Nonlinear force addition of newly recruited motor units in the cat hindlimb

Cadence Paradox in Cycling-Part 2: Theory and Simulation of Maximal Lactate Steady State and Carbohydrate Utilization Dependent on Cycling Cadence

PURPOSE

To develop and evaluate a theory on the frequent observation that cyclists prefer cadences (RPMs) higher than those considered most economical at submaximal exercise intensities via modeling and simulation of its mathematical description.

METHODS

The theory combines the parabolic power-to-velocity (v) relationship, where v is defined by crank length, RPM-dependent ankle velocity, and gear ratio, RPM effects on the maximal lactate steady state (MLSS), and lactate-dependent carbohydrate oxidation (CHO). It was tested against recent experimental results of 12 healthy male recreational cyclists determining the v-dependent peak oxygen uptake (VO2PEAKv), MLSS (MLSSv), corresponding power output (PMLSSv), oxygen uptake at PMLSSv (VO2MLSSv), and CHOMLSSv-management at 100 versus 50 per minute, respectively. Maximum RPM (RPMMAX) attained at minimized pedal torque was measured. RPM-specific maximum sprint power output (PMAXv) was estimated at RPMs of 100 and 50, respectively.

RESULTS

Modeling identified that MLSSv and PMLSSv related to PMAXv (IPMLSSv) promote CHO and that VO2MLSSv related to VO2PEAKv inhibits CHO. It shows that cycling at higher RPM reduces IPMLSSv. It suggests that high cycling RPMs minimize differences in the reliance on CHO at MLSSv between athletes with high versus low RPMMAX.

CONCLUSIONS

The present theory-guided modeling approach is exclusively based on data routinely measured in high-performance testing. It implies a higher performance reserve above IPMLSSv at higher RPM. Cyclists may prefer high cycling RPMs because they appear to minimize differences in the reliance on CHO at MLSSv between athletes with high versus low RPMMAX.

Muscle contractile properties directly influence shared synaptic inputs to spinal motor neurons

Alpha band oscillations in shared synaptic inputs to the alpha motor neuron pool can be considered an involuntary source of noise that hinders precise voluntary force production. This study investigated the impact of changing muscle length on the shared synaptic oscillations to spinal motor neurons, particularly in the physiological tremor band. Fourteen healthy individuals performed low-level dorsiflexion contractions at ankle joint angles of 90° and 130°, while high-density surface electromyography (HDsEMG) was recorded from the tibialis anterior (TA). We decomposed the HDsEMG into motor units spike trains and calculated the motor units' coherence within the delta (1-5 Hz), alpha (5-15 Hz), and beta (15-35 Hz) bands. Additionally, force steadiness and force spectral power within the tremor band were quantified. Results showed no significant differences in force steadiness between 90° and 130°. In contrast, alpha band oscillations in both synaptic inputs and force output decreased as the length of the TA was moved from shorter (90°) to longer (130°), with no changes in delta and beta bands. In a second set of experiments (10 participants), evoked twitches were recorded with the ankle joint at 90° and 130°, revealing longer twitch durations in the longer TA muscle length condition compared to the shorter. These experimental results, supported by a simple computational simulation, suggest that increasing muscle length enhances the muscle's low-pass filtering properties, influencing the oscillations generated by the Ia afferent feedback loop. Therefore, this study provides valuable insights into the interplay between muscle biomechanics and neural oscillations. KEY POINTS: We investigated whether changes in muscle length, achieved by changing joint position, could influence common synaptic oscillations to spinal motor neurons, particularly in the tremor band (5-15 Hz). Our results demonstrate that changing muscle length from shorter to longer induces reductions in the magnitude of alpha band oscillations in common synaptic inputs. Importantly, these reductions were reflected in the oscillations of muscle force output within the alpha band. Longer twitch durations were observed in the longer muscle length condition compared to the shorter, suggesting that increasing muscle length enhances the muscle's low-pass filtering properties. Changes in the peripheral contractile properties of motor units due to changes in muscle length significantly influence the transmission of shared synaptic inputs into muscle force output. These findings prove the interplay between muscle mechanics and neural adaptations.

Using force or EMG envelope as feedback signal for motor control system

PURPOSE

This work studied muscle neuro-mechanics during symmetrical up-going ramp (UGR) and down-going ramp (DGR).

AIM

to evaluate during the modulation of muscular action the outcome of force feedback (FF) or neural feedback (NF) on the behavior of the trailing signals - i.e. the EMG envelope (eEMG) for FF or force signal for NF.

METHOD

Subjects: 20. Investigated muscles: dorsal interosseous (FDI) and tibialis anterior (TA). Detected signals: force and EMG. Visual feedback: force (FF), eEMG (NF). Effort triangles: ramps duration 7.5 s, vertex at 50 and 100 % of the maximal voluntary action. Eventually, each subject performed FF50%, FF100%, NF50% and NF100% per each muscle. In each condition the areas beneath the force and eEMG signals were computed to calculate the ratios between the DGR and UGR values during the different tasks (force area DGR / force area UGR; eEMG area DGR / eEMG area UGR). Electro-mechanical coupling efficiency (EMCE) was estimated through the eEMG area / force area ratio for both UGR and DGR in each condition.

RESULTS

a) FF. FDI: eEMG area ratio was 0.84 ± 0.15 and 0.73 ± 0.17 for FF50% and FF100%, respectively. TA: eEMG area ratio was 0.88 ± 0.11 and 0.91 ± 0.17 for FF50% and FF100%, respectively. b) NF: FDI: force area ratio was 1.18 ± 0.13 and 1.17 ± 0.13 for NF50% and NF100%, respectively. TA: force area ratio was 1.17 ± 0.21 and 1.07 ± 0.19 for NF50% and NF100%, respectively. c) DGR EMCE was greater than UGR EMCE in all four tasks.

CONCLUSION

The influence of UGR on deployed EMCE in the following force decrement phase underpins the changes of trailing signals area during DGR. This underlines the necessity of a careful evaluation of the features of FF or NF for experimental studies or rehabilitation purposes involving the motor control system.

Non-Linearity in Motor Unit Velocity Twitch Dynamics: Implications for Ultrafast Ultrasound Source Separation

Ultrasound (US) muscle image series can be used for peripheral human-machine interfacing based on global features, or even on the decomposition of US images into the contributions of individual motor units (MUs). With respect to state-of-the-art surface electromyography (sEMG), US provides higher spatial resolution and deeper penetration depth. However, the accuracy of current methods for direct US decomposition, even at low forces, is relatively poor. These methods are based on linear mathematical models of the contributions of MUs to US images. Here, we test the hypothesis of linearity by comparing the average velocity twitch profiles of MUs when varying the number of other concomitantly active units. We observe that the velocity twitch profile has a decreasing peak-to-peak amplitude when tracking the same target motor unit at progressively increasing contraction force levels, thus with an increasing number of concomitantly active units. This observation indicates non-linear factors in the generation model. Furthermore, we directly studied the impact of one MU on a neighboring MU, finding that the effect of one source on the other is not symmetrical and may be related to unit size. We conclude that a linear approximation is partly limiting the decomposition methods to decompose full velocity twitch trains from velocity images, highlighting the need for more advanced models and methods for US decomposition than those currently employed.

