Changes In Motor Unit Recruitment Research Articles

Effects Of Fatigue On Hamstring:quadricep Ratio And Neuromuscular Activation During Closed Kinetic Chain Exercises

PURPOSE: To examine the effects of fatigue on the hamstring:quadricep ratio and motor unitrecruitment during closed kinetic chain exercises. METHODS: Ten males (20.1 + 1.1 yrs) and ten females ( 21.2 + 2.1 yrs) cycled at 110% of theirPhysical Working Capacity Fatigue Threshold (PWCFT) until exhaustion (TTE). Participants alsoperformed three closed kinetic chain (CKC) exercises before (PRE) and after (POST) the TTE, includinga front lunge (FL), side lunge (SL), and single leg squat (SLSQ). Each of the four CKC exercises wereperformed twice PRE and twice POST. Electromyography (EMG) of the vastus lateralis (VL) and bicepsfemoris (BF) was recorded on the dominant leg. Raw EMG recordings were normalized to maximalvoluntary isometric contractions (MVICs) and analyzed for root mean square (RMS). Thehamstring:quadricep ratio (H:Q) was calculated using RMS as:BF/VL. Data were analyzed usingmixed-design repeated-measures ANOVA. RESULTS: During the FL, a main effect of time was observed for the H:Q (F = 10.199;p = 0.005) andRMS of the VL (F = 17.462;p < 0.001). Specifically, H:Q was lower at POST (0.388 + 0.081) comparedto PRE (0.451 + 0.091) and RMS of VL was higher at POST (33.435 + 3.113) compared to PRE (27.937+ 3.004). No main effect of time (F = 6.275;p = 0.22) for RMS of the BF was observed. During theSLSQ, a main effect of time was observed for the H:Q (F = 7.498;p = 0.014) and RMS of the VL (F = 13.380;p = 0.002). Specifically, H:Q was lower at POST (0.392 + 0.087) compared to PRE (0.527 + 0.119) and RMS of VL was higher at POST (50.683 + 4.056) compared to PRE (41.457 + 4.391). Nomain effect of time (F = 0.238;p = 0.632) was observed for RMS of the BF During the SL, a main effectof time was observed for H:Q (F = 7.526;p = 0.013) and RMS of the VL (F = 9.548;p = 0.006).Specifically, H:Q was higher at POST (0.493 + 0.097) compared to PRE (0.354 + 0.098) and RMS of VLwas lower at POST (37.24 + 1.072) compared to PRE (49.644 + 1.083). No main effect of time (F = 0.766;p = 0.392) was observed for RMS of the BF. CONCLUSIONS: Fatigue may impact the H:Q ratio during closed-kinetic chain exercises These datasuggest that changes in the H:Q ratio may be dependent on fatigue-related changes in motor unitrecruitment of quadricep muscles.

Mind-Muscle Connection: Limited Effect of Verbal Instructions on Muscle Activity in a Seated Row Exercise.

Verbal instruction increases electromyographic (EMG) activity in the first three repetitions of an exercise, but its effect on an entire exercise set until failure is unknown. Once there are changes in motor unit recruitment due to fatigue, the effect of verbal instructions can change during different intervals of a set. This study analyzed whether verbal instruction emphasized the contraction of back muscles (i.e., myoelectric activity) during initial, intermediate, and final exercise repetitions performed until failure. Twenty participants with little or no experience in strength training performed a seated row exercise with and without verbal instruction. Surface electrodes were fixed over the latissimus dorsi, teres major, biceps brachii, and posterior deltoid (PD) muscles. Myoelectric activity was computed by mean amplitude and by the median frequency. We analyzed data with repeated measures multivariate analysis of variance and found that, with verbal instruction, there was increased EMG mean amplitude in the latissimus dorsi (15.21%, p = .030) and reduced EMG mean amplitude in the PD (14.39%, p = .018) on initial repetitions. Other muscle EMG amplitudes did not change. On intermediate repetitions, there was reduced signal amplitude only in the PD (15.03%, p = .022). The verbal instruction did not interfere with signal amplitude on final repetitions nor in the median frequency throughout the series. Verbal instruction seems to have little effect on increasing myoelectric activity of these targeted muscles in an entire set of a resistance training.

