Maximal Isometric Force Research Articles

A portable articulated dynamometer for ankle dorsiflexion and plantar flexion strength measurement: a design, validation, and user experience study

Monitoring ankle strength is crucial for assessing daily activities, functional ability, and preventing lower extremity injuries. However, the current methods for measuring ankle strength are often unreliable or not easily portable to be used in clinical settings. Therefore, this study proposes a portable dynamometer with high reliability capable of measuring ankle dorsiflexion and plantar flexion. The proposed portable dynamometer comprised plates made of aluminum alloy 6061 and a miniature tension–compression load cell. A total of 41 healthy adult participants applied maximal isometric dorsiflexor and plantar flexor forces on a Lafayette Handheld Dynamometer (HHD) and the portable dynamometer. The results were cross-validated, using change in mean, and two independent examiners evaluated the inter-rater and test–retest reliabilities in separate sessions using intraclass correlation coefficients, standard error of measurement, and minimal detectable change. Both dorsiflexion and plantar flexion measurements demonstrated a strong correlation with the HHD (r = 0.827; r = 0.973) and showed high inter-rater and test–retest reliabilities. Additionally, the participant responses to the user experience questionnaire survey indicated vastly superior positive experiences with the portable dynamometer. The study findings suggest that the designed portable dynamometer can provide accurate and reliable measurements of ankle strengths, making it a potential alternative to current methods in clinical settings.

Sensitivity Analysis of Upper Limb Musculoskeletal Models During Isometric and Isokinetic Tasks.

Sensitivity coefficients are used to understand how errors in subject-specific musculoskeletal model parameters influence model predictions. Previous sensitivity studies in the lower limb calculated sensitivity using perturbations that do not fully represent the diversity of the population. Hence, the present study performs sensitivity analysis in the upper limb using a large synthetic dataset to capture greater physiological diversity. The large dataset (n = 401 synthetic subjects) was created by adjusting maximum isometric force, optimal fiber length, pennation angle, and bone mass to induce atrophy, hypertrophy, osteoporosis, and osteopetrosis in two upper limb musculoskeletal models. Simulations of three isometric and two isokinetic upper limb tasks were performed using each synthetic subject to predict muscle activations. Sensitivity coefficients were calculated using three different methods (two point, linear regression, and sensitivity functions) to understand how changes in Hill-type parameters influenced predicted muscle activations. The sensitivity coefficient methods were then compared by evaluating how well the coefficients accounted for measurement uncertainty. This was done by using the sensitivity coefficients to predict the range of muscle activations given known errors in measuring musculoskeletal parameters from medical imaging. Sensitivity functions were found to best account for measurement uncertainty. Simulated muscle activations were most sensitive to optimal fiber length and maximum isometric force during upper limb tasks. Importantly, the level of sensitivity was muscle and task dependent. These findings provide a foundation for how large synthetic datasets can be applied to capture physiologically diverse populations and understand how model parameters influence predictions.

The effect of changing the length of the components of the muscle-tendon unit of the knee joint on its isometric force and torque