Skeletal muscle models composed of motor units: A review

The mathematical muscle models should include several aspects of muscle structure and physiology. First, muscle force is the sum of forces of multiple motor units (MUs), which have different contractile properties and play different roles in generating muscle force. Second, whole muscle activity is an effect of net excitatory inputs to a pool of motoneurons innervating the muscle, which have different excitability, influencing MU recruitment. In this review, we compare various methods for modeling MU twitch and tetanic forces and then discuss muscle models composed of different MU types and number. We first present four different analytical functions used for twitch modeling and show limitations related to the number of twitch describing parameters. We also show that a nonlinear summation of twitches should be considered in modeling tetanic contractions. We then compare different muscle models, most of which are variations of Fuglevand's model, adopting a common drive hypothesis and the size principle. We pay attention to integrating previously developed models into a consensus model based on physiological data from in vivo experiments on the rat medial gastrocnemius muscle and its respective motoneurons. Finally, we discuss the shortcomings of existing models and potential applications for studying MU synchronization, potentiation, and fatigue.

Sex differences in the modulation of the motor unit discharge rate leads to reduced force steadiness

The purpose of this study was to evaluate the relationship between the variability in the motor unit inter-pulse interval and force steadiness at submaximal and maximal force outputs between the sexes. Twenty-four male and 24 female participants were recruited to perform isometric dorsiflexion contractions at 20, 40, 60, 80, and 100% maximum voluntary contraction. Tibialis anterior myoelectric signal was recorded by an intramuscular electrode. Females had lower force steadiness (coefficient of variation of force (CoV-Force), 27.3%, p < 0.01) and a greater coefficient of variation of motor unit action potential inter-pulse interval (CoV-IPI), compared with males (9.6%, p < 0.01). There was no significant correlation between the normalized CoV-IPI and CoV-Force (r = 0.19, p > 0.01), but there was a significant repeated measures correlation between the raw scores for root-mean-square force error and the standard deviation of motor unit discharge rate (r = 0.65, p < 0.01). Females also had a greater incidence of doublet discharges on average across force levels (p < 0.01). The sex differences may result from motor unit behaviours (i.e., doublet and rapid discharges, synchronization, rate coding or recruitment), leading to lower force steadiness and greater CoV-IPI in females. Novelty: Sex differences in force steadiness may be due to neural strategies. Females have lower force steadiness compared with males. Greater incidence of doublet discharges in females may result in lesser force steadiness.

Transitory force decrease following a sudden reduction in stimulation frequency in motor units of rat medial gastrocnemius

Effects of a sudden decrease in the stimulation frequency for motor unit force were studied in rat medial gastrocnemius. For 161 functionally isolated single motor units of three types (S, FR, FF), unfused tetanic contractions were evoked by three-phase trains of stimuli (low-high-low frequency). The course of the tetanus at the onset of the third phase of the force recording was analyzed in tetani with variable fusion degree. For 78 units within the third phase of tetanus, a transitory force decrease to a level lower than in the first phase (identical frequency), was observed. This phenomenon was more frequent for fast fatigue resistant (65.9%) than for fast fatigable and slow motor units (27.1% and 35.5%, respectively). Moreover, the force decrease was strongest for fast resistant motor units (up to 36.5%) and when contractions evoked at variable frequencies of stimulation were compared, the highest amplitudes of the studied force decrease were noted for middle-fused tetani (0.50-0.90). A new phenomenon of transitory force decrease in tetanic contractions of motor units with a decrease in stimulation frequency was found. Most probably, the phenomenon is dependent on disturbances in the force transmission by collagen surrounding active muscles fibers.

Morphometric properties and innervation of muscle compartments in rat medial gastrocnemius

The rat medial gastrocnemius (MG) muscle is composed of the proximal and distal compartments. In this study, morphometric properties of the compartments and their muscle fibres at five levels of the muscle length and the innervation pattern of these compartments from lumbar segments were investigated. The size and number of muscle fibres in the compartments were different. The proximal compartment at the largest cross section (25% of the muscle length) had 34% smaller cross-sectional area but contained a slightly higher number of muscle fibres (max. 5521 vs. 5360) in comparison to data for the distal compartment which had the largest cross-sectional area at 40% of the muscle length. The muscle fibre diameters revealed a clear tendency within both compartments to increase along the muscle (from the knee to the Achilles tendon) up to 46.9 μm in the proximal compartment and 58.4 μm in the distal one. The maximal tetanic and single twitch force evoked by stimulation of L4, L5, and L6 ventral roots in whole muscle and compartments were measured. The MG was innervated from L4 and L5, only L5, or L5 and L6 segments. The proximal compartment was innervated by axons from L5 or L5 and L4, and the distal one from L5, L5 and L6, or L5 and L4 segments. The forces produced by the compartments summed non-linearly. The tetanic forces of the proximal and distal compartments amounted to 2.24 and 4.86 N, respectively, and their algebraic sums were 11% higher than the whole muscle force (6.37 N).

Increased intensity and reduced frequency of EMG signals from feline self-reinnervated ankle extensors during walking do not normalize excessive lengthening

Kinematics of cat level walking recover after elimination of length-dependent sensory feedback from the major ankle extensor muscles induced by self-reinnervation. Little is known, however, about changes in locomotor myoelectric activity of self-reinnervated muscles. We examined the myoelectric activity of self-reinnervated muscles and intact synergists to determine the extent to which patterns of muscle activity change as almost normal walking is restored following muscle self-reinnervation. Nerves to soleus (SO) and lateral gastrocnemius (LG) of six adult cats were surgically transected and repaired. Intramuscular myoelectric signals of SO, LG, medial gastrocnemius (MG), and plantaris (PL), muscle fascicle length of SO and MG, and hindlimb mechanics were recorded during level and slope (±27°) walking before and after (10-12 wk postsurgery) self-reinnervation of LG and SO. Mean myoelectric signal intensity and frequency were determined using wavelet analysis. Following SO and LG self-reinnervation, mean myoelectric signal intensity increased and frequency decreased in most conditions for SO and LG as well as for intact synergist MG (P < 0.05). Greater elongation of SO muscle-tendon unit during downslope and unchanged magnitudes of ankle extensor moment during the stance phase in all walking conditions suggested a functional deficiency of ankle extensors after self-reinnervation. Possible effects of morphological reorganization of motor units of ankle extensors and altered sensory and central inputs on the changes in myoelectric activity of self-reinnervated SO and LG are discussed.

