Force Variability Research Articles

Techno-economic Analysis of Optimal Grid-Connected Renewable Electricity and Hydrogen-to-Power Dispatch

One of the significant challenges in renewable integration is balancing supply and demand. The variability in generation and demand forces the grid to experience significant market price volatility. Moreover, electricity curtailment is adhered to during low-demand periods. Hydrogen Energy storage systems (HESS) can provide power dispatch flexibility and facilitate the reduction in curtailment. Unlike other storage systems such as batteries, the energy and power capacities for HESS design can be decoupled, resulting in a long-duration storage solution. In our paper, we perform electricity dispatch optimization from renewable sources such as solar and wind to the electricity market, where hydrogen is optimally produced using electrolysis, stored during low electricity prices, and converted to electricity using fuel cells to support the grid. The capital cost optimization suggests high profits for investors leveraging market price volatility even with low HESS round-trip efficiency and high upfront costs.

Muscle contributions to reduced ankle joint contact force during drop vertical jumps in patients with chronic ankle instability

Chronic ankle instability is a condition linked to progressive early ankle joint degeneration. Patients with chronic ankle instability exhibit altered biomechanics during gait and jump landings and these alterations are believed to contribute to aberrant joint loading and subsequent joint degeneration. Musculoskeletal modeling has the capacity to estimate joint loads from individual muscle forces. However, the influence of chronic ankle instability on joint contact forces remains largely unknown. The objective of this study was to compare tri-axial (i.e., compressive, anterior-posterior, and medial–lateral) ankle joint contact forces between those with and without chronic ankle instability during the ground contact phase of a drop vertical jump. Fifteen individuals with and 15 individuals without chronic ankle instability completed drop vertical jump maneuvers in a research laboratory. We used those data to drive three-dimensional musculoskeletal simulations and estimate muscle forces and tri-axial joint contact force variables (i.e., peak and impulse). Compared to those without chronic ankle instability, the ankles of patients with chronic ankle instability underwent lower compressive ankle joint contact forces as well as lower anterior-posterior and medial–lateral shearing forces during the weight acceptance phase of landing (p <.05). These findings suggest that patients with chronic ankle instability exhibit lower ankle joint loading patterns than uninjured individuals during a drop vertical jump, which may be considered in rehabilitation to potentially reduce the risk of early onset of ankle joint degeneration.

Contribution of FDI, fiscal policy, and labor force on inequality and economic growth: A case study in South Sulawesi, Indonesia

South Sulawesi Province faces a significant problem: economic growth that has not been inclusive. This study analyzes the relationship between foreign investment contribution, fiscal policy, and labor force to inclusive economic growth in South Sulawesi, Indonesia. The results of this study show two relationships, namely direct and indirect relationships. Judging from the direct influence. First, FDI and local native income positively affect the level of inequality. However, in contrast to general allocation funds and labor force variables, each negatively affects inequality. Meanwhile, FDI, general allocation funds, and local original revenues positively affect economic growth. However, in contrast to the variables labor force and inequality, each negatively affects the inequality level. When viewed from its indirect influence, FDI, general allocation funds, labor force, and local native income each positively impact economic growth through levels of inequality. The implication of this study is the importance of governments allocating FDI investments for infrastructure development that supports inclusive growth.

Simulation Research on Non-Bevel Gear Transmission Mechanism

Abstract: There are problems with vibration and impact in the meshing transmission process of non-bevel gear. Therefore, a simulation control method of non-bevel gear transmission based on ADAMS and MATLAB is proposed. First of all, the design modeling of non-bevel gear is briefly described, and the influence of order and eccentricity on its transmission ratio is analyzed; Then, the established non-bevel gear model is imported into ADAMS, and the constraint, motion pair, driving force, input and output variables are set. The data transfer relationship is established by using the ADAMS/Control module and MATLAB/Simulink interface, and the non-bevel gear mechanical system module is generated in MATLAB/Simulink; Finally, its joint simulation control system is built in the MATLAB/Simulink module, and its control simulation analysis is carried out. The research results show that by comparing and analyzing the simulation results of only constant value input and adding the PI control module, the amplitude, and frequency of the angular velocity and angular acceleration curves of the driven wheel are significantly reduced, that is, the vibration and impact in the non-bevel gear transmission are reduced, which verifies the correctness and feasibility of the control strategy, It provides a new method for the control design and development of the vibration reduction performance of non-bevel gear transmission; The result is beneficial to the application and popularization of non-bevel gears.

