Assessment of lower-back muscle fatigue using electromyography, mechanomyography, and near-infrared spectroscopy

Gender Comparisons of Mechanomyographic Amplitude and Mean Power Frequency versus Isometric Torque Relationships

This study compared the patterns of mechanomyographic (MMG) amplitude and mean power frequency vs. torque relationships in men and women during isometric muscle actions of the biceps brachii. Seven men (mean age 23.9 ± 3.5 yrs) and 8 women (mean 21.0 ± 1.3 yrs) performed submaximal to maximal isometric muscle actions of the dominant forearm flexors. Following determination of the isometric maximum voluntary contraction (MVC), they randomly performed submaximal step muscle actions in 10% increments from 10% to 90% MVC. Polynomial regression analyses indicated that the MMG amplitude vs. isometric torque relationship for the men was best fit with a cubic model (R2= 0.983), where MMG amplitude increased slightly from 10% to 20% MVC, increased rapidly from 20% to 80% MVC, and plateaued from 80% to 100% MVC. For the women, MMG amplitude increased linearly (r2= 0.949) from 10% to 100% MVC. Linear models also provided the best fit for the MMG mean power frequency vs. isometric torque relationship in both the men (r2= 0.813) and women (r2= 0.578). The results demonstrated gender differences in the MMG amplitude vs. isometric torque relationship, but similar torque-related patterns for MMG mean power frequency. These findings suggested that the plateau in MMG amplitude at high levels of isometric torque production for the biceps brachii in the men, but not the women, may have been due to greater isometric torque, muscle stiffness, and/or intramuscular fluid pressure in the men, rather than to differences in motor unit activation strategies for modulating isometric torque production.

Effects of blackcurrant anthocyanin intake on peripheral muscle circulation during typing work in humans

This double-blind, placebo-controlled, crossover study investigated the effect of blackcurrant anthocyanin (BCA) intake on peripheral circulation during rest and during typing work by using near-infrared spectroscopy (NIRS), and it also assessed improvement in shoulder stiffness caused by poor local circulation. In a resting circulation study, nine healthy male subjects took capsules of BCA at a dosage of 17 mg kg(-1) or placebo (isoenergetic sugar). NIRS was used to measure left forearm blood flow (FBF) following venous occlusion and muscle oxygen consumption following arterial occlusion prior to and hourly for 4 h after ingestion of BCA. Plasma anthocyanin concentration was measured prior to ingestion and 1, 2, and 4 h later. FBF increased significantly 2 h after BCA ingestion [BCA 1.22 (0.13)-fold increase relative to pre-values vs placebo 0.83 (0.06) of pre-values; P < 0.05] and then tended to increase for a further 3 h after ingestion [BCA 1.26 (0.15)-fold increase relative to pre-values vs placebo 0.82 (0.07) of pre-values; P = 0.078]. There was, however, no significant difference in muscle oxygen consumption between BCA and placebo intake at any time point. In a typing work study, 11 healthy subjects took capsules of BCA (7.7 mg kg(-1)) or placebo (isoenergetic sugar) daily for 2 weeks. The subjects then performed intermittent typing workload for 30 min in order to induce acute shoulder stiffness. During the workload, total hemoglobin and oxygenated hemoglobin (oxy-Hb) were determined using NIRS and myoelectric signals measured in the right trapezius muscle using electromyography (EMG). The viscoelasticity of the trapezius muscle was also evaluated using a muscle stiffness meter before and after the typing workload. BCA intake prevented the decrease in oxy-Hb significantly (P < 0.05), and also tended to alleviate the increase in root mean square (RMS) of the EMG during the typing workload, and also muscle stiffness after the workload. There was no improvement in typing performance with BCA intake. The results of this study suggest that intake of BCA may improve shoulder stiffness caused by typing work by increasing peripheral blood flow and reducing muscle fatigue.

Mechanomyographic Responses of the Vastus Medialis to Isometric and Eccentric Muscle Actions

PURPOSE

The mechanomyographic (MMG) signal may be used to examine the motor control strategies used to modulate torque during various types of muscle actions. Therefore, the purpose of this study was to examine the MMG amplitude and mean power frequency (MPF) versus torque relationships during isometric and eccentric isokinetic muscle actions.

METHODS

Eleven adults (mean age +/- SD = 20.8 +/- 1.4 yr) volunteered to perform isometric and eccentric isokinetic leg extension muscle actions at 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100% of peak torque on a Cybex 6000 dynamometer. A piezoelectric crystal contact sensor was placed on the vastus medialis to detect the MMG signal.