Summation of slow motor unit forces at constant and variable interpulse intervals in rat soleus muscle

Effects of the summation of forces generated by functionally isolated slow-twitch motor units (MU) of the rat soleus muscle were examined in this study. Initially, the twitch, fused tetanic and unfused tetanic contractions evoked by trains of stimuli at variable interpulse intervals were recorded for each MU. Then, two, three or four MUs were co-activated, and the recorded forces were compared to the algebraic sum of the forces of individual MUs. The mean cumulative force of twitches and the mean cumulative force of fused tetani were not statistically different from the respective algebraic sums of forces, which revealed a high degree of linearity in the summation. However, relaxation of the recorded tetanic contractions (either fused or unfused) was faster than that predicted by the linear summation of individual contractions. Moreover, for twitch and tetanic contractions, a tendency to shorten relaxation with an increasing number of co-active MUs was noted. The results indicate that forces of rat soleus slow MUs sum up more linearly than in the respective cat muscle as well as more linearly than for fast MUs in the medial gastrocnemius muscle.

Synchronization of low- and high-threshold motor units

INTRODUCTION

We examined the degree of synchronization for both low- and high-threshold motor unit (MU) pairs at high force levels.

METHODS

MU spike trains were recorded from the quadriceps during high-force isometric leg extensions. Short-term synchronization (between -6 and 6 ms) was calculated for every unique MU pair for each contraction.

RESULTS

At high force levels, earlier recruited motor unit pairs (low-threshold) demonstrated relatively low levels of short-term synchronization (approximately 7.3% extra firings than would have been expected by chance). However, the magnitude of synchronization increased significantly and linearly with mean recruitment threshold (reaching 22.1% extra firings for motor unit pairs recruited above 70% MVC).

CONCLUSIONS

Three potential mechanisms that could explain the observed differences in synchronization across motor unit types are proposed and discussed.

Motor unit

Movement is accomplished by the controlled activation of motor unit populations. Our understanding of motor unit physiology has been derived from experimental work on the properties of single motor units and from computational studies that have integrated the experimental observations into the function of motor unit populations. The article provides brief descriptions of motor unit anatomy and muscle unit properties, with more substantial reviews of motoneuron properties, motor unit recruitment and rate modulation when humans perform voluntary contractions, and the function of an entire motor unit pool. The article emphasizes the advances in knowledge on the cellular and molecular mechanisms underlying the neuromodulation of motoneuron activity and attempts to explain the discharge characteristics of human motor units in terms of these principles. A major finding from this work has been the critical role of descending pathways from the brainstem in modulating the properties and activity of spinal motoneurons. Progress has been substantial, but significant gaps in knowledge remain.

Extra-torque of human tibialis anterior during electrical stimulation with linearly varying frequency and amplitude trains

This work aimed to characterise the whole human muscle input/output law during electrical stimulation with triangular varying frequency and amplitude trains through combined analysis of torque, mechanomyogram (MMG) and electromyogram (EMG). The tibialis anterior (TA) of ten subjects (age 23-35 years) was investigated during static contraction obtained through neuromuscular electrical stimulation. After potentiation, TA underwent two 15s stimulation patterns: (a) frequency triangle (FT): 2 > 35 > 2 Hz at Vmax (amplitude providing full motor unit recruitment); (b) amplitude triangle (AT): Vmin > Vmax > Vmin (Vmin providing TA least mechanical response) at 35 Hz. 2 > 35 Hz or Vmin > Vmax as well as 35 > 2 Hz or Vmax > Vmin were defined as up-going ramp (UGR) and down-going ramp (DGR), respectively. TA torque, MMG and EMG were detected by a load cell, an optical laser distance sensor and a probe with two silver bar electrodes, respectively. For both FT and AT, only the two mechanical signals resulted always larger in DGR than in UGR, during AT extra-torque and extra-MMG were present even in the first 1/3 of the amplitude range where EMG data presented no significant differences between DGR and UGR. Our data suggest that extra-torque and extra-displacement are evident for both FT and AT, being mainly attributed to an intrinsic muscle property.

Electromyogram and force fluctuation during different linearly varying isometric motor tasks

The purpose of this work was to verify if deviation from the mirror-like behaviour of the motor units activation strategy (MUAS) and de-activation strategy (MUDS) and the degree of the error of the motor control system, during consecutive linearly increasing-decreasing isometric tension tasks, depend on the maximum reached tension and/or on the rate of tension changes. In 12 male subjects the surface EMG and force produced by the first dorsal interosseus activity were recorded during two (a and b) trapezoid isometric contractions with different plateau (a: 50% maximal voluntary contraction (MVC) and b: 100% MVC) and rate of tension changes (a: 6.7% MVC/s and b: 13.3% MVC/s) during up-going (UGR) and down-going (DGR) ramps. Ten steps (ST) 6s long at 5, 10, 20, 30, 40, 50, 60, 70, 80 and 90% MVC were also recorded. The root mean square (RMS) and mean frequency (MF) from EMG and the relative error of actual force output with respect to the target (% ERR) were computed. The EMG-RMS/% MVC and EMG-MF/% MVC relationships were not overlapped when the ST and DGR as well as the UGR and DGR data were compared. The % ERR/% MVC relationships during a and b contractions differed from ST data only below 20% MVC. It can be concluded that MUAS and MUDS are not mirroring one each other because MU recruitment or de-recruitment threshold may be influenced by the maximum effort and by the % MVC/s of UGR and DGR. The role of MUs mechanical and/or central nervous system hysteresis on force decrement control is discussed.

Summation of motor unit forces in rat medial gastrocnemius muscle

The summation of contractile forces of motor units (MUs) was analyzed by comparing the recorded force during parallel stimulation of two and four individual MUs or four groups of MUs to the algebraic sum of their individual forces. Contractions of functionally-isolated single MUs of the medial gastrocnemius muscle were evoked by electrical stimulation of thin filaments of the split L5 or L4 ventral roots of spinal nerves. Additionally, contractions of large groups of MUs were evoked by stimuli delivered to four parts of the divided L5 ventral root. Single twitches, 40Hz unfused tetani, and 150Hz fused maximum tetani were recorded. In these experimental situations the summation was more effective for unfused tetani than for twitches or maximum tetani. The results obtained for pairs of MUs were highly variable (more- or less-than-linear summation), but coactivation of more units led to progressively weaker effects of summation, which were usually less-than-linear in comparison to the algebraic sums of the individual forces. The variability of the results highlights the importance of the structure of the muscle and the architecture of its MUs. Moreover, the simultaneous activity of fast and slow MUs was considerably more effective than that of two fast units.

