Amplitude Of Force Fluctuations Research Articles

Influence of visual feedback and cognitive challenge on the age-related changes in force steadiness.

Force steadiness can be influenced by visual feedback as well as presence of a cognitive tasks and potentially differs with age and sex. This study determined the impact of altered visual feedback on force steadiness in the presence of a difficult cognitive challenge in young and older men and women. Forty-nine young (19-30yr; 25 women, 24 men) and 25 older (60-85yr; 15 women; 10 men) performed low force (5% of maximum) static contractions with the elbow flexor muscles in the presence and absence of a cognitive challenge (counting backwards by 13) either with low or high visual feedback gain. The cognitive challenge reduced force steadiness (increased force fluctuation amplitude) particularly in women (cognitive challenge × sex: P < 0.05) and older individuals (cognitive challenge × age: P < 0.05). Force steadiness improved with high-gain visual feedback compared with low-gain visual feedback (P < 0.01) for all groups (all interactions: P > 0.05). Manipulation of visual feedback had no influence on the reduced force steadiness in presence of the cognitive challenge for all groups (all P > 0.05). These findings indicate that older individuals and women have greater risk of impaired motor performance of the upper extremity if steadiness is required during a low-force static contraction. Manipulation of visual feedback had minimal effects on the reduced force steadiness in presence of a difficult cognitive challenge.

Investigating the stick-slip vibration behavior of a locomotive with adhesion control in a curve

In order to explain the wheel-rail stick-slip vibration phenomenon of alocomotive in a curve, a co-simulation dynamic model taking into account adhesion control was established to reproduce the locomotive wheel-rail curve stick-slip vibration behavior, and the effect of parameters such as creep threshold, descent slope, rail surface condition and track curve radius on the stick-slip vibration behavior was measured by the traction force and the overall dispersion of adhesion coefficient. The results illustrate that the wheel-rail curve stick-slip vibration is caused by the dynamic traction force fluctuation under the adhesion control, and the increase of creep threshold will lead to the decrease of tractive force fluctuation amplitude and the decrease of stick-slip vibration intensity, which will increase the adhesion utilization rate, However, the increase of descent slope, the decrease of track curve radius and wheel-rail friction coefficient have the opposite effect on stick-slip vibration behavior. This phenomenon can be eliminated by improving the rail surface condition, expanding the track curve radius and lowering the locomotive traction force.

Log Unloading at Logging Enterprise Berths

In the development of market economy in Russia, the main trend in the log processing industry is the implementation of the cutting edge technology and effective innovation in order to boost the labor productivity and reduce the production cost on the basis of environmentally friendly and rational use of timber resources. Taking into account the important role of water log transport used for timber rafting, the problem of efficiency of loading and unloading of flat rafts at log unloading berths is quite urgent. The current research considers the ways of increasing unloading efficiency of multi-row flat rafts at logging enterprise berths. The aim of this research is to present the results of the experiments of unloading multi-row flat rafts at logging enterprise berth. In these experiments, a load lifting crane equipped with special load grippers was used for unloading multi-row flat rafts at log unloading berth. During the experiments, the duration of unloading sequence from water to the unloading berth was defined and applied to multi-row flat rafts calculated by Northern (Arctic) Federal University named after M.V. Lomonosov and Volga State University of Technology based on the results of the laboratory experiments and statistical processing of the obtained data. We also carried out a strength test of the crane load gripper used for transposition of lengthy flat rafts in timber onshore yards and log unloading berths. The research was carried out on lifting force fluctuation amplitude and oscillation period of the load gripper during the process of flat raft unloading from water on log unloading berth. The fluctuation amplitude of the lifting force of the load-gripper from the ground is higher by 11–33% than from the water, facilitated by the water damping capacity. Based on the laboratory research, the durability of load grippers, patented in the Russian Federation under No. 2476366 and No. 2526767, was experimentally tested. The hypothesis of theoretical calculations of the special load gripper based on the equitability of gross load distribution on the gripper applicable to general purpose lifting apparatus for four choker grapples was experimentally confirmed. The certainty assumption amounted to 97.6%.

