Effects of series compliance on twitches superimposed on voluntary contractions.

The activation of skeletal muscle during voluntary isometric contraction has been assessed by measuring the increase in force caused by a superimposed maximal shock to the motor nerve (the twitch-interpolation technique). When the muscle is held isometric, the increase in force with stimulation (superimposed twitch force) decreases with increasing voluntary force, and a line fit through the data can be extrapolated to maximal voluntary force at the zero twitch force axis. In a previous paper we questioned the applicability of this technique in situations where a high series compliance allows the muscle to shorten during the superimposed twitch. To explore effects of series compliance, we measured force of the adductor pollicis during voluntary isometric contractions with noncompliant and compliant loading devices. With the compliant loading device, superimposed twitch force was systematically less than with the noncompliant device, and the plot of superimposed twitch force vs. voluntary force was often concave upward, preventing easy extrapolation to maximal voluntary force. These findings are consistent with force-velocity characteristics of muscle and suggest that twitch-interpolation data must be interpreted with caution when the muscle is not held isometric during the superimposed twitch.     

Variability in the interpolated twitch torque for maximal and submaximal voluntary contractions.

The superimposed twitch technique is frequently used to study the degree of motor unit activation during voluntary effort. This technique is one of the preferred methods to determine the activation deficit (AD) in normal, athletic, and patient populations. One of the limitations of the superimposed twitch technique is its variability under given contractile conditions. The objective of this research was to determine the source(s) of variability in the superimposed twitch force (STF) for repeat measurements. We hypothesized that the variability in the AD measurements may be caused by the timing of the twitch force relative to the onset of muscle activation, by force transients during the twitch application, by small variations in the actual force from the nominal target force, and by variations in the resting twitch force. Twenty-eight healthy subjects participated in this study. Sixteen of these subjects participated in a protocol involving contractions at 50% of their maximal voluntary contraction (MVC) effort, whereas the remaining 12 participated in a protocol involving contractions at 100% of their MVC. Doublet-twitch stimuli were superimposed onto the 50 and 100% effort knee extensor muscle contractions, and the resting twitch forces, voluntary knee extensor forces, and STFs were then measured. The mean resting twitch forces obtained before and after 8 s of 50% of MVC were the same. Similarly, the mean STFs determined at 1, 3, 5, and 7 s into the 50% MVC were the same. The variations in twitch force were significantly smaller after accounting for the actual force at twitch application than those calculated from the prescribed forces during the 50% MVC protocol (P < 0.05). Furthermore, the AD and the actual force showed statistically significant negative correlations for the 50% MVC tests. The interpolated twitch torque determined for the maximal effort contractions ranged from 1 to 70%. In contrast to the protocol at 50% of MVC, negative correlations were only observed in 5 of the 12 subjects during the 100% effort contractions. These results suggest that small variations in the actual force from the target force can account for the majority of the variations in the STFs for submaximal but not maximal effort contractions. For the maximal effort contractions, large variations in the STF exist due to undetermined causes.     

Alterations of neuromuscular function after an ultramarathon.

Neuromuscular fatigue of the knee extensor (KE) and plantar flexor (PF) muscles was characterized after a 65-km ultramarathon race in nine well-trained runners by stimulating the femoral and tibial nerves, respectively. One week before and immediately after the ultramarathon, maximal twitches were elicited from the relaxed KE and PF. Electrically evoked superimposed twitches of the KE were also elicited during maximal voluntary contractions (MVCs) to determine maximal voluntary activation. MVC and maximal voluntary activation decreased significantly after the ultramarathon (-30.2 +/- 18.0% and -27.7 +/- 13.0%, respectively; P < 0.001). Surprisingly, peak twitch increased after the ultramarathon from 15.8 +/- 6.3 to 19.7 +/- 3.3 N. m for PF (P < 0.01) and from 131.9 +/- 21.2 to 157.1 +/- 35.9 N for KE (P < 0.05). Also, shorter contraction and half-relaxation times were observed for both muscles. The compound muscle action potentials (M wave) were not significantly altered by the ultramarathon with the exception of the soleus, which showed a slightly higher M-wave amplitude after the running. From these results, it can be concluded that 65 km of running 1) severely depressed the maximal voluntary force capacity mainly because of a decrease in maximal voluntary activation, 2) potentiated the twitch mechanical response, and 3) did not change significantly the M-wave characteristics.     

