Recruitment Of Fast Motor Units Research Articles

Human myoelectric spatial patterns differ among lower limb muscles and locomotion speeds.

The spatial distribution of myoelectric activity within lower limb muscles is often nonuniform and can change during different stationary tasks. Recent studies using high‐density electromyography (EMG) have suggested that spatial muscle activity may also differ among muscles during locomotion, but contrasting electrode array sizes and experimental designs have limited cross‐study comparisons. Here, we sought to determine if spatial EMG patterns differ among lower limb muscles and locomotion speeds. We recorded high‐density EMG from the vastus medialis, tibialis anterior, biceps femoris, medial gastrocnemius, and lateral gastrocnemius muscles of 11 healthy subjects while they walked (1.2 and 1.6 m/s) and ran (2.0, 3.0, 4.0, and 5.0 m/s) on a treadmill. To overcome the detrimental effects of cable, electrode, and soft tissue movements on high‐density EMG signal quality during locomotion, we applied multivariate signal cleaning methods. From these data, we computed the spatial entropy and center of gravity from the total myoelectric activity within each recording array during the stance or swing phases of the gait cycle. We found heterogeneous spatial EMG patterns evidenced by contrasting spatial entropy among lower limb muscles. As locomotion speed increased, mean entropy values decreased in four of the five recorded muscles, indicating that EMG signal amplitudes were more spatially heterogeneous, or localized, at faster speeds. The EMG center of gravity location also shifted in multiple muscles as locomotion speed increased. Contrasting myoelectric spatial distributions among muscles likely reflect differences in muscle architecture, but increasingly localized activity and spatial shifts in the center of gravity location at faster locomotion speeds could be influenced by preferential recruitment of faster motor units under greater loads.

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.

Load‐sensitive adhesion factor expression in the elderly with skiing: relation to fiber type and muscle strength

We hypothesized that 12 weeks of downhill skiing mitigates the functional deficits of knee extensor muscles in elderly subjects due to the specific recruitment of fast motor units during forceful turns on the slope. Downhill skiing led to a 1.4-fold increase in the mean cross-sectional area of slow (P=0.04)- and fast (P=0.08)-type muscle fibers. Fold changes in the expression of the structural component of focal adhesions, gamma-vinculin, were correlated with alterations in concentric force (r=0.64). Hypertrophy of fast fibers was more pronounced in women than in men (1.7 vs 1.1). Gender-specific structural-functional adjustments of knee extensor muscles and attached patellar tendon were reflected by altered expression of pro- vs de-adhesive proteins and a number of correlations. The de-adhesive protein tenascin-C was selectively increased in women compared with men (1.7 vs 1.1) while the content of the adhesive collagen XII was specifically reduced in women. The pro-adhesive focal adhesion kinase showed a specific increase in men compared with women (1.9 vs 1.1). Our findings indicate that quantitatively matched adaptations in slow and fast motor units of extensor muscle underlie the preventive effect of skiing against sarcopenia and support that hypertrophy and reinforcement of fiber adhesion operate in the improvement of muscle strength.

