muscles examined. The median frequency of the biceps and triceps EMG decreased with increasing muscle length, possibly due to relative changes in electrode position or a decrease in muscle fibre diameter. The relationship between EMG amplitude and force, normalised with respect to its maximum force at each angle, did not vary with joint angle in the biceps or brachioradialis muscles over all angles, or in the triceps between 45° and 120° of flexion. These results suggest that the neural excitation level to each muscle is determined by the required percentage of available force rather than the absolute force required. It is, therefore, recommended that when using surface EMG to estimate muscle excitation, force should be normalised with respect to its maximum value at each angle.

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.

Can supraspinal/central fatigue explain the lesser muscle endurance of men compared with women?

Several studies have shown that women can present with greater muscle endurance (less fatigability) compared with men ([3][1], [5][2], [10][3], [12][4]), although this is not always the case ([4][5], [8][6]). Most of these studies have focused on peripheral physiological mechanisms, mostly those

The effect of sustained low‐intensity contractions on supraspinal fatigue in human elbow flexor muscles

Subjects quickly fatigue when they perform maximal voluntary contractions (MVCs). Much of the loss of force is from processes within muscle (peripheral fatigue) but some occurs because voluntary activation of the muscle declines (central fatigue). The role of central fatigue during submaximal contractions is not clear. This study investigated whether central fatigue developed during prolonged low-force voluntary contractions. Subjects (n=9) held isometric elbow flexions of 15% MVC for 43 min. Voluntary activation was measured during brief MVCs every 3 min. During each MVC, transcranial magnetic stimulation (TMS) was followed by stimulation of either brachial plexus or the motor nerve of biceps brachii. After nerve stimulation, a resting twitch was also evoked before subjects resumed the 15% MVC. Perceived effort, elbow flexion torque and surface EMG from biceps, brachioradialis and triceps were recorded. TMS was also given during the sustained 15% MVC. During the sustained contraction, perceived effort rose from approximately 2 to approximately 8 (out of 10) while ongoing biceps EMG increased from 6.9+/-2.1% to 20.0+/-7.8% of initial maximum. Torque in the brief MVCs and the resting twitch fell to 58.6+/-14.5 and 58.2+/-13.2% of control values, respectively. EMG in the MVCs also fell to 62.2+/-15.3% of initial maximum, and twitches evoked by nerve stimulation and TMS grew progressively. Voluntary activation calculated from these twitches fell from approximately 98% to 71.9+/-38.9 and 76.9+/-18.3%, respectively. The silent period following TMS lengthened both in the brief MVCs (by approximately 40 ms) and in the sustained target contraction (by approximately 18 ms). After the end of the sustained contraction, the silent period recovered immediately, voluntary activation and voluntary EMG recovered over several minutes while MVC torque only returned to approximately 85% baseline. The resting twitch showed no recovery. Thus, as well as fatigue in the muscle, the prolonged low-force contraction produced progressive central fatigue, and some of this impairment of the subjects' ability to drive the muscle maximally was due to suboptimal output from the motor cortex. Although caused by a low-force contraction, both the peripheral and central fatigue impaired the production of maximal voluntary force. While central fatigue can only be demonstrated during MVCs, it may have contributed to the disproportionate increase in perceived effort reported during the prolonged low-force contraction.

Maximal force, voluntary activation and muscle soreness after eccentric damage to human elbow flexor muscles

Muscle damage reduces voluntary force after eccentric exercise but impaired neural drive to the muscle may also contribute. To determine whether the delayed-onset muscle soreness, which develops approximately 1 day after exercise, reduces voluntary activation and to identify the possible site for any reduction, voluntary activation of elbow flexor muscles was examined with both motor cortex and motor nerve stimulation. We measured maximal voluntary isometric torque (MVC), twitch torque, muscle soreness and voluntary activation in eight subjects before, immediately after, 2 h after, 1, 2, 4 and 8 days after eccentric exercise. Motor nerve stimulation and motor cortex stimulation were used to derive twitch torques and measures of voluntary activation. Eccentric exercise immediately reduced the MVC by 38 +/- 3% (mean +/- s.d., n = 8). The resting twitch produced by motor nerve stimulation fell by 82 +/- 6%, and the estimated resting twitch by cortical stimulation fell by 47 +/- 15%. While voluntary torque recovered after 8 days, both measures of the resting twitch remained depressed. Muscle tenderness occurred 1-2 days after exercise, and pain during contractions on days 1-4, but changes in voluntary activation did not follow this time course. Voluntary activation assessed with nerve stimulation fell 19 +/- 6% immediately after exercise but was not different from control values after 2 days. Voluntary activation assessed by motor cortex stimulation was unchanged by eccentric exercise. During MVCs, absolute increments in torque evoked by nerve and cortical stimulation behaved differently. Those to cortical stimulation decreased whereas those to nerve stimulation tended to increase. These findings suggest that reduced voluntary activation contributes to the early force loss after eccentric exercise, but that it is not due to muscle soreness. The impairment of voluntary activation to nerve stimulation but not motor cortical stimulation suggests that the activation deficit lies in the motor cortex or at a spinal level.

Eccentric torque-velocity relationship of the elbow flexors

This study verified the eccentric torque-velocity relationship of human elbow flexors by considering muscle damage induced in maximal eccentric torque measurements. Twenty subjects (26.4 ± 6.2 yrs) were tested twice, separated by 7 days, for maximal voluntary isometric torque at 90 ◦ (1.57 rad) of elbow flexion (ISO) and isokinetic eccentric torque (ECC) at velocities of 30 ◦ ·s −1 followed by 90 ◦ , 150 ◦ , 210 ◦ ·s −1 , and a repeated 30 ◦ ·s −1 using a range of movement from 60 ◦ to 140 ◦ (180 ◦ : full extension). ISO preceded each ECC measure to assess the effects of muscle damage and/or fatigue on the measures. The difference in ECC at 30 ◦ ·s −1 between the first and second attempts was used to adjust the torque values. The reliability of the measurements was supported by high intra-class correlation coefficient (0.96-0.99) and low coefficient of variation (6.3-9.1%). Peak ECC at all velocities were significantly ( p< 0.05) greater than ISO (14-16%), but no significant differences were evident among velocities. The second ECC at 30 ◦ ·s −1 was significantly ( p< 0.05) lower (∼10%) than the first, and ISO decreased significantly ( p< 0.05) over the measurements (∼10%). Following adjustment, no significant differences in ECC torque among velocities were still evident. It is concluded that eccentric torque is approximately 15% higher than isometric torque without influence of angular velocity for the elbow flexors.