Effect of number of stimuli and timing of twitch application on variability in interpolated twitch torque.

Application of a supramaximal electrical twitch to the voluntarily contracted muscle is used to assess the level of muscle activation. Large variability in the interpolated twitch torque (ITT) has been observed when repeated stimulations are performed. It is hypothesized that this variability in ITT is caused by the stochastic nature of the timing of twitch application relative to pulses of voluntary excitation trains. Two experiments were performed on 12 subjects each to test this hypothesis. For the first experiment, a single twitch was superimposed on a train stimulation at different time intervals relative to the train pulses. For the second experiment, single, double, triple, or quadruple twitches were applied on a voluntarily contracted muscle. The ITT critically depended on the time point of twitch application: a single pulse applied 5 ms before a train pulse consistently evoked higher ITTs than all other stimulation conditions. Furthermore, variability of the ITT decreased as the number of applied twitches increased. The results support the hypothesis that a large part of the variability in the ITT may be caused by the timing of the superimposed twitch relative to the motor unit trains. The variability may be reduced by increasing the number of superimposed twitches.     

Twitch contractile properties of plantar flexor muscles in power and endurance trained athletes

This study compared twitch contractile properties of plantar flexor muscles among three groups of 12 subjects each: endurance and power trained athletes and untrained subjects. The posterior tibial nerve was stimulated by supramaximal square wave pulses of 1-ms duration. Power trained athletes had higher twitch maximal force, maximal rates of force development and relaxation and also maximal voluntary contraction (MVC) force. The trained subjects had a smaller twitch maximal force: MVC force ratio and shorter twitch contraction and half-relaxation times than the untrained subjects with no significant differences between the two groups. Thus, the short time for evoked twitches in the athletes compared to the untrained subjects would seem unrelated to the type of training. It is concluded that power training induces a more evident increase of muscle force-generating capacity and speed of contraction and relaxation than endurance training. Accepted: 24 April 1999     

Postactivation potentiation, fiber type, and twitch contraction time in human knee extensor muscles.

In small mammals, muscles with shorter twitch contraction times and a predominance of fast-twitch, type II fibers exhibit greater posttetanic twitch force potentiation than muscles with longer twitch contraction times and a predominance of slow-twitch, type I fibers. In humans, the correlation between potentiation and fiber-type distribution has not been found consistently. In the present study, postactivation potentiation (PAP) was induced in the knee extensors of 20 young men by a 10-s maximum voluntary isometric contraction (MVC). Maximal twitch contractions of the knee extensors were evoked before and after the MVC. A negative correlation (r = -0. 73, P < 0.001) was found between PAP and pre-MVC twitch time to peak torque (TPT). The four men with the highest (HPAP, 104 +/- 11%) and lowest (LPAP, 43 +/- 7%) PAP values (P < 0.0001) underwent needle biopsies of vastus lateralis. HPAP had a greater percentage of type II fibers (72 +/- 9 vs. 39 +/- 7%, P < 0.001) and shorter pre-MVC twitch TPT (61 +/- 12 vs. 86 +/- 7 ms, P < 0.05) than LPAP. These data indicate that, similar to the muscles of small mammals, human muscles with shorter twitch contraction times and a higher percentage of type II fibers exhibit greater PAP.     

Modeling of summation of individual twitches into unfused tetanus for various types of rat motor units.