Background. A knee contracture developed due to immobilization after extra-articular injuries leads to lameness, impaired walking, and the appearance of pain in other joints of the lower limb. Long-term immobilization of the joint leads to weakening and increasing stiffness of muscles, changes in the length of tendons and myofibrils, loss of cushioning properties of cartilage, inhibition of blood circulation processes in the periarticular tissues. Adequate rehabilitation and recovery of the knee joint function after immobilization is a medical and social problem in terms of reducing disability. The goal of the work: to study the influence of changes in strength, length of muscles and tendons of the knee joint on torque. Materials and methods. The OpenSim Leg39.osim base model was used for simulation. The parameters of the muscles and tendons were changed in accordance with the conditions of immobilization for 45 days at the angle of preferential position of the limb — 15°. Three models were analyzed: a basic model without changes in muscle parameters, a model with a change in the length of the muscle-tendon unit, and a model with an additional change in muscle strength (according to calculations). Results. Changing the length of the components of the muscle-tendon unit of the flexors has little effect on both the isometric strength of the muscles and the knee joint torques produced by these muscles. A decrease in the maximum isometric force of the flexor muscles leads to a decrease in torque. When the length of the extensors changes, there is a sharp increase in the isometric force and torques of the knee joint and a change in the direction of the active torque trajectory. A decrease in muscle strength leads to a decrease in the level of torques approaching normal, but the direction remains unchanged. Conclusions. Mathematical modeling of the effect of changes in the components of the muscle-tendon unit of the muscles responsible for the movement of the knee joint showed that minor alterations in their length lead to noticeable violations of both the torque of the joint and changes in the isometric strength of the muscle during movements. Predicting the change in knee joint torque can be useful in studying specific clinical situations with contractures caused by a long-term immobilization after extra-articular injuries. But it cannot be fully transferred into practice due to the significant conditionality of model parameters. The used modeling method can show trends in the change of muscle functioning parameters when changing their geometry, and can be extended to study of other muscle and joint parameters.

Upper Extremity Muscle Activation Pattern Prediction Through Synergy Extrapolation and Electromyography-Driven Modeling.

Patients with neuromuscular disease fail to produce necessary muscle force and have trouble maintaining joint moment required to perform activities of daily living. Measuring muscle force values in patients with neuromuscular disease is important but challenging. Electromyography (EMG) can be used to obtain muscle activation values, which can be converted to muscle forces and joint torques. Surface electrodes can measure activations of superficial muscles, but fine-wire electrodes are needed for deep muscles, although it is invasive and require skilled personnel and preparation time. EMG-driven modeling with surface electrodes alone could underestimate the net torque. In this research, authors propose a methodology to predict muscle activations from deeper muscles of the upper extremity. This method finds missing muscle activation one at a time by combining an EMG-driven musculoskeletal model and muscle synergies. This method tracks inverse dynamics joint moments to determine synergy vector weights and predict muscle activation of selected shoulder and elbow muscles of a healthy subject. In addition, muscle-tendon parameter values (optimal fiber length, tendon slack length, and maximum isometric force) have been personalized to the experimental subject. The methodology is tested for a wide range of rehabilitation tasks of the upper extremity across multiple healthy subjects. Results show this methodology can determine single unmeasured muscle activation up to Pearson's correlation coefficient (R) of 0.99 (root mean squared error, RMSE = 0.001) and 0.92 (RMSE = 0.13) for the elbow and shoulder muscles, respectively, for one degree-of-freedom (DoF) tasks. For more complicated five DoF tasks, activation prediction accuracy can reach up to R = 0.71 (RMSE = 0.29).

Changes in the Force-Velocity Relationship of Knee Muscles After Anterior Cruciate Ligament Reconstruction Using the Isokinetic 2-Point Model.

After anterior cruciate ligament reconstruction (ACL-R), knee muscle strength symmetry is used as part of the return-to-sport criteria. However, little is known about the changes in the force-velocity (F-V) relationship, which could affect athletic performance. This study investigated the F-V relationship of knee muscles at 4 and 8months after ACL-R, using the 2-point method tested by isokinetic dynamometry. A total of 103 physically trained individuals (24.6 [9.3]y, 59.2% male) who underwent primary ACL-R were included. Demographic information and surgery characteristics were collected at 6weeks postoperatively. Isokinetic knee flexors' and extensors' peak torques were measured at 60° and 240° per second in the concentric mode at 4 and 8 monthspostoperative. Peak torques and angular velocities were converted to force and linear velocity for calculating maximum isometric force (F0) and the slope of the regression line (F-V slope). At 4 and 8 months postoperative, F0 was significantly lower and F-V slope was significantly less steep (less negative) on the operated leg compared with the nonoperated leg for knee extensors (P < .001) and flexors (P < .001-.002). The limb symmetry index calculated using F0 was lower than the limb symmetry indexes assessed at 60° and 240° per second, especially for knee flexors (P < .001). The use of patellar tendon grafts was associated with lower F0 and a less steep F-V slope compared with hamstring tendon grafts (P < .010). The isokinetic 2-point model assessing the F-V relationship provides additional and relevant insight for evaluating knee muscle strength after ACL-R.