Investigation on the hydrodynamic load fluctuations through passive flexible leading edge

The study presents a numerical investigation of two-dimensional partly flexible plate dynamics. The structure is immersed in a turbulent fluid flow with a Reynolds number based on its chord of 104. The plate is animated by a forced pitching movement. The flexibility effects of the plate's leading edge are analyzed, as it deforms under the hydrodynamic loads. The fluid–structure interaction effects are considered by solving a coupled problem using a strong implicit procedure. Both fluid and solid dynamics are solved. The numerical results of the present study are validated with experimental ones with a good agreement between both approaches for the lower reduced frequencies. Differences are observable for high frequency that could be imputable to the three-dimensional aspects of the experiment. It has been shown that with an appropriate choice of the rigidity of the structure, it is possible to mitigate the unsteady load fluctuations without affecting the load mean values too much. Indeed, at low pitching frequency (drag mode), the leading-edge vortex generation is impacted by the flexible leading edge. As a result, it tends to decrease the hydrodynamic force fluctuation amplitude without really impacting the mean force value. Conversely, at high pitching frequency (propulsive mode), it was found that a flexible leading edge tends to increase both the magnitudes of the hydrodynamic forces and their mean values. Finally, it is shown that the load fluctuation mitigation, or amplification, is maximum for a specific flexibility value depending on the pitching frequency.

Integration of a dissipative contact force model into vehicle–track dynamics for analysing wheel–rail dynamic interaction under short-wavelength irregularity

Wheel–rail dynamic interaction under short-wavelength irregularity which has a detrimental effect on the service life of vehicle and track components, is of crucial importance to be estimated accurately. Aiming at this, dissipative contact force models are integrated into a validated vehicle–track coupled dynamics model, which enables to consider the energy dissipation at the wheel–rail interface. Wheel–rail interactions for different dissipative contact force models are evaluated, and the Hu–Guo model is selected as the most suitable one for wheel–rail interaction analysis. The wheel–rail interactions for the Hu–Guo model and Hertz theory are analysed under different track irregularities including sinusoidal rail corrugation and rail welded joints. The simulation results indicate that the Hu–Guo model has little effect on the normal interaction at low frequencies, but reduces the contact force fluctuation amplitude obviously at high frequencies above 600 Hz, with a more significant reduction at the resonance peak of high frequency range. The Hu–Guo model has an obvious contact force phase lead compared with the Hertz theory at higher frequencies above 1250 Hz. The Hu–Guo dissipative contact force model is needed for the wheel–rail interaction analysis at high frequencies above 600 Hz.

Streamline Computation Study on Rotation Aerodynamic Noise Prediction of Cross-flow Fan

The approach of resolving analysis the cross-flow relative motion streamline on the cross-flow straight blade is presented to predict the rotation aerodynamic noise performance of cross-flow fan by solving the dipole source term of Ffowcs Williams and Hawkings equation, while the cross-flow fan is applied in the indoor unit of split-type air-conditioner and operated in the rated condition. The calculating results of the method are respectively drawn by programming in Matlab computational language, and compared with the results of CAA numerical simulation and noise experiment. There are some differences in the distribution condition of aerodynamic force fluctuation amplitude on the blade surface, and sound pressure in the related frequency on the indoor unit casing, or on the sphere far field, between the streamline computation approach and numerical simulation. The orders of magnitudes of these calculating results solved by the analysing streamline method are similar with that attained by the numerical simulation. The resolving analysis approach has the characteristics of decreasing the computing cost and not constructing the acoustics grid model of fan, compared with the numerical simulation. The error between the result of numerical simulation and noise test is larger than that between the result of theoretical calculation and noise test, so the approach could be used for the rotation aerodynamic noise analysis of the cross-flow fan.

High-reynolds number flow around coated symmetrical hydrofoil: effect of streamwise slip on drag force and vortex structures

Effects of slip on the flow around a symmetrical hydrofoil with a blunt trailing-edge are numerically investigated at Reynolds numbers of $$Re=5 \times {10^6}$$ , $$12.5 \times {10^6}$$ and $$25 \times {10^6}$$ based on the free-stream velocity and chord length. The simulations are performed by applying a two-dimensional Unsteady Reynolds–Averaged Navier–Stokes (URANS) approach and SST k–ω turbulence model. Furthermore, the Navier boundary condition with different slip lengths ( $${L_{\text{s}}}=\;{\text{2}},{\text{ 35 and 7}}0$$ µm) is employed on the surfaces. The results indicate that the Ls has a considerable effect on the integral parameters and a moderate influence on the wake flow structure. As such, a massive drag reduction (up to 47%) is observed, and an increase of Ls causes the increase of both frictional and pressure drag reduction rate. It is shown that a noticeable drag reduction can be achieved when the non-dimensional slip length is larger than one. The increment of slip length also leads to increase the amplitude of force fluctuations and frequency of vortex shedding; besides, the strength of vortex structures and the turbulence intensities are augmented.