Repeated stimulation of motor units (MUs) causes an increase of the force output that cannot be explained by linear summation of equal twitches evoked by the same stimulation pattern. To explain this phenomenon, an algorithm for reconstructing the individual twitches, that summate into an unfused tetanus is described in the paper. The algorithm is based on an analytical function for the twitch course modeling. The input parameters of this twitch model are lead time, contraction and half-relaxation times and maximal force. The measured individual twitches and unfused tetani at 10, 20, 30 and 40 Hz stimulation frequency of three rat motor units (slow, fast resistant to fatigue and fast fatigable) are processed. It is concluded that: (1) the analytical function describes precisely the course of individual twitches; (2) the summation of equal twitches does not follow the results from the experimentally measured unfused tetani, the differences depend on the type of the MU and are bigger for higher values of stimulation frequency and fusion index; (3) the reconstruction of individual twitches from experimental tetanic records can be successful if the tetanus is feebly fused (fusion index up to 0.7); (4) both the maximal forces and time parameters of individual twitches subtracted from unfused tetani change and influence the course of each tetanus. A discrepancy with respect to the relaxation phase was observed between experimental results and model prediction for tetani with fusion index exceeding 0.7. This phase was predicted longer than the experimental one for better fused tetani. Therefore, a separate series of physiological experiments and then, more complex model are necessary for explanation of this distinction.     

The dependence of the twitch course of medial gastrocnemius muscle of the rat and its motor units on stretching of the muscle.

The relationship between the force of a single twitch of the medial gastrocnemius muscle of the rat and contraction and half-relaxation times, on one hand, and the load of the muscle on the other, was studied. Twitches of the whole muscle and its individual motor units were induced. The optimal load, at which the majority of motor units reached the greatest twitch force, was 10 G. Mean optimal loads for twitches of different types of motor units were very similar. Slow motor units reached a slightly greater twitch force at greater loads (12.5 G) than at 10 G. However, the optimal load for the twitch of the whole muscle was much greater. It was 47 G on the average. The contraction and half-relaxation times of motor units, as well as of the whole muscle, became longer as the force stretching the muscle increased. Half-relaxation time changed more rapidly than contraction time. Both parameters were undergoing the greatest changes in slow motor units.     

In vivo behaviour of human muscle architecture and mechanomyographic response using the interpolated twitch technique

This study investigated the origin of curvilinear change in the superimposed mechanomyogram (MMG) amplitude of the human medial gastrocnemius muscle (MG) with increasing contraction intensity. The superimposed twitch amplitude, the superimposed MMG amplitude and the extent of fascicle shortening were measured using ultrasonic images of electrical stimulation during isometric plantar flexions at levels 20%, 40%, 60%, 80%, and 100% of the maximal voluntary contraction (MVC). The superimposed twitch amplitude, the superimposed MMG amplitude and the extent of fascicle shortening decreased with increasing contraction intensity. The superimposed MMG amplitude and the extent of fascicle shortening showed a curvilinear decrease, while the superimposed twitch amplitude showed a linear decrease at levels up to 80% of the MVC. There was a linear relationship between the superimposed MMG amplitude and the extent of fascicle shortening at different contraction intensities. These results indicate that the superimposed MMG amplitude reflects changes in the extent of fascicle shortening at different contraction intensities better than the superimposed twitch amplitude. Our study suggests that the origin of the curvilinear decrease of superimposed MMG amplitude is associated with a curvilinear decrease of the extent of fascicle shortening with increasing contraction intensity in the human MG.     

Reproducible measurement of voluntary activation of human elbow flexors with motor cortical stimulation.

Voluntary activation of muscle is commonly quantified by comparison of the extra force added by motor nerve stimulation during a contraction [superimposed twitch (SIT)] with that produced at rest by the same stimulus (resting twitch). An inability to achieve 100% voluntary activation implies that failure to produce maximal force output from the muscle must have occurred at a site at or above the level of the motoneurons. We have used cortical stimulation to quantify voluntary activation. Here, incomplete activation implies a failure at or above the level of motor cortical output. With cortical stimulation, it is inappropriate to compare extra force evoked during a contraction with the twitch evoked in resting muscle because motor cortical and spinal cord excitability both increase with activity. However, an appropriate "resting twitch" can be estimated. We previously estimated its amplitude by extrapolation of the linear relation between SIT amplitude and voluntary torque calculated from 35 contractions of >50% maximum (Todd G, Taylor JL, and Gandevia SC. J Physiol 551: 661-671, 2003). In this study, we improved the utility of this method to enable evaluation of voluntary activation when it may be changing over time, such as during the development of fatigue, or in patients who may be unable to perform large numbers of contractions. We have reduced the number of contractions required to only three. Estimation of the resting twitch from three contractions was reliable over time with low variability. Furthermore, its reliability and variability were similar to the resting twitch estimated from 30 contractions and to that evoked by conventional motor nerve stimulation.     

