Agonist-antagonist Co-activation Research Articles

Is the rate of force development scaling factor associated with the rate of agonist muscle activation scaling factor and the level of agonist-antagonist coactivation?

BACKGROUND: Rate of force development scaling factor (RFD-SF) is used to assess neuromuscular quickness across submaximal levels. It has been suggested that such metric would be associated with the magnitude of the agonist muscle activation and with the level of simultaneous antagonist muscle group activation, i.e., agonist-antagonist coactivation. AIM: To examine the associations between 1) the RFD-SF and the rate of agonist muscle activation scaling factor (RAgMA-SF) and 2) the RFD-SF and the agonist-antagonist coactivation. METHOD: Eleven healthy and physically active subjects (7 men and 4 women, aged between 18 and 22 years) performed maximal strength tests of the knee extensors and flexors and a test that involved the production of brief submaximal force pulses of the knee extensors. The force exerted was recorded by a load cell and the electromyographic activities of the vastus lateralis and biceps femoris muscles were obtained during the tests. In the brief force pulses test, individuals were asked to produce around 120 submaximal force pulses that ranged between 20 to 80% of their maximum force value, while trying to produce each force pulse as fast as possible. The values of RFD-SF, RAgMA-SF of the vastus lateralis muscle, and a coactivation index (ICOA) were computed. RESULTS: The RFD-SF is strongly associated with the RAgMA-SF (r=0.74; p=0.009) but not with the ICOA (r=0.29; p=0.38). CONCLUSION: The RFD-SF is directly associated with the rate of agonist muscle activation and is not affected by the degree of coactivation between agonist and antagonist muscles.

Training History-Dependent Functional Role of EMG Model-Predicted Antagonist Moments in Knee Extensor Moment Generation in Healthy Young Adults

Resistance training (RT) improves the skeletal muscle’s ability to generate maximal voluntary force and is accompanied by changes in the activation of the antagonist muscle which is not targeted primarily by RT. However, the nature and role of neural adaptation to RT in the antagonist muscle is paradoxical and not well understood. We compared moments, agonist muscle activation, antagonist activation, agonist-antagonist coactivation, and electromyographic (EMG) model-predicted moments generated by antagonist hamstring muscle coactivation during isokinetic knee extension in leg strength-trained (n = 10) and untrained (n = 11) healthy, younger adults. Trained vs. untrained adults were up to 58% stronger. During knee extension, hamstring activation was 1.6-fold greater in trained vs. untrained adults (p = 0.022). This hamstring activation produced 2.6-fold greater model-predicted antagonist moments during knee extension in the trained (42.7 ± 19.55 Nm) vs. untrained group (16.4 ± 12.18 Nm; p = 0.004), which counteracted (reduced) quadriceps knee extensor moments ~43 Nm (0.54 Nm·kg−1) and by ~16 Nm (0.25 Nm·kg−1) in trained vs. untrained. Antagonist hamstring coactivation correlated with decreases and increases, respectively, in quadriceps moments in trained and untrained. The EMG model-predicted antagonist moments revealed training history-dependent functional roles in knee extensor moment generation.

Laws of nature that define biological action and perception.

We describe a physical approach to biological functions, with the emphasis on the motor and sensory functions. The approach assumes the existence of biology-specific laws of nature uniting salient physical variables and parameters. In contrast to movements in inanimate nature, actions are produced by changes in parameters of the corresponding laws of nature. For movements, parameters are associated with spatial referent coordinates (RCs) for the effectors. Stability of motor actions is ensured by the abundant mapping of RCs across hierarchical control levels. The sensory function is viewed as based on an interaction of efferent and afferent signals leading to an iso-perceptual manifold where percepts of salient sensory variables are stable. This approach offers novel interpretations for a variety of known neurophysiological and behavioral phenomena and makes a number of novel testable predictions. In particular, we discuss novel interpretations for the well-known phenomena of agonist-antagonist co-activation and vibration-induced illusions of both position and force. We also interpret results of several new experiments with unintentional force changes and with analysis of accuracy of perception of variables produced by elements of multi-element systems. Recently, this approach has been expanded to interpret motor disorders including spasticity and consequences of subcortical disorders (such as Parkinson's disease). We suggest that the approach can be developed for cognitive functions.