Changes in VO2 Kinetics After Elevated Baseline Do Not Necessarily Reflect Alterations in Muscle Force Production in Both Sexes.

A link between muscle fatigue, decreased efficiency and the slow component of oxygen uptake (VO2sc) has been suggested. However, a cause-effect relationship remains to be elucidated. Although alterations in VO2 kinetics after elevated baseline work rate have previously been reported, to date no study has observed the effect on muscle force production (MFP) behavior considering physiological differences between male and female subjects. This study investigated the effect of elevated baseline work rate on the VO2 kinetics and MFP in 10 male and 10 female healthy subjects. Subjects performed 4 transitions of very-heavy (VH) intensity cycling in a randomized order after unloaded (U-VH) or moderate (M-VH) exercise. Maximal isokinetic efforts (MIE) were performed before and after each condition at two different cadences (60 or 120 rpm). Whereas baseline VO2 and time constant (τ) were significantly higher in M-VH compared to U-VH, the fundamental amplitude and the VO2 slow component (VO2sc) were significantly lower in M-VH (p < 0.05) in both sexes. Blood lactate concentration ([La]) and rate of perceived exertion (RPE) were not influenced by condition or sex (p > 0.05). The MFP post-exercise was not significantly influenced by condition in both sexes and cadences (Δtorque for males: at 60 rpm in U-VH = 13 ± 10 Nm, in M-VH = 13 ± 9 Nm; at 120 rpm in U-VH = 22 ± 14 Nm, in M-VH = 21 ± 12 Nm; for females: at 120 rpm in U-VH = 10 ± 9 Nm, in M-VH = 12 ± 8 Nm; p > 0.05), with the exception that female subjects presented smaller decreases in M-UH at 60 rpm compared to U-VH (11 ± 13 vs. 18 ± 14 Nm, respectively, p < 0.05). There was no correlation between the decrease in torque production and VO2 kinetics parameters (p > 0.05). The alterations in VO2 kinetics which have been suggested to be linked to changes in motor unit recruitment after elevated baseline work rate did not reflect alterations in MFP and fatigue in both sexes.

Changes In Motor Unit Recruitment And De-recruitment Strategies Are Not Associated With The Repeated-bout Effect

The “repeated bout effect” (RBE) is an adaption that attenuates muscle damage following eccentric exercise. Several neural adaptations have been proposed to underlie the RBE. PURPOSE: This study used decomposition of surface EMG signals (dEMG) to examine the relationship between recruitment (RT) and de-recruitment thresholds (DRT) and changes in firing rates or motor units during recruitment and de-recruitment prior to and following eccentric exercise resulting in the RBE. METHODS: Nine participants performed 5 sub-maximal isometric trapezoid contractions at force levels corresponding to 50% and 80% of maximal isometric strength (MVC). Eccentric exercise was then performed until biceps brachii MVC had decreased by ~40%. MVC, range-of-motion (ROM), and delayed onset muscle soreness (DOMS) were measured 24-hours, 72-hours, and 1-week following eccentric exercise. Three weeks later all procedures were repeated. EMG signals of the biceps brachii were decomposed into individual motor-unit action potential trains. The relationship between RT and DT was examined using linear regression. The slope of the change in mean firing rate (MFR) during the ramp-up and ramp-down phase of the trapezoid contraction was also examined. RESULTS: No changes were found in the slope of the RT vs DT relationship for 50% MVC (1.13±0.17 vs 1.29±0.43; p=0.42) and 80% MVC (1.09±0.18 vs 1.25±0.41; p=0.25). There were also no changes in the y-intercept of the RT vs DT relationship at 50% (-13.79±6.77 vs -15.37±17.07; p=0.80) and at 80% (-9.29±10.71 vs -23.07±20.28; p=0.06) of MVC. The mean slope of the increase in firing rate during recruitment did not change between bouts 10.2±1.8 vs 10.5±2.2 pps/s (p=0.77) and 8.4±0.7 vs 9.0±1.6 pps/s (p=0.28) for 50% and 80% of MVC, respectively. The slope of the decrease in firing rate during de-recruitment did not differ at 50% of MVC -9.7±1.5 vs -10.2±1.8 pps/s (p=0.48), but became steeper during contractions at 80% of MVC -7.3±0.9 vs -8.7±1.7 pps/s (p=0.04). However, no relationship was observed between the change in DT slope and the magnitude of the RBE. CONCLUSION: A bout of eccentric exercise conferred protection from a subsequent identical bout. Few changes in motor-unit recruitment and de-recruitment behavior were observed suggesting changes in these parameters are not responsible for the RBE.