Experimental Investigation on the Dynamic Icing Process over a Rotating Propeller Model

An experimental study was conducted to investigate the dynamic ice accretion process over the blade surfaces of a rotating propeller model for unmanned-aerial-system (UAS) applications. In addition to revealing the transient ice accretion process over the rotating propeller surfaces, the dynamic thrust force generated by the propeller model was also measured simultaneously along with the required power inputs to drive the propeller model. Because of the combined effects of aerodynamics forces and the centrifugal force associated with the rotation motion, the ice accretion process over the rotating propeller surfaces was found to become very complicated. The ice accretion over the rotating propeller surfaces was found to become more preferable along the radial direction with the formation of lobster-tail-like ice structures extruding out from the propeller blade surfaces. The aerodynamic performance of the propeller model was also found to degrade tremendously due to the ice accretion, causing a significant reduction (i.e., up to 70% reduction) in mean thrust generation and a dramatic increase in force fluctuation amplitude (i.e., up to 250% increase). Despite of different types of ice accretion, the propeller model was always found to consume more power operating under icing conditions (i.e., up to 250% more power consumption under glaze icing conditions).

Brain areas associated with force steadiness and intensity during isometric ankle dorsiflexion in men and women.

Although maintenance of steady contractions is required for many daily tasks, there is little understanding of brain areas that modulate lower limb force accuracy. Functional magnetic resonance imaging was used to determine brain areas associated with steadiness and force during static (isometric) lower limb target-matching contractions at low and high intensities. Fourteen young adults (6 men and 8 women; 27.1 ± 9.1 years) performed three sets of 16-s isometric contractions with the ankle dorsiflexor muscles at 10, 30, 50, and 70 % of maximal voluntary contraction (MVC). Percent signal changes (PSCs, %) of the blood oxygenation level-dependent response were extracted for each contraction using region of interest analysis. Mean PSC increased with contraction intensity in the contralateral primary motor area (M1), supplementary motor area, putamen, pallidum cingulate cortex, and ipsilateral cerebellum (p < 0.05). The amplitude of force fluctuations (standard deviation, SD) increased from 10 to 70 % MVC but relative to the mean force (coefficient of variation, CV %) was greatest at 10 % MVC. The CV of force was associated with PSC in the ipsilateral parietal lobule (r = -0.28), putamen (r = -0.29), insula (r = -0.33), and contralateral superior frontal gyrus (r = -0.33, p < 0.05). There were minimal sex differences in brain activation across the isometric motor tasks indicating men and women were similarly motivated and able to activate cortical motor centers during static tasks. Control of steady lower limb contractions involves cortical and subcortical motor areas in both men and women and provides insight into key areas for potential cortical plasticity with impaired or enhanced leg function.

Sex-Related Effects in the Superhydrophobic Properties of Damselfly Wings in Young and Old Calopteryx splendens

Numerous sex-related morphological adaptations are connected to reproductive behavior in animals. For example, females of some insect species can submerge during oviposition, which may lead to sex-related adaptations in the hydrophobicity (water-repellency) due to specialization of certain morphological structures. On the other hand, ageing can cause changes in hydrophobicity of the surface, because the morphological structures can wear with age. Here, we investigated sex-and age-related differences in wing hydrophobicity and in morphology (spine density, wax cover characteristics, size of females' pseudopterostigma) potentially related to hydrophobicity of Calopteryx splendens damselflies. Hydrophobicity was measured with two methods, by measuring the contact angle (CA) between a wing and water droplet, and by dipping a wing into water and measuring forces needed to submerge, withdraw, and pull-out a wing from water. We found that C. splendens wings are superhydrophobic, having mean CAs of 161°. The only sex and age related difference in the hydrophobicity measurements was that young females had stronger amplitude of force fluctuations during withdrawal of wings from water than young males. This suggests that young females may form less uniform air pockets on their wings while submerged. From the morphological structures measured here, the only sex related finding was that old females had denser spine cover than young females in their wing veins. The difference may be explained by better survival of females with denser spine cover. The most important morphological character that predicted superhydrophobicity was the prevalence of long wax rods on wing veins. In addition, female pseudopterostigma area (a trait present only in females) was negatively related to pull-out force, suggesting that large pseudopterostigmas might help females to emerge from water following oviposition. The subtle sex-related differences in hydrophobicity could be explained by the fact that both sexes must resist rain, and males are occasionally in contact with water.