Estimation of Muscle Co-Activations in Wrist Rehabilitation After Stroke is Sensitive to Motor Unit Distribution and Action Potential Shapes.

We evaluated different muscle excitation estimation techniques, and their sensitivity to Motor Unit (MU) distribution in muscle tissue. For this purpose, the Convolution Kernel Compensation (CKC) method was used to identify the MU spike trains from High-Density ElectroMyoGrams (HDEMG). Afterwards, Cumulative MU Spike Train (CST) was calculated by summing up the identified MU spike trains. Muscle excitation estimation from CST was compared to the recently introduced Cumulative Motor Unit Activity Index (CAI) and classically used Root-Mean-Square (RMS) amplitude envelop of EMG. To emphasize their dependence on the MU distribution further, all three muscle excitation estimates were used to calculate the agonist-antagonist co-activation index. We showed on synthetic HDEMG that RMS envelopes are the most sensitive to MU distribution (10 % dispersion around the real value), followed by the CST (7 % dispersion) and CAI (5 % dispersion). In experimental HDEMG from wrist extensors and flexors of post-stroke subjects, RMS envelopes yielded significantly smaller excitations of antagonistic muscles than CST and CAI. As a result, RMS-based co-activation estimates differed significantly from the ones produced by CST and CAI, illuminating the problem of large diversity of muscle excitation estimates when multiple muscles are studied in pathological conditions. Similar results were also observed in experimental HDEMG of six intact young males.

Agonist-Antagonist Coactivation Enhances Corticomotor Excitability of Ankle Muscles.

Spinal pathways underlying reciprocal flexion-extension contractions have been well characterized, but the extent to which cortically evoked motor-evoked potentials (MEPs) are influenced by antagonist muscle activation remains unclear. A majority of studies using transcranial magnetic stimulation- (TMS-) evoked MEPs to evaluate the excitability of the corticospinal pathway focus on upper extremity muscles. Due to functional and neural control differences between lower and upper limb muscles, there is a need to evaluate methodological factors influencing TMS-evoked MEPs specifically in lower limb musculature. If and to what extent the activation of the nontargeted muscles, such as antagonists, affects TMS-evoked MEPs is poorly understood, and such gaps in our knowledge may limit the rigor and reproducibility of TMS studies. Here, we evaluated the effect of the activation state of the antagonist muscle on TMS-evoked MEPs obtained from the target (agonist) ankle muscle for both tibialis anterior (TA) and soleus muscles. Fourteen able-bodied participants (11 females, age: 26.1 ± 4.1 years) completed one experimental session; data from 12 individuals were included in the analysis. TMS was delivered during 4 conditions: rest, TA activated, soleus activated, and TA and soleus coactivation. Three pairwise comparisons were made for MEP amplitude and coefficient of variability (CV): rest versus coactivation, rest versus antagonist activation, and agonist activation versus coactivation. We demonstrated that agonist-antagonist coactivation enhanced MEP amplitude and reduced MEP CVs for both TA and soleus muscles. Our results provide methodological considerations for future TMS studies and pave the way for future exploration of coactivation-dependent modulation of corticomotor excitability in pathological cohorts such as stroke or spinal cord injury.

Torque steadiness and neuromuscular responses following fatiguing concentric exercise of the knee extensor and flexor muscles in young and older individuals