Spike shape analysis for the surface and needle electromyographic interference pattern

The ability of surface and needle electromyographic (EMG) spike shape measures to match changes in motor unit recruitment, firing rate, and synchronization during force gradation, were compared. The purpose of the study was to determine the force level at which the surface EMG spike shape measures no longer parallel their indwelling analogues. Secondarily, the impact of the noise rejection criterion on the sensitivity of the spike shape measures was examined. Maximal isometric elbow flexion ramp contractions were performed while recording surface and needle EMG from the biceps brachii. Spike shape measures were calculated in 500 ms epochs over the duration of the ramp contraction. The spike threshold for needle EMG spike detection was varied to examine the effect of the algorithm’s selectivity. The pattern of change across force levels between surface and needle EMG measures was compared. Spike detection resulted in the same pattern of change for both surface and needle amplitude measures over the gradation of force. Frequency measures and mean number of peaks per spike (MNPPS) were affected by electrode-source distance and spike threshold. Surface and needle frequency measures changed in parallel to 50% MVC while the MNPPS plateaued at 50–55% MVC. Spike shape analysis of surface EMG can track changes in the interference pattern produced by recruitment and rate-coding up to 50% MVC.

The effect of inter-electrode distance on the electric field distribution during transcutaneous lumbar spinal cord direct current stimulation.

Previous studies have indicated potential neuromodulation of the spinal circuitry by transcutaneous spinal direct current stimulation (tsDCS), such as changes in motor unit recruitment, shortening of the peripheral silent period and interference with supraspinal input to lower motor neurons. All of these effects were dependent on the polarity of the electrodes. The present study investigates how the distance between the electrodes during tsDCS influences the electric field's (E-field) spatial distribution in the lumbar and sacral spinal cord (SC). The electrodes were placed longitudinally along the SC, with the target electrode over the lumbar spine, and the return electrode above the former, considering four different distances (4, 8, 12 and 16 cm from the target). A fifth configuration was also tested with the return electrode over the right deltoid muscle. Peak values of the E-field's magnitude are found in the lumbo-sacral region of the SC for all tested configurations. Increasing the distance between the electrodes results in a wider spread of the E-field magnitude distribution along the SC, with larger maximum peak values and a smoother variation. The fifth configuration does not present the highest maximum values when compared to the other configurations. The results indicate that the choice of the return electrode position relative to the target can influence the distribution and the range of values of the E-field magnitude in the SC. Possible clinical significance of the observed effects will be discussed.

Recruitment of faster motor units is associated with greater rates of fascicle strain and rapid changes in muscle force during locomotion

Animals modulate the power output needed for different locomotor tasks by changing muscle forces and fascicle strain rates. To generate the necessary forces, appropriate motor units must be recruited. Faster motor units have faster activation-deactivation rates than slower motor units, and they contract at higher strain rates; therefore, recruitment of faster motor units may be advantageous for tasks that involve rapid movements or high rates of work. This study identified motor unit recruitment patterns in the gastrocnemii muscles of goats and examined whether faster motor units are recruited when locomotor speed is increased. The study also examined whether locomotor tasks that elicit faster (or slower) motor units are associated with increased (or decreased) in vivo tendon forces, force rise and relaxation rates, fascicle strains and/or strain rates. Electromyography (EMG), sonomicrometry and muscle-tendon force data were collected from the lateral and medial gastrocnemius muscles of goats during level walking, trotting and galloping and during inclined walking and trotting. EMG signals were analyzed using wavelet and principal component analyses to quantify changes in the EMG frequency spectra across the different locomotor conditions. Fascicle strain and strain rate were calculated from the sonomicrometric data, and force rise and relaxation rates were determined from the tendon force data. The results of this study showed that faster motor units were recruited as goats increased their locomotor speeds from level walking to galloping. Slow inclined walking elicited EMG intensities similar to those of fast level galloping but different EMG frequency spectra, indicating that recruitment of the different motor unit types depended, in part, on characteristics of the task. For the locomotor tasks and muscles analyzed here, recruitment patterns were generally associated with in vivo fascicle strain rates, EMG intensity and tendon force. Together, these data provide new evidence that changes in motor unit recruitment have an underlying mechanical basis, at least for certain locomotor tasks.