Effect of acute severe hypoxia on peripheral fatigue and endurance capacity in healthy humans

We hypothesized that severe hypoxia limits exercise performance via decreased contractility of limb locomotor muscles. Nine male subjects [mean +/- SE maximum O(2) uptake (Vo(2 max)) = 56.5 +/- 2.7 ml x kg(-1) x min(-1)] cycled at > or =90% Vo(2 max) to exhaustion in normoxia [NORM-EXH; inspired O(2) fraction (Fi(O(2))) = 0.21, arterial O(2) saturation (Sp(O(2))) = 93 +/- 1%] and hypoxia (HYPOX-EXH; Fi(O(2)) = 0.13, Sp(O(2)) = 76 +/- 1%). The subjects also exercised in normoxia for a time equal to that achieved in hypoxia (NORM-CTRL; Sp(O(2)) = 96 +/- 1%). Quadriceps twitch force, in response to supramaximal single (nonpotentiated and potentiated 1 Hz) and paired magnetic stimuli of the femoral nerve (10-100 Hz), was assessed pre- and at 2.5, 35, and 70 min postexercise. Hypoxia exacerbated exercise-induced peripheral fatigue, as evidenced by a greater decrease in potentiated twitch force in HYPOX-EXH vs. NORM-CTRL (-39 +/- 4 vs. -24 +/- 3%, P < 0.01). Time to exhaustion was reduced by more than two-thirds in HYPOX-EXH vs. NORM-EXH (4.2 +/- 0.5 vs. 13.4 +/- 0.8 min, P < 0.01); however, peripheral fatigue was not different in HYPOX-EXH vs. NORM-EXH (-34 +/- 4 vs. -39 +/- 4%, P > 0.05). Blood lactate concentration and perceptions of limb discomfort were higher throughout HYPOX-EXH vs. NORM-CTRL but were not different at end-exercise in HYPOX-EXH vs. NORM-EXH. We conclude that severe hypoxia exacerbates peripheral fatigue of limb locomotor muscles and that this effect may contribute, in part, to the early termination of exercise.     

Twitch potentiation after fatiguing exercise in man

Twitch potentiation was studied in the human triceps surae complex before and after intermittent maximal voluntary contractions or electrical stimulation at 20 Hz. Both forms of exercise were conducted with intact circulation for a maximum of 10 min or with circulatory occlusion until force output declined 50%. The relative potentiation was determined when a control twitch was compared to a twitch obtained after 5 s of maximal voluntary plantar flexion. The unpotentiated twitch torque (PT) and potentiated twitch torque (PT*) were reduced most severely after voluntary ischemic exercise (63.2% and 52.5% respectively, (P less than 0.001)). However, the relative potentiation (PT*/PT) immediately after voluntary ischemic exercise increased to 1.65 +/- 0.18 from 1.22 +/- 0.13 at rest. Both PT and PT* recovered quickly after exercise. At rest, twitch contraction time (CT) and one-half relaxation time (1/2 RT) in the unpotentiated twitch were longer than that of contraction (CT*) and one-half relaxation time (1/2 RT*) in the potentiated twitch. Following non-occluded exercise, CT, CT*, 1/2 RT and 1/2 RT* were shortened relative to rest. After ischemic exercise CT and CT* were shortened although 1/2 RT and 1/2 RT* increased relative to rest. Both CT* and 1/2 RT* quickly recovered to pre-exercise values by 5 min post-exercise. Ratios of potentiated/control twitch parameters were not altered after nonoccluded exercise, but were increased after ischemic exercise. These results suggest that the mechanisms of fatigue which depress voluntary torque and twitch and potentiated twitch torques, do not interfere with the extent of potentiation after fatiguing exercise.     

