Total Force Production Research Articles

Two aspects of feed-forward control of action stability: effects of action speed and unexpected events.

We explored two types of anticipatory synergy adjustments (ASA) during accurate four-finger total force production task. The first type is a change in the index of force-stabilizing synergy during a steady state when a person is expecting a signal to produce a quick force change, which is seen even when the signal does not come (steady-state ASA). The other type is the drop in in the synergy index prior to a planned force change starting at a known time (transient ASA). The subjects performed a task of steady force production at 10% of maximal voluntary contraction (MVC) followed by a ramp to 20% MVC over 1s, 3s, and as a step function (0s). In another task, in 50% of the trials during the steady-state phase, an unexpected signal could come requiring a quick force pulse to 20% MVC (0-surprise). Inter-trial variance in the finger force space was used to quantify the index of force-stabilizing synergy within the uncontrolled manifold hypothesis. We observed significantly lower synergy index values during the steady state in the 0-ramp trials compared to the 1-ramp and 3-ramp trials. There was also larger transient ASA during the 0-ramp trials. In the 0-surprise condition, the synergy index was significantly higher compared to the 0-ramp condition whereas the transient ASA was significantly larger. The finding of transient ASA scaling is of importance for clinical studies, which commonly involve populations with slower actions, which can by itself be associated with smaller ASAs. The participants varied the sharing pattern of total force across the fingers more in the task with "surprises". This was coupled to more attention to precision of performance, i.e., inter-trial deviations from the target as reflected in smaller variance affecting total force, possibly reflecting higher concentration on the task, which the participants perceived as more challenging compared to a similar task without surprise targets.

Enhanced boxing punch impact with silicone cushioning.

Elastic cushioning materials protect human tissue from injury by absorbing impact energy and delaying its transfer. However, the potential compromise in force delivery to the hitting target remains unknown. To examine if silicone cushioning compromises punch force delivery to a hitting target, a double-blind crossover trial with 12 elite boxers was conducted following material tests. Each boxer delivered five maximal punches under two conditions: silicone-hand wrapping and gauze-hand wrapping, in counterbalanced order, with a 3-day interval between sessions. Force distribution along the Z-axis indicated the punch's intended direction, while forces along the X and Y axes represented force dissipation toward unwanted direction. The material tests (based on ASTM International, West Conshohocken, PA, USA) demonstrated substantially higher compression to disruption for silicone than gauze of similar thickness. During the punching trials, the silicon-based hand wrapping exhibited slightly higher total force production (436 ± 33 N vs. 372 ± 12 N, p < 0.001) than the gauze-based hand wrapping. Moreover, force wastage, calculated as the sum of forces along the X and Y axes vs. the total force produced in percentage, was notably lower for silicone material (2.0% wastage) compared to gauze (3.8% wastage) (p < 0.001). The use of silicone materials lengthened the contact time between the punching fist and the hitting target from 35 ms to 50 ms (p < 0.001). The elastic cushion does not compromise the force delivery of the boxing glove to the hitting target. Instead, it appears to allow for additional maneuvering time for alignment during the fist-target contact with higher impact.