Characteristics of Unsteady Excitation Induced by Cavitating Flow in Centrifugal Pumps

Unsteady excitation induced by cavitating flow is one of the main factors causing the vibration and noise of the pump.In order to study the characteristics of unsteady excitation,the unsteady numerical simulation of the cavitating flow in the flow-passage of a centrifugal pumps is carried out by using the full cavitation model and the mixed two-phase flow model.Simulation results indicates that the cavitation bubbles generate at the suction side of blades near the leading edge,the region of bubbles extends with the reduction of inlet total pressure.When cavitation becomes serious,low-speed vortex is observed in the flow-passage.The frequency of the pressure fluctuations has the obvious discrete characteristics and the blade passing frequency dominates the pressure fluctuations.Along with the development of cavitation,cavitating flow can induce the increase of pressure fluctuation amplitude in the flow-passage,and there is a corresponding relationship between them.The cavitation can also induce the increase of unsteady radial force fluctuation amplitude and substantial increase of axial force.So cavitation affects operation stability and reliability of pumps.

Muscle fatigue increases the amplitude of fluctuations of tangential forces during isometric contractions

The orientation of the resultant force during a given contraction depends on the architecture of the muscles involved, and cooperation among muscles may induce force fluctuations in multiple directions. A novel setup for recording three-dimensional forces was developed to assess the effects of muscle fatigue on submaximal contractions. Fifteen subjects performed isometric dorsiflexions, elbow flexions, knee extensions, plantarflexions, and trunk extensions (12s, 2.5–80% of maximal voluntary force) before and after a fatigue protocol. Electromyography (EMG) was recorded from relevant muscles. The standard deviation of force in the three directions and the agonist EMG amplitude increased with target force (p<.045), while the coefficient of variation of force and total excursions of the center of pressure showed maximal values at low target forces, reaching a plateau at moderate forces. Fatigue induced higher force fluctuations in task-related and tangential directions and increased agonist EMG amplitude for all muscle groups (p<.04). Fluctuations of tangential forces were greater during knee extensions compared with other muscle groups (p<.007). The present data demonstrate that fatigue increases the amplitude of force fluctuations in task-related and tangential forces. Moreover, alternation of activity between multiple synergist muscles is associated with increased fluctuations of tangential forces, especially during fatigue.

Numerical Simulation of Cutting Force and Temperature Field in High Speed Machining of Titanium Alloys

Improvements in manufacturing technologies require better modeling and simulation of metal cutting processes. A fully thermal-mechanical coupled finite element analysis (FEA) was applied to model and simulate the high speed machining of TiAl6V4. The development of serrated chip formation during high speed machining was simulated. The effects of rake angle on chip morphology, cutting force and the evolution of the maximum temperature at the tool rake were analyzed with the finite element model. The simulation results show that the segmented chip formation results in cutting force fluctuation. Although the segmentation frequency of the chip increases with the increase of the rake angle, the degree of segmentation becomes weaker and the cutting force fluctuation amplitude decreases. The predicted temperature distribution during the cutting process is consistent with the experimental results given in a literature.

The effect of contraction intensity on force fluctuations and motor unit entrainment in individuals with essential tremor

Quantify the effect of increasing contraction intensity on the amplitude of force fluctuations and neuromuscular and force tremor spectral power. Twenty-one subjects with essential tremor (ET) and 22 healthy controls applied isometric wrist extension contractions. Various sub-maximal contraction intensities were evaluated (5%-, 10%-, 20%- and 30%-MVC). Force fluctuations and wrist extensor neuromuscular activity were recorded using a load cell and electromyography (EMG). Higher contraction intensities were associated with larger amplitude force fluctuations and greater neuromuscular activation. However, spectral power associated with tremor peaks remained relatively constant (EMG) or decreased (force) with increasing contraction intensity. Motor unit entrainment associated with centrally generated oscillatory inputs does not increase with greater levels of muscle activation. Rather than influencing a constant proportion of active motor units, abnormal oscillatory drive influences a relative constant number of total motor units. When combined with the findings from our previous study on postural tremor, the present results provide preliminary evidence that abnormal stretch reflex activity may contribute to this motor unit entrainment.