The purpose of this study was to investigate the age-related alterations in the ability to exert maximal and to sustain submaximal isometric muscle torques after a fatiguing concentric exercise conducted with knee extensor (KE) and flexor (KF) muscles. Sixteen young (aged 19-30 years; 8 women) and 17 older (aged 65-75 years; 9 women) volunteers participated. The following tasks were performed before and immediately after 22 maximal concentric efforts of the right KE and KF at 1.05 rad/s: (1) a maximal voluntary isometric contraction (MVIC) task involving both KE and KF; and (2) a KE torque-steadiness task at a submaximal target contraction intensity (20% MVIC). During the dynamometric tests, surface EMG was recorded simultaneously from the KE and KF muscles. Fatigue-induced reductions in knee extension MVIC were similar (~15%) between groups, but young participants showed more pronounced declines in agonist (i.e. quadriceps) EMG responses in both time (RMS amplitude; ~15% vs. ~10%, p < 0.001) and frequency (median frequency; ~14% vs. ~8%, p < 0.01) domains. Torque steadiness exhibited a similar post-fatigue decrease in the two age groups (p < 0.01), but interestingly agonist activation (~17%; p < 0.001) and antagonist (i.e. hamstrings) co-activation (~16%; p < 0.001) declined only in the older participants. These findings suggest that the fatiguing concentric KE and KF exercise results in similar relative reductions (%) in maximal torque and steadiness of the KE in young and older individuals, but they are sustained by different age-related neuromuscular strategies.

Changes in knee joint angle affect torque steadiness differently in young and older individuals

PurposeThis study investigated the effect of knee joint angle and age on torque steadiness of knee extensors (KE) at varying submaximal isometric contractions. Methods22 young (24.0 ± 2.6 years; 11 women) and 22 older (69.4 ± 2.4 years; 10 women) healthy men and women participated. Maximal voluntary isometric contraction (MVIC) of the KE and flexors was examined at 90° and 60° of knee flexion (0° = full extension). At each angle, participants performed KE isometric contractions (20%, 50% and 80% MVIC) to evaluate torque steadiness, whereas surface EMG was concurrently acquired from the vastus lateralis and biceps femoris muscles. ResultsAge-related impairment in torque steadiness was observed only at 20% MVIC (p < 0.01). A lower level of steadiness (p < 0.001) with a higher level of agonist and antagonist activations (all p < 0.01) was observed at 90° of knee flexion in comparison with contractions at 60°. Young participants were less steady when performing contractions at 90° compared with contractions at 60° of flexion (p < 0.001), whereas there was no difference for older participants. Furthermore, a significant effect of agonist activation and antagonist co-activation on torque steadiness was observed, but only in young participants (all p < 0.05). ConclusionsThese findings suggest that: (1) knee joint angle affects torque steadiness differently in young and older individuals; (2) an age-related impairment in steadiness was only presented at the lowest considered contraction intensity; and (3) the muscle activation responses were not associated with steadiness in older individuals.

Effects of Voluntary Agonist-Antagonist Coactivation on Stability of Vertical Posture.

We explored the effects of voluntary coactivation of agonist-antagonist leg and trunk muscles on stability of vertical posture. Young healthy subjects performed several tasks while standing with no additional muscle coactivation, low coactivation, and high coactivation. Postural stability was estimated using indices of postural sway and of intertrial variance in the space of muscle groups with parallel scaling of activation levels (M-modes). An increase in coactivation led to a significant increase in the postural sway speed reflected in faster rambling and trembling trajectories. Coactivation also led to a relative drop in the variance component that had no effects on the center of pressure coordinate and an increase in the component that shifted the center of pressure. We conclude that additional muscle coactivation does not help to stabilize vertical posture and is more likely to lead to postural destabilization. The results are consistent with an earlier hypothesis on muscle coactivation ensuring abundance (excessive degrees of freedom) at the level of control variables.

Activation of elbow extensors during passive stretch of flexors in patients with post-stroke spasticity