Similar alteration of motor unit recruitment strategies during the anticipation and experience of pain

A motor unit consists of a motoneurone and the multiple muscle fibres that it innervates, and forms the final neural pathway that influences movement. Discharge of motor units is altered (decreased discharge rate and/or cessation of firing; and increased discharge rate and/or recruitment of new units) during matched-force contractions with pain. This is thought to be mediated by nociceptive (pain) input on motoneurones, as demonstrated in animal studies. It is also possible that motoneurone excitability is altered by pain related descending inputs, that these changes persist after noxious stimuli cease, and that direct nociceptive input is not necessary to induce pain related changes in movement. We aimed to determine whether anticipation of pain (descending pain related inputs without nociceptor discharge) alters motor unit discharge, and to observe motor unit discharge recovery after pain has ceased. Motor unit discharge was recorded with fine-wire electrodes in the quadriceps of 9 volunteers. Subjects matched isometric knee-extension force during anticipation of pain (anticipation: electrical shocks randomly applied over the infrapatellar fat-pad); pain (hypertonic saline injected into the fat-pad); and 3 intervening control conditions. Discharge rate of motor units decreased during pain (P<.001) and anticipation (P<.01) compared with control contractions. De-recruitment of 1 population of units and new recruitment of another population were observed during both anticipation and pain; some changes in motor unit recruitment persisted after pain ceased. This challenges the fundamental theory that pain-related changes in muscle activity result from direct nociceptor discharge, and provides a mechanism that may underlie long-term changes in movement/chronicity in some musculoskeletal conditions.

Neuromuscular Responses of the Vastus Lateralis to Slow Walking With Vascular Restriction

Motor unit recruitment of individual muscles could substantially differ with changes in speed and load. However, the recruitment patterns associated with low intensity walk training with vascular restriction at sustained loads and constant speed could be different than normal walking. Vascular restricted (VR) walk training is an effective method for increasing muscular strength; however the changes in motor unit recruitment and firing frequency have not been investigated to understand the mechanisms underlying the neuromuscular adaptations. PURPOSE: The purpose of this study was to analyze differences in electrical activity of the vastus lateralis during VR and non vascular restricted (non-VR) walking sessions. METHODS: Seven healthy males [means ± (SD): age 22.7±(6.8) yrs, height 178±(5.4) cm, weight 181.7±(16.9) lbs.] performed both a VR walking session and a non-VR walking session for 20 minutes on a treadmill at a speed of 80.4 m-min[[Unsupported Character - &#713;]]1. During each walking period the subjects wore surface EMG electrodes that were placed along the longitudinal axis of the vastus lateralis (VL) of the right thigh at a distance of 33.3% between the lateral femoral epicondyle and the greater trochanter. A repeated measures ANOVA was used to determine the differences in both EMG amplitude (RMS) and median frequency of firing (MDF). RESULTS: The VR walking session resulted in significantly greater RMS values compared to the non-VR walking session (p < 0.05); however there was no main effect for time or interaction between condition and time. For EMG MDF, there were no significant main effects for condition or time, and no significant interaction. CONCLUSIONS: The results indicate that walking during VR of the lower limbs may have a greater impact on neural adaptation of the VL due to increased motor unit activation as indicated by an increased RMS value. This might occur because blood flow restriction affects motor unit recruitment patterns resulting in the recruitment of more high-threshold, fast-twitch muscle fibers.