MUSCLE ACTIVATION VARIABILITY AS A DETERMINANT OF GAIT VARIABILITY IN OLDER ADULTS

Abstract Gait variability is the step-to-step fluctuations in spatial, temporal, and kinetic gait parameters that reduces the rhythmicity of walking and places older adults at greater risk for falling. Gait variability is influenced by the senses, balance, executive function, strength, and potentially the consistency of neuromuscular activation. The purpose of this study was to test whether older adults with high muscle activation (HMA) variability during walking have greater kinetic, spatial and temporal gait variability than those with low activation variability (LMA). Twenty-seven men and women 65-85 yr walked at normal and maximal speeds on an instrumented treadmill that recorded gait parameters while the electromyogram (EMG) was recorded from vastus lateralis and gastrocnemius lateralis muscles. EMG was recorded over 10 steps and its coefficient of variation was used to identify HMA and LMA groups. Repeated measures ANOVA tested the effects of group (HMA, LMA) and speed (normal, maximal) on gait variability. A significant main effect of speed existed as weight acceptance peak force variability increased 21% (p = 0.014), push off force variability increased 29% (p = 0.015) and step length variability increased 20% (p = 0.02) at maximal speed. A significant interaction existed between speed and group for weight acceptance peak force variability (p = 0.03) as HMA increased 50% while LMA changed little (-4%). Gait variability increased with speed in older men and women, more so for those who exhibit greater muscle activation variability. This suggests the consistency of neuromuscular activation is a factor that affects the rhythmicity of walking.

Motor unit activity and synaptic inputs to motoneurons in the caudal part of the injured spinal cord.

Spinal cord injury (SCI) disrupts neuronal function below the lesion epicenter, causing disuse muscle atrophy. We investigated motor unit (MU) activity and synaptic inputs to motoneurons in the caudal region of the injured spinal cord. Participants with C4-C7 cervical injuries were studied. The extensor digitorum communis (EDC) muscle, which is mainly innervated by C8, was assessed for disuse muscle atrophy. Using advanced electromyography and signal-processing techniques, we examined the concurrent activation of a substantial population of MUs during force-tracking tasks. We found that in participants with SCI (n = 9), both MU discharge rates and the amplitudes of MU action potentials were significantly lower than in controls (n = 9). After SCI, MUs were recruited in a limited force range as the strength of muscle contractions increased, implying a disruption in the orderly MU recruitment pattern. Coherence analysis revealed reduced synaptic inputs to motoneurons in the delta band (0.5-5 Hz) for participants with SCI, suggesting diminished common synaptic inputs to the EDC muscle. In addition, participants with SCI exhibited greater muscle force variability. Using principal component analysis on low-frequency MU discharge rates, we found that the first common component (FCC) captured the most discharge variability in participants with SCI. The coefficients of variation (CV) of the FCC correlated with force signal CVs, suggesting force variability mainly results from common synaptic inputs to the EDC muscle after SCI. These results advance our understanding of the neurophysiology of disuse muscle atrophy in human SCI, paving the way for therapeutic interventions to restore muscle function.NEW & NOTEWORTHY This study analyzed motor unit (MU) function below the lesion epicenter in patients with spinal cord injury (SCI). We found reduced MU discharge rates and action potential amplitudes in participants with SCI compared with controls. The strength of common synaptic inputs to motoneurons was reduced in patients with SCI, with increased force variability primarily due to low-frequency oscillations of common inputs. This study enhances understanding of neurophysiological and behavioral changes in disuse muscle atrophy post-SCI.

Advanced Force Coordination of Lower Extremities During Visuomotor Control Task in Soccer Players

ABSTRACT Purpose: This study is aimed to determine specific bilateral lower extremity motor performances and coordination patterns in soccer players with healthy controls using the bilateral force control paradigm. We hypothesized that soccer players would show more advanced bilateral force control performances than untrained controls. Methods: Participants were 13 university soccer players and 13 healthy controls. Each group performed bilateral ankle dorsiflexion force control tasks across two vision conditions (i.e., vision and no-vision) and two targeted force levels (i.e., 10% and 40% maximum voluntary contraction). We calculated force accuracy, variability, and symmetry to assess force control performances. To estimate bilateral force coordination, we calculated Pearson’s correlation coefficients between feet-within a single trial and considered uncontrolled manifold variables across multiple trials. In the no-vision condition, we performed secondary analyses for initial force control patters after removal of visual feedback. Results: There were no significant group differences in bilateral force accuracy and variability but, compared to the control group, soccer players showed higher force symmetry between left and right ankle dorsiflexion forces. For force coordination between feet, soccer players revealed more negative values of the correlation coefficient and greater good variability from the uncontrolled manifold analysis than those for the control group. The secondary analysis revealed no significant group differences in the time until force drift and amount of force adjustments. Conclusions: Soccer players have more compensatory and flexible interlimb force coordination strategies between feet.