RESULTS

Polynomial regression analyses indicated that for the isometric muscle actions, the relationships for MMG amplitude (r2 = 0.984) and MPF (r2 = 0.989) versus torque were linear. For the eccentric isokinetic muscle actions, the relationships for MMG amplitude (r2 = 0.580) and MPF (r2 = 0.961) versus torque were also linear.

CONCLUSION

The patterns for MMG amplitude and MPF may reflect the motor control strategies that modulate torque production for isometric and eccentric isokinetic muscle actions. Based on the results of this and previous studies, it appears that for the vastus medialis, torque is modulated in a similar manner for isometric, concentric, and eccentric isokinetic muscle actions. Specifically, these findings suggest that gradation of torque involves increases in recruitment and firing rate to 100% voluntary torque production.

Neck muscle fatigue and spatial orientation during stepping in place in humans

Neck proprioceptive input, as elicited by muscle vibration, can produce destabilizing effects on stance and locomotion. Neck muscle fatigue produces destabilizing effects on stance, too. Our aim was to assess whether neck muscle fatigue can also perturb the orientation in space during a walking task. Direction and amplitude of the path covered during stepping in place were measured in 10 blindfolded subjects, who performed five 30-s stepping trials before and after a 5-min period of isometric dorsal neck muscle contraction against a load. Neck muscle electromyogram amplitude and median frequency during the head extensor effort were used to compute a fatigue index. Head and body kinematics were recorded by an optoelectronic system, and stepping cadence was measured by sensorized insoles. Before the contraction period, subjects normally stepped on the spot or drifted forward. After contraction, some subjects reproduced the same behavior, whereas others reduced their forward progression or even stepped backward. The former subjects showed minimal signs of fatigue and the latter ones marked signs of fatigue, as quantified by the dorsal neck electromyogram index. Head position and cadence were unaffected in either group of subjects. We argue that the abnormal fatigue-induced afferent input originating in the receptors transducing the neck muscle metabolic state can modulate the egocentric spatial reference frame. Notably, the effects of neck muscle fatigue on orientation are opposite to those produced by neck proprioception. The neck represents a complex source of inputs capable of modifying our orientation in space during a locomotor task.

Mechanomyographic amplitude and mean power frequency versus torque relationships during isokinetic and isometric muscle actions of the biceps brachii

The purpose of this investigation was to determine the mechanomyographic (MMG) amplitude and mean power frequency (MPF) versus torque (or force) relationships during isokinetic and isometric muscle actions of the biceps brachii. Ten adults (mean +/- SD age = 21.6 +/- 1.7 years) performed submaximal to maximal isokinetic and isometric muscle actions of the dominant forearm flexors. Following determination of isokinetic peak torque (PT) and the isometric maximum voluntary contraction (MVC), the subjects randomly performed submaximal step muscle actions in 10% increments from 10% to 90% PT and MVC. Polynomial regression analyses indicated that MMG amplitude increased linearly with torque during both the isokinetic (r2 = 0.982) and isometric (r2 = 0.956) muscle actions. From 80% to 100% of isometric MVC, however, MMG amplitude appeared to plateau. Cubic models provided the best fit for the MMG MPF versus isokinetic (R2 = 0.786) and isometric (R2 = 0.940) torque relationships, although no significant increase in MMG MPF was found from 10% to 100% of isokinetic PT. For the isometric muscle actions, however, MMG MPF remained relatively stable from 10% to 50% MVC, increased from 50% to 80% MVC, and decreased from 80% to 100% MVC. The results demonstrated differences in the MMG amplitude and MPF versus torque relationships between the isokinetic and isometric muscle actions. These findings suggested that the time and frequency domains of the MMG signal may be useful for describing the unique motor control strategies that modulate dynamic versus isometric torque production.

Monitoring Individual Erector Spinae Fatigue Responses Using Electromyography and Near Infrared Spectroscopy

This study examined the utility of electromyography and near-infrared spectroscopy (NIRS) in assessing m. erector spinae activity during the Biering-Sørensen Back Muscle Endurance (BSME) test. Six men and four women (27.0 ± 7.1 years of age) performed the BSME test (time = 131.5 ± 43.5 s). EMG was used to quantify neuromuscular activity of the right and left side at the L3 level, and root mean square was scaled for maximum value at the start of the exercise. NIRS was used to evaluate blood volume (BV) and oxygenation (OX) simultaneous with EMG bilaterally at the L3 level. There was a decrease to 49 ± 8% of initial median frequency (mean = 83 Hz) on both right and left sides when the exercise was 90% complete, and the slope of the median frequency/time relationship was significantly related to BSME time (r = 0.82). Group means for BV increased during back exercise while OX decreased and was significantly different between right and left sides of the lower back. However, large OX response differences among individuals and between right and left sides were noted. OX and median frequency were moderately related (r = 0.27-0.38). It appears that NIRS combined with EMG is a promising tool for assessing localized metabolic and neuromuscular activity during static contractions of the lower back. Key words: back endurance, back health, muscle oxygenation, blood volume