P-022: THERAPEUTIC EFFECTS OF GBT1118 ON SKELETAL MUSCLE ENERGETICS AND FUNCTION IN SICKLE CELL MICE

Purpose: Sickle cell disease (SCD) is an hemoglobinopathy ultimately resulting red blood cells sickling. SCD patients suffer from anemia, arterial oxyhemoglobin desaturation, hematologic and hemodynamic abnormalities, all of which disrupt oxygen supply and can reduce exercise capacity and quality of life. Strenuous exercise in SCD patients may induce hemolysis, an early lactate accumulation, an intracellular acidosis, all of which are potential triggering factors for sickling and vaso-occlusive crises. GBT1118 is a HbS polymerization inhibitor which can increase hemoglobin affinity for oxygen and the corresponding supply to tissues. On that basis, we hypothesized that GBT1118 could dampen skeletal muscle energetics and function. Materials and methods: Skeletal muscle energetics and function of sixteen Townes mice (2 ± 0.5 month-old) were assessed in response to a standardized Rest-Stimulation(1Hz)-Recovery protocol before and after an 8-week period (8WP). Investigations were conducted using a setup designed to be operational inside a 7T horizontal MRI scanner (PharmaScan-Bruker). The setup allows (i) magnetic resonance imaging, (ii) muscle force measurements and (iii) 31P-magnetic resonance spectroscopy. Assessments conducted before and after the 8WP were identified with number 1 (SCDGBT1) and two (SCDGBT2). Ten mice received a chronic supplementation of GBT1118 (4g/kg in chow) during the 8WP (SCDGBT), whereas six mice had access to not supplemented chow (SCDCON). Results: The specific peak force (sPf) measured at the beginning of stimulation significantly increased over the 8WP (SCDCON2 +68%, p < 0.05). This increased initial sPf was also present in SCDGBT2 as compared to SCDGBT1 (+ 30%, p < 0.05). Over the entire stimulation session, the overall specific [TM1] force production (sTFP) significantly increased over the 8WP (SCDCON2 +32%, p < 0.05, Figure 1). The increased sTFP was also present in SCDGBT2 as compared to SCDGBT1 (+ 25 %, p < 0.05). However, the sTFP-normalized pHi reduction and sTFP-normalized phosphocreatine (PCr) breakdown significantly decreased in SCDGBT2 as compared to SCDGBT1 (p < 0.05, Table 1). Conclusion: In the control condition, the 8-week period was accompanied by a significant increase in specific total force production. Such an increase has been previously reported from 3 to 10 weeks of age (+ 33%). Overall, the increased specific total force production we reported was related to time and cannot be accounted for by GBT supplementation. In SCDGBT, the expected increased force production related to time was associated to a reduced acidosis and PCr breakdown which were not found in the control condition. For a given amount of force, SCDGBT mice muscle would experience a lower net proton accumulation and a reduced net PCr consumption. This reduction of both acidosis and PCr consumption could illustrate a diminished cost of contraction in response to GBT1118 supplementation which could be related to an improved contractile efficiency as previously described in humans. SCD mice treated with GBT1118 exhibit a reduced acidosis for a given force. Given that acidosis is a potential triggering factor of sickling, GBT would protect SCD muscle against the consequences of sickling. One could speculate that SCD patients receiving GBT1118 might be able to perform exercise with a reduced exposure to sickling.Specific total force production in SCDCON and SCDGBT before and after the 8-week period. Each point represents a single measurement and lines are linking consecutive points. p-values are mentioned on the top of the figure.Metabolic indices in SCDCON and SCDGBT before and after the 8-week period. The authors do not declare any conflict of interest

Effects of Hydroxyurea on Skeletal Muscle Energetics and Function in a Mildly Anemic Mouse Model.

Hydroxyurea (HU) is a ribonucleotide reductase inhibitor most commonly used as a therapeutic agent in sickle cell disease (SCD) with the aim of reducing the risk of vaso-occlusion and improving oxygen transport to tissues. Previous studies suggest that HU may be even beneficial in mild anemia. However, the corresponding effects on skeletal muscle energetics and function have never been reported in such a mild anemia model. Seventeen mildly anemic HbAA Townes mice were subjected to a standardized rest-stimulation (transcutaneous stimulation)-protocol while muscle energetics using 31Phosphorus magnetic resonance spectroscopy and muscle force production were assessed and recorded. Eight mice were supplemented with hydroxyurea (HU) for 6 weeks while 9 were not (CON). HU mice displayed a higher specific total force production compared to the CON, with 501.35 ± 54.12 N/mm3 and 437.43 ± 57.10 N/mm3 respectively (+14.6%, p < 0.05). Neither the total rate of energy consumption nor the oxidative metabolic rate were significantly different between groups. The present results illustrated a positive effect of a HU chronic supplementation on skeletal muscle function in mice with mild anemia.

How does switching synchronization of pitching parallel foils from out-of-phase to in-phase change their wake dynamics?