Influence of horizontal resistance loads on spatiotemporal and ground reaction force variables during maximal sprint acceleration

This study aimed to elucidate the influence of horizontal resistance loads on the spatiotemporal and ground reaction force (GRF) variables during maximal sprint acceleration. Nine male sprinters (20.2 ± 1.2 years; 175.3 ± 4.5 cm, 69.7 ± 6.1 kg) performed sprint-running with six loading conditions of one unresisted and five resisted loads of 4, 6, 8, 10, and 12 kg using a resistance training device with intelligent drag technology. During the trials, the GRFs for all steps were determined using a 50-m force plate system. The spatiotemporal and GRF variables at running velocity of every 0.5 m/s were obtained and compared across the loading conditions. The maximal running velocity under 0, 4, 6, 8, 10, and 12 kg loading conditions were 9.84 ± 0.41, 8.55 ± 0.41, 8.09 ± 0.33, 7.62 ± 0.34, 7.11 ± 0.31, and 6.71 ± 0.29 m/s, respectively. ANOVA revealed significant main effects of load on the measured variables (η2 = 0.236–0.715, p &lt; 0.05), except for stance-averaged anteroposterior GRF and braking impulse. However, the observed differences between the loading conditions were small, with approximately 4% (1.3–7.5%) for the GRF variables and approximately 9% (1.2–22.3%) for the spatiotemporal variables. The present study indicates that horizontal resistance load in sprint acceleration has little impact on the spatiotemporal and GRF variables at a given running velocity. In contrast to a general recommendation, one should adopt a heavy load in resisted sprint aiming to improve performance in the earlier stage of maximal sprint acceleration.

The Contribution of the Agricultural Labour Force in Romania to the Sector’s Economic Performance

This study aims to assess the quantitative and, above all, the qualitative potential of the agricultural labour force in Romania. In the first part of this paper, analyses will be carried out on the agricultural labour force to identify the potential of countries that attach particular importance to the subject studied and the inter-relationships between them, where agriculture is a major employer. In the second part of the paper, an empirical analysis of labour force statistics will be carried out to assess its potential, both quantitatively and qualitatively. Finally, we want to determine the influence of certain population characteristics on the qualitative potential of the labour force. The bibliometric analysis provides a historical, contextual and evolutionary perspective on previous research and problematic factors in the field, linking the “agricultural labour force” to other issues such as climate change, economic growth, sustainability, education and income. It provides a comprehensive framework and links between the various elements influencing the agricultural sector, strengthening the understanding of the interconnections between labour force and other key variables in the sector.

Kinetic modeling of fluid-induced interactions in compressible, rarefied gas flows for aerodynamically interacting particles

In the study of gas-particulate multiphase systems, the flow of high-speed gas through a distribution of solid particulates is of utmost importance. While these aerodynamically interacting systems have been extensively studied for low-speed gas flows in the gas continuum regime, less attention has been given to high-speed systems where non-continuum effects are significant due to the high flow gradients. To address this, the flow of rarefied gas through an aerodynamically interacting monodisperse spherical particle system is studied using the Direct Simulation Monte Carlo (DSMC) gas-kinetic approach. Since the method provides the best resolution of shocks at supersonic Mach numbers it is used to classify the weak separated shocks and strong collective shocks in these systems based on particle spacing in a two-particulate system at different orientation angles. The study used the two-particle system to help analyze more complex particle distributions of volume fractions, 1%, 5%, and 15%, exposed to gas flows in the slip and transitional gas regime for a free-stream Mach number range of 0.2<Ma∞<2.0. We observe that the weak separated shocks in the 1% distribution allow a higher degree of gas penetration and shock-particle interactions or “hypersonic-surfing”, exposing a major fraction of the particulates to higher force magnitudes. In contrast, the strong collective shock in the 5% and 15% distributions only generates high particulate forces on the flow-facing particles. Finally, a simple stochastic model is proposed for use in large-scale Eulerian–Lagrangian simulations that captures the non-monotonic behavior of average drag and force variability generated by the complicated gas particulate interactions in the compressible gas regime.