Load Type Influences Motor Unit Recruitment in Biceps Brachii During a Sustained Contraction

Twenty subjects participated in four experiments designed to compare time to task failure and motor-unit recruitment threshold during contractions sustained at 15% of maximum as the elbow flexor muscles either supported an inertial load (position task) or exerted an equivalent constant torque against a rigid restraint (force task). Subcutaneous branched bipolar electrodes were used to record single motor unit activity from the biceps brachii muscle during ramp contractions performed before and at 50 and 90% of the time to failure for the position task during both fatiguing contractions. The time to task failure was briefer for the position task than for the force task (P=0.0002). Thirty and 29 motor units were isolated during the force and position tasks, respectively. The recruitment threshold declined by 48 and 30% (P=0.0001) during the position task for motor units with an initial recruitment threshold below and above the target force, respectively, whereas no significant change in recruitment threshold was observed during the force task. Changes in recruitment threshold were associated with a decrease in the mean discharge rate (-16%), an increase in discharge rate variability (+40%), and a prolongation of the first two interspike intervals (+29 and +13%). These data indicate that there were faster changes in motor unit recruitment and rate coding during the position task than the force task despite a similar net muscle torque during both tasks. Moreover, the results suggest that the differential synaptic input observed during the position task influences most of the motor unit pool.

Task failure during fatiguing contractions performed by humans

By comparing the physiological adjustments that occur when two similar fatiguing contractions are performed to failure, it is possible to identify mechanisms that limit the duration of the more difficult task. This approach has been used to study two fatiguing contractions, referred to as the force and position tasks, which differed in the type of feedback given to the subject and the amount of support provided by the surroundings. Even though the two tasks required a similar net muscle torque during submaximal isometric contractions, the duration that the position task could be sustained was consistently much briefer than that for the force task. The position task involved a greater rate of increase in EMG activity and more marked changes in motor unit recruitment and rate coding compared with the force task. These observations are consistent with the hypothesis that the motor unit pool was recruited more rapidly during the position task. The difference in motor unit behavior appeared to be caused by variation in synaptic input, likely involving heightened sensitivity of the stretch reflex during the position task. Upon repeat performances of the two fatiguing contractions, some subjects were able to increase the time to failure for the force task but not the position task. Furthermore, the time to failure for the position task could be influenced by the postural demands associated with maintaining the position of the limb, and the difference in the two durations was enhanced when the postural activity evoked a pressor response. These observations indicate that the difference in the duration of the two fatiguing contractions was attributable to differences in the control strategy used to sustain the tasks and the magnitude of the associated postural activity.

Sex differences in muscle fatigability and activation patterns of the human quadriceps femoris

The purposes of this study were to determine if the fatigability of the quadriceps femoris varies by biological sex under conditions of normal muscle blood flow and ischemia, and if differences in neuromuscular activation patterns exist. Young men and women (n = 11/group; age 20-39 years) performed a sustained knee extension contraction at 25% of maximal force under conditions of occluded (OCC) and normal muscle blood flow (NON-OCC). Electromyographic (EMG) activity was recorded from the vastus lateralis (VL), rectus femoris (RF), vastus medialis (VM) and biceps femoris (BF) muscles, and analyzed for fatigue-induced changes in the amplitude and burst rate and duration (transient changes in motor unit recruitment) of the signal. Additionally, force fluctuations during the sustained contraction were quantified. Women had a longer time to task failure during the NON-OCC task [214.9 +/- 20.5 vs. 169.1 +/- 20.5 (SE) s] (P = 0.02), but not during the OCC task (179.6 + 19.6 vs. 165.2 +/- 19.6 s). EMG data demonstrated sex differences in the neuromuscular activation pattern of the RF muscle and the collectively averaged QF muscles. During the NON-OCC and OCC tasks women achieved a higher relative activation of the RF at task failure than men (NON-OCC: 40.68 +/- 4.57 vs. 24.49 +/- 4.19%; OCC: 36.80 +/- 5.45 vs. 24.41 +/- 2.12%) (P = 0.02 and 0.05, respectively). Also, during both tasks, they demonstrated a greater relative activation at task failure than men when an average of the VL, VM and RF was considered. Additionally, women exhibited a greater coefficient of variation in force fluctuations during the last-third of the fatiguing NON-OCC task (6.21 +/- 0.567 vs. 4.56 +/- 0.56%) (P = 0.001). No sex differences in EMG burst rate or duration were observed, although there was a trend towards greater EMG burst rate of the RF in association with muscle fatigue in the women (P = 0.09). Interestingly, the only neuromuscular activation variable that displayed a significant relationship with the time to task failure was the average relative EMG of the QF at task failure, and this relationship was observed under both experimental blood flow conditions (NON-OCC: r = 0.47, P = 0.03; OCC: r = 0.44, P = 0.04). These results indicate that sex differences in muscle blood flow and/or muscle metabolism are in part responsible for the female advantage in fatigue-resistance. Additionally, these findings suggest that men synergistically recruit the RF compartment to a lesser extent than women in association with muscle fatigue, and that women achieve an overall greater relative activation of the QF at task failure than men. However, the implications of these sex differences in neuromuscular activation patterns during fatiguing muscular contractions on the ability to withstand muscle fatigue (prolonged time to task failure) does not appear to be causally related.