Alterations to the unsteady wake dynamics imposed by abrupt changes in the phase angle between two pitching side-by-side foils are computationally examined at the Reynolds number of 1000 and 4000 and Strouhal number of 0.25–0.5. Four hybrid modes are considered in this study inspired by the swimming habits of red nose tetra fish and burst-and-coast swimming phenomenon. At the higher Strouhal number of 0.50, abrupt changes in the phase angle result in the formation and growth of a secondary vortex street between the two primary streets, which enable and maintain a split-wake configuration. Furthermore, phase switching alters pressure levels on the top and bottom surfaces of both foils to similar levels, which attribute to lowering the side-force. The growth rate of the secondary vortex street remains consistent for all four hybrid modes. At lower Strouhal numbers (0.25–0.4), however, the abrupt change in the phase angle converts the wake to a single vortex street. Thus, this indicates that the wake reactions for such cases in synchronization substantially change at lower Strouhal number. Although a different behavior of total side force production is observed at a lower Reynolds number for Strouhal number of 0.50, the wake dynamics implies that phase alterations act as a similar flow control mechanism to stabilize the wake. Finally, it is identified that the suspension of oscillations significantly limits the implications of initiation of oscillations on wake dynamics and performance following abrupt changes in the phase angle.

Reliability of bilateral and shear components in a two-legged counter-movement jump

Strength asymmetry can be detrimental to athlete performance and may lead to injury. The countermovement jump (CMJ) can be used to measure strength asymmetry via shear force production. The reliability of parameters and effects of asymmetry and shear force production on vertical CMJ performance were evaluated in a study with 15 university-level sprint and high jump athletes ( m = 11, f = 4). The athletes performed three CMJs on two occasions, separated by 1 week. Tri-axial ground reaction force (GRF) was recorded using two force platforms embedded within a bespoke weight training area. Key performance metrics were calculated in real-time describing total CMJ performance, asymmetry and shear force production. Changes in the means and coefficients of variation (CV) were used to express reliability. Twenty-six parameters from the Total analysis and 21 Asymmetry analysis parameters showed a CV lower than 10%. Temporal and kinetic variables describing Asymmetry analysis highlight a lower CV compared with equivalent parameters derived from Total analysis. Shear parameters show high levels of CV compared with Total analysis and Asymmetry analysis. The measures of asymmetry calculated using methods described in this work were shown to be reliable for monitoring CMJ performance. No significant negative relationships were found between measures of asymmetry or shear force and traditional performance metrics in the CMJ (e.g. jump height, specific peak power and peak force). Further work is required to identify the potential of reducing asymmetry on CMJ performance.

Is the Rear Foot Elevated Split Squat Unilateral? An Investigation Into the Kinetic and Kinematic Demands.

Helme, M, Emmonds, S, and Low, C. Is the rear foot elevated split squat unilateral? An investigation into the kinetic and kinematic demands. J Strength Cond Res 36(7): 1781-1787, 2022-The purpose of the study was to determine the unilateral nature of the rear foot elevated split squat (RFESS). Specifically, the production of force by the rear leg was examined to better understand its role, if any, toward successful completion of the exercise. Male volunteers were recruited (n = 26, age = 23.8 ± 4.6 years, body mass = 88.1 ± 10.7 kg, and height = 1.79 ± 0.1 m), who were recreationally trained and engaged in a structured strength and conditioning program, including both bilateral and unilateral exercises, and had at least 2 years supervised training experience. Subjects participated in an incremental 5 repetition maximum protocol after familiarization. Kinetic data were recorded through 2 independent force plates, 1 integral to the floor and the second mounted on top of solid weightlifting blocks. Kinematic data were captured through 3-dimensional motion analysis. A total of 715 repetitions were analyzed, and the mean contribution of the lead foot to total vertical force production was 84.36 ± 3.6%. An almost certainly small positive correlation (rho = 0.25, confidence interval [CI] 0.18-0.33) was found between the percentage of force produced by the lead foot, with increasing exercise intensity. A most likely trivial, nonsignificant correlation (rho = -0.01, CI -0.09 to 0.06) with rear foot force production, representing the mass of the rear leg. Data from this study do not indicate that the rear foot contributes to the kinetic demands of the exercise and therefore suggests that the RFESS is a valid unilateral exercise.