ObjectivesDeficits in regulation of tonic stretch reflex thresholds (TSRTs) after stroke occur in elbow flexors and extensors leading to spasticity in specific joint ranges. Threshold deregulation may also be responsible for other deficits such as abnormal activation of passively shortening muscles. Goals were to characterize activation of shortening elbow extensors during passive elbow flexor stretch in individuals with stroke, and identify its relationship to upper-limb motor impairment. MethodsThirty-three participants with unilateral stroke participated. TSRTs in elbow flexors were measured by stretching passive elbow flexors at different velocities. EMG responses were recorded from stretched agonist (biceps) and shortened antagonist (triceps) muscles. ResultsTriceps activation during passive biceps stretch occurred in all but 4 participants simultaneously with, before or after biceps activation onset. Biceps and triceps activation onsets and durations decreased with stretch velocity. Biceps TSRT and triceps activation magnitude did not correlate with sensorimotor impairment but greater stroke chronicity tended to be related to higher biceps TSRTs (r = 0.406, p = 0.041). ConclusionsStroke may result in both limitations in reciprocal inhibition and excessive agonist-antagonist co-activation, likely from deficits in TSRT modulation in both muscle groups. SignificanceSince both reciprocal inhibition and co-activation are fundamental to normal motor control, their cooperative action should be considered in designing interventions to increase the ranges of regulation of TSRTs in flexors and extensors to enhance upper limb functional recovery.

Neither total muscle activation nor co-activation explains the youthful walking economy of older runners

BackgroundOlder adults (≥65 years) exhibit greater metabolic rates during walking (worse walking economy) compared to young adults. Yet, previous research suggests that habitual running, but not habitual walking, exercise mitigates the age-related deterioration of walking economy. Research questionDoes total leg muscle activation and/or agonist-antagonist co-activation explain the superior walking economy of older runners versus older walkers? MethodsWe quantified metabolic power, leg muscle activation, and co-activation in older walkers and older runners during walking at 0.75, 1.25, and 1.75 m/s. ResultsWhile accounting for multiple comparisons, neither total stride (each speed p ≥ 0.024), stance- (each speed p ≥ 0.217), nor swing- (each speed p ≥ 0.170) phase EMG amplitude differed between older walkers and older runners at 0.75, 1.25, or 1.75 m/s. Stride averaged medial gastrocnemius and biceps femoris activation was lower in older runners than older walkers at 1.25 and 1.75 m/s (all p ≤ 0.025). We also calculated shank, thigh, and overall (shank and thigh) agonist-antagonist leg muscle co-activation over each stride, and the only difference between groups was a greater shank co-activation in older runners at 0.75 m/s (p = 0.024). Across groups, stride, stance-, and swing-phase total muscle activation positively correlated with gross metabolic power (R2 = 0.58–0.66; all p < 0.001). Paradoxically, across groups, stride, stance-, and swing-phase muscle co-activation indices were negatively correlated with gross metabolic power (R2 = 0.08–0.29; all p ≤ 0.007). SignificanceNeither total leg muscle activation nor co-activation explains the superior walking economy of older runners versus older walkers.

Running Economy: Neuromuscular and Joint-Stiffness Contributions in Trained Runners.

It is debated whether running biomechanics make good predictors of running economy, with little known about the neuromuscular and joint-stiffness contributions to economical running gait. To understand the relationship between certain neuromuscular and spatiotemporal biomechanical factors associated with running economy. Thirty trained runners performed a 6-min constant-speed running set at 3.3m·s-1, where oxygen consumption was assessed. Overground running trials were also performed at 3.3m·s-1 to assess kinematics, kinetics, and muscle activity. Spatiotemporal gait variables, joint stiffness, preactivation, and stance-phase muscle activity (gluteus medius, rectus femoris, biceps femoris, peroneus longus, tibialis anterior, and gastrocnemius lateralis and medius) were variables of specific interest and thus determined. In addition, preactivation and ground contact of agonist-antagonist coactivation were calculated. More economical runners presented with short ground-contact times (r = .639, P < .001) and greater stride frequencies (r = -.630, P < .001). Lower ankle and greater knee stiffness were associated with lower oxygen consumption (r = .527, P = .007 and r = .384, P = .043, respectively). Only lateral gastrocnemius-tibialis anterior coactivation during stance was associated with lower oxygen cost of transport (r = .672, P < .0001). Greater muscle preactivation and biarticular muscle activity during stance were associated with more economical runners. Consequently, trained runners who exhibit greater neuromuscular activation prior to and during ground contact, in turn optimizing spatiotemporal variables and joint stiffness, will be the most economical runners.