Inhomogeneities in muscle activation reveal motor unit recruitment

This paper presents a novel method to quantify spatial changes in muscle activation pattern by multi-channel surface electromyography (MCSEMG) in order to investigate motor unit recruitment variation. The method is based on non-uniform distributions of motor units that cause spatial inhomogeneous muscle activation. To evaluate the method, 15 subjects performed three isometric elbow flexion contractions consisting of slow sinusoidal changes in force ranging from 0% to 80% of the maximal voluntary contraction. MCSEMG electrodes were placed in a 10 × 13 grid over the biceps brachii muscle. From all channels, root mean square (RMS) values of bipolar leadings were computed over 0.5 s epochs over the whole recording. Thereafter, correlation coefficients were calculated between the RMS values at one epoch, with the RMS values at another epoch. Results showed consistent spatial changes in the distribution of RMS at different contraction levels up to 80% of maximal voluntary contraction and when comparing increasing and decreasing contractions at the same force level. These findings are reproducible within and between subjects, and in agreement with physiological phenomena and therefore indicate that the spatial inhomogeneities of motor unit properties in the biceps brachii muscle can be used to study changes in motor unit recruitment.

Sex Differences in Muscle Fatigability and Neuromuscular Activation Patterns of the Human Quadriceps Femoris

1892 PURPOSE: To determine if the fatigability of the quadriceps femoris varies by biological sex under ischemic and perfused conditions; and 2) to determine if differences in neuromuscular activation patterns (NMAP) exist. METHODS: Young males and females (n = 11/group; ages 20–39 years) performed a sustained contraction at 25% of maximal force under conditions of ischemia (ISCH) and normal muscle blood flow (NON-ISCH). For the NON-ISCH condition electromyographic (EMG) activity was recorded from the vastus lateralis (VL), rectus femoris (RF), vastus medialis (VM) and biceps femoris (BF) muscles, and analyzed for fatigue-induced changes in the amplitude (RMS EMG) and bursting activity (transient changes in motor unit recruitment) of the signal. Additionally, torque fluctuations (tremor) were quantified as an indirect measure of excitatory drive. RESULTS: Females had a longer endurance time during NON-ISCH (214.9 ± 20.5 s vs 169.1 ± 20.5 s, p = 0.02), but not during ISCH (179.6 + 19.6 v s 165.2 ± 19.6 s). EMG data demonstrated fatigueinduced sex differences in the NMAP of the RF. The most noticeable differences were the males failing to recruit the RF as the fatiguing contraction progressed and females achieving a greater relative activation (p<0.05). Additionally, during the NON-ISCH condition females exhibited a greater tremor during the last-third of the contraction (6.17 ± 0.57% vs. 4.26 ± 0.60%, p<0.00), and had a tendency for increased bursting activity (P = 0.08). CONCLUSIONS: Sex differences in intramuscular blood flow and/or muscle metabolism are in part responsible for the female advantage in fatigue-resistance. Additionally, fatigue-induced sex differences were observed in the NMAP of the quadriceps femoris. These findings suggest there are fatigue-induced sex differences in supraspinal descending drive, and/or excitability of the human neuromuscular system.