Gender, muscle, and velocity comparisons of mechanomyographic and electromyographic responses during isokinetic muscle actions

The purpose of this study was to examine the responses of peak torque (PT), mean power output (MP), mechanomyographic (MMG) and electromyographic (EMG) amplitude and mean power frequency (MPF) of the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) in males and females during maximal, concentric isokinetic muscle actions. Subjects performed maximal leg extensions at 60 degrees s(-1), 120 degrees s(-1), 180 degrees s(-1), 240 degrees s(-1), 300 degrees s(-1), 360 degrees s(-1), 420 degrees s(-1), and 480 degrees s(-1). No gender differences were observed, but there were muscle-specific differences for the patterns of MMG MPF, EMG amplitude, and EMG MPF. The MP and MMG amplitude increased to 180-240 degrees s(-1), plateaued, and then decreased to 480 degrees s(-1). MMG MPF for the VL and VM remained unchanged to 300 degrees s(-1), but then increased to 480 degrees s(-1). The EMG amplitude for the RF and EMG MPF for the VL decreased across velocity. Overall, these findings indicated that there were muscle-specific, velocity-related differences in the associations among motor control strategies (EMG amplitude and MPF) and the mechanical aspects of isokinetic muscular activity (MMG amplitude and MPF).

Neck muscle fatigue affects postural control in man

We hypothesised that, since anomalous neck proprioceptive input can produce perturbing effects on posture, neck muscle fatigue could alter body balance control through a mechanism connected to fatigue-induced afferent inflow. Eighteen normal subjects underwent fatiguing contractions of head extensor muscles. Sway during quiet stance was recorded by a dynamometric platform, both prior to and after fatigue and recovery, with eyes open and eyes closed. After each trial, subjects were asked to rate their postural control. Fatigue was induced by having subjects stand upright and exert a force corresponding to about 35% of maximal voluntary effort against a device exerting a head-flexor torque. The first fatiguing period lasted 5 min (F1). After a 5-min recovery period (R1), a second period of fatiguing contraction (F2) and a second period of recovery (R2) followed. Surface EMG activity from dorsal neck muscles was recorded during the contractions and quiet stance trials. EMG median frequency progressively decreased and EMG amplitude progressively increased during fatiguing contractions, demonstrating that muscle fatigue occurred. After F1, subjects swayed to a larger extent compared with control conditions, recovering after R1. Similar findings were obtained after F2 and after R2. Although such behaviour was detectable under both visual conditions, the effects of fatigue reached significance only without vision. Subjective scores of postural control diminished when sway increased, but diminished more, for equal body sway, after fatigue and recovery. Contractions of the same duration, but not inducing EMG signs of fatigue, had much less influence on body sway or subjective scoring. We argue that neck muscle fatigue affects mechanisms of postural control by producing abnormal sensory input to the CNS and a lasting sense of instability. Vision is able to overcome the disturbing effects connected with neck muscle fatigue.

Evidence of long term muscle fatigue following prolonged intermittent contractions based on mechano- and electromyograms

The focus of the present study is the long term element of muscle fatigue provoked by prolonged intermittent contractions at submaximal force levels and analysed by force, surface electromyography (EMG) and mechanomyogram (MMG). It was hypothesized that fatigue related changes in mechanical performance of the biceps muscle are more strongly reflected in low than in high force test contractions, more prominent in the MMG than in the EMG signal and less pronounced following contractions controlled by visual compared to proprioceptive feedback. Further, it was investigated if fatigue induced by 30 min intermittent contractions at 30% as well as 10% of maximal voluntary contraction (MVC) lasted more than 30 min recovery. In six male subjects the EMG and MMG were recorded from the biceps brachii muscle during three sessions with fatiguing exercise at 10% with visual feedback and at 30% MVC with visual and proprioceptive feedback. EMG, MMG, and force were evaluated during isometric test contractions at 5% and 80% MVC before prolonged contraction and after 10 and 30 min of recovery. MVC decreased significantly after the fatiguing exercise in all three sessions and was still decreased even after 30 min of recovery. In the time domain significant increases after the fatiguing exercise were found only in the 5% MVC tests and most pronounced for the MMG. No consistent changes were found for neither EMG nor MMG in the frequency domain and feedback mode did not modify the results. It is concluded that long term fatigue after intermittent contractions at low force levels can be detected even after 30 min of recovery in a low force test contraction. Since the response was most pronounced in the MMG this may be a valuable variable for detection of impairments in the excitation-contraction coupling.