Oxygen Kinetics During Specific Intermittent Work

PURPOSE: The time (t) discriminates the speed of activation of the oxidations mechanisms, during an intermittent work (IW) 95 - 100% of the VO2 Max. The deficit of oxygen (DO2) of the fast phase of the O2 kinetics (O2K) represents the anaerobic work participation.1 PURPOSE: to analyzer the DO2 utility during specific intermittent work (SIW) 2,3. METHODS: Olympic athletes: 4 boxers and 4 judokas. Age (18-24 years); weight (60-75 kg) and height (170-185 cm). SIW on treadmill (slope 2%; heating 2 min; 4 repetitions of 2 min by 1 of rest). Each repetition represents one round and in the rest the subject remains seated; thus the test is considered specific for boxing. VO2 Max was measured with K4b2 and DO2 by COSMED software. It was applied ANOVA TWO WAY with and without replications. RESULTS: In boxing: DO2 = 1905±597 ml F NS between three evaluations. The DO2 between subjects (BS) and rounds was different F= 3.24 p<0.003 and F=4.83 p<0.003 respectively; interaction NS. In judo DO2 = 2050±769 ml BS F=6.48 p<0.007; between repetitions N.S. CONCLUSIONS: the DO2 differences BS, demonstrate that the functional answer would be modulated by the characteristics of the box and by the individual variability. The fast phase of the O2 kinetics (O2K) is considered dependent of the type of muscular fiber with recruitment of fast motor units, we could think that the results, express the functional answer to specific metabolic requirements of box. The SIW designed originally for boxing, demonstrated a relative utility in judo,possibly due to that competition times are different. 1 Barstow, Thomas J., Andrew M. Jones, Paul H. Nguyen, and Richard Casaburi. Influence of muscle fiber type and pedal frequency on oxygen uptake kinetics of heavy exercise. J. Appl. Physiol. 81(4): 1642-1650, 1996. 2 Narváez Pérez G.E. et al. O2 participation during the 40-s work. Medicine and Science in Sports and exercise, Vol., 27:5, Supplement. 1995 3 Narváez P.G.E. and H. Rodríguez P. Improper weight loss in amateur boxers and disorders of the C.N.S. Proceedings of International Medical Symposium "The Safety of the Athlete in Olympic Sports" (Ed) A. Francone. Assisi. Oct.1990, pp.55-56.

Relationship between Torque and Angular Velocity of Human Dorsiflexors in vivo

Numerous studies have investigated torque-angular velocity relation of human muscles in vivo, by using isokinetic strength measurements. However, the experimental data in the low-torque, high-velocity region are typically missing, primarily because of large inertial mass and limited range of motion in the human limbs. PURPOSE: To describe the whole picture of torque-angular velocity relation of human dorsiflexors in vivo, by combining isokinetic strength measurement with the slack test, a method for determining unloaded shortening velocity of muscle (Edman, 1979; Sasaki & Ishii, 2005). METHODS: Fifteen healthy students (age: 19.5 ± 0.8 years, height: 164.4 ± 9.1 cm, body mass: 53.9 ± 8.4 kg) participated in this study. In the slack-test measurement, the quick release of ankle joint was applied while the subjects exerted an isometric torque at 15, 50, and 85% of maximal voluntary contraction (MVC). Since the relation between the angular displacement of quick release (ÄL) and the time required to redevelop the torque (Ät) was approximately linear, the unloaded shortening velocity of dorsiflexors was determined as the linear regression slope of ÄL-Ät relation. In the isokinetic measurement, peak torque was determined at angular velocities of 0, 0.26, 0.52, 1.05, 2.09, and 3.14 rad s−1. Surface electromyographic activities (EMGs) of tibialis anterior (TA) and soleus (SOL) muscles during contraction were also examined. RESULTS: The results from the slack test showed that the unloaded shortening velocity of dorsiflexors increased with the contraction intensity (R2 = 0.37, P < 0.0001), suggesting an additional recruitment of fast motor units at higher contraction intensities (i.e. 50 and 85% of MVC). When combined with the unloaded shortening velocity determined at 85% of MVC (13.6 ± 9.1 rad s−1), the isokinetic torque-angular velocity relation had a hyperbolic shape, in which the shape parameter ranged between 0.09 and 1.38. In addition, EMGs of TA and SOL during contraction were consistent across different contraction velocity (Friedman test: P < 0.24, TA; P < 0.26, SOL). CONCLUSION: Torque-angular velocity relation of human dorsiflexors in vivo had hyperbolic shape, in which the shape parameter was consistent with the other muscles of humans and animals.