An exploration of the mechanistic link between the enhanced paper grip test and the risk of falling

The enhanced paper grip test (EPGT) offers an easy-to-use measure of hallux plantar-flexion strength that does not need expensive specialised equipment. Literature suggests that it could be a useful screening tool to assess the risk of falling in older people. However, research on a specific mechanistic link to the risk of falling is lacking. It is hypothesised here that muscle weakening (assessed by the EPGT) is indicative of impaired ability to recover balance after a slip or a trip. To get an initial assessment of validity of the above hypothesis, the EPGT is compared against an established lab-based measure of lower-limb strength that is capable of assessing a person’s ability to recover balance after a slip or a trip: maximum isometric leg press push-off force (leg press force). A gender-balanced sample of twenty people (median age=34 y) was recruited. Two different but equaly valid techniques of administering the EPGT were included regarding whether the participants’ ankle was supported by the examiner or not. Results for the two EPGT techniques differed susbtantialy but they were both significantly associated with leg press force and therefore linked to better ability to maintain balance after a slip or a trip. The “ankle not held” EPGT technique was more strongly correlated to leg press force (r(18) = 0.652, p = 0.002) than the “ankle held” (r(18) = 0.623, p = 0.003) and appears to be the more favourable technique to administer the EPGT. These findings offer new insight on a potential mechanistic link between the EPGT and the risk of falling and support its optimal use in future research involving older people.

A longer Achilles tendon moment arm length is not associated with superior hopping performance.

Variability in musculoskeletal and lower leg structure has the potential to influence hopping height. Achilles tendon moment arm length and plantarflexor muscle strength can influence ankle joint torque development and, consequently, hopping performance. While most studies have examined the connection of the Achilles tendon moment arm with hopping performance including the resting length, in this study we attempted to explore how the changes in Achilles tendon moment arm are related to hopping performance. Therefore, the purpose of this study was to test for correlations between foot and lower leg muscle structure parameters (i.e., muscle mass, volume, cross-sectional area and Achilles tendon moment arm length) and hopping height performance in relation to changes in Achilles tendon moment arm length. Eighteen participants (10 males 8 female) performed repetitive bilateral hopping on a force platform while sagittal plane kinematics of the lower leg were recorded. Additionally, maximal isometric plantarflexion was measured. To obtain structural parameters of the lower leg, the right lower leg of each participant was scanned with magnetic resonance imaging. The cross-sectional areas of the Achilles tendon, soleus, lateral and medial gastrocnemius were measured, while muscle volumes, muscle mass, and Achilles tendon moment arm length were calculated. Contrary to our initial assumption, longer Achilles tendon moment arm did not result in superior hopping performance. Interestingly, neither maximal isometric plantarflexion force nor muscle size correlated with repetitive bilateral hopping performance. We can assume that the mechanical characteristics of the tendon and the effective utilization of the stored strain energy in the tendon may play a more important role in repetitive hopping than the structural parameters of the lower leg.

FIM: A fatigued-injured muscle model based on the sliding filament theory

Skeletal muscle modeling has a vital role in movement studies and the development of therapeutic approaches. In the current study, a Huxley-based model for skeletal muscle is proposed, which demonstrates the impact of impairments in muscle characteristics. This model focuses on three identified ions: H+, inorganic phosphate Pi, and Ca2+. Modifications are made to actin-myosin attachment and detachment rates to study the effects of H+ and Pi. Additionally, an activation coefficient is included to represent the role of calcium ions interacting with troponin, highlighting the importance of Ca2+. It is found that maximum isometric muscle force decreases by 9.5% due to a reduction in pH from 7.4 to 6.5 and by 47.5% in case of the combination of a reduction in pH and an increase of Pi concentration up to 30 mM, respectively. Then the force decline caused by a fall in the active calcium ions is studied. When only 15% of the total calcium in the myofibrillar space is able to interact with troponin, up to 80% force drop is anticipated by the model. The proposed fatigued-injured muscle model is useful to study the effect of various shortening velocities and initial muscle-tendon lengths on muscle force; in addition, the benefits of the model go beyond predicting the force in different conditions as it can also predict muscle stiffness and power. The power and stiffness decrease by 40% and 6.5%, respectively, due to the pH reduction, and the simultaneous accumulation of H+ and Pi leads to a 50% and 18% drop in power and stiffness.