Residual force enhancement due to active muscle lengthening allows similar reductions in neuromuscular activation during position- and force-control tasks

BackgroundResidual torque enhancement (rTE) is the increase in torque observed during the isometric steady state following active muscle lengthening when compared with a fixed-end isometric contraction at the same muscle length and level of neuromuscular activation. In the rTE state, owing to an elevated contribution of passive force to total force production, less active force is required, and there is a subsequent reduction in activation. In vivo studies of rTE reporting an activation reduction are often performed using a dynamometer, where participants contract against a rigid restraint, resisting a torque motor. rTE has yet to be investigated during a position task, which involves the displacement of an inertial load with positional control. MethodsA total of 12 participants (6 males, 6 females; age = 22.8 ± 1.1 years, height = 174.7 ± 8.6 cm, mass = 82.1 ± 37.7 kg; mean ± SD) completed torque- and position-matching tasks at 60% maximum voluntary contraction for a fixed-end isometric contraction and an isometric contraction following active lengthening of the ankle dorsiflexors. ResultsThere were no significant differences in activation between torque- and position-matching tasks (p = 0.743), with ∼27% activation reduction following active lengthening for both task types (p < 0.001). ConclusionThese results indicate that rTE is a feature of voluntary, position-controlled contractions. These findings support and extend previous findings of isometric torque-control conditions to position-controlled contractions that represent different tasks of daily living.

Cross-Bridge Cycling Kinetics Slow at Longer Muscle Length in Tetanic Contracting Mouse Soleus Muscle

Muscle contraction results from force-generating cross-bridge interactions between myosin and actin. Cross-bridge cycling kinetics underlie fundamental properties of muscle contraction, such as total force production and energetics, and factors that influence cross-bridge kinetics at the molecular level can propagate to critically alter whole-muscle function. Reciprocally, whole-muscle properties, including movement and the length of the muscle, can influence kinetics on the cross-bridge level. Previous research using single-molecule and single-fiber experiments has suggested that cross-bridge cycling is strain-dependent, and that increasing the strain on cross-bridges may slow their cycling rate and prolong their attachment duration. However, whether this property is maintained in whole-muscle to affect cross-bridge behavior under various strains and muscle lengths remains unclear. Here, we estimated cross-bridge cycling kinetics using small-amplitude step-length perturbations (1% total muscle-tendon length) in electrically-activated mouse soleus muscles. We separated the resulting force response into distinct phases associated with cross-bridge detachment and cross-bridge recruitment rates. The initial, fast single-exponential force decay is dominated by myosin detachment events and the slower force recovery phase is dominated by cross-bridge recruitment events. We found that cross-bridge detachment kinetics were ∼30% slower when muscles were stretched 10% longer than optimal length (Lo) (t1/2=8.75 ms vs 6.65 ms at Lo, 17°C). Detachment kinetics were ∼35% faster at 10% shorter length (t1/2=4.25ms at 90% of Lo). These findings suggest that cross-bridge kinetics vary with muscle length during intact, tetanic contraction in skeletal muscle, which may follow from strain-dependent mechanisms of cross-bridge cycling and could intrinsically modulate whole muscle force-generation and energetics during locomotion. This study advances our understanding of the classic Length-Tension relationship of skeletal muscle and implies that strain-dependent cross-bridge kinetics contribute another feedback pathway between whole-muscle function and cross-bridge activity.

Force-Velocity-Power Profiling During Weighted-Vest Sprinting in Soccer

Purpose: To describe the load-velocity relationship and the effects of increasing loads on spatiotemporal and derived kinetic variables of sprinting using weighted vests (WV) in soccer players and determining the load that maximizes power output. Methods: A total of 23 soccer players (age 20.8 [1.5]y) performed 10 maximal 30-m sprints wearing a WV with 5 different loads (0%, 10%, 20%, 30%, and 40% body mass [BM]). Sprint velocity and time were collected using a radar device and wireless photocells. Mechanical outputs were computed using a recently developed valid and reliable field method that estimates the step-averaged ground-reaction forces during overground sprint acceleration from anthropometric and spatiotemporal data. Raw velocity-time data were fitted by an exponential function and used to calculate the net horizontal ground-reaction forces and horizontal power output. Individual linear force-velocity relationships were then extrapolated to calculate the theoretical maximum horizontal force (F0) and velocity and the ratio of force application (proportion of the total force production that is directed forward at sprint start). Results: Magnitude-based inferences showed an almost certain decrease in F0 (effect size = 0.78-3.35), maximum power output (effect size = 0.78-3.81), and maximum ratio of force (effect size = 0.82-3.87) as the load increased. The greatest changes occurred with loads heavier than 20% BM, especially in ratio of force. In addition, the maximum power was achieved under unloaded conditions. Conclusions: Increasing load in WV sprinting affects spatiotemporal and kinetic variables. The greatest change in ratio of force happened with loads heavier than 20% BM. Thus, the authors recommend the use of loads ≤20% BM for WV sprinting.