Task-specific recruitment of motor units for vibration damping

Vibrations occur within the soft tissues of the lower extremities due to the heel-strike impact during walking. Increases in muscle activity in the lower extremities result in increased damping to reduce this vibration. The myoelectric intensity spectra were compared using principal component analysis from the tibialis anterior and lateral gastrocnemius of 40 subjects walking with different shoe conditions. The soft insert condition resulted in a significant, simultaneous increase in muscle activity with a shift to higher myoelectric frequencies in the period 0–60 ms after heel-strike which is the period when the greater vibration damping occurred. These increases in myoelectric frequency match the spectral patterns which indicate increases in recruitment of faster motor units. It is concluded that fast motor units are recruited during the task of damping the soft-tissue resonance that occurs following heel-strike.

Motor unit activation patterns during isometric, concentric and eccentric actions at different force levels

Motor unit activation patterns were studied during four different force levels of concentric and eccentric actions. Eight male subjects performed concentric and eccentric forearm flexions with the movement range from 100° to 60° in concentric and from 100° to 140° elbow angle in eccentric actions. The movements were started either from zero preactivation or with isometric preactivation of the force levels of 20, 40, 60 and 80% MVC. The subjects were then instructed to maintain the corresponding relative force levels during the dynamic actions. Intramuscular and surface EMG was recorded from biceps brachii muscle. Altogether 28 motoneuron pools were analyzed using the intramuscular spike-amplitude frequency (ISAF) analysis technique of Moritani et al. [1]. The mean spike amplitude was lower and the mean spike frequency higher in the isometric preactivation phase than in the consequent concentric and eccentric actions. When the movements started with isometric preactivation the mean spike amplitude increased significantly ( P<0.001) up to 80% in isometric and concentric actions but in eccentric actions the increase continued only up to 60% ( P<0.01). The mean spike frequency in isometric preactivation and in concentric action with preactivation was lower only at the 20% force level ( P<0.01) as compared to the other force levels while in eccentric action with preactivation the increase between the force levels was significant ( P<0.01) up to 60%. When the movement was started without preactivation the mean spike amplitude at 20% and at 40% force level was higher ( P<0.01) in eccentric action than in concentric actions. It was concluded that the recruitment threshold may be lower in dynamic as compared to isometric actions. The recruitment of fast motor units may continue to higher force levels in isometric and in concentric as in eccentric actions which, on the other hand, seems to achieve the higher forces by increasing the firing rate of the active units. At the lower force levels mean spike amplitude was higher in eccentric than in concentric actions which might indicate selective activation of fast motor units. This was, however, the case only when the movements were started without isometric preactivation.

EMG power spectrum and features of the superimposed M-wave during voluntary eccentric and concentric actions at different activation levels.

Muscle fiber conduction velocity (CV) may be affected by the level of voluntary activation and by the diameter of the fiber. Both the frequency component of the electromyography (EMG) power spectrum, such the as median frequency (MF) or mean power frequency, and the duration of muscle compound action potential to single supramaximal electrical impulse (maximal M-wave) may be related to CV. The aim of the present study was to examine how changes in the activation level in lengthening and in shortening conditions would affect the EMG power spectrum during voluntary effort, and compare them to changes in M-wave shape in similar conditions. Ten male subjects performed eccentric and concentric knee extensions at force levels of 40%, 60%, 80% and 100% of maximal eccentric and concentric knee extension force (maximum voluntary contraction, MVC) at an angular velocity of 2 rad.s(-1). In order to measure the M-wave at each force level and in a relaxed condition, a supramaximal electrical stimulus was given to the femoral nerve. The surface EMG was recorded from the vastus lateralis, vastus medialis, and rectus femoris muscles, and the average EMG (aEMG) and MF were calculated. The results show that although the absolute force was greater, the aEMG was generally lower in eccentric as compared to concentric actions at all of the force levels tested. Although the aEMG increased as force increased, no consistent differences were observed in the amplitude of the maximal M-wave in any of the conditions, or in the duration of the M-wave between eccentric and concentric actions. However, as the force level increased the duration of the M-wave decreased significantly ( P<0.01) for both eccentric and concentric actions. On average, no major differences were observed in MF between eccentric and concentric actions or between the force levels in either type of contraction, although rather large variations were observed throughout the motions. In voluntary situations, the recruitment of fast motor units with higher muscle fiber CVs and the increased firing rate of the active units increases the muscle fiber CV as the activation level increases. Even though in conditions of supramaximal electrical nerve stimulation all motor units should be activated simultaneously, the duration of the M-wave in the present study decreased as the force level increased. Possible candidates for the change in the CV may be increased activation of the Na(+)/K(+) pump resulting from the activity in the muscle preceding the electrical stimulation and/or changes in the muscle fiber length between different force levels.