Multi-Site Identification and Generalization of Clusters of Walking Behaviors in Individuals With Chronic Stroke and Neurotypical Controls

Background Walking patterns in stroke survivors are highly heterogeneous, which poses a challenge in systematizing treatment prescriptions for walking rehabilitation interventions. Objectives We used bilateral spatiotemporal and force data during walking to create a multi-site research sample to: (1) identify clusters of walking behaviors in people post-stroke and neurotypical controls and (2) determine the generalizability of these walking clusters across different research sites. We hypothesized that participants post-stroke will have different walking impairments resulting in different clusters of walking behaviors, which are also different from control participants. Methods We gathered data from 81 post-stroke participants across 4 research sites and collected data from 31 control participants. Using sparse K-means clustering, we identified walking clusters based on 17 spatiotemporal and force variables. We analyzed the biomechanical features within each cluster to characterize cluster-specific walking behaviors. We also assessed the generalizability of the clusters using a leave-one-out approach. Results We identified 4 stroke clusters: a fast and asymmetric cluster, a moderate speed and asymmetric cluster, a slow cluster with frontal plane force asymmetries, and a slow and symmetric cluster. We also identified a moderate speed and symmetric gait cluster composed of controls and participants post-stroke. The moderate speed and asymmetric stroke cluster did not generalize across sites. Conclusions Although post-stroke walking patterns are heterogenous, these patterns can be systematically classified into distinct clusters based on spatiotemporal and force data. Future interventions could target the key features that characterize each cluster to increase the efficacy of interventions to improve mobility in people post-stroke.

Design and application of a flexible fabric transfer manipulator

ABSTRACT The pick-up and transfer of easily deformable multi-layer fabrics is a challenge in the automation process of the textile industry. A transfer manipulator using interfacial adhesion forces has potential applications in the pick-up and transfer of fabrics. However, rapid adhesion and detachment cannot be achieved because of the small variability of adhesion and detachment forces between the fabric and the adhering material, which results in difficulty of picking and placing of fabric. To address this problem, in this study, an innovative pressure and strip manipulator structure is designed, utilizing the fabric gesture presented during the fabric peeling process to adjust the peel angle, and thereafter affect the adhesion force. The factors affecting the adhesion force, such as peeling speed, duration, and preload are analyzed, and a control method to regulate the adhesion force by peeling speed and the gap between the adhesion element and positioning block is proposed. Based on the interface adhesion force, the transfer manipulator can realize low-cost, high-stability, and high-efficiency fabric picking, which has good prospects for application in fabric production.

Effect of extension of the ulnar fingers on force control and muscle activity of the hand during a precision pinch.

Some individuals extend the three ulnar fingers when performing a precision pinch. The aim of the present study was to investigate the mechanisms and effect of the extension of the ulnar fingers during a pinch. When performing a pulp pinch task with the ulnar fingers in two positions (extension and flexion), 27 participants maintained 5% of their maximum force. The mean pinch force, force variability and time taken to reach the targeted force (reaching time) were calculated. Muscle activity was simultaneously measured, using surface electromyography, for nine muscles: the flexor pollicis brevis; abductor pollicis brevis; flexor pollicis longus; first lumbrical; first dorsal interosseous; flexor digitorum superficialis of the index finger; extensor indicis; and extensor digitorum of the index and ring fingers. No significant differences in the mean pinch force or force variability were found. However, the reaching time was significantly shorter (approximately 20% reduction) in the extension position and the activities in the flexor pollicis brevis, first lumbrical, extensor indicis and extensor digitorum of the ring finger were significantly higher. These findings suggest that extending the ulnar fingers during pinching enhances the activity of key muscles involved in the movement and allows for more rapid force exertion.

Improving the Grasping Force Behavior of a Robotic Gripper: Model, Simulations, and Experiments

Robotic grippers allow industrial robots to interact with the surrounding environment. However, control architectures of the grasping force are still rare in common industrial grippers. In this context, one or more sensors (e.g., force or torque sensors) are necessary. However, the incorporation of such sensors might heavily affect the cost of the gripper, regardless of its type (e.g., pneumatic or electric). An alternative approach could be open-loop force control strategies. Hence, this work proposes an approach for optimizing the open-loop grasping force behavior of a robotic gripper. For this purpose, a specialized robotic gripper was built, as well as its mathematical model. The model was employed to predict the gripper performance during both static and dynamic force characterization, simulating grasping tasks under different experimental conditions. Both simulated and experimental results showed that by managing the mechanical properties of the finger–object contact interface (e.g., stiffness), the steady-state force variability could be greatly reduced, as well as undesired effects such as finger bouncing. Further, the object’s size is not required unlike most of the grasping approaches for industrial rigid grippers, which often involve high finger velocities. These results may pave the way toward conceiving cheaper and more reliable open-loop force control techniques for use in robotic grippers.