Exercise, but Not Metformin Prevents Muscle Function Loss due to Doxorubicin in Mice Using an In‐Situ Model

Doxorubicin (DOX) is a clinically relevant chemotherapeutic known to cause severe side effects in heart and skeletal muscle. The deleterious effects DOX elicits on skeletal muscle are concerning due to the high rates of severe fatigue experienced by chemotherapy patients. Identifying treatments that prevent this skeletal muscle dysfunction is critically important. Exercise (EX) and metformin (MET) have both been suggested as treatments to reduce the toxic effects of DOX. However, their effects on muscle function in the gastrocnemius‐plantaris‐soleus (GPS) complex are unknown. Additionally, the mechanism by which DOX decreases muscle function are still under investigation. DOX can cause mitochondrial disruption, but the direct effects of mitochondrial function on muscle function have not been studied in this model. The purpose of this study is three fold: (1) to determine if DOX decreased muscle function of the GPS complex using an in‐situ muscle fatigue protocol, (2) determine if EX or MET treatment can prevent muscle function loss due to DOX, and (3) determine if muscle function loss is due to a decrease in mitochondrial respiratory function. C57BL/6 mice were randomly assigned to 4 groups: CON, DOX, DOX+EX, or DOX+MET. DOX was administered as a single intraperitoneal injection at 15 mg/kg body weight 3 days before sacrifice. Moderate EX treatment was done by treadmill running for 60 minutes for 5 days at 70% max prior to DOX injection. MET treatment was administered via oral gavage at 500 mg/kg body weight for 5 days beginning 2 days before DOX injection. GPS complex muscle function was measured in‐situ using tetanic contractions elicited by direct stimulation in a 6 minute fatigue protocol. Real time mitochondrial respiratory function was measured in gastrocnemius muscle. We report that DOX treatment caused ~20% body weight loss at 3 days, an effect that was partially prevented in DOX+EX (p&lt;0.05) or DOX+MET (p=0.06). A small decrease in total force production was observed with DOX, which trended towards significance (0.08≥p≤0.04). DOX+EX treatment restored force production back to CON levels, but DOX+MET did not. The half relaxation rate was impaired by an average of 18% with DOX treatment (p&lt;0.05). DOX+EX prevented this impairment of the half relaxation rate (p&lt;0.05). DOX+MET did not restore impaired half relaxation rate caused by DOX. Mitochondrial respiration was not different between the groups, suggesting that the changes in muscle function occur independent of mitochondrial function. In conclusion, DOX treatment decreased muscle function in the GPS complex by impairing half relaxation rates. EX but not MET treatment prevented impaired half relaxation rate. These changes occurred independent of mitochondrial respiration.Support or Funding InformationMentoring Environment Grant, Brigham Young UniversityThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Endurance training reduces exercise-induced acidosis and improves muscle function in a mouse model of sickle cell disease

Sickle cell disease (SCD) mice (Townes model of SCD) presented exacerbated exercise-induced acidosis and fatigability as compared to control animals. We hypothesize that endurance training could represent a valuable approach to reverse these muscle defects. Endurance-trained HbAA (HbAA-END, n=10), HbAS (HbAS-END, n=11) and HbSS (HbSS-END, n=8) mice were compared to their sedentary counterparts (10 HbAA-SED, 10 HbAS-SED and 9 HbSS-SED mice) during two rest – exercise – recovery protocols during which muscle energetics and function were measured. In vitro analyses of some proteins involved in muscle energetics, pH regulation and oxidative stress were also performed. Exercise-induced acidosis was lower in HbSS-END mice as compared to their sedentary counterparts during both moderate (p<0.001) and intense (p<0.1) protocols. The total force production measured during both protocols was higher in trained mice compared to sedentary animals. In vitro analyses revealed that enolase/citrate synthase ratio was reduced in HbSS-END (p<0.001) and HbAS-END (p<0.01) mice compared to their sedentary counterparts. In addition, malondialdehyde concentration was reduced in trained mice (p<0.05). In conclusion, endurance training would reverse the more pronounced exercise-induced acidosis, reduce oxidative stress and ameliorate some of the muscle function parameters in SCD mice.