Motor variability: within-subject correlations during separate and simultaneous contractions

To determine the similarity of motor variability in proximal muscles, young and elderly adults performed steady elbow flexor (EF) and knee extensor (KE) contractions separately (SEP; at 2.5, 30, and 65% of maximum) and simultaneously (SIM; at 2.5 and 30% of maximum), with (VIS) and without (NVIS) visual feedback. Between-muscle correlations of fluctuation amplitude (SD, CV of force), time-based cross-correlations (CC), force power spectra, and frequency-based coherence (COH) values were computed from the concurrent force records. Correlations of fluctuation amplitude ranged from r = 0.34 to 0.86 (P < 0.05) across forces, SEP/SIM, and vision conditions, but were absent for 2.5% NVIS. The relatively low CC values for SIM (r = 0.22-0.33) were stronger for elderly than young adults. The vast majority of the power in the force fluctuations was <4 Hz for all records. Weak COH peaks were only observed <2 Hz for elderly and between 3 and 4 Hz for young, and COH was slightly stronger for elderly below 3 Hz for the 30% MVC target force. The correlations in force fluctuation amplitude suggest that the EF and KE motor neuron pools similarly transform the oscillating synaptic input and may influence each other. The cross-correlations suggest the remote motor neuron pools are influenced similarly in time by a common source of excitation, perhaps more coherently for elderly adults at low frequencies.

Force control is impaired in the ankle plantarflexors of elderly adults

This study determined the amplitude of force fluctuations for the ankle dorsiflexor (DF) and plantarflexor (PF) muscles of young and elderly adults. Maximal voluntary contraction (MVC) force and isometric DF and PF steadiness (2.5-80% MVC) was assessed in 11 young (23+/-3 years, 5 women, 6 men) and 10 elderly (73+/-6 years, 5 women, 5 men) adults. The coefficient of variation (CV) and power spectrum of the force was measured from the steadiness trials. MVC force was lower for elderly adults for PF (38% lower, P=0.002) but not DF (20% lower, P=0.14). For PF, the CV of force was greater for elderly than young adults at 2.5% (2.64 vs. 1.71%) and 5% MVC (1.78 vs. 1.24%), similar at 10, 50, and 80% MVC, and greater for young than elderly at the 30% MVC target force. For DF, the CV of force was similar for young and elderly at all target forces (P>0.05). The CV of force was 49% lower for the PF compared with DF muscles across all target forces (P<0.0001). This difference was significantly greater at the 2.5 (58%), 5 (58%), and 10% MVC (44%) target forces compared with higher target forces. The power spectra of the force fluctuations for both muscles were consistently dominated by frequencies below 2 Hz. For elderly adults, the neuromuscular factors that underlie both muscle strength and force fluctuations during low-force contractions are impaired in the ankle plantarflexors but not the dorsiflexors.

Variability of motor unit discharge and force fluctuations across a range of muscle forces in older adults

Variability of motor unit discharge is a likely contributor to the greater force fluctuations observed in old adults at low muscle forces. We sought to determine whether the variability of motor unit discharge rate underlies the fluctuations in force during steady contractions across a range of forces in young (n = 11) and old (n = 14) adults. The coefficient of variation (CV) for discharge rate and force were measured during a force-matching task as the first dorsal interosseous muscle performed isometric contractions. The recruitment thresholds of the 78 motor units ranged from 0.04% to 34% of maximal voluntary contraction (MVC) force. The CV for discharge rate ranged from 7.6% to 46.2% and was greater (P < 0.05) for old adults (21.5% +/- 7.7%) than young adults (17.3% +/- 8.1%). Although the CV for force was similar for young and old subjects (2.53% +/- 1.6%) across all target forces, it was greater for old adults at the lowest forces. Furthermore, there was a positive relation (r2 = 0.20, P < 0.001) between the CV for force and the CV for discharge rate across the range of recruitment thresholds. This relation was significant for old adults (r2 = 0.30, P < 0.001), but not for young adults (r2 = 0.06, P > 0.05). Thus, the normalized variability (CV) of motor unit discharge was greater in old adults and was related to the amplitude of force fluctuations across a broader range of forces than previously examined. These findings underscore the contribution of variability of motor unit activity to motor output in normal human aging.