Mechanomyographic and electromyographic responses during submaximal cycle ergometry.

The purpose of this study was to examine the mechanomyographic (MMG) and electromyographic (EMG) responses during continuous, cycle ergometer workbouts performed at constant power outputs. Eight adults [mean (SD) age, 21.5 (1.6) years] volunteered to perform an incremental test to exhaustion for the determination of peak power (Wpeak) and four, 15-min (or to exhaustion) rides at constant power outputs of 50%, 65%, 80%, and 95% Wpeak. Piezoelectric crystal contact sensors were placed on the vastus lateralis (VL) and vastus medialis (VM) muscles to record the MMG signals. Bipolar surface electrode arrangements were placed on the VL and VM to record the EMG signals. Five-second samples of the MMG and EMG signals were recorded every 30 s at power outputs of 50%, 65%, and 80% Wpeak, and every 15 s at 95% Wpeak. The amplitudes of the selected portions of the signals were normalized to the first values recorded during the continuous rides, and regression analyses were used to determine whether the slope coefficients for the MMG and EMG versus time relationships were significantly (P < 0.05) different from zero. The results indicate that EMG amplitude increased (range of slope coefficients: 0.03-0.56) during the continuous rides for both muscles at all four power outputs (except the VM at 50% Wpeak), while MMG amplitude increased (slope coefficient at 95% Wpeak for VM = 0.19), decreased (range of slope coefficients for VL and VM at 50% and 65% Wpeak = -0.14 to -0.24), or remained unchanged (range of slope coefficients for VL and VM at 80% Wpeak and VL at 95% peak = -0.06 to 0.12) depending on the power output. The patterns of the MMG responses, however, were similar for the VL and VM muscles, except at 95% Wpeak. Fatigue-induced changes in motor-unit recruitment and discharge rates, or muscular compliance may explain the differences between power outputs in the patterns of the MMG amplitude responses.

Evaluation of patterned stimulation for use in surface functional electrical stimulation systems

The rapid onset of muscle fatigue is one of the major limiting factors when using electrical stimulation to restore functional movement in individuals with a neurological deficit. It has been proposed that muscle efficiency can be improved by using variable frequency pulse trains (VFT), consisting of repeated doublets, rather than a constant frequency train (CFT) of stimulating impulses. The aim of this study was to test this hypothesis. Experiments were conducted on fifteen non-impaired and one spinal cord injured subjects. It was shown that a VFT delivering the same electrical energy with a CFT produces a relative increase in muscle output, attaining a maximum for VFTs composed of doublets with an inter-pulse interval of 5 ms. It was also shown that this effect is highly dependent on the experimental conditions and especially the intensity of applied stimulation and this effect diminishes at high intensity levels. We suggest that any improvement in muscle output observed when using a VFT can be attributed to changes in motor unit recruitment and is not an intrinsic property of the muscle. Our results show that there is practically no benefit in using VFTs for alleviating the problem of fatigue in functional electrical stimulation systems.

Simultaneous 31P-NMR spectroscopy and EMG in exercising and recovering human skeletal muscle: a correlation study

A large number of studies have shown amplitude and spectral changes of the electromyogram during exercise, leading to several theories of how these changes might be related to the underlying metabolic changes. The amplitude and spectral changes are generally interpreted as changes in motor unit recruitment and a reduction of the muscle fiber conduction velocity due to proton or lactate accumulation. This study focuses on the causality of spectral changes of the surface electromyogram and proton or lactate accumulation and how the changes in motor unit recruitment are related to the metabolic status of the muscle. Simultaneous 31P-nuclear magnetic resonance spectroscopy and surface electromyography were performed during sustained static exercise and recovery in healthy volunteers and a patient with McArdle's disease. A clear dissociation between the median power frequency of the surface electromyogram and pH was seen in the healthy volunteers during recovery and during exercise in the patient with McArdle's disease. The results indicate that proton or lactate accumulation is not primarily responsible for the spectral changes of the surface electromyogram as previously suggested. The motor unit recruitment (as judged by the root mean square of the surface electromyogram) increased hyperbolically during the submaximal static exercise, with decreasing phosphocreatine-to-P(i) ratio reaching maximum at 0.6 (exhaustion), and seems to constitute a consistent metabolic limit to the exercise. The increased myoelectrical activity seen after exercise is not caused by an incomplete recovery of phosphorous metabolism, pH, or lactate but could probably be an impairment of the excitation-contraction coupling.