Motor unit synchronization in FDI and biceps brachii muscles of strength-trained males

Motor unit (MU) synchronization is the simultaneous or near-simultaneous firing of two MUs which occurs more often than would be expected by chance. The present study sought to investigate the effects of exercise training, muscle group, and force level, by comparing the magnitude of synchronization in the biceps brachii (BB) and first dorsal interosseous (FDI) muscles of untrained and strength-trained college-aged males at two force levels, 30% of maximal voluntary contraction (MVC) and 80% MVC. MU action potentials were recorded directly via an intramuscular needle electrode. The magnitude of synchronization was assessed using previously-reported synchronization indices: k', E, and CIS. Synchronization was significantly higher in the FDI than in the BB. Greater synchronization was observed in the strength-trained group with CIS, but not with E or k'. Also, synchronization was significantly greater at 80% MVC than at 30% MVC with E, but only moderately greater with CIS and there was no force difference with k'. Synchronization prevalence was found to be greater in the BB (80.1%) than in the FDI (71.5%). Thus, although the evidence is a bit equivocal, it appears that MU synchronization is greater at higher forces, and greater in strength-trained individuals than in untrained subjects.

Determining the extent of neural activation during maximal effort

PURPOSE

The purpose of this study was to compare the extent of neural activation assessed by the central activation ratio (CAR) versus activation estimated from T2 magnetic resonance imaging (MRI) and neuromuscular electrical stimulation (NMES).

METHODS

Seven college-age individuals volunteered for this study. CAR was determined by manually superimposing a train of NMES (50 Hz, 450-mus biphasic pulses) for 1 s during a maximal voluntary effort. The MRI-NMES method assessed activation by stimulating the knee extensors for 3 min in a 2 s on, 2 s off cycle. T2 MR images were taken at rest and after NMES was administered. Theoretical maximal torque (TMT) of the knee extensors was calculated based on the MRI-NMES activation data. The TMT was then divided by the maximal voluntary isometric contraction (MVIC) of each subject to determine the extent of neural activation during a MVIC.

RESULTS

The results for CAR reveal the percent activation (mean +/- SD) of the quadriceps femoris during a MVIC was 92 +/- 7% for the right thigh and 96 +/- 4% for the left thigh. The MRI-NMES method estimated that MVIC could be achieved if 75 +/- 14% of the knee extensors on the right thigh and 74 +/- 14% on the left thigh were activated. These results are similar to findings that showed MVIC could be achieved by stimulating 71% of the knee extensors.

CONCLUSIONS

We conclude that CAR overestimates the extent of neural activation during an MVIC because the 3D shape of the thigh is altered. This will change electric current flow to the axonal motor neuron branches and limit the artificially evoked torque, thereby resulting in an overestimation of CAR.

Extra force from asynchronous stimulation of cat soleus muscle results from minimizing the stretch of the common elastic elements

Rack and Westbury showed that low-frequency asynchronous stimulation of a muscle produces greater force compared with synchronous stimulation. This study tested the hypothesis that the difference results from the dynamic stretch of the common elastic elements. In eight anesthetized cats, the soleus was attached to a servomechanism to control muscle length and record force. The ventral roots were divided into four bundles so each innervated approximately 1/4 of the soleus. The elasticity shared by each part of the muscle was estimated and the servomechanism programmed to compensate for its stretch. At each test frequency (5, 7.5, and 10 Hz), the muscle was stimulated by asynchronous stimulation, synchronous stimulation, summation of force with each part stimulated individually, and summation with each part stimulated individually and the servomechanism mimicking tendon stretch during asynchronous stimulation. Muscle length was isometric except for the last protocol. The observed differences were small. The greatest difference occurred during stimulation at 5 Hz with muscle length on the ascending limb of the length-tension curve. Here, the average forces, normalized by asynchronous force, were asynchronous, 100%; synchronous, 73%; summation, 110%; and summation with stretch compensation, 98%. The results support the hypothesis and suggest that the common elasticity can be used to predict force gains from asynchronous stimulation.

Neural Adaptations to Resistive Exercise

It is generally accepted that neural factors play an important role in muscle strength gains. This article reviews the neural adaptations in strength, with the goal of laying the foundations for practical applications in sports medicine and rehabilitation. An increase in muscular strength without noticeable hypertrophy is the first line of evidence for neural involvement in acquisition of muscular strength. The use of surface electromyographic (SEMG) techniques reveal that strength gains in the early phase of a training regimen are associated with an increase in the amplitude of SEMG activity. This has been interpreted as an increase in neural drive, which denotes the magnitude of efferent neural output from the CNS to active muscle fibres. However, SEMG activity is a global measure of muscle activity. Underlying alterations in SEMG activity are changes in motor unit firing patterns as measured by indwelling (wire or needle) electrodes. Some studies have reported a transient increase in motor unit firing rate. Training-related increases in the rate of tension development have also been linked with an increased probability of doublet firing in individual motor units. A doublet is a very short interspike interval in a motor unit train, and usually occurs at the onset of a muscular contraction. Motor unit synchronisation is another possible mechanism for increases in muscle strength, but has yet to be definitely demonstrated. There are several lines of evidence for central control of training-related adaptation to resistive exercise. Mental practice using imagined contractions has been shown to increase the excitability of the cortical areas involved in movement and motion planning. However, training using imagined contractions is unlikely to be as effective as physical training, and it may be more applicable to rehabilitation. Retention of strength gains after dissipation of physiological effects demonstrates a strong practice effect. Bilateral contractions are associated with lower SEMG and strength compared with unilateral contractions of the same muscle group. SEMG magnitude is lower for eccentric contractions than for concentric contractions. However, resistive training can reverse these trends. The last line of evidence presented involves the notion that unilateral resistive exercise of a specific limb will also result in training effects in the unexercised contralateral limb (cross-transfer or cross-education). Peripheral involvement in training-related strength increases is much more uncertain. Changes in the sensory receptors (i.e. Golgi tendon organs) may lead to disinhibition and an increased expression of muscular force. Agonist muscle activity results in limb movement in the desired direction, while antagonist activity opposes that motion. Both decreases and increases in co-activation of the antagonist have been demonstrated. A reduction in antagonist co-activation would allow increased expression of agonist muscle force, while an increase in antagonist co-activation is important for maintaining the integrity of the joint. Thus far, it is not clear what the CNS will optimise: force production or joint integrity. The following recommendations are made by the authors based on the existing literature. Motor learning theory and imagined contractions should be incorporated into strength-training practice. Static contractions at greater muscle lengths will transfer across more joint angles. Submaximal eccentric contractions should be used when there are issues of muscle pain, detraining or limb immobilisation. The reversal of antagonists (antagonist-to-agonist) proprioceptive neuromuscular facilitation contraction pattern would be useful to increase the rate of tension development in older adults, thus serving as an important prophylactic in preventing falls. When evaluating the neural changes induced by strength training using EMG recording, antagonist EMG activity should always be measured and evaluated.