Muscle coactivation: definitions, mechanisms, and functions.

The phenomenon of agonist-antagonist muscle coactivation is discussed with respect to its consequences for movement mechanics (such as increasing joint apparent stiffness, facilitating faster movements, and effects on action stability), implication for movement optimization, and involvement of different neurophysiological structures. Effects of coactivation on movement stability are ambiguous and depend on the effector representing a kinematic chain with a fixed origin or free origin. Furthermore, coactivation is discussed within the framework of the equilibrium-point hypothesis and the idea of hierarchical control with spatial referent coordinates. Relations of muscle coactivation to changes in one of the basic commands, the c-command, are discussed and illustrated. A hypothesis is suggested that agonist-antagonist coactivation reflects a deliberate neural control strategy to preserve effector-level control and avoid making it degenerate and facing the necessity to control at the level of signals to individual muscles. This strategy, in particular, allows stabilizing motor actions by covaried adjustments in spaces of control variables. This hypothesis is able to account for higher levels of coactivation in young healthy persons performing challenging tasks and across various populations with movement impairments.

Effect of Knee Joint Angle and Contraction Intensity on Hamstrings Coactivation.

This study investigated the effect of knee joint angle and contraction intensity on the coactivation of the hamstring muscles (when acting as antagonists to the quadriceps) in young and older individuals of both sexes. A total of 25 young (24 ± 2.6 yr) and 26 older (70 ± 2.5 yr) healthy men and women participated. Maximal voluntary isometric contraction of the knee extensors and flexors was assessed at two knee joint angles (90° and 60°, 0° = full extension). At each angle, participants performed submaximal contractions of the knee extensors (20%, 50%, and 80% maximal voluntary isometric contraction), whereas surface EMG was simultaneously acquired from the vastus lateralis and biceps femoris muscles to assess the level (EMG root-mean-square) of agonist activation and antagonist coactivation. Subcutaneous adipose tissue in the areas corresponding to surface EMG electrode placements was measured via ultrasonography. The contractions performed at 90° knee flexion demonstrated higher levels of antagonist coactivation (all P < 0.01) and agonist activation (all P < 0.01) as a function of contraction intensity compared with the 60° knee flexion. Furthermore, after controlling for subcutaneous adipose tissue, older participants exhibited a higher level of antagonist coactivation at 60° knee flexion compared with young participants (P < 0.05). The results of the present study suggest that 1) the antagonist coactivation is dependent on knee joint angle and contraction intensity and 2) subcutaneous adipose tissue may affect the measured coactivation level likely because of a cross-talk effect. Antagonist coactivation may play a protective role in stabilizing the knee joint and maintaining constant motor output.

Gluteus medius coactivation response in field hockey players with and without low back pain

ObjectivesTo examine the effect of prolonged standing on gluteus medius coactivation and to observe whether the changes in gluteus medius coactivation over time were related to the development of low back pain in elite female field hockey players. DesignProspective cohort design. MethodsParticipants were 39 elite female field hockey players (14 with a history of low back pain). Before the prolonged stand, maximal hip abduction strength, side bridge hold endurance and hip abduction range of motion were measured bilaterally. Surface electromyography was collected from the gluteus medius for coactivation analysis during a prolonged stand for 70 min. Low back pain was rated every 10 min on a visual analogue scale. ResultsFourteen of 39 participants developed low back pain. The Time effect was significant for gluteus medius coactivation response (p = 0.003) and visual analogue scale score (p < 0.001). There were no significant group × time interactions. Yet athletes who developed pain had higher coactivation for the majority of the stand task. ConclusionsWhile female field hockey players have high agonist-antagonist coactivation patterns during prolonged standing, stand task is a useful tool to predict low back pain occurrence in players with and without history of pain.