Model-generated decomposition of unfused tetani of motor units evoked by random stimulation

Unfused tetani of motor units (MUs) evoked by stimulation at variable interpulse intervals at mean frequencies of 20, 25, 33, 40 and 50 Hz were studied using ten functionally isolated fast-type MUs from the medial gastrocnemius muscle of adult Wistar rats. A previously proposed algorithm and computer program for mathematical decomposition of unfused tetani into a series of twitches, representing responses to individual pulses, were used. Analysis of the parameters of the decomposed twitches showed considerable variability in force of successive contractions. These twitches were extremely variable with up to 2-fold higher forces and longer contraction times than a single twitch evoked by one stimulus. However, when the stimulation frequency was decreased, the decomposed twitches became similar to the single twitch with respect to amplitude and contraction time. It was found that the basic contractile parameters of decomposed twitches could be predicted with high accuracy on the basis of the tetanus force level at which the next contraction begins. This analysis of the parameters of decomposed twitches demonstrated that the contractile responses of the muscle fibers to successive action potentials generated by motoneurons are highly variable and depend on the previous MU state.     

Twitch interpolation technique in testing of maximal muscle strength: Influence of potentiation, force level, stimulus intensity and preload

The aim was to study the methodological aspects of the muscle twitch interpolation technique in estimating the maximal force of contraction in the quadriceps muscle utilizing commercial muscle testing equipment. Six healthy subjects participated in seven sets of experiments testing the effects on twitch size of potentiation, time lag after potentiation, magnitude of voluntary force, stimulus amplitude, stimulus duration, angle of the knee, and angle of the hip. In addition, the consequences of submaximal potentiation on the estimation of maximal force from twitch sizes were studied in five healthy subjects. We found an increase in twitch size with increasing levels of potentiation and twitch size decreased exponentially following potentiation. We found a curvilinear relationship between twitch size and voluntary force, and these properties were more obvious when the stimulation intensity of the preload was reduced. The relationship between twitch size and force was only linear, for force levels greater than 25% of maximum. It was concluded that to achieve an accurate estimate of true maximal force of muscle contraction, it would be necessary for the subject to be able to perform at least 75% of the true maximal force.     

Passive and dynamic muscle architecture during transverse loading for gastrocnemius medialis in man

External forces from our environment impose transverse loads on our muscles. Studies in rats have shown that transverse loads result in a decrease in the longitudinal muscle force. Changes in muscle architecture during contraction may contribute to the observed force decrease. The aim of this study was to quantify changes in pennation angle, fascicle dimensions, and muscle thickness during contraction under external transverse load.Electrical stimuli were elicited to evoke maximal force twitches in the right calf muscles of humans. Trials were conducted with transverse loads of 2, 4.5, and 10 kg. An ultrasound probe was placed on the medial gastrocnemius in line with the transverse load to quantify muscle characteristics during muscle twitches.Maximum twitch force decreased with increased transverse muscle loading. The 2, 4.5, and 10 kg of transverse load showed a 9, 13, and 16% decrease in longitudinal force, respectively. Within the field of view of the ultrasound images, and thus directly beneath the external load, loading of the muscle resulted in a decrease in the muscle thickness and pennation angle, with higher loads causing greater decreases. During twitches the muscle transiently increased in thickness and pennation angle, as did fascicle thickness. Higher transverse loads showed a reduced increase in muscle thickness. Smaller increases in pennation angle and fascicle thickness strain also occurred with higher transverse loads.This study shows that increased transverse loading caused a decrease in ankle moment, muscle thickness, and pennation angle, as well as transverse deformation of the fascicles.     

Excitation-contraction coupling model to estimate the recirculating fraction of activator calcium in intact cardiac muscle.

