Knee extensor fatigue resistance of young and older men and women performing sustained and brief intermittent isometric contractions

Introduction: Susceptibility to muscle fatigue during aging could depend on muscle activation patterns. Methods: Young (mean age, 22 years) and older (mean age 70 years) men and women completed two fatigue tests of knee extensor muscles using voluntary and electrically stimulated contractions. Results: Older subjects displayed a shift to the left of the torque‐frequency relationship and held a sustained voluntary isometric contraction at 50% maximal strength for significantly longer than young (P < 0.001). Young and old showed similar fatigue during electrically induced, intermittent isometric contractions (1‐s on, 1‐s off for 2 min), but women fatigued less than men (P = 0.001). Stronger muscles fatigued more quickly, and slower contractile properties were associated with longer sustained contractions. Conclusions: The slowing and weakness of older muscle was associated with superior fatigue resistance during sustained isometric contractions. Young and old showed similar fatigue following a series of brief, intermittent contractions, but women fatigued less than men. Muscle Nerve 50 : 393–400, 2014     

Fatigue and recovery from dynamic contractions in men and women differ for arm and leg muscles

Introduction: Whether there is a gender difference in fatigue and recovery from maximal velocity fatiguing contractions and across muscles is not understood. Methods: Sixteen men and 19 women performed 90 isotonic contractions at maximal voluntary shortening velocity (maximal velocity concentric contractions, MVCC) with the elbow flexor and knee extensor muscles (separate days) at a load equivalent to 20% maximal voluntary isometric contraction (MVIC). Results: Power (from MVCCs) decreased similarly for men and women for both muscles (P > 0.05). Men and women had similar declines in MVIC of elbow flexors, but men had greater reductions in knee extensor MVIC force and MVIC electromyogram activity than women (P < 0.05). The decline in MVIC and power was greater, and force recovery was slower for the elbow flexors compared with knee extensors. Conclusions: The gender difference in muscle fatigue often observed during isometric tasks was diminished during fast dynamic contractions for upper and lower limb muscles. Muscle Nerve 48 : 436–439, 2013     

Muscular adaptations and insulin‐like growth factor‐1 responses to resistance training are stretch‐mediated

Introduction: Modulation of muscle characteristics was attempted through altering muscle stretch during resistance training. We hypothesized that stretch would enhance muscle responses. Methods: Participants trained for 8 weeks, loading the quadriceps in a shortened (SL, 0–50° knee flexion; n = 10) or lengthened (LL, 40–90°; n = 11) position, followed by 4 weeks of detraining. Controls (CON; n = 10) were untrained. Quadriceps strength, vastus lateralis architecture, anatomical cross‐sectional area (aCSA), and serum insulin‐like growth factor‐1 (IGF‐1) were measured at weeks 0, 8, 10, and 12. Results: Increases in fascicle length (29 ± 4% vs. 14 ± 4%), distal aCSA (53 ± 12% vs. 18 ± 8%), strength (26 ± 6% vs. 7 ± 3%), and IGF‐1 (31 ± 6% vs. 7 ± 6%) were greater in LL compared with SL muscles (P < 0.05). No changes occurred in CON. Detraining decrements in strength and aCSA were greater in SL than LL muscles (P < 0.05). Conclusions: Enhanced muscle in vivo (and somewhat IGF‐1) adaptations to resistance training are concurrent with muscle stretch, which warrants its inclusion within training. Muscle Nerve 49 : 108–119, 2014     

Effect of training on contractile and metabolic properties of wrist extensors in spinal cord-injured individuals