Stability of hand force production. I. Hand level control variables and multifinger synergies.

We combined the theory of neural control of movement with referent coordinates and the uncontrolled manifold hypothesis to explore synergies stabilizing the hand action in accurate four-finger pressing tasks. In particular, we tested a hypothesis on two classes of synergies, those among the four fingers and those within a pair of control variables, stabilizing hand action under visual feedback and disappearing without visual feedback. Subjects performed four-finger total force and moment production tasks under visual feedback; the feedback was later partially or completely removed. The "inverse piano" device was used to lift and lower the fingers smoothly at the beginning and at the end of each trial. These data were used to compute pairs of hypothetical control variables. Intertrial analysis of variance within the finger force space was used to quantify multifinger synergies stabilizing both force and moment. A data permutation method was used to quantify synergies among control variables. Under visual feedback, synergies in the spaces of finger forces and hypothetical control variables were found to stabilize total force. Without visual feedback, the subjects showed a force drift to lower magnitudes and a moment drift toward pronation. This was accompanied by disappearance of the four-finger synergies and strong attenuation of the control variable synergies. The indexes of the two types of synergies correlated with each other. The findings are interpreted within the scheme with multiple levels of abundant variables.NEW & NOTEWORTHY We extended the idea of hierarchical control with referent spatial coordinates for the effectors and explored two types of synergies stabilizing multifinger force production tasks. We observed synergies among finger forces and synergies between hypothetical control variables that stabilized performance under visual feedback but failed to stabilize it after visual feedback had been removed. Indexes of two types of synergies correlated with each other. The data suggest the existence of multiple mechanisms stabilizing motor actions.

Sarcomere and Inter-Sarcomere Dynamics within Skeletal Muscle Myofibrils

Introduction: Inter-sarcomere dynamics within muscle myofibrils underlie basic mechanisms of contraction. In this study, we used microfluidic perfusions to control the environment of sarcomeres within isolated myofibrils to directly investigate inter-sarcomere dynamics. Methods: Pre-calibrated micro-needles were used to hold single myofibrils from rabbit skeletal muscles. A micropipette was used to locally control individual sarcomeres or groups of sarcomeres. The experimental were recorded in real time and analyzed for measurements of force and changes in length in individual sarcomeres within a myofibril. Results:(A) We tested length dependence of inter-sarcomere dynamics. Three initial average sarcomere lengths (SLi) were tested: (I) between 2.4-2.65μm, (II) between 2.65-2.9μm, and (III) above 2.9μm. Activation of the target sarcomere induced the displacement of the adjacent sarcomeres and the micro-needles towards the activation point. Force produced by the myofibril, and the displacements of sarcomeres situated adjacent to local activation were larger at increasing SLi ((I) 1.01±0.03μm, (II) 1.17±0.03μm, (III) 1.28±0.04μm). (B) The displacement of adjacent sarcomeres was further increased when the myofibril was tested in rigor conditions (1.08±0.03μm) in comparison with relaxing conditions (0.92±0.04μm), suggesting that the interaction among sarcomeres is regulated by myofibril stiffness. (C) All sarcomeres comprising a myofibril produced similar active forces (21.07±0.56nN/μm2) in a given SLi. (D) Long myofibrils produce higher forces than short myofibrils, and also presented higher levels of non-uniformity among sarcomeres, affecting the final magnitude of myofibril shortening. (E) Sarcomeres contracting along the descending limb of the force-sarcomere length relation negatively affect the total myofibril shortening and force production. Conclusion: Inter-sarcomere dynamics requires the cooperative work of the contractile and elastic elements in the in order to transmit force from sarcomere to sarcomere.