Changes in contractile properties and neuromuscular propagation evaluated by simultaneous mechanomyogram and electromyogram during experimentally induced hypothermia

The present study examined, whether or not mechanomyogram (MMG) amplitude and frequency component could reflect the contractile properties of the triceps surae muscles, composed of relatively slow soleus (SOL) and fast medial gastrocnemius (MG), during experimentally induced hypothermia condition. In eight male subjects, lying in prone position, supramaximal single twitch and repetitive electrical stimulations at 10 Hz were applied at the intramuscular temperatures of control (34 degrees C), 15, 20, and 25 degrees C, respectively. The hypothermia induced substantial reduction in muscle contractile properties, e.g. prolonged twitch contraction and half relaxation times, resulted in a highly significant reduction in the fluctuation of force signal during the repetitive stimulations. These changes were almost mirrored by the similar and significant reductions in the MMG amplitude in both SOL and MG. Power spectrum analysis revealed that peak frequency components of MMG and fluctuation of force were almost matched with the applied stimulation frequencies, independent of the temperature condition. These results strongly suggest that MMG analysis could be employed to study muscle contractile properties varying across different physiological conditions.

Muscle oxygen consumption, determined by NIRS, in relation to external force and EMG

Local oxygen consumption in a muscle (VO(2)) can be determined by near infrared spectroscopy (NIRS). In principle it should be possible to use this measure to validate musculoskeletal models. However, the relationship between VO(2) and external force, or between VO(2) and surface EMG, as a measure for muscle activity, is hardly known. The aim of this study was: (1) to evaluate the characteristics of the relationship between VO(2) and external moments and (2) to determine whether differences exist between the EMG-moment relationship and the VO(2)-moment relationship. Subjects (n=5) were asked to perform isometric contractions exerting combinations of elbow flexion and pro/supination moments at force levels up to 70% of their maximum. Simultaneous surface-EMG and NIRS measurements were performed on the m. biceps breve (BB) and the m. brachioradialis (BR). A linear relationship was found between EMG and VO(2). For the BB VO(2) and EMG were linearly related to both the flexion moment and the pro/supination moment. However, for the BR only a linear relationship with flexion moment was found. As expected, based on the findings above, the relationship between VO(2) and elbow flexion moment can be described by a linear equation, under the conditions of this study (isometric, and force levels up to 70%). These findings suggest that load sharing is independent of force level and that next to EMG, VO(2) can be used for the validation of musculoskeletal models.

Characteristics of surface mechanomyogram are dependent on development of fusion of motor units in humans

The purpose of this study was to test whether surface mechanomyogram (MMG) recorded on the skin reflects the contractile properties of individual motor units in humans. Eight motor units in the medial gastrocnemius muscle were identified, and trains of stimulation at 5, 10, 15, and 20 Hz were delivered to each isolated motor unit. There was a significant positive correlation between the duration of MMG and twitch duration. MMG amplitude decreased with increasing stimulation frequency. Reductions in MMG amplitude were in parallel with the reductions in force fluctuations, and the rate of change in both was positively correlated across the motor units. Rate of change in MMG amplitude against force was negatively correlated to half relaxation time and twitch duration. Similar negative correlations were found between force fluctuations and contractile properties. These results provide evidence supporting a direct relation between MMG and contractile properties of individual motor units within the gastrocnemius muscle, indicating that surface MMG is dependent on the contractile properties of the activated motor units in humans.

Endothelium-dependent vasorelaxation induced by black currant concentrate in rat thoracic aorta

We investigated the effect of black currant (BC) concentrate on smooth muscle in rat thoracic aorta. BC concentrate dose-dependently relaxed the norepinephrine (0.1 microM)-precontracted aorta, and the response was abolished after endothelium removal. Both oxyhemoglobin (1 microM), a nitric oxide (NO) scavenger, and IH-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ, 0.5 microM), an inhibitor of guanylyl cyclase (GC), inhibited the relaxing effect of BC concentrate. NG-nitro-L-arginine methyl ester (L-NAME, 10 microM), a nitric oxide synthase (NOS) inhibitor, inhibited the relaxation, and the subsequent addition of L-arginine (1 mM), a NOS substrate, reversed the inhibitory effects of L-NAME. Neither indomethacin (10 microM), an inhibitor of cyclooxygenase, nor atropine (1 microM), an antagonist of muscarinic receptors, modified the effect of BC concentrate. Diphenhydramine (3 microM) and chlorpheniramine (2 microM), selective antagonists of H1-receptors, inhibited the relaxation, but cimetidine (0.3 mM), a selective antagonist of H2-receptors, did not affect the relaxation. These results indicate that, in the rat aorta, BC concentrate enhances synthesis of NO, which subsequently induces the endothelium-dependent vasorelaxation via the H1-receptors on the endothelium.