Paretic human muscle rapidly loses strength and oxidative endurance, and electrical stimulation training may partly reverse this. We evaluated the effects of two training protocols on the contractile and metabolic properties of the wrist extensor in 12 C-5/6 tetraplegic individuals. The wrist extensor muscles were stimulated for 30 min/day, 5 days/week, for 12 weeks, using either a high-resistance (Hr) or a low-resistance (Lr) protocol. Total work output was similar in both protocols. The nontrained arm was used as a control. Maximum voluntary torque increased in the Hr (P < 0.05) but not the Lr group. Electrically stimulated peak tetanic torque at 15 HZ, 30 HZ, and 50 HZ were unchanged in the Lr group and tended to increase only at 15 HZ (P < 0.1) in the Hr group. Resistance to fatigue, however, increased (P < 0.05) in both Hr (42%) and Lr (41%) groups. Muscle metabolism was evaluated by (31)P nuclear magnetic resonance spectroscopy ((31)P-NMRS) during and following a continuous 40-s 10-HZ contraction. In the Hr group the cost of contraction decreased by 38% (P < 0.05) and the half-time of phosphocreatine (PCr) recovery was shortened by 52% (P < 0.05). Thus, long-term electrically induced stimulation of the wrist extensor muscles in spinal cord injury (SCI) increases fatigue resistance independent of training pattern. However, only the Hr protocol increased muscle strength and was shown to improve muscle aerobic metabolism after training. Muscle Nerve 27: 72-80, 2003     

Evoked EMG and Muscle Fatigue During Isokinetic FES-Cycling in Individuals With SCI

Purpose: This study investigated whether muscle fatigue during functional electrical stimulation (FES)‐induced cycling was associated with changes occurring in evoked electromyographic signals (eEMG, M‐waves) in individuals with spinal cord injury. We also explored the effects of recovery intervals between exercise sessions on the relationship between eEMG and muscle torque. Methods: Eight individuals with spinal cord injury performed three FES‐cycling sessions of 15‐min duration, with 5 min of recovery between them. The quadriceps muscles were electrically stimulated as the prime agonist to produce cycling. Pedal torques and surface eEMG signals were synchronously processed and recorded for offline analysis. Results: Large Torque decreases (20–44%) were observed in the first 5 min of cycling during the three exercise bouts, while changes of similar magnitude did not occur on any of the M‐wave time‐series (less than 19%). Between 5 and 15 min of cycling, muscle fatigue lowered the plateau baselines of Torque (ranging from 41% to 62%), M‐wave peak‐to‐peak amplitude (PtpA) and Area (ranging from 60% to 98%) time‐series, yet the magnitudes of these reductions were not consistent between them. Conclusion: We concluded that muscle fatigue during FES‐cycling was not associated with, nor could be predicted by, eEMG signals. Nonetheless, the consistency between M‐waves and Torque time‐curves in their direction of change clearly warrants further investigation.     

Muscle fiber velocity and electromyographic signs of fatigue in fibromyalgia

Introduction: Fibromyalgia (FM) is a disorder of widespread muscular pain. We investigated possible differences in surface electromyography (sEMG) in clinically unaffected muscle between patients with FM and controls. Methods: sEMG was performed on the biceps brachii muscle of 13 women with FM and 14 matched healthy controls during prolonged dynamic exercises, unloaded, and loaded up to 20% of maximum voluntary contraction. The sEMG parameters were: muscle fiber conduction velocity (CV); skewness of motor unit potential (peak) velocities; peak frequency (PF) (number of peaks per second); and average rectified voltage (ARV). Results: There was significantly higher CV in the FM group. Although the FM group performed the tests equally well, their electromyographic fatigue was significantly less expressed compared with controls (in CV, PF, and ARV). Conclusion: In the patients with FM, we clearly showed functional abnormalities of the muscle membrane, which led to high conduction velocity and resistance to fatigue in electromyography. Muscle Nerve 46: 738–745, 2012     

Skeletal muscle fatigue and physical endurance of young and old mice

In order to evaluate the role played by muscular and extramuscular factors in the development of fatigue in old age, the time course of fatigue in isolated skeletal muscles and spontaneous motor activity and endurance of whole animals were monitored using young (3–6 months) and old (34–36 months) CF57BL/6J mice. The isolated extensor digitorum longus (EDL) and soleus muscles from old mice had smaller (P < 0.05) mass and developed lower (P < 0.02) maximal tetanic tension at 100-Hz stimulation than the muscles of young mice. During stimulation at 30 Hz every 2.5 s, a 50% decline in original tetanic tension occurred by 109 s in young EDL and 129 s in old EDL, but by 482 s in young soleus and 1134 s (projected) in old soleus, indicating more (P < 0.05) resistance to fatigue in old than young soleus. However, the old mice showed significantly fewer (P < 0.002) spontaneous ambulatory movements than the young mice. On a treadmill with a belt speed of 10 m/min at an inclination of 0°, the old mice could only run for 22 min compared to 39 min ran by young mice (P < 0.02). They took more rest periods (P < 0.02) than the young mice. In a quantitative swimming monitor, the old mice swam for a shorter (P < 0.05) time than young mice (20.4 min compared to 28.6 min). Integrated swimming activity at 20 min was smaller (P < 0.05) in old mice than in young mice (413 g/s compared to 628 g/s). Hence increased fatigue in old age is not caused by impairment of processes within the muscles, but by impairment of central or extramuscular processes. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21: 1729–1739, 1998     