Damage characteristics and material removal mechanisms of CVD-SiC in nanotests: A comparative with sintered SiC

High sensitivity to crack and damage of chemical vapor-deposited silicon carbide (CVD- SiC) restricts the high-efficiency and low-damage manufacturing of CVD-SiC components. This paper aims to reveal the damage and material removal mechanisms of CVD-SiC after being scratched by diamond abrasives. The mechanical properties, scratching force, depth of ductile-brittle transition, residual depth and material removal damages of the CVD-SiC were discussed by the indentation, nano-scratching experiments, and compared with sintered silicon carbide (SSiC) ceramics. The results demonstrated that the mechanical properties of CVD-SiC are better than SSiC. In terms of hardness and fracture toughness, CVD-SiC demonstrates a 10– 20% and 20– 30% higher level of performance than SSiC, respectively. SSiC exhibits a brittleness level approximately 30% higher than that of CVD-SiC. SSiC ceramics are prone to crack deflection, intergranular fracture and transgranular fracture during indentation, indicating a higher propensity for brittle fracture in SSiC. The crack propagation in CVD-SiC is relatively regular. Both materials undergo the ductile, ductile-brittle transition and brittle stage, and the fluctuation variability of CVD-SiC material is notably lower than that of SSiC during the brittle stage. Compared with SSiC, the residual depth, groove width and transverse force of CVD-SiC are smaller under the same normal load. The critical depth of ductile-brittle transition of CVD-SiC is about 2 times that of SSiC in experimental and theoretical, indicating that CVD-SiC easily achieves ductile grinding. The mechanical properties, crack system and crystal structure of the CVD-SiC and SSiC materials cause the divergences in brittle removal. The radial-median crack system dominates the material removal of CVD-SiC. The disparity of deformations on the inside and outside of the crystal in the brittle stage leads to significant fluctuation variability of force and depth of SSiC during the scratching.

Integral Method for Determining the Stress State of an Anisotropic Reinforced Concrete Shell

The article deals with the issue of estimating the strength and stress-strain state of a reinforced concrete shell by the method of membrane theory and the theory of infinitesimal bending. A brief description of the methodology consists in the fact that the ground stress state of the shell is allocated to an independent problem, when, without introducing boundary effects, two of the four boundary conditions of the general moment theory are distinguished, which, together with the equations of the momentless theory, determine the ground stress state, and then boundary effects are superimposed. The equilibrium equations of the moment theory in forces and moments, geometric equations, components of tangential deformation and displacement, and the physical equations of state connecting them, expressing forces and moments through the components of deformation, are presented, since the characteristics of the stress-strain state of the shell depend not only on the variability of external influences and forces, but also on the length of the structure. The provisions laid down in the article shall be retained in the case of casings made of anisotropic material, provided that the presented elastic ratio is met. A system of equations of the generalized semi-membrane (semi-bending) state of an arbitrary shell of zero curvature, which determines the accuracy of this approach, is solved. The equations of the semi-torque theory for a circular cylindrical shell are presented, as well as describing the semi-membrane stress state of a long shell of zero curvature. The most important result of the study is the method of constructing integrals of the ground stress state of the shell based on the method of simple iterations, which expands the possibilities of the reliability theory, which makes it possible to build the foundations for the practical calculation of reinforced concrete shells according to the membrane theory and the theory of infinitesimal bending.

Strength constrained topology optimization of hyperealstic structures with large deformation-induced frictionless contact

This paper developed a strength constrained topology optimization method for hyperelastic structures with large deformation-induced frictionless contact. The Neo-Hookean hyperelastic constitutive equation is adopted as the material model that incorporates both material and geometric nonlinearity. The large deforming contact is described by the node-to-segment algorithm. The topology optimization model is developed under the SIMP framework. Given the SIMP-related fictitious domain, we develop a nodal variable-based interpolation scheme to build smooth correlation between the contact force and nodal density variables, thus enabling the smooth transition and variation of contact conditions. Moreover, to guarantee the structure strength, local strain energy constraints are formulated and aggregated by the P-norm function. The details of sensitivity analysis are explicitly explained. At last, several benchmark examples are conducted that validate the proposed method.