Potentiated contractions were evoked with rapid pace pause maneuver in 14 length-clamped ferret papillary muscles paced 12 times/min at 25 degrees C. At 1.25 mM [Ca2+]o the average steady-state force was 2.94 +/- 1.08 g/mm2 and the potentiated contraction averaged 10.96 +/- 1.61 g/mm2. At 5.0 mM [Ca2+]o the steady-state force increased to 6.18 +/- 1.23 g/mm2 and the potentiated contraction averaged 12.08 +/- 1.15 g/mm2. Under the conditions of these experiments the potentiated contraction obtained at 5.0 mM [Ca2+]o is equal to the maximum twitch tension (Fmax) these muscles can generate. We have previously shown that Fmax is an equivalent of maximal calcium activated force. Since there is a beat to beat nearly exponential decay of the evoked potentiation, the fraction (= fraction x) of the potentiation that is not dissipated with each beat is nearly constant. Using an excitation-contraction coupling model we have previously found that x reflects a measure of the recirculating fraction of activator calcium. Because the tension-calcium relationship is better characterized by a sigmoidal curve, we have now incorporated the Hill equation in the model. To account for the inverse relationship between [Ca2+]i and the magnitude of the slow inward current, a term for negative feedback (h) was also included. We have determined the quantity (x-h) because x and h could not be determined separately. The quantity (x-h) was denoted as x'. The average values of x' at 1.25 and 5.0 mM [Ca2+]o were significantly different (p less than 0.0001), approximately 20% at the lower [Ca2+]o and about 50% at the higher [Ca2+]o.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in the time course of potentiated and fatigued contractions of fast motor units in rat muscle

The contraction and relaxation times of the twitches and the last contractions within 32 unfused tetani of FF and 27 unfused tetani of FR motor units in the rat medial gastrocnemius muscle were studied during prolonged activity. The pattern of the MU stimulation included single pulses (to evoke twitches) and series of three trains of stimuli at 40, 50 and 60 Hz (to evoke unfused tetani), repeated 30 times. The analysis concerned changes of force and time parameters at the beginning of activity, during the potentiation and then during the fatigue. It was found that changes of force during the potentiation and the fatigue were mainly accompanied by changes in the course of relaxation. The significant prolongation of the half-relaxation time during the potentiation of either twitches or unfused tetani was revealed in both types of fast MU. The twitch contraction time did not change markedly, whereas significantly shortened in the last contractions of unfused tetani during the potentiation. These changes of time parameters correlated to the increase of the fusion degree. During the fatigue, the time parameters shortened, however, changes of the half-relaxation times were remarkably higher. The shortening of relaxation was responsible for the decrease of the fusion degree. Changes of the fusion index exceeding 0.75 during the potentiation or decreasing below this value during the fatigue, were accompanied by respective appearance or disappearance of the biphasic relaxation.     

The effects of Ramadan intermittent fasting on the underlying mechanisms of force production capacity during maximal isometric voluntary contraction

The aim of the present study was to investigate the effects of Ramadan intermittent fasting (RIF) on the underlying mechanisms of force production capacity during maximal voluntary isometric contraction (MVIC) using the superimposed twitch technique. Ten healthy male physical education students performed three MVIC of the knee extensor superimposed with nerve electrical stimulation during four testing phases: one week before Ramadan (BR), at the end of the first week of Ramadan (R-1), during the fourth week of Ramadan (R-4) and two weeks after Ramadan (AR). This study was performed during Ramadan 2016. MVIC values, voluntary activation level (VAL), potentiated resting twitch and electromyography signals were recorded during each MVIC. The French version of the Profile of Mood States questionnaire (POMS-f) was used to evaluate the subjective mood states in each testing session. The results showed that MVIC values (890.57 ± 67.90 vs. 816.46 ± 54.72 N) and VAL (87.73 ± 3.27 vs. 77.32 ± 7.87%) decreased at R-1 compared to BR (p < 0.001). However, the neuromuscular efficiency and the potentiated resting twitch remained unchanged during Ramadan (R). Results showed that depression (p < 0.01; 6.3 ± 1.57 vs. 4.7 ± 1.25), fatigue (p < 0.001; 9.2 ± 1.93 vs. 4.6 ± 2.01) and anxiety (p < 0.001; 6.4 ± 1.51 vs. 11.8 ± 1.23) scores of POMS-f were higher during R-1 compared to BR. In conclusion, RIF-related impairment of maximal muscle force seems to be related to nervous alterations of the VAL, whereas the RIF did not adversely affect peripheral mechanisms.