Repeated high-force eccentric exercise: effects on muscle pain and damage.

Five women and three men (aged 24-43 yr) performed maximal eccentric contractions of the elbow flexors (for 20 min) on three occasions, spaced 2 wk apart. Muscle pain, strength and contractile properties, and plasma creatine kinase (CK) were studied before and after each exercise bout. Muscle tenderness was greatest after the first bout and thereafter progressively decreased. Very high plasma CK levels (1,500-11,000 IU/l) occurred after the first bout, but the second and third bouts did not significantly affect the plasma CK. After each bout the strength was reduced by approximately 50% and after 2 wk had only recovered to 80% of preexercise values. Each exercise bout produced a marked shift of the force-frequency curve to the right which took approximately 2 wk to recover. The recovery rate of both strength and force-frequency characteristics was faster after the second and third bouts. Since the adaptation occurred after the performance of maximal contractions it cannot have been a result of changes in motor unit recruitment. The observed training effect of repeated exercise was not a consequence of the muscle becoming either stronger or more resistant to fatigue.

Intramuscular and surface electromyogram changes during muscle fatigue.

Twelve male subjects were tested to determine the effects of motor unit (MU) recruitment and firing frequency on the surface electromyogram (EMG) frequency power spectra during sustained maximal voluntary contraction (MVC) and 50% MVC of the biceps brachii muscle. Both the intramuscular MU spikes and surface EMG were recorded simultaneously and analyzed by means of a computer-aided intramuscular spike amplitude-frequency histogram and frequency power spectral analysis, respectively. Results indicated that both mean power frequency (MPF) and amplitude (rmsEMG) of the surface EMG fell significantly (P less than 0.001) together with a progressive reduction in MU spike amplitude and firing frequency during sustained MVC. During 50% MVC there was a significant decline in MPF (P less than 0.001), but this decline was accompanied by a significant increase in rmsEMG (P less than 0.001) and a progressive MU recruitment as evidenced by an increased number of MUs with relatively large spike amplitude. Our data suggest that the surface EMG amplitude could better represent the underlying MU activity during muscle fatigue and the frequency powers spectral shift may or may not reflect changes in MU recruitment and rate-coding patterns.

Sleep influences on diaphragmatic motor unit discharge

The discharge of motor units in the costal and crural regions of the diaphragm was studied in unrestrained cats during defined sleep and waking states. Changes in motor unit recruitment and pattern of activity were observed for different sleep-waking states. During periods of active waking, many motor units were recruited and discrimination of single-unit waveforms was difficult. However, in the transition from active to quiet waking, many diaphragmatic motor units ceased discharging, thus making it possible to discern single-unit waveforms. This cessation of motor unit activity continued during the transition from quiet waking (AW) to quiet sleep (QS) and from QS to rapid eye movement (REM) sleep. In some diaphragmatic motor units, discharge stopped completely during REM sleep and resumed only upon arousal. Thus, it was apparent that the recruitment of diaphragmatic motor units was markedly affected by the sleep-waking state. Changes in the patterns of discharge of diaphragmatic motor units also occurred during different sleep-waking states. Motor unit firing rate was typically slower in QS than AW. The rate of motor unit discharge during REM sleep varied considerably. Autocorrelation histograms demonstrated periodic patterns of motor unit discharge within inspiratory bursts. Intervals of repetitive peaks in the autocorrelation histograms ranged from 40 to 110 ms. These periodic patterns were strongest during AW and REM sleep and generally attenuated during QS. Cross-correlation histograms revealed the presence of synchronous activity between motor units in the diaphragm. This coupling of diaphragmatic motor unit discharge was also attenuated during QS compared with AW and REM sleep. Together, these results demonstrated a marked influence of the sleep-waking state on the neuromotor control of the diaphragm.