Dynamic Seeding versus Microinjection of Adipose-Derived Mesenchymal Stem Cells to Acellular Nerve Allograft Reconstructions.

Functional recovery after acellular nerve allograft (ANA) reconstruction remains inferior to that after autologous nerve grafting, but improved outcomes have been demonstrated with the addition of adipose-derived mesenchymal stem cells (MSCs). Controversy exists regarding the optimal cell-delivery method to enhance ANA reconstructions. The authors investigated the functional recovery of ANAs after dynamic seeding versus microinjection of MSCs. Forty Lewis rats underwent reconstruction of a 10-mm sciatic nerve defect. Animals were divided into 4 groups: reversed autograft, ANA alone, dynamically seeded ANA, or ANA injected with MSCs. During the survival period, ultrasound measurements of the tibialis anterior muscle cross-sectional area were performed. At 12 weeks, functional recovery was evaluated using measurements of ankle contracture, compound muscle action potential, maximum isometric tetanic force, muscle mass, histomorphometry, and immunofluorescence. The dynamic seeding and microinjection groups demonstrated higher cross-sectional tibialis anterior muscle area recovery than autografts and ANAs alone at week 8 and weeks 4 and 8, respectively. The ankle contracture and compound muscle action potential amplitude recovery were superior in autografts and both seeding methods compared with ANAs alone. The microinjection group demonstrated significantly higher isometric tetanic force, muscle mass, and number of axons compared with ANAs alone. Both seeding methods showed higher CD34 densities compared with ANAs alone. No significant differences between dynamic seeding and microinjection were observed in functional or histologic outcomes. The addition of MSCs to ANAs demonstrated earlier motor regeneration compared with autografts and ANAs alone. Both seeding methods improved functional outcomes in the rat sciatic nerve defect model.

Consequences of sarcolemma fatigue on maximal muscle strength production in patients with myalgic encephalomyelitis/chronic fatigue syndrome

BackgroundMyalgic encephalomyelitis is an invalidating chronic disease often associated with exercise-induced alterations of muscle membrane excitability (M wave). No simultaneous measurements of maximal isometric force production and sarcolemma fatigue in the same muscle group have been previously reported. We hypothesized that M wave alterations could be partly responsible for the reduced muscle force present in this invalidating disease. MethodsThis retrospective study compared two groups of patients who presented (n = 30) or not (n = 28) alterations of M waves evoked by direct muscle stimulation during and after a cycling exercise bout. The maximal handgrip strength was measured before and after exercise, concomitantly with electromyogram recordings from flexor digitorum longus muscle. The patients also answered a questionnaire to identify eventual exacerbation of their clinical symptoms following the exercise test. FindingsThe M wave amplitude significantly decreased in muscles and the M wave duration significantly increased in the group of patients with M wave alterations after exercise. Resting values of handgrip were significantly lower in patients with exercise-induced M-wave alterations than in patients without M-wave abnormalities. In patients with exercise-induced M-wave alterations, handgrip significantly decreased after exercise and the changes in handgrip and M wave were positively correlated. The frequency of post-exertion malaise, increased fatigue, myalgia, headache and cognitive dysfunction was significantly higher in patients with M-wave alterations and variations in handgrip after exercise. InterpretationThese data suggest that post-exercise sarcolemma fatigue often measured in patients with myalgic encephalomyelitis could be the cause of muscle failure.

Validity and reliability of upper body push and pull tests to determine one-repetition maximum.