Aging, resistance training, and motor unit discharge behavior

Researchers have alluded the existence of "neural factors" in the expression and development of muscular strength. Candidate neural factors including motor unit recruitment, rate coding, double firing, and motor unit synchronization are discussed in this review. Aging is generally accompanied by lower motor unit discharge rates. However, both young and older adults exhibit rapid changes in muscular strength with repeated strength testing. These strength changes occur with concomitant albeit transient increase in motor unit discharge rate. These and other neural factors may contribute to the initial increase in muscular strength observed during the early phases of resistance exercise training.

Summation of Motor Unit Force in Passive and Active Muscle

Nonlinear summation of force has been observed between motor units. The complex structure of muscle suggests many reasons why this could happen. When large portions of the muscle are active, however, the nonlinearities are small, and generally explained by stretch of the common elasticity. This suggests that the type of nonlinearity observed between motor units may not be physiologically significant.

Intraneural microstimulation of motor axons in the study of human single motor units

Single motor unit activity has been studied in depth since the first intramuscular electrodes were developed more than 70 years ago. Many techniques have been combined or used in isolation since then. Intraneural motor axon microstimulation allows the detailed study of single motor units in awake human subjects in a manner most analogous to that used in reduced animal preparations. A microelectrode, inserted percutaneously into a peripheral nerve, stimulates the axon of a single alpha-motoneuron at a site remote from the contracting muscle, allowing detailed analyses of the contractile properties of a single motor unit in an otherwise quiescent muscle, that is, without interference of simultaneously active motor units or the presence of an electrode within the muscle. The methods and results obtained using this technique are described and compared to those of other studies of single motor units in human subjects. Differences have been found between human and animal motor units and between motor units of various muscles. Studying human and animal motor units using an analogous technique provides insight into the interpretation of human data when results differ from animal data, and when human motor units cannot be examined in the same way, or at a similar level of detail, as animal motor units.

Multiple features of motor-unit activity influence force fluctuations during isometric contractions

To identify the mechanisms responsible for the fluctuations in force that occur during voluntary contractions, experimental measurements were compared with simulated forces in the time and frequency domains at contraction intensities that ranged from 2 to 98% of the maximum voluntary contraction (MVC). The abduction force exerted by the index finger due to an isometric contraction of the first dorsal interosseus muscle was measured in 10 young adults. Force was simulated with computer models of motor-unit recruitment and rate coding for a population of 120 motor units. The models varied recruitment and rate-coding properties of the motor units and the activation pattern of the motor-unit population. The main finding was that the experimental observations of a minimum in the coefficient of variation (CV) for force (1.7%) at approximately 30% MVC and a plateau at higher forces could not be replicated by any of the models. The model that increased the level of short-term synchrony with excitatory drive provided the closest fit to the experimentally observed relation between the CV for force and the mean force. In addition, the results for the synchronization model extended previous modeling efforts to show that the effect of synchronization is independent from that of discharge-rate variability. Most of the power in the force power spectra for the models was contained in the frequency bins below 5 Hz. Only a model that included a low-frequency oscillation in excitation, however, could approximate the experimental finding of peak power at a frequency below 2 Hz: 38% of total power at 0.99 Hz and 43% at 1.37 Hz, respectively. In contrast to the experimental power spectra, all model spectra included a second peak at a higher frequency. The secondary peak was less prominent in the synchronization model because of greater variability in discharge rate. These results indicate that the variation in force fluctuations across the entire operating range of the muscle cannot be explained by a single mechanism that influences the output of the motor-unit population.

Factors affecting the stability of the spike-triggered averaged force in the human first dorsal interosseus muscle

The reproducibility of motor unit twitches obtained using spike-triggered averaging (STA) was examined in the human first dorsal interosseus. For each motor unit (30 total) a series of STA twitches was derived using a 30 s averaging window. Within each averaging window, eight independent measures characterizing motor unit discharge and whole muscle force properties were recorded. These included the mean and standard deviation (S.D.) of the interspike interval (ISI), the mean and S.D. of pre and post-trigger ISIs used in averaging, and mean and S.D. of whole muscle force. To determine the relative importance of the independent variables on twitch reproducibility, the variables were used in a multiple regression analysis performed on STA twitch peak force (PF), time to peak force (TTP) and time of half-force decay (HFD). It was found that PF was significantly correlated to the mean and S.D. of whole muscle force, and mean post-trigger ISI. TTP was significantly correlated to the S.D. of the post-trigger ISI and mean whole muscle force while HFD was related to the mean and S.D. of the pre-trigger ISI and the mean post-trigger ISI. It was concluded that by minimizing whole muscle force variability and the mean and S.D. of acceptable ISIs used in the STA process, the reproducibility of the STA twitch is improved.

Nonlinear summation of force in cat tibialis anterior: a muscle with intrafascicularly terminating fibers

The complex connective tissue structure of muscle and tendon suggests that forces from two parts of a muscle may not summate linearly, particularly in muscles with intrafasciculary terminating fibers, such as cat tibialis anterior (TA). In four anesthetized cats, the TA was attached to a servomechanism to control muscle length and record force. The ventral roots were divided into two bundles, each innervating about half the TA, so the two parts could be stimulated alone or together. Nonlinear summation of force (F(nl)) was measured during isometric contractions. F(nl) was small and negative, indicating less than linear summation of the parts, which is consistent with the predicted F(nl) of muscle fibers connected in series. F(nl) was more significant when smaller parts of the muscle were tested (21.8 vs. 8% for whole muscle). These data were fit to a model where both parts of the muscle were assumed to stretch a common elasticity. Compensatory movements of the servomechanism showed the common elasticity is very stiff, and the model cannot account for F(nl) in cat TA.

Intramuscular force transmission

The architectural form of skeletal muscle, the pattern of activity/usage between neighbouring fibres, and the pathways for lateral and lengthwise tension delivery are all of interest in understanding muscle function and dysfunction. We have attempted to contribute to understanding of intramuscular force transmission by investigating the functional relationships between coactive motor units, and by examining the detailed molecular and morphological features at sites of tension transfer. We found that tension delivery is modulated by interaction between active and inactive fibres, that many muscle fibre terminations feature structural coupling between fibres, and that sites of tension delivery feature a variety of proteins including acetylcholinesterase, NCAM, dystrophin and two splice variants of the alpha7 integrins. We conclude that structural and molecular pathways exist to deliver force within, along, and between muscle fibres, and that the quality/quantity of tension delivered from any single fibre is at least partly a consequence of whether its neighbouring fibres are synchronously coactive.