Anticipatory synergy adjustments reflect individual performance of feedforward force control

We grasp and dexterously manipulate an object through multi-digit synergy. In the framework of the uncontrolled manifold (UCM) hypothesis, multi-digit synergy is defined as the coordinated control mechanism of fingers to stabilize variable important for task success, e.g., total force. Previous studies reported anticipatory synergy adjustments (ASAs) that correspond to a drop of the synergy index before a quick change of the total force. The present study compared ASA’s properties with individual performances of feedforward force control to investigate a relationship of those. Subjects performed a total finger force production task that consisted of a phase in which subjects tracked target line with visual information and a phase in which subjects produced total force pulse without visual information. We quantified their multi-digit synergy through UCM analysis and observed significant ASAs before producing total force pulse. The time of the ASA initiation and the magnitude of the drop of the synergy index were significantly correlated with the error of force pulse, but not with the tracking error. Almost all subjects showed a significant increase of the variance that affected the total force. Our study directly showed that ASA reflects the individual performance of feedforward force control independently of target-tracking performance and suggests that the multi-digit synergy was weakened to adjust the multi-digit movements based on a prediction error so as to reduce the future error.

Endurance Training Improves Muscle Function And Metabolism In A Sickle Cell Disease Mouse Model

Sickle cell disease (SCD) is an inherited disorder resulting in the synthesis of an abnormal hemoglobin (HbS) and associated with the occurrence of painful vaso-occlusive crisis (VOC). While it has been recently suggested that oxygen supply and use might be impaired due to muscle remodeling, the corresponding metabolic effects in exercising muscle are still unknown. Considering that regular moderate-intensity exercise is known to be beneficial on muscle structure and energetics, endurance training could alleviate the muscle remodelling described in SCD and improve muscle function. PURPOSE: The aim of this study was to characterize in a mouse model of SCD i) the potential abnormalities in muscle energetics and function, and ii) the potential beneficial effects of endurance training. METHODS: Sedentary control (HbAA-SED, n=10), heterozygous (HbAS-SED, n=10) and sickle cell (HbSS-SED, n=10) mice and endurance-trained (8-weeks treadmill running) healthy (HbAA-END, n=10), heterozygous (HbAS-END, n=11) and sickle cell (HbSS-END, n=8) mice were placed in a home-built device designed for a strictly non-invasive study of posterior hindlimb muscles function and metabolism by NMR during a standardized rest - stimulation - recovery protocol. RESULTS: During the stimulation period, HbSS mice developed less force as compared to HbAS mice (p<0.01) and HbAS mice displayed a reduced total force production as compared to HbAA mice (p<0.05). Whatever the genotype, training resulted in a significantly increased force production (p<0.001). Regarding muscle energetics, the exercise-induced phosphocreatine (PCr) depletion and pH decrease were significantly (p < 0.05) larger in HbSS mice as compared to HbAA and HbAS mice. Interestingly, training resulted in a significant (p<0.05) reduction in PCr depletion and intracellular acidosis. CONCLUSIONS: We demonstrated in the present study that SCD was associated with an impaired muscle force production and a larger exercise-induced metabolic consumption. The larger acidosis, which could trigger VOC, might be explained by both an increased non-oxidative glycolytic energy production and an altered protons handling. Interestingly, endurance training improved muscle function and reduced metabolic abnormalities, including acidosis.

The effects of grip width on sticking region in bench press

The aim of this study was to examine the occurrence of the sticking region by examining how three different grip widths affect the sticking region in powerlifters’ bench press performance. It was hypothesised that the sticking region would occur at the same joint angle of the elbow and shoulder independent of grip width, indicating a poor mechanical region for vertical force production at these joint angles. Twelve male experienced powerlifters (age 27.7 ± 8.8 years, mass 91.9 ± 15.4 kg) were tested in one repetition maximum (1-RM) bench press with a narrow, medium and wide grip. Joint kinematics, timing, bar position and velocity were measured with a 3D motion capture system. All participants showed a clear sticking region with all three grip widths, but this sticking region was not found to occur at the same joint angles in all three grip widths, thereby rejecting the hypothesis that the sticking region would occur at the same joint angle of the elbow and shoulder independent of grip width. It is suggested that, due to the differences in moment arm of the barbell about the elbow joint in the sticking region, there still might be a poor mechanical region for total force production that is joint angle-specific.

Changes in skeletal muscle and tendon structure and function following genetic inactivation of myostatin in rats.