The purpose of this study was to explore the validity and reliability of three different strength testing approaches to determine one-repetition maximum (1RM) in the bench press and prone bench pull. Twenty-eight recreationally active subjects (25 ± 2 years, 178 ± 8 cm, 78 ± 9 kg) were assessed for load-velocity (L-V) relationship, 1RM, maximal isometric force (MIF), and maximal repetitions to failure (MRF) in a Smith Machine on three separated sessions. Linear regression was used for L-V relationship, MIF, and MRF to predict 1RM. Level of significance was set to ρ ≤ 0.05. Reliability analyses of the varying 1RM estimations revealed mean differences from 0.6 to -1.3 kg (mainly trivial effects) between test days 1 and 2, intraclass correlation coefficient was > 0.96, and coefficient of variation (CV) was in the range 2.3-8.3% for all tests. Regarding validity, all 1RM predictions exhibited a mean difference ≤ 1.3 kg (trivial), except for the L-V relationship method that underestimated the predicted 1RM by 5 kg (small) compared to the actual bench press 1RM. However, the L-V relationship method showed the least mean absolute errors. CVs were in the range 4.5-13.2%. Standard error of the estimate was in the range 3.2-9.7 kg. Change scores for all tests were significantly correlated with change scores in actual 1RM, except for MIF in the prone bench pull. Smallest deviations in 1RM predictions were observed for the L-V relationship approach. All 1RM prediction methods were highly comparable to the traditional 1RM test. However, given the high variability associated with individual predictions for each method, they cannot be used interchangeably.

Valproic acid reduces muscle susceptibility to contraction-induced functional loss but increases weakness in two murine models of Duchenne muscular dystrophy.

Skeletal muscles in animal models of Duchenne muscular dystrophy (DMD) are more susceptible to contraction-induced functional loss, which is not related to fatigue. Valproic acid (VPA) reportedly improves serological and histological markers of damage in dystrophin-deficient murine muscle. Here, we tested whether VPA would reduce the susceptibility to contraction-induced functional loss in two murine DMD models. Adult female mdx (mild) and D2-mdx (severe) DMD murine models were administered VPA (240 mg/kg) or saline for 7 days. Some VPA-treated mdx mice also performed voluntary running in a wheel, which is known to reduce the susceptibility to contraction-induced functional loss; that is, isometric force drop following eccentric contractions. In situ muscle function was assessed before, during and after eccentric contractions. Muscle utrophin and desmin expression were also evaluated using immunoblotting. Interestingly, VPA reduced the isometric force drop following eccentric contractions in both murine models, without change in the relative eccentric maximal force and in the expression of utrophin and desmin. VPA for 7 days combined with voluntary running had no additive effect compared to VPA alone. Furthermore, VPA reduced the absolute isometric maximal force before eccentric contractions in both murine models. The results of our study indicated that VPA in both murine DMD models reduced the susceptibility to contraction-induced functional loss but increased muscle weakness.

Phosphorylation Mimetic of Myosin Regulatory Light Chain Mitigates Cardiomyopathy-Induced Myofilament Impairment in Mouse Models of RCM and DCM.