Summation of forces from multiple motor units in the cat soleus muscle

Nearly all muscle models and most motor control concepts assume that forces from individual muscle fibers and motor units sum in an additive manner once effects of in-series tendon compliance are taken into account. Due to the numerous mechanical linkages between individual fibers, though, it is unclear whether this assumption is warranted. This work examined motor unit force summation over a wide range of muscle forces in the cat soleus. Nonadditive summation implies a nonlinear summation of motor unit forces. Summation nonlinearities were quantified during interactions of 10 individual motor units and 4 motor unit bundles containing approximately 10 units each. These protocols allowed motor unit force summation to be examined from approximately 0 to 25% of tetanic muscle force. Nonlinear summation was assessed by comparing the actual forces to the algebraic sum of individual units and bundles stimulated in isolation. Superadditive summation meant that the actual force exceeded the algebraic sum, whereas subadditive summation meant that the actual force was smaller than the algebraic sum. Experiments tested the hypothesis that superadditive summation occurs at low force levels when few motor units are recruited, whereas subadditive summation prevails above 10% of tetanic force. Results were consistent with this hypothesis. As in previous studies, nonlinear summation in the soleus was modest, but a clear transition from predominately superadditive to predominantly subadditive summation occurred in the range of 6-8% of tetanic force. The largest nonlinearities were transient and appeared at the onset of recruitment and derecruitment of groups of motor units. The results are discussed in terms of the mechanical properties of the connective tissue forming the tendon and linking muscle fibers.

Motor-unit synchronization alters spike-triggered average force in simulated contractions

The purpose of the study was to quantify the effect of motor-unit synchronization on the spike-triggered average forces of a population of motor units. Muscle force was simulated by defining mechanical and activation characteristics of the motor units, specifying motor neuron discharge times, and imposing various levels of motor-unit synchronization. The model comprised 120 motor units. Simulations were performed for motor units 5-120 to compare the spike-triggered average responses in the presence and absence of motor-unit synchronization with the motor-unit twitch characteristics defined in the model. To synchronize motor-unit activity, selected motor-unit discharge times were adjusted; this kept the number of action potentials constant across the three levels of synchrony for each motor unit. Because there was some overlap of motor-unit twitches even at minimal discharge rates, the simulations indicated that spike-triggered averaging underestimates the twitch force of all motor units and the contraction time of motor units with contraction times longer than 49 ms. Although motor-unit synchronization increased the estimated twitch force and decreased the estimated contraction time of all motor units, spike-triggered average force changed systematically with the level of synchrony in motor units 59-120 (upper 90% of the range of twitch forces). However, the reduction in contraction time was similar for moderate and high synchrony. In conclusion, spike-triggered averaging appears to provide a biased estimate of the distribution of twitch properties for a population of motor units because twitch fusion causes an underestimation of twitch force for slow units and motor-unit synchronization causes an overestimation of force for fast motor units.

Modeling variability of force during isometric contractions of the quadriceps femoris

The authors modeled variability of force during continuous isometric contractions of the quadriceps femoris. Twenty adults (aged 25 +/- 6 years old) performed isometric leg extensions. Target forces were 11 percentages of maximum voluntary contraction (%MVC), ranging from 2 to 95%, and 5 absolute levels, from 25 to 225 N. The authors used standard deviation of absolute force, coefficient of variation, and signal-to-noise ratio as measures of variability. The results suggested a nonlinear relationship between variability and level of force, which could best be expressed as %MVC and not as absolute force. Variability for continuous isometric contractions was described best by a sigmoidal logistic function. The sigmoidal pattern of variability as a function of %MVC is consistent with physiological mechanisms.

Spinal and supraspinal factors in human muscle fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.

Contractile properties of human motor units in health, aging, and disease

The primary function of skeletal muscle is to produce force for postural control and movement. Although the contractile properties of the whole muscle are useful functional indicators, they do not accurately reflect the heterogeneity of the constituent motor units (MUs) and their changes in health and disease. However, data on the contractile properties of human MUs, in comparison to other animal species, are relatively sparse. This, in part, is due to greater methodological challenges of in vivo studies of MUs in the human. The purpose of this review is to critically appraise the methods used in humans; to describe the normative data from different muscle groups; to discuss differences between data from healthy humans and other animal species; and, last, to characterize changes of the MU contractile properties in aging, disease, and in response to intervention. Because the spike-triggered averaging technique can only be used to study the twitch properties, other methods were subsequently developed to measure a wider range of contractile properties. Although there is general agreement between human data and those from other animal species, major differences do exist. Potential reasons for these discrepancies include true biological differences, but differences in the techniques used may also be responsible. Although limited, measurement of MU contractile properties in humans has provided insight into the changes associated with aging and motoneuronal diseases and provides a means of gauging their adaptive capacity for training and immobilization.

Role of motor unit structure in defining function

Motor units, defined as a motoneuron and all of its associated muscle fibers, are the basic functional units of skeletal muscle. Their activity represents the final output of the central nervous system, and their role in motor control has been widely studied. However, there has been relatively little work focused on the mechanical significance of recruiting variable numbers of motor units during different motor tasks. This review focuses on factors ranging from molecular to macroanatomical components that influence the mechanical output of a motor unit in the context of the whole muscle. These factors range from the mechanical properties of different muscle fiber types to the unique morphology of the muscle fibers constituting a motor unit of a given type and to the arrangement of those motor unit fibers in three dimensions within the muscle. We suggest that as a result of the integration of multiple levels of structural and physiological levels of organization, unique mechanical properties of motor units are likely to emerge.

Nonlinear summation of force in cat soleus muscle results primarily from stretch of the common-elastic elements

The complex connective tissue structure of muscle and tendon suggests that forces from two parts of a muscle may not summate linearly. This study measured the nonlinear summation of force (F(nl)) in whole cat soleus during isometric and ramp movements. In six anesthetized cats, the soleus was attached to a servomechanism to control muscle length and record force. The ventral roots were divided into two bundles, each innervating about half the soleus; thus the two parts could be stimulated alone or together. In all experiments, F(nl) was small (<6% of maximum tetanic tension). Peak F(nl) occurred during changes in muscle force, either as a result of imposed muscle movement or the onset or offset of a stimulus train. The data were fit to a model in which both parts of the muscle were assumed to stretch to a common elasticity. The servomechanism was programmed to compensate for reduced stretch of the common elasticity during partial compared with whole muscle activation. These compensatory movements showed how the model could account for most, but not all, of F(nl).

