Isometric Force Production Research Articles

Efficacy of High-Definition Transcranial Alternating Current Stimulation (HD-tACS) at the M1 Hotspot Versus C3 Site in Modulating Corticospinal Tract Excitability.

Previous studies have shown that beta-band transcranial alternating current stimulation (tACS) applied at the M1 hotspot can modulate corticospinal excitability. However, it remains controversial whether tACS can influence motor unit activities at the spinal cord level. This study aims to compare the efficacy of applying tACS over the hotspot versus the conventional C3 site on motor unit activities and subsequent behavioral changes. This study used a randomized crossover trial design, where fifteen healthy participants performed a paced ball-squeezing exercise while receiving high-definition tACS (HD-tACS) at 21 Hz and 2 mA for 20 min. HD-tACS targeted either the flexor digitorum superficialis (FDS) hotspot or the C3 site, with the order of stimulation randomized for each participant and a 1-week washout period between sessions. Motor unit activities were recorded from the FDS. HD-tACS intervention significantly reduced the variability of motor unit firing rates and increased force variability during isometric force production. The significant modulation effects were seen only when the intervention was