Sarcolemmal excitability changes in normal human aging

Introduction: The exact mechanisms underlying the loss of skeletal muscle bulk and power with normal human aging are not well established. Recording of muscle velocity recovery cycles (MVRCs) is an in‐vivo neurophysiologic technique we employed to assess the impact of age on sarcolemmal excitability. Methods: MVRC recordings were obtained from tibialis anterior (n = 74) and rectus femoris (n = 32) muscles in 74 healthy subjects (18–84 years, median age 35 years, interquartile range 29–55 years). Results: Increasing age was linearly associated with longer muscle relative refractory period (MRRP) and reduced early supernormality (ESN) in both tibialis anterior (MRRP: r² = 0.38, P < 0.001; ESN: r² = 0.33, P < 0.001) and rectus femoris (MRRP: r² = 0.30, P = 0.002; ESN: r² = 0.19, P = 0.01) muscles. Discussion: The results are consistent with progressive depolarization of the resting sarcolemmal potential with normal aging. This may be an important mechanism in explaining age‐related muscle decline. Muscle Nerve 57 : 981–988, 2018     

Surface electromyography features in manual workers affected by carpal tunnel syndrome

Introduction: Alterations in surface electromyographic (sEMG) signals of the abductor pollicis brevis muscle were evaluated in 24 non‐manual workers and 40 manual workers (25 asymptomatic and 15 reporting CTS symptoms). Methods: The initial value (IV) and the normalized rate of change (NRC) of average rectified value (ARV), mean frequency of the power spectrum (MNF), and muscle fiber conduction velocity (MFCV) were calculated during contractions at 20% and 50% of maximal voluntary contraction (MVC). Neuromuscular efficiency (NME) and kurtosis of the sEMG amplitude distribution were estimated. Results: With respect to controls, manual workers showed higher NME, lower ARV IV, and reduced myoelectric manifestations of fatigue (lower MNF NRC for both contraction levels, and lower MFCV NRC at 50% MVC). Kurtosis at 20% MVC showed higher values in symptomatic manual workers than in the other two groups. Conclusions: Kurtosis seems to be a promising parameter for use in monitoring individuals who develop CTS. Muscle Nerve 45: 873–882, 2012     

Aging impairs regulation of ryanodine receptors from extensor digitorum longus but not soleus muscles

ABSTRACT: Introduction: Because impaired excitation‐contraction coupling and reduced sarcoplasmic reticulum (SR) Ca²⁺ release may contribute to the age‐associated decline in skeletal muscle strength, we investigated the effect of aging on regulation of the skeletal muscle isoform of the ryanodine receptor (RyR1) by physiological channel ligands. Methods: [³H]Ryanodine binding to membranes from 8‐ and 26‐month‐old Fischer 344 extensor digitorum longus (EDL) and soleus muscles was used to investigate the effects of age on RyR1 modulation by Ca²⁺ and calmodulin (CaM). Results: Aging reduced maximal Ca²⁺‐stimulated binding to EDL membranes. In 0.3 μM Ca²⁺, age reduced binding and CaM increased binding to EDL membranes. In 300 μM Ca²⁺, CaM reduced binding, but the age effect was not significant. Aging did not affect Ca²⁺ or CaM regulation of soleus RyR1. Discussion: In aged fast‐twitch muscle, impaired RyR1 Ca²⁺ regulation may contribute to lower SR Ca²⁺ release and reduced muscle function. Muscle Nerve 57 : 1022–1025, 2018     