Myostatin is a negative regulator of skeletal muscle and tendon mass. Myostatin deficiency has been well studied in mice, but limited data are available on how myostatin regulates the structure and function of muscles and tendons of larger animals. We hypothesized that, in comparison to wild-type (MSTN(+/+) ) rats, rats in which zinc finger nucleases were used to genetically inactivate myostatin (MSTN(Δ/Δ) ) would exhibit an increase in muscle mass and total force production, a reduction in specific force, an accumulation of type II fibres and a decrease and stiffening of connective tissue. Overall, the muscle and tendon phenotype of myostatin-deficient rats was markedly different from that of myostatin-deficient mice, which have impaired contractility and pathological changes to fibres and their extracellular matrix. Extensor digitorum longus and soleus muscles of MSTN(Δ/Δ) rats demonstrated 20-33% increases in mass, 35-45% increases in fibre number, 20-57% increases in isometric force and no differences in specific force. The insulin-like growth factor-1 pathway was activated to a greater extent in MSTN(Δ/Δ) muscles, but no substantial differences in atrophy-related genes were observed. Tendons of MSTN(Δ/Δ) rats had a 20% reduction in peak strain, with no differences in mass, peak stress or stiffness. The general morphology and gene expression patterns were similar between tendons of both genotypes. This large rodent model of myostatin deficiency did not have the negative consequences to muscle fibres and extracellular matrix observed in mouse models, and suggests that the greatest impact of myostatin in the regulation of muscle mass may not be to induce atrophy directly, but rather to block hypertrophy signalling.

Aerodynamic forces and flow structures of the leading edge vortex on a flapping wing considering ground effect

The aim of this work is to provide an insight into the aerodynamic performance of the beetle during takeoff, which has been estimated in previous investigations. We employed a scaled-up electromechanical model flapping wing to measure the aerodynamic forces and the three-dimensional flow structures on the flapping wing. The ground effect on the unsteady forces and flow structures were also characterized. The dynamically scaled wing model could replicate the general stroke pattern of the beetle's hind wing kinematics during takeoff flight. Two wing kinematic models have been studied to examine the influences of wing kinematics on unsteady aerodynamic forces. In the first model, the angle of attack is asymmetric and varies during the translational motion, which is the flapping motion of the beetle's hind wing. In the second model, the angle of attack is constant during the translational motion. The instantaneous aerodynamic forces were measured for four strokes during the beetle's takeoff by the force sensor attached at the wing base. Flow visualization provided a general picture of the evolution of the three-dimensional leading edge vortex (LEV) on the beetle hind wing model. The LEV is stable during each stroke, and increases radically from the root to the tip, forming a leading-edge spiral vortex. The force measurement results show that the vertical force generated by the hind wing is large enough to lift the beetle. For the beetle hind wing kinematics, the total vertical force production increases 18.4% and 8.6% for the first and second strokes, respectively, due to the ground effect. However, for the model with a constant angle of attack during translation, the vertical force is reduced during the first stroke. During the third and fourth strokes, the ground effect is negligible for both wing kinematic patterns. This finding suggests that the beetle's flapping mechanism induces a ground effect that can efficiently lift its body from the ground during takeoff.

Effect of fatigue on force production and force application technique during repeated sprints

We investigated the changes in the technical ability of force application/orientation against the ground vs. the physical capability of total force production after a multiple-set repeated sprints series. Twelve male physical education students familiar with sprint running performed four sets of five 6-s sprints (24 s of passive rest between sprints, 3 min between sets). Sprints were performed from a standing start on an instrumented treadmill, allowing the computation of vertical ( F V), net horizontal ( F H) and total ( F Tot) ground reaction forces for each step. Furthermore, the ratio of forces was calculated as RF= F H F Tot −1, and the index of force application technique ( D RF) representing the decrement in RF with increase in speed was computed as the slope of the linear RF-speed relationship. Changes between pre- (first two sprints) and post-fatigue (last two sprints) were tested using paired t-tests. Performance decreased significantly (e.g. top speed decreased by 15.7±5.4%; P<0.001), and all the mechanical variables tested significantly changed. F H showed the largest decrease, compared to F V and F Tot. D RF significantly decreased ( P<0.001, effect size=1.20), and the individual magnitudes of change of D RF were significantly more important than those of F Tot (19.2±20.9 vs. 5.81±5.76%, respectively; P<0.01). During a multiple-set repeated sprint series, both the total force production capability and the technical ability to apply force effectively against the ground are altered, the latter to a larger extent than the former.