Examination of intrafascicular muscle fiber terminations: implications for tension delivery in series-fibered muscles

Mammalian skeletal muscles with long fascicle lengths are predominantly composed of short muscle fibers that terminate midbelly with no direct connection to the muscle origin or insertion. The manner in which these short fibers terminate and transmit tension through the muscle to their tendons is poorly understood. We made an extensive morphological study of a series-fibered muscle, the guinea pig sternomastoid, in order to define the full range of structural specializations for tension transmission from short fibers within this muscle. Terminations were examined in single fibers, teased small bundles of fibers, and in sections at both the light and electron microscopic level. In many cases, sites of fiber termination were defined by reactivity for the enzyme acetylcholinesterase, which also marks myotendinous junctions. Additionally, transport of the lipophilic fluorescent dye, DiI, or injection of Lucifer Yellow were used to visualize undisturbed fiber terminations in whole muscles using confocal and fluorescence microscopy. At the light microscopic level, we find that intrafascicularly terminating fibers end about equally often in either a long progressive taper, or in a series of small or larger blunt steps. Combinations of these two morphologies are also seen. However, when analyzed at higher resolution with confocal or electron microscopy, the apparently smooth progressive tapers appear also to be predominantly composed of a series of fine stepped terminations. Stepwise terminations in most cases join face-to-face with complementary endings of neighboring muscle fibers, some via an extended collagenous bridge and others at close interdigitating myomyonal junctions. These muscle-to-muscle junctions show many of the features of myotendinous junctions, including dense subsarcolemmal plaques in regions of myofibrillar termination and we suggest that they serve to pass tension from fiber to fiber along the longitudinal axis of the muscle. In addition, we observe regions of apparent side-to-side adhesion between neighboring fibers at sites where there is no apparent fiber tapering or structural specialization typical of myofibril termination. These sites show acetylcholinesterase reactivity, and large numbers of collagen fibers passing laterally from fiber to fiber. These latter connections seem most likely to be involved in lateral transmission of tension, either from fiber to fiber, or from fiber to endomysium. Overall, our results suggest that tension from intrafascicularly terminating fibers is likely to be passed along the muscle to the tendon using both in-series and in-parallel arrangements. The results are discussed in light of current theories of tension delivery within the series-fibered muscles typical of large, nonprimate mammals.

Localization and contractile properties of intrinsic longitudinal motor units of the rat tongue

Tongue dysfunction is a hallmark of many human clinical disorders, yet we lack even a rudimentary understanding of tongue neural control. Here, the location and contractile properties of intrinsic longitudinal motor units (MUs) of the rat tongue body are described to provide a foundation for developing and testing theories of tongue motor control. One hundred and sixty-five MUs were studied by microelectrode penetration and stimulation of individual motor axons coursing in the terminal portion of the lateral (retrusor) branch of the hypoglossal nerve in the rat. Uniaxial MU force was recorded by a transducer attached to the protruded tongue tip, and MU location was estimated by electromyographic (EMG) electrodes implanted into the anterior, middle, and posterior portions of the tongue body. All MUs produced retrusive force. MU twitch force ranged from 2-129 mg (mean = 35 mg) and tetanic force ranged from 9-394 mg (mean = 95 mg). MUs reached maximal twitch force in 8-33 ms (mean = 15 ms) and were resistant to fatigue; following 2 min of stimulation, MUs (n = 11) produced 78-131% of initial force. EMG data were collected for 105 MUs. For 65 of these MUs, the EMG response was confined to a single electrode location: for 26 MUs to the anterior, 21 MUs to the middle, and 18 MUs to the posterior portion of the tongue. Of the remaining MUs, EMG responses were observed in two (38/40) or all three (2/40) tongue regions. These data provide the first contractile measures of identified intrinsic tongue body MUs and the first evidence that intrinsic longitudinal MUs are restricted to a portion of tongue length. Localization of MU territory suggests a role for intrinsic MU in the regional control of the mammalian tongue observed during feeding and speech.

Motor-unit synchronization increases EMG amplitude and decreases force steadiness of simulated contractions

The purpose of the study was to determine the effect of motor-unit synchronization on the surface electromyogram (EMG) and isometric force using a computer model of muscle contraction. The EMG and force were simulated by generating muscle fiber action potentials, defining motor-unit mechanical characteristics and territories, estimating motor-unit action potentials, specifying motor-unit discharge times, and imposing various levels of motor-unit synchronization. The output (EMG and force) was simulated at 11 levels of excitation, ranging from 5 to 100% of maximum. To synchronize motor-unit activity, selected motor-unit discharge times were adjusted; however, the number of motor units recruited and the average discharge rate of each unit was constant across synchronization conditions for a given level of excitation. Two levels of synchronization were imposed on the discharge times: a moderate and a high level, which approximated the experimentally observed range of motor-unit synchronization. The moderate level of synchrony caused the average EMG to increase by approximately 65%, whereas the high level caused a 130% increase in the EMG with respect to the no-synchrony condition. Neither synchrony condition influenced the magnitude of the average force. However, motor-unit synchronization did increase the amplitude of the fluctuations in the simulated force, especially at intermediate levels of excitation. In conclusion, motor-unit synchronization increased the amplitude of the average rectified EMG and decreased the steadiness of the force exerted by the muscle in simulated contractions.

Transmission of forces within mammalian skeletal muscles

Most models of in vivo musculoskeletal function fail to take into account the diversity of force trajectories defined by muscle fiber architecture. It has been shown for many muscles, across species, that muscle fibers commonly end within muscle fascicles without reaching a myotendinous junction, and that many of these fibers show a progressive decline in cross-sectional area along the length of the muscle. The significance of these anatomical observations is that the tapering would seem to preclude forces generated at the largest cross-sectional area of the fibers being transmitted to the sarcomeres toward the ends of the tapered fiber. If all of the forces are transmitted via the sarcomeres arranged in series, those few sarcomeres at the smaller ends of the fibers must tolerate the stress exerted by the more numerous sarcomeres arranged in parallel at the portions of the fiber with larger cross-sectional areas. A logical alternative would be for forces to be transmitted laterally along the length of a fiber to the cell membrane and the extracellular matrix. Such a structural arrangement would permit an alternative force transmission vector and minimize the necessity for a precise level of force to be generated along the entire length of a fiber. There are cytoarchitectural and biochemical data demonstrating the presence of a subcellular network which is appropriately located to transmit forces from the active intracellular contractile elements to the extracellular intramuscular connective tissues. However, to fully comprehend how forces are transmitted from individual cross bridges to the tendon, it will be necessary to understand the interactions of all of the components of the muscle tendon complex from the molecular to the multicellular level. It is insufficient to know the physiology of the individual components in a restricted experimental paradigm and assume that these conditions account for the functional characteristics in vivo. Thus, the challenge is to understand how the sarcomeres and all of the associated structures transmit the forces of the whole muscle to its attachments.