The Correlation Between Elbow Flexor And Knee Extensor Steadiness Is Modified By Visual Feedback

The neural mechanisms that underlie the correlation in force fluctuations between the elbow flexor (EF) and knee extensor (KE) muscles are unknown. PURPOSE To determine the effect of visual feedback and target force on the correlation between the steadiness of contractions of the EF and KE muscles. METHODS Healthy, minimally active young (N = 9; 23 ± 3 yrs) and old adults (N = 17; 75 ± 7 yrs) underwent assessment of maximum voluntary contraction (MVC) and force fluctuations during submaximal constant-force contractions of the EF and KE muscles. Constant-force contractions were performed in the EF and KE separately (SEP) at target forces of 2.5, 30, and 65% MVC and simultaneously (SIMUL) in the EF and KE at 2.5 and 30% of MVC. The SEP contractions were performed with and without visual feedback of the force (VIS, NOVIS). SIMUL contractions were performed only with NOVIS. DC drift in the force (< 0.05Hz) was removed before measuring the fluctuations. The coefficient of variation (CV) of force was measured similarly for all conditions (SD of force/mean force for the segment). Correlations were calculated between the EF and KE for the CV of force for each target force and condition. RESULTS MVC force was correlated between EF and KE (r = 0.82, P <0.001). The CV of force was 23% lower in the NOVIS condition (P <0.001). SEP condition: The CV of force was positively correlated between EF and KE for the VIS condition at 2.5% (r = 0.46, P = 0.02) and 30% MVC (r = 0.8, P <0.001), with a trend at 65% MVC (r = 0.32, P = 0.1). With NOVIS the correlations persisted strongly at 30 and 65% MVC (0.67 and 0.77, P <0.001), but not at the lowest target force of 2.5% MVC (r = 0.06, P = 0.76). SIMUL condition: The CV of force was strongly correlated between EF and KE at 30% MVC (r = 0.86, P <0.001) but was uncorrelated at the lowest target force of 2.5% MVC (r = 0.02, P = 0.9). CONCLUSION Visual correction of force substantially increases the amplitude of force fluctuations during isometric force-matching tasks with the knee extensors and elbow flexors. The correlation in steadiness between the EF and KE corroborates our previous findings. Removal of visual feedback eliminates the between-muscle correlation only at low muscle forces in both SEP and SIMUL. This finding suggests that the neural factors that underlie the correlation in steadiness between these two muscle groups at low activation levels are influenced by visual feedback. Supported by NIH AG19171 to BLT.

Aeroacoustic performance evaluation of axial flow fans based on the unsteady pressure field on the blade surface

Because of the European and global regulation concerning acoustic emission, the goal of manufacturers is to substantially decrease the noise radiated by turbomachines, and in particular axial fans, without degrading their aerodynamic performances. High rotation speed and increasing power add to the overall difficulties. The theoretical study of this paper consists of two parts: (1) an aerodynamic approach based on the vortex surface method and (2) an aeroacoustic approach which mainly concerns the prediction of the tonal noise using the Ffowcs Williams and Hawkings (FW–H) equation. One of the main goals is the evaluation of the unsteady aerodynamic forces applied on the fan blades. A 2D software analysis, based on the vortex surface method (or potential flow method), was carried out. That process enabled evaluation of the potential flow around a mobile grid; first in a steady mode, and secondly in an unsteady mode by introducing an upstream disturbance in the form of an inlet velocity variation. The sources of noise corresponding to the zones with high force fluctuation amplitude are located initially on the blade surface. These fluctuating forces are used to predict the tonal noise radiated by the fan in far field by means of the FW-H equation. Two axial flow fans were used in this study. The theoretical results will be compared to the experimental ones in order to evaluate the aeroacoustic performances of the fans.

Mechanisms that contribute to differences in motor performance between young and old adults

This paper examines the physiological mechanisms responsible for differences in the amplitude of force fluctuations between young and old adults. Because muscle force is a consequence of motor unit activity, the potential mechanisms include both motor unit properties and the behavior of motor unit populations. The force fluctuations, however, depend not only on the age of the individual but also on the muscle group performing the task, the type and intensity of the muscle contraction, and the physical activity status of the individual. Computer simulations and experimental findings performed on tasks that involved single agonist and antagonist muscles suggest that differences in force fluctuations are not attributable to motor unit twitch force, motor unit number, or nonuniform activation of the agonist muscle, but that they are influenced by the variability and common modulation of motor unit discharge in both the agonist and antagonist muscles. Because the amplitude of the force fluctuations does not vary linearly with muscle activation, these results suggest that multiple mechanisms contribute to the differences in force fluctuations between young and old adults, although the boundary conditions for each mechanism remain to be determined.