This study focuses on mimicking constitutive phosphorylation in the N-terminus of the myosin regulatory light chain (S15D-RLC) as a rescue strategy for mutation-induced cardiac dysfunction in transgenic (Tg) models of restrictive (RCM) and dilated (DCM) cardiomyopathy caused by mutations in essential (ELC, MYL3 gene) or regulatory (RLC, MYL2 gene) light chains of myosin. Phosphomimetic S15D-RLC was reconstituted in left ventricular papillary muscle (LVPM) fibers from two mouse models of cardiomyopathy, RCM-E143K ELC and DCM-D94A RLC, along with their corresponding Tg-ELC and Tg-RLC wild-type (WT) mice. The beneficial effects of S15D-RLC in rescuing cardiac function were manifested by the S15D-RLC-induced destabilization of the super-relaxed (SRX) state that was observed in both models of cardiomyopathy. S15D-RLC promoted a shift from the SRX state to the disordered relaxed (DRX) state, increasing the number of heads readily available to interact with actin and produce force. Additionally, S15D-RLC reconstituted with fibers demonstrated significantly higher maximal isometric force per cross-section of muscle compared with reconstitution with WT-RLC protein. The effects of the phosphomimetic S15D-RLC were compared with those observed for Omecamtiv Mecarbil (OM), a myosin activator shown to bind to the catalytic site of cardiac myosin and increase myocardial contractility. A similar SRX↔DRX equilibrium shift was observed in OM-treated fibers as in S15D-RLC-reconstituted preparations. Additionally, treatment with OM resulted in significantly higher maximal pCa 4 force per cross-section of muscle fibers in both cardiomyopathy models. Our results suggest that both treatments with S15D-RLC and OM may improve the function of myosin motors and cardiac muscle contraction in RCM-ELC and DCM-RLC mice.

High Prevalence of Non-Responders Based on Quadriceps Force after Pulmonary Rehabilitation in COPD.

Pulmonary rehabilitation (PR) in patients with COPD improves quality of life, dyspnea, and exercise tolerance. However, 30 to 50% of patients are "non-responders" (NRs) according to considered variables. Surprisingly, peripheral muscle force is never taken into account to attest the efficacy of PR, despite its major importance. Thus, we aimed to estimate the prevalence of force in NRs, their characteristics, and predictors of non-response. In total, 62 COPD patients were included in this retrospective study (May 2019 to December 2020). They underwent inpatient PR, and their quadriceps isometric maximal force (QMVC) was assessed. The PR program followed international guidelines. Patients with a QMVC increase <7.5 N·m were classified as an NR. COPD patients showed a mean improvement in QMVC after PR (10.08 ± 12.97 N·m; p < 0.001). However, 50% of patients were NRs. NRs had lower pre-PR values for body mass, height, body mass index, PaO2, and QMVC. Non-response can be predicted by low QMVC, high PaCO2, and gender (when male). This model has a sensitivity of 74% and specificity of 81%. The study highlights the considerable number of NRs and potential risk factors for non-response. To systematize the effects, it may be interesting to implement blood gas correction and/or optimize the programs to enhance peripheral and central effects.

A Quantitative Investigation of Mental Fatigue Elicited during Motor Imagery Practice: Selective Effects on Maximal Force Performance and Imagery Ability.

In the present study, we examined the development of mental fatigue during the kinesthetic motor imagery (MI) of isometric force contractions performed with the dominant upper limb. Participants (n = 24) underwent four blocks of 20 MI trials of isometric contractions at 20% of the maximal voluntary contraction threshold (20% MVCMI) and 20 MI trials of maximal isometric contractions (100% MVCMI). Mental fatigue was assessed after each block using a visual analogue scale (VAS). We assessed maximal isometric force before, during and after MI sessions. We also assessed MI ability from self-report ratings and skin conductance recordings. Results showed a logarithmic pattern of increase in mental fatigue over the course of MI, which was superior during 100% MVCMI. Unexpectedly, maximal force improved during 100% MVCMI between the 1st and 2nd evaluations but remained unchanged during 20% MVCMI. MI ease and vividness improved during 100% MVCMI, with a positive association between phasic skin conductance and VAS mental fatigue scores. Conversely, subjective measures revealed decreased MI ability during 20% MVCMI. Mental fatigue did not hamper the priming effects of MI on maximal force performance, nor MI's ability for tasks involving high physical demands. By contrast, mental fatigue impaired MI vividness and elicited boredom effects in the case of motor tasks with low physical demands.

Influence of Force-Length Relationship and Task-Specific Constraints on Finger Force-Generating Capacities.