Nonlinear tension summation of different combinations of motor units in the anesthetized cat peroneus longus muscle

The purpose of this study was to examine the linearity of summation of the forces produced by the stimulation of different combinations of type identified motor units (MUs) in the cat peroneus longus muscle (PL) under isometric conditions. The muscle was fixed at its twitch optimal length, and the tension produced by the single MU was recorded during 24- and 72-Hz stimulation. The summation analysis was first carried out for MUs belonging to the same functional group, and then different combinations of fast fatigable (FF) MUs were added to the nonfatigable slow (S) and fatigue resistant (FR) group. The tension resulting from the combined stimulation of increasing numbers of MUs (measured tension) was evaluated and compared with the linearly predicted value, calculated by adding algebraically the tension produced by the individual MUs assembled in the combination (calculated tension). Tension summation displayed deviations from linearity. S and FR MUs mainly showed marked more than linear summation; FF MUs yielded either more or less than linear summation; and, when the FF units were recruited after the S and FR MUs, less than linear summation always occurred. The magnitude of the nonlinear summation appeared stimulus frequency dependent for the fatigable FF and FI group. The relationship between measured tension and calculated tension for each MU combination was examined, and linear regression lines were fitted to each set of data. The high correlation coefficients and the different slope values for the different MU-type combinations suggested that the nonlinear summation was MU-type specific. The mechanisms of nonlinear summations are discussed by considering the consequences of internal shortening and thus the mechanical interactions among MUs and shifts in muscle fiber length to a more or less advantageous portion of single MU length-tension curves.

Independence of motor unit recruitment and rate modulation during precision force control

The vertebrate motor system chiefly employs motor unit recruitment and rate coding to modulate muscle force output. In this paper, we studied how the recruitment of new motor units altered the firing rate of already-active motor units during precision force production in the first dorsal interosseous muscle. Six healthy adults performed linearly increasing isometric voluntary contractions while motor unit activity and force output were recorded. After motor unit discharges were identified, motor unit firing rates were calculated before and after the instances of new motor unit recruitment. Three procedures were applied to compute motor unit firing rate, including the mean of a fixed number of inter-spike intervals and the constant width weighted Hanning window filter method, as well as a modified boxcar technique. In contrast to previous reports, the analysis of the firing rates of over 200 motor units revealed that reduction of the active firing rates was not a common mechanism used to accommodate the twitch force produced by the recruitment of a new motor unit. Similarly, during de-recruitment there was no tendency for motor unit firing rates to increase immediately following the cessation of activity in other motor units. Considerable consistency in recruitment behavior was observed during repeated contractions. However, firing rates during repeated contractions demonstrated considerably more fluctuation. It is concluded that the neuromuscular system does not use short-term preferential motor unit disfacilitation to effect precise regulation of muscular force output.

Contractile Properties of Human Motor Units: Is Man a Gat?

A major goal in neuroscience is to understand how the CNS controls posture and movement in humans. This requires an understanding of individual human motor unit properties and how they interact within the muscle to perform different tasks. This article describes differences and similarities between the contractile properties of human motor units and those of the cat prototype medial gastrocnemius (MG) muscle, on which so many studies have been conducted. The article describes the methods available for measuring human motor unit properties and their limitations, and it discusses how far the behavior of whole muscles can be predicted from their histochemistry. It questions the extent to which human motor units conform to the conventional criteria by which S (slow, fatigue resistant), FR (fast but fatigue resistant) and FF (fast, fatigable) unit types are usually classified. An important difference between human and cat MG data is that weak human motor units are not necessarily slow, nor strong ones fast; that is, generally, human unit force is not correlated with contractile speed. Also, unlike cat MG, the few human muscles studied so far contain few if any FF units but a high proportion of units with intermediate fatigue resistance (Flnt). These apparently aberrant human properties, however, are also found in other cat and rat muscles. Thus, cat MG may not be the best model for motor unit behavior generally. Finally, the influence of human motor unit properties on force output by recruitment and/or rate coding is discussed.

Resistance training and human cervical muscle recruitment plasticity

This study examined cervical neuromuscular adaptations to resistance training. The ResX group performed conventional resistance training plus head-extension exercise. Another group performed only conventional resistance training, and the control group performed no resistance exercise. Muscle use during head extension was determined by quantifying shifts in T2 in serial-transaxial magnetic resonance images of the neck. ResX was the only group that showed a training effect. Training decreased (P < 0.05) the cross-sectional area (CSA) of cervical muscle used to perform submaximal head extension by 31%. This reflected a decrease (P < 0.05) in relative use of the splenius capitis, semispinalis capitis, and semispinalis cervicis and multifidus muscles by about one-third; their percentage of CSA showing contrast shift was reduced from 60 to 40% on average. This same exercise evoked no contrast shift in the levator scapulae, longissimus capitis and cervicis, and scalenus medius and anterior muscles posttraining, yet 20% or more of their CSA was engaged pretraining. The relative CSA of cervical musculature that was used to perform maximal head extension was increased (P < 0.05) 16% by training. The findings suggest functional redundancy of neck musculature that can be modified by training; submaximal tasks can be performed despite cessation of recruitment of individual muscles, yet recruitment can be increased for maximal efforts. These results also suggest that neuromuscular adaptations to training require a specific cervical exercise.

Summation of extraocular motor unit tensions in the lateral rectus muscle of the cat

Contractile measures on 67 single muscle units in the cat lateral rectus muscle were made in response to motoneuron stimulation. Simultaneous activation of four to five additional units, using muscle nerve stimulation, allowed an examination of unit force summation. Linear force addition was found in 73% of the units, while 25% added only about half of their twitch force to the twitch force of the nerve-activated units. "Nonadditive" units had significantly weaker twitch tensions than the units which added linearly. Lengthening or shortening the whole muscle, from maximal isometric settings, reduced whole muscle twitch tension as well as muscle unit tension. Injury to the lateral rectus muscle did not significantly alter whole muscle tension. These findings suggest that the known serial and branching arrangement of these muscle fibers, as well as the complex interfiber matrix, may help explain the force reduction in some muscle units and the whole muscle's resistance to insult.

Effects of asynchronous stimulation on the human quadriceps femoris muscle

OBJECTIVE

Examine the effects of asynchronously activating motor units of the human quadriceps femoris muscle during electrical stimulation.

METHODS

Two stimulation channels were used to activate the muscle. One pair of electrodes was placed over the vastus medialis (Channel 1) and another pair was placed over the vastus lateralis (Channel 2). All contractions were isometric. The stimulation frequency was 10 Hz for each channel and the effect of varying the delay between the two channels was tested.

RESULTS

Synchronous activation of both channels produced the greatest forces and stimulating the muscle 180 degrees out of phase (ie, with a 50-msec delay) produced the least forces.

CONCLUSIONS

Activating motor units asynchronously under certain conditions will be less efficient than synchronous activation. This study, therefore, helps to define the boundary conditions when asynchronous stimulation may be helpful.