Timing of Muscle Response to a Sudden Leg Perturbation: Comparison between Adolescents and Adults with Down Syndrome

Movement disturbances associated with Down syndrome reduce mechanical stability, worsening the execution of important tasks such as walking and upright standing. To compensate these deficits, persons with Down syndrome increase joint stability modulating the level of activation of single muscles or producing an agonist-antagonist co-activation. Such activations are also observed when a relaxed, extended leg is suddenly released and left to oscillate passively under the influence of gravity (Wartenberg test). In this case, the Rectus femoris of adults with Down syndrome displayed peaks of activation after the onset of the first leg flexion. With the aim to verify if these muscular reactions were acquired during the development time and to find evidences useful to give them a functional explanation, we used the Wartenberg test to compare the knee joint kinematics and the surface electromyography of the Rectus femoris and Biceps femoris caput longus between adolescents and adults with Down syndrome. During the first leg flexion, adolescents and adults showed single Rectus femoris activations while, a restricted number of participants exhibited agonist-antagonist co-activations. However, regardless the pattern of activation, adults initiated the muscle activity significantly later than adolescents. Although most of the mechanical parameters and the total movement variability were similar in the two groups, the onset of the Rectus femoris activation was well correlated with the time of the minimum acceleration variability. Thus, in adolescents the maximum mechanical stability occurred short after the onset of the leg fall, while adults reached their best joint stability late during the first flexion. These results suggest that between the adolescence and adulthood, persons with Down syndrome explore a temporal window to select an appropriate timing of muscle activation to overcome their inherent mechanical instability.

Association between physical activity levels and physiological factors underlying mobility in young, middle-aged and older individuals living in a city district.

Maintaining adequate levels of physical activity is known to preserve health status and functional independence as individuals grow older. However, the relationship between determinants of physical activity (volume and intensity) and physiological factors underlying mobility (cardio-respiratory fitness, neuromuscular function and functional abilities) is still unclear. The aim of this study was to investigate the association between objectively quantified physical activity and a spectrum of physiological factors underlying mobility in young, middle-aged and older individuals living in a city district. Experiments were carried out on 24 young (28±2 years), 24 middle-aged (48±2 years) and 24 older (70±3 years) gender-matched volunteers. Physical activity was monitored by a wearable activity monitor to quantify volume and intensity of overall physical activity and selected habitual activities over 24 hours. Ventilatory threshold was assessed during an incremental cycling test. Torque, muscle fiber conduction velocity and agonist-antagonist coactivation were measured during maximal voluntary contraction of knee extensors and flexors. Ground reaction forces were measured during sit-to-stand and counter-movement jump. K-means cluster analysis was used to classify the participants’ physical activity levels based on parameters of volume and intensity. Two clusters of physical activity volume (i.e., high and low volume) and three clusters of physical activity intensity (i.e. high, medium and low intensity) were identified in all participants. Cardio-respiratory fitness was associated with volume of overall physical activity as well as lying, sitting, standing, walking and stair climbing. On the other hand, neuromuscular function and functional abilities showed a significant association with intensity of overall physical activity as well as postural transition, walking and stair climbing. As a practical application, the relative role played by volume and intensity of overall physical activity and selected habitual activities should be taken into account in the design of preventative training interventions to preserve mobility as individuals grow older.

Cutaneous sensory feedback plays a critical role in agonist–antagonist co-activation

The purpose of this study was to investigate the role of cutaneous feedback in the agonist-antagonist co-activation mechanism during maximum voluntary force (MVF) production by the fingers. Seventeen healthy male subjects (age: 23.8 ± 1.0 years) were asked to press with maximal effort at their fingertips. Finger forces at the fingertips and muscle activities of the flexor digitorum superficialis (FDS, agonist) and extensor digitorum communis (EDC, antagonist) were recorded using force sensors and electromyography, respectively. There were two experimental conditions: with and without administration of a ring block to the fingers (i.e., anesthesia and normal conditions, or AC and NC, respectively). The ring block was used to deprive cutaneous feedback. Consistent with previous studies, finger MVF decreased significantly in AC compared with NC. Moreover, the force production of non-task fingers significantly increased in AC. Muscle activity of the EDC was significantly lower in AC than in NC; no significant changes in the FDS muscle were observed. The findings of this study show that cutaneous feedback not only increases MVF and force accuracy, but facilitates agonist-antagonist co-activation by increasing antagonist muscle activation. The results of this study imply that cutaneous feedback is linked to both primary and adjacent motor neurons.