Abbreviations’ List: ANOVA: Analysis of variance; AR: After Ramadan; BMI: Body-mass index; BR: Before Ramadan; EMG: Electromyography; ER: End of Ramadan; MF: Mean frequency; M_max: Peak-to-peak M-wave amplitudes; MVIV: Maximal voluntary isometric contraction; NES: Nerve electrical stimulation; NME: Neuromuscular efficiency; POMS-f: French version of the Profile of Mood States questionnaire; R: Ramadan; R-1: First week of Ramadan; R-4: Fourth week of Ramadan; RF: Rectus femoris; RIF: Ramadan intermittent fasting; RMS: Root mean square; VAL: Voluntary activation level; VL: Vastus lateralis; VM: Vastus medialis.     

Differences in the log-transformed electromyographic–force relationships of the plantar flexors between high- and moderate-activated subjects

The present study examined the log-transformed electromyographic amplitude (EMG) versus force relationships for the medial gastrocnemius (MG) and soleus (SOL) in high- and moderate-activated subjects. Twenty-five (age = 21 ± 2 year; mass = 62 ± 12 kg) participants performed six submaximal contractions (30–90% maximal voluntary contraction [MVC]) with the interpolated twitch technique (ITT) performed at 90% MVC to calculate percent voluntary activation (% VA). Sixteen participants with > 90% VA at 90% MVC were categorized high-activated group; the remaining nine were the moderate-activated group. Linear regression models were fit to the log-transformed EMG–force relationships. The slope (b value) and the antilog of the Y-intercept (a value) were calculated. The b values from the MG EMG–force relationships were higher (P < 0.05) for the high-activated group (1.27 ± 0.13) than the moderate-activated group (0.88 ± 0.06). The a values and p–p M-wave amplitude values (collapsed across twitches [superimposed and potentiated]) were larger (P < 0.05) for the MG (1.17 ± 0.40 and 8.98 ± 0.46 mV) than the SOL (0.24 ± 0.07 and 4.48 ± 0.20 mV) when collapsed across groups. The b value from the log-transformed EMG–force relationships is an attractive model to determine if a subject has the ability to achieve high activation of their MG without muscle or nerve stimulation.     

Initial force production before sag is enhanced by prior contraction followed by a 3-minute rest period in fast motor units of the rat medial gastrocnemius

Unfused tetanic contractions evoked in fast motor units exhibit extra-efficient force production at the onset of contraction, an effect called “boost”. Boost is diminished in subsequent contractions if there is a short rest period between contractions, but can be re-established with a longer period of rest. We tested the hypothesis that contractile activity and rest could enhance boost-related metrics. Two sets of 3 unfused tetani were evoked 3 min apart in fast fatigable (FF) and fast fatigue-resistant (FR) motor units of the rat medial gastrocnemius. The greatest changes occurred in the first unfused tetanic contractions. Relative to the first contraction in the first set, the first contraction in the second set exhibited higher peak force during boost in a subset of motor units (76% of FF and 48% of FR). Enhanced force during boost was influenced by interaction of slowing of twitch contraction time (up to 20% and 25%, for FF and FR motor units, respectively), half-relaxation time (up to 37% and 49% for FF and FR motor units, respectively), and potentiation of the first twitch (up to 13% and 5% for FF and FR motor units, respectively). Examination of twitches evoked between sets suggested opportunity for greater enhancement of boost with shorter intervening rest periods. The phenomenon of enhanced boost following motor unit activity may interest sports scientists.