Grip strength loss in extended and flexed wrist postures has been explained by reduced force-generating capacities of extrinsic finger flexor resulting from non-optimal length, owing to the force-length relationship. Recent works suggested that other muscles, especially wrist extensors, participate in this grip strength loss. The objective of this study was to clarify the role of the force-length relationship in finger force production. 18 participants performed maximal isometric finger force production during pinch grip (Pinch) and four-finger pressing (Press) tasks in four different wrist postures (extended, flexed, neutral, spontaneous). The maximum finger force (MFF), finger and wrist joint angles, as well as activation of four muscles were determined using dynamometry, motion capture, and electromyography. The force and length of the four muscles were estimated from joint angles and muscle activation using a musculoskeletal model. MFF decreased for flexed wrist during Pinch but remained stable across wrist postures during Press. The results suggested that the loss of pinch grip force in deviated wrist posture is partially related to force-length relationship of finger extensors. In opposition, MFF during Press was not influenced by the modulation of muscle capacities but was probably first limited by mechanical and neural factors related to finger interdependence.

Neuromuscular electrical stimulation improves reaction time and execution time of roundhouse kick in highly skilled martial arts athletes

ABSTRACT The purpose of the study was to evaluate the influence of neuromuscular electrical stimulation (NMES) on a roundhouse kick (RHK) and the rate of force development (RFD) and peak force during maximal isometric contractions with the knee extensors. Sixteen martial arts athletes were randomly assigned to either a training group (NMES + martial arts) or a control group (martial arts). The RHK was performed to a fixed target located approximately 1.5 m away from the athlete. The reaction time and execution time were quantified with a light-sensor system. Participants were tested before and after completing 15 sessions of training (5 weeks, 3 sessions/week, 90 min/session). In addition, the training group performed another 15 sessions (3 sessions/week, 30 min/session) in which electrical stimulation was superimposed on maximal isometric contractions of the quadriceps (100 Hz, 450 μs). There were no statistically significant changes in either RFD or maximal isometric force for either group (p > 0.05). However, the training group experienced significant reductions (p < 0.05) in both reaction time (−9.2%) and the execution time (−5.9%). The findings indicate that supplemental NMES training can improve sport-specific movements, such as the RHK, without any changes in maximal force capabilities in skilled martial arts athletes.

Analysis of Muscle Strength and Electromyographic Activity during Different Deadlift Positions

The aim of the study was to analyze muscle activation in the three positions of the deadlift (DL). Twenty male participants (33.4 ± 3.9 years; 42.2 ± 9.1 months of experience with DL; 91.0 ± 14.8 kg; and 1.78 ± 0.06 m) pulled a bar through isometric actions in three DL positions: lift-off, mid-pull, and lockout. Isometric strength, knee angle, and activation of the rectus femoris (RF), biceps femoris (BF), lateral gastrocnemius (GAL), and erector spinae (ERE) muscles were collected. The analysis of variance showed that the maximum isometric force presented differences between the positions (p = 0.001; η2 = 0.973) considered large with higher values at the mid-pull position. Interactions were found between muscles and position (p = 0.001; η2 = 0.527) considered large. The RF and ERE showed greater activation in the lift-off position, while in the mid-pull position, there was greater activation of the BF and GAL muscles. The DL positions produce different activations in the bi-articular and uni-articular muscles. The lift-off requires more activation from the RF and ERE positions. The mid-pull position, despite generating greater force, presented greater activations in the BF and GAL. The ERE showed higher activations as the external torque was greater.

Voltage sensor current, SR Ca2+ release, and Ca2+ channel current during trains of action potential-like depolarizations of skeletal muscle fibers.