Muscle Activation Patterns in Infants With Myelomeningocele Stepping on a Treadmill

To characterize how infants with myelomeningocele (MMC) activate lower limb muscles over the first year of life, without practice, while stepping on a motorized treadmill. Twelve infants with MMC were tested longitudinally at 1, 6, and 12 months. Electromyography was used to collect data from the tibialis anterior, lateral gastrocnemius, rectus femoris, biceps femoris. Across the first year, infants showed no electromyographic activity for approximately 50% of the stride cycle with poor rhythmicity and timing of muscles, when activated. Single muscle activation predominated; agonist-antagonist coactivation was low. Probability of individual muscle activity across the stride decreased with age. Infants with MMC show high variability in timing and duration of muscle activity, few complex combinations, and very little change over time.

Keynote

The challenges associated with maintaining an active lifestyle change across the lifespan. To characterize these adaptations, the presentation will describe several cross-sectional studies that identify factors constraining physical performance at three time points from childhood to senescence: 1) children during the initial years of puberty; 2) middle-aged adults who have not yet experienced substantial declines in muscle strength; and 3) the decrease in mobility exhibited by healthy older adults. In a cross-sectional study of 72 healthy children (39 boys and 33 girls; 8–14 yrs), we found that the differences in daily levels of physical activity between girls and boys at three Tanner stages (T1–T3) were strongly associated (R2 > 0.85) with neuromuscular characteristics (leg strength and endurance) for both sexes and with percent body fat for the T3 girls (Rudroff et al. submitted). In contrast to the association between muscle strength and physical activity in children, middle-aged adults (51 ± 6 yrs) exhibited longer times (65.7 ± 8.6 s vs 59.3 ± 6.0 s) to complete a test of manual dexterity (Grooved pegboard test) than young adults (26 ± 4 yrs), despite there being no differences between the two groups in hand strength (handgrip, pinch grip, and index finger abduction). Moreover, multiple regression analysis identified steadiness as the better predictor of dexterity than the strength measures (Marmon et al. Med Sci Sports Exerc 43: 560–567, 2011). The capacity to modulate muscle activation continues to decline with advancing years and is manifested as a greater reliance on coactivation of agonst and antagonist muscles with a reduced capacity to incorporate feedback control in the performance of voluntary actions. Healthy older adults (74 ± 3 yrs), for example, prefer to accommodate changes in mechanical stability by increasing agonist-antagonist coactivation rather than adopting the strategy used by young adults to modulate spinal reflex pathways (Baudry et al. J Neurophysiol 103: 623–631, 2010). Moreover, participation in physical activity by older adults can be further compromised by heightened sensations of fatigue that are unrelated to measures of performance fatigability. These studies demonstrate that in addition to the development and maintenance of muscle mass, physical performance capabilities across the lifespan are equally dependent on the capacity to provide appropriate activation signals to the involved muscles.

Child—Adult Differences in Muscle Activation — A Review

Children differ from adults in many muscular performance attributes such as size-normalized strength and power, endurance, fatigability and the recovery from exhaustive exercise, to name just a few. Metabolic attributes, such as glycolytic capacity, substrate utilization, and VO2 kinetics also differ markedly between children and adults. Various factors, such as dimensionality, intramuscular synchronization, agonist-antagonist coactivation, level of volitional activation, or muscle composition, can explain some, but not all of the observed differences. It is hypothesized that, compared with adults, children are substantially less capable of recruiting or fully employing their higher-threshold, type-II motor units. The review presents and evaluates the wealth of information and possible alternative factors in explaining the observations. Although conclusive evidence is still lacking, only this hypothesis of differential motor-unit activation in children and adults, appears capable of accounting for all observed child-adult differences, whether on its own or in conjunction with other factors.