Electromyogram power spectrum and features of the superimposed maximal M-wave during voluntary isometric actions in humans at different activation levels.

The frequency characteristics of the electromyogram (EMG) power spectrum, such as the median or the mean power frequency, as well as the duration of the muscle compound action potential response to a single supramaximal electrical stimulus (maximal M-wave) may both be related to the conduction velocity (CV) of the muscle fibre. To investigate this further, we studied in ten male subjects: the EMG of the vastus lateralis, vastus medialis and rectus femoris muscles during maximal isometric knee extensions at 40%, 60%, 80% and 100% of maximal voluntary contraction and also the maximal M-wave, elicited by a single supramaximal stimulus to the femoral nerve, of the same muscles at rest or superimposed on the same levels of voluntary contraction. The EMG was recorded during the constant force phase of the voluntary contractions, the duration of which was 2.5-4 s, with a 1.5 min pause between contractions. The average EMG (aEMG) and the median frequency (MF) were then calculated. The results indicated that as aEMG increased with increase in force, MF remained unchanged. However, while the amplitude of the M-wave was not affected, the duration of the M-wave was shorter as the force level increased. The duration of the M-wave may be affected by recruitment of faster motor units, by increased firing rate of the active units and by changes in the muscle fibre length. The shorter duration of the M-wave observed at higher force levels was not, however, accompanied by a corresponding increase in MF. The MF could not therefore be used as a parameter to reflect the changes in voluntary muscle activation and CV. When MF was calculated by overlapping the fast Fourier transform (FFT) windows (0.4 s long window moved data point by data point to the right) for 1 s periods of the isometric plateau phase at each force level, the difference between the lowest and the highest MF was quite substantial. These variations suggest that FFT window placement, i.e. which part of the signal is chosen for the analyses, may play an important role even in isometric situations.

Decreased EMG median frequency during a second bout of eccentric contractions.

Others have reported preferential recruitment of fast motor units in muscles during performance of eccentric contractions and there is evidence that fast muscle fibers are more susceptible to eccentric contraction-induced injury. We tested the hypothesis that during a second bout of maximal eccentric contractions 1 wk after the first, there would be a reduction in the electromyographic (EMG) median frequency (MF) with minimal change in the EMG root-mean-square (RMS), indicating greater reliance on slower motor units. This could provide an explanation for the enhanced resistance to eccentric contraction-induced injury after a single bout of eccentric exercise. Human subjects performed 50 maximal voluntary eccentric (N = 10) or concentric (N = 10) contractions of the anterior crural muscles on two occasions separated by 1 wk. To determine whether MF changes during the second bout could be a consequence of injury to fibers in fast motor units, the anterior crural muscles of mice were electrically stimulated to perform 50 maximal eccentric (N = 10) or concentric (N = 9) contractions on two occasions separated by 1 wk. In both the humans and mice, torque production and tibialis anterior muscle RMS and MF were measured during the two exercise bouts. In human tibialis anterior muscle, MF was 30% lower (P < 0.01) during the second eccentric bout although RMS was the same. In the mice, RMS and MF were unchanged at any time after the first eccentric bout despite torque deficits similar to those observed in the humans. The data indicate that with repetition of maximal voluntary eccentric contractions, there is an increased activation of slow motor units and a concomitant decrease in activation of fast units.