Neuromuscular fatigue after low‐ and medium‐frequency electrical stimulation in healthy adults

Introduction: In this study we investigated fatigue origins induced by low‐frequency pulsed current (PC) and medium‐frequency current (MF) neuromuscular electrical stimulation (NMES) after a clinical‐like session. Methods: Eleven healthy men randomly underwent 2 NMES sessions, PC and MF, on quadriceps muscle (15‐minute duration, 6 seconds on and 18 seconds off). Maximal voluntary contraction (MVC), central activation ratio (CAR), vastus lateralis electromyographic activity (EMG), and evoked contractile properties were determined before and after the sessions. Evoked torque and discomfort during the sessions were also measured. Results: Both currents produced decreases in MVC, EMG, and evoked contractile properties after the sessions. No difference was found between currents for all variables (P > 0.05). Evoked torque during sessions decreased (P < 0.05). No difference was observed in mean evoked torque and discomfort (P > 0.05). Discussion: Both currents induced similar neuromuscular fatigue. Clinicians can choose either PC or MF and expect similar treatment effects when the goal is to generate gains in muscle strength. Muscle Nerve 58 : 293–299, 2018     

Oscillatory interaction between the hand area of human primary motor cortex and finger muscles during steady-state isometric contraction

Objective

To compare the magnitudes of β-band coherence between the primary motor cortex (M1) and electromyogram (EMG) for finger muscles, and to determine whether M1–EMG coherence is related to the stability of muscle contraction.

Methods

Cortical signals and EMG during steady-state isometric contraction of right thumb muscle (flexor pollicis brevis (FPB)) or right little finger muscle (flexor digiti minimi brevis (FDMB)) were recorded simultaneously with magnetoencephalography system from 13 right-handed healthy subjects.

Results

The magnitudes of β-band M1–EMG coherence and spectral power in the M1 for the FPB muscle were greater than that for the FDMB muscle (P < 0.001 and P < 0.005, respectively). The stability of EMG for the FPB was higher than that for FDMB (P < 0.001). Greater levels of β-band M1–EMG coherence were associated with higher levels of EMG stability (P < 0.05). The mean dipole sources of the FPB muscle were located more laterally, inferiorly and anteriorly than that of FDMB in the M1 hand area (P < 0.005).

Conclusions

The strength of β-band M1–EMG coherence would play an important role in the stability level of finger-muscle contraction.

Significance

The β-band M1–EMG coherence may reflect effective oscillatory interaction between the M1 and finger muscle during steady-state motor output.     

Shifts in EMG spectral power during fatiguing dynamic contractions

Introduction: We examined the etiology of the electromyographic (EMG) spectral shift during dynamic fatigue. Methods: Nineteen subjects (mean ± SD age = 22.4 ± 1.6 years) performed 50 consecutive maximal concentric isokinetic contractions of dominant leg extensors. Surface EMG signals were detected from the vastus lateralis, rectus femoris, and vastus medialis during each contraction, processed with a wavelet analysis, and the resulting spectra were decomposed with a nonparametric spectral decomposition procedure. Results: The results indicated that the decreases in EMG frequency during the 50 contractions were generally due to reductions in high‐frequency power and increases in low‐frequency power. In addition, the spectral shifts were most pronounced for the rectus femoris, followed by the vastus lateralis, and then the vastus medialis. Conclusions: The spectral decomposition procedure is much more sensitive for tracking dynamic fatigue than is EMG mean frequency or median frequency. Muscle Nerve 50 : 95–102, 2014     

A dynamic network involving M1-S1, SII-insular, medial insular, and cingulate cortices controls muscular activity during an isometric contraction reaction time task