Hip and knee kinematics, center of pressure position, and ground reaction force are associated with Achilles tendon force during jump landing.

Jump-landing exercises are often performed during the rehabilitation of Achilles tendon (AT) injuries. However, the factors that affect the AT force (ATF) during landing are unclear. This study aimed to determine the kinematics and ground reaction force (GRF) variables associated with the peak ATF during a drop vertical jump (DVJ). The landing phase of DVJ was evaluated in 101 healthy participants (46 males, age: 21.2 ± 1.4 years old) using a three-dimensional motion analysis system with two force plates. ATF was estimated from the ankle flexion angle and moment. Univariate and multivariate regression analyses were performed with the peak ATF as the dependent variable. The vertical GRF (VGRF), center of pressure (COP), forward trunk leaning, hip/knee/ankle joint angles at peak ATF, and sex were used as independent variables. In the univariate regression analysis, larger VGRF (β = 0.813), more anterior COP position (β = 0.214), smaller knee flexion (β = -0.251) and adduction (β = -0.252), smaller hip flexion (β = -0.407), smaller forward trunk lean (β = -0.492), and male sex (β = -0.282) were significantly associated with a larger peak ATF. Multivariate analysis revealed that larger VGRF (β = 1.018), more anterior COP position (β = 0.320), a larger knee (β = 0.442), and smaller hip flexion (β = -0.205) were associated with the larger peak ATF. The VGRF, COP position, and knee and hip flexion were independently associated with ATF. Modifying these factors may be useful in managing tendon loading during jump-landing exercises.

A comparison of load cell and pressure sensors to measure in-water force in young competitive swimmers

The purpose of this study was to compare the in-water force of young competitive swimmers using tethered swimming and differential pressure sensors. Thirty-one swimmers (16 girls and 15 boys) were randomly assigned to perform two in-water tests. Swimmers completed two maximum bouts of 25 m front crawl with a differential pressure system and a 30 s maximum bout with an attached load cell (tethered-swimming). The peak force (FPEAK, in N) of dominant and non-dominant upper limbs was retrieved for further analysis. Comparison between methods revealed significant differences in all force variables (p ≤ 0.05) and the biases (mean differences) were large in girls (FPEAK dominant, 45.89 N; FPEAK non-dominant, 43.79 N) and boys (FPEAK dominant, 67.26 N; FPEAK non-dominant, 61.78 N). Despite that, simple linear regression models between the two methods showed significant relationships with a moderate effect in all variables for girls, whereas in boys a high and moderate effect was verified for FPEAK of dominant and non-dominant limbs (respectively). It seems that using pressure sensors and tethered swimming leads to different FPEAK values in young competitive, where correction factors are needed to compare data between both methods.

Increasing load carriage and running speed differentially affect the magnitude, variability and coordination patterns of muscle forces

The study aims to investigate the effects of different loads and speed during running on inter- and intra-individual muscle force amplitudes, variabilities and coordination patterns. Nine healthy participants ran on an instrumentalized treadmill with an empty weight vest at two velocities (2.6 m/s and 3.3 m/s) or while carrying three different loads (4.5, 9.1, 13.6 kg) at 2.6 m/s while kinematics and kinetics were synchronously recorded. The major lower limb muscle forces were estimated using a musculoskeletal model. Muscle force amplitudes and variability, as well as coordination patterns were compared at the group and at the individual level using respectively statistical parametric mapping and covariance matrices combined with multidimensional scaling. Increasing the speed or the load during running increased most of the muscle force amplitudes (p < 0.01). During the propulsion phase, increasing the load increased muscle force variabilities around the ankle joint (modification of standard deviation up to 5% of body weight (BW), p < 0.05) while increasing the speed decreased variability for almost all the muscle forces (up to 10% of BW, p < 0.05). Each runner has a specific muscle force coordination pattern signature regardless of the different experimental conditions (p < 0.05). Yet, this individual pattern was slightly adapted in response to a change of speed or load (p < 0.05). Our results suggest that adding load increases the amplitude and variability of muscle force, but an increase in running speed decreases the variability. These findings may help improve the design of military or trail running training programs and injury rehabilitation by progressively increasing the mechanical load on anatomical structures.