In skeletal muscle, CaV 1.1 serves as the voltage sensor for both excitation-contraction coupling (ECC) and L-type Ca2+ channel activation. We have recently adapted the technique of action potential (AP) voltage clamp (APVC) to monitor the current generated by the movement of intramembrane voltage sensors (IQ ) during single imposed transverse tubular AP-like depolarization waveforms (IQAP ). We now extend this procedure to monitoring IQAP , and Ca2+ currents during trains of tubular AP-like waveforms in adult murine skeletal muscle fibers, and compare them with the trajectories of APs and AP-induced Ca2+ release measured in other fibers using field stimulation and optical probes. The AP waveform remains relatively constant during brief trains (<1 sec) for propagating APs in non-V clamped fibers. Trains of 10 AP-like depolarizations at 10 Hz (900 ms), 50 Hz (180 ms), or 100 Hz (90 ms) did not alter IQAP amplitude or kinetics, consistent with previous findings in isolated muscle fibers where negligible charge immobilization occurred during 100 ms step depolarizations. Using field stimulation, Ca2+ release did exhibit a considerable decline from pulse to pulse during the train, also consistent with previous findings, indicating that the decline of Ca2+ release during a short train of APs is not correlated to modification of charge movement. Ca2+ currents during single or 10 Hz trains of AP-like depolarizations were hardly detectable, were minimal during 50 Hz trains, and became more evident during 100 Hz trains in some fibers. Our results verify predictions on the behavior of the ECC machinery in response to AP-like depolarizations and provide a direct demonstration that Ca2+ currents elicited by single AP-like waveforms are negligible, but can become more prominent in some fibers during short high-frequency train stimulation that elicits maximal isometric force.

Lack of Sod1 induces disruption of innervation accompanied by Schwann cell proliferation in skeletal muscle

The neuromuscular junction (NMJ) is an excitatory synapse that constitutes the primary site of communication between the nervous system and skeletal muscle and therefore any degenerative changes at the NMJ have the potential to impair muscle function. The primary cellular components of the NMJ are the motor neuron, the muscle fiber and terminal Schwann cells (tSC) that surround the NMJ. The importance of redox homeostasis for the maintenance of NMJ integrity is supported by work from our group showing degenerative changes at NMJs in mice lacking copper zinc superoxide dismutase (CuZnSOD; Sod1 -/- ) as early as 4 months of age. Prior work from others suggests that tSCs contribute trophic support for the NMJ and facilitate motor unit remodeling, but changes in tSC structure and function under conditions of elevated oxidative stress have not been thoroughly explored. Using in vitro assays, we previously showed that primary Schwann cells treated with a sublethal dose of the reactive oxygen species inducing agent Paraquat (PQ) resulted in increased Schwann cell number, ki67+ staining, and gene expression of trophic factors and the critical NMJ organizing protein Agrin. Furthermore, differentiated C2C12 myotubes treated with conditioned media from PQ treated Schwann cell cultures showed increased number of acetylcholine receptor (AChR) plaques. The goal of the current study was to characterize NMJs in young Sod1 -/- mice (2-months) to determine the early effects of elevated oxidative stress on NMJ structure in vivo with a specific focus on tSCs. We hypothesized that loss of Sod1 would lead to aberrant tSC and NMJ structure associated with decreased muscle function. We assessed maximum isometric force, NMJ morphology, and number of Schwann cells in gastrocnemius muscles of WT (n = 3) and Sod1 -/- (n = 3) mice. In Sod1 -/- mice, muscle force was reduced by 26% ( p = 2e-4) with nerve stimulation compared to direct muscle stimulation, and overlap between motor nerve terminals and AChRs was reduced by nearly 60% compared to WT controls. Post-synaptic AChR area was increased by 37% in Sod1 -/- mice, and tSC area and numbers per NMJ were increased by 60% and 155%, respectively in Sod1 -/- mice. Finally, Schwann cells proximal to NMJs showed increased Ki67+ labeling in Sod1 -/- mice compared to WT mice. This analysis demonstrates that early changes in both pre- and post-synaptic NMJ components and reduced neurotransmission due to loss of Sod1 are associated with marked increases in tSC proliferation. Our data implicate a protective role of muscle-resident Schwann cells in preserving NMJ integrity in response to alterations in redox homeostasis that may be mediated in part through activation and proliferation of these cells. These findings indicate that further investigation of the dynamics between Schwann cells and NMJ function is important to increase understanding of neuromuscular degenerativeconditions that involve altered redox homeostasis such as sarcopenia. Supported by the University of Michigan Medical School Office of Research, the American Physiological Society, and NIH AR069620. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.