Magnetoencephalographic, electromyographic (EMG), work, and reaction time (RT) were recorded from nine subjects during visually triggered intermittent isometric contractions of the middle finger under two conditions: unloaded and loaded (30% of maximal voluntary contraction). The effect of muscle fatigue was studied over three consecutive periods under both conditions. In the loaded condition, the motor evoked field triggered by the EMG onset decreased with fatigue, whereas movement-evoked fields (MEFs) increased (P < 0.01). Fatigue was demonstrated in the loaded condition, since (i) RT increased due to an increase in the electromechanical delay (P < 0.002); (ii) work decreased from Periods 1 to 3 (P < 0.005), while (iii) the myoelectric RMS amplitude of both flexor digitorum superficialis and extensor muscles increased (P < 0.003) and (iv) during Period 3, the spectral deflection of the EMG median frequency of the FDS muscle decreased (P < 0.001). In the unloaded condition and at the beginning of the loaded condition, a parallel network including M1-S1, posterior SII-insular, and posterior cingulate cortices accounted for the MEF activities. However, under the effect of fatigue, medial insular and posterior cingulate cortices drove this network. Moreover, changes in the location of insular and M1-S1 activations were significantly correlated with muscle fatigue (increase of RMS-EMG; P < 0.03 and P < 0.01, respectively). These results demonstrate that a plastic network controls the strength of the motor command as fatigue occurs: sensory information, pain, and exhaustion act through activation of the medial insular and posterior cingulate cortices to decrease the motor command in order to preserve muscle efficiency and integrity.     

Whole-Body Muscle Magnetic Resonance Imaging in Glycogen-Storage Disease Type III

Introduction: The main objective of this study was to describe muscle involvement on whole-body magnetic resonance imaging scans in adults at different stages of glycogen-storage disease type III (GSDIII). Methods: Fifteen patients, 16–59 years of age, were examined on a 3-T system. The examinations consisted of coronal and axial T1-weighted images or fat images with a Dixon technique, and were scored for 47 muscles using Mercuri's classification. Muscle changes consisted of internal bright signals of fatty replacement. Results: Distribution across muscles showed predominant signal alteration in the lower limbs and postural muscles. This finding is consistent with the overall clinical presentation of GSDIII and the results of heatmap scores. Review of the MRI scans provided new information regarding recurrent muscle changes, particularly in the soleus, gastrocnemius medial head, and thoracic extensor muscles. Discussion: Whole-body muscle imaging provides clinically relevant information regarding muscle involvement in GSDIII. A severity score may contribute to improved patient management. Muscle Nerve, 2019     

Mechanical properties of the plantarflexor musculotendinous unit during passive dorsiflexion in children with cerebral palsy compared with typically developing children

Aim To examine the passive length–tension relations in the myotendinous components of the plantarflexor muscles of children with and without cerebral palsy (CP) under conditions excluding reflex muscle contraction. Method A cross‐sectional, non‐interventional study was conducted in a hospital outpatient clinic. Passive torque–angle characteristics of the ankle were quantified from full plantarflexion to full available dorsiflexion in 26 independently ambulant children with CP (11 females, 15 males; mean age: 6y 11mo, range 4y 7mo–9y 7mo) and 26 age‐matched typically developing children (18 females, 8 males; mean age 7y 2mo, range 4y 1mo–10y 4mo). In the children with CP, the affected (hemiplegia; n=21) or more affected (diplegia; n=5) leg was tested; in typically developing children, the leg tested was randomly selected. Gross Motor Function Classification System levels were I (n=15) and II (n=11). Care was taken to eliminate active or reflex muscle contribution to the movement, confirmed by the absence of electromyographic activity. Results There were small but significant differences between the two groups for maximum ankle dorsiflexion (p=0.003), but large and significant differences in the torques required to produce the same displacement (p<0.001). Further, the hysteresis of the average loading cycle in the children with CP was over three times that of the typically developing children (p<0.001). Interpretation We believe that the plantarflexor muscles of children with CP are stiffer and intrinsically more resistant to stretch, even though they retain near normal excursion. This increased stiffness is a non‐neurally‐mediated feature demonstrated by these children. The extent to which it influences function and predisposes the children to development of soft tissue contracture is unknown.     

Metabolic and nonmetabolic components of fatigue monitored with 31P-NMR

The goal of this study was to determine the roles of metabolic and nonmetabolic factors in muscle fatigue. Rat gastrocnemius muscles were fatigued by stimulation of the nerve (n = 6) or muscle (n = 4, after 2 days of denervation). ³¹Phosphorus nuclear magnetic resonance spectroscopy was used to measure levels of intracellular inorganic phosphate (Pi) and hydrogen ions (H⁺) (which are thought to inhibit contraction) and the high-energy phosphates, phosphocreatine (PCr), and ATP. For both indirect and direct stimulation, with fatigue to ≈60% initial tetanic force, [Pi] increased from ≈3.5 mmol/L to ≈20 mmol/L and [PCr] decreased from ≈27 mmol/L to ≈9 mmol/L. However, with continued fatigue to 25–35% initial tetanic force, neither [Pi] or [PCr] changed further. [ATP] and pH changed only slightly during fatigue. The results are consistent with early fatigue arising from metabolic inhibition of contraction; but later fatigue arising independent of metabolites, due to impaired activation beyond the neuromuscular junction. © 1994 John Wiley & Sons, Inc.     

Smooth muscle electromyography from rat urethra

Electrical signals recorded from the penis have been suggested as reflecting electromyographic activity in the underlying smooth muscles. In order to verify this assertion, we manipulated the signal recorded from rat urethra surface. Stimulation of the pelvic nerve brought about a reduction of activity (965 ± 826 to 166 ± 143 μV, root mean square of the power at range 0.005–1 Hz, P = 0.008), with a significant frequency-response relationship (P = 0.0002). This effect was not altered by temporary closure of the aorta (P = 0.89), thus ruling out hemodynamic artifact as a possible cause of signal change during stimulation. Our findings support the assertion that the signal indeed reflects activity in smooth muscle. © 1997 John Wiley & Sons, Inc. Muscle Nerve 20: 1497–1501, 1997     

Prior heat stress effects fatigue recovery of the elbow flexor muscles

Introduction: Long‐lasting alterations in hormones, neurotransmitters, and stress proteins after hyperthermia may be responsible for the impairment in motor performance during muscle fatigue. Methods: Subjects (n = 25) performed a maximal intermittent fatigue task of elbow flexion after sitting in either 73° or 26°C to examine the effects of prior heat stress on fatigue mechanisms. Results: The heat stress increased the tympanic and rectal temperatures by 2.3° and 0.82°C, respectively, but there was full recovery prior to the fatigue task. Although prior heat stress had no effects on fatigue‐related changes in volitional torque, electromyographic (EMG) activity, torque relaxation rate, motor evoked potential (MEP) size, and silent period (SP) duration, prior heat stress acutely increased the pre‐fatigue relaxation rate and chronically prevented long‐duration fatigue (P < 0.05). Conclusions: These findings indicate that prior passive heat stress alone does not alter voluntary activation during fatigue, but prior heat stress and exercise produce longer‐term protection against long‐duration fatigue. Muscle Nerve 44: 115–125, 2011     

Location and severity of spasticity in the first 1–2 weeks and at 3 and 18 months after stroke

Background and purpose: There is no consensus concerning the location or severity of spasticity, or how this changes with time after stroke. The purpose was to describe: the location and severity of spasticity, in different muscle groups, during the first 1–2 weeks and at 3 and 18 months after stroke; the association between the severity of spasticity and control of voluntary movements; and the occurrence of spasticity in younger versus older patients. Methods: In a cohort of consecutive patients, the following parameters were assessed during the first 1–2 weeks (n = 109) and at 3 (n = 95) and 18 (n = 66) months after first‐ever stroke: spasticity, by the Modified Ashworth Scale in different muscle groups; plantar‐flexor clonus, by physical examination; and movement function, by the Lindmark Motor Assessment Scale. Results: During the first 1–2 weeks and at 3 months after stroke, spasticity was most common in the anti‐gravity muscles. The severity of upper extremity spasticity increased over time (P < 0.05). Upper extremity spasticity and movement scores were moderately associated (r = ?0.61, P < 0.05). At 3 months, spasticity was more common amongst the younger patients (P < 0.05). Conclusions: The results confirm that spasticity is most common in the anti‐gravity muscles and is associated with the control of voluntary movements. As the severity of spasticity also increased after 3 months, when neurally mediated spasticity is expected to have passed its peak, intrinsic muscle changes may play a larger role than neural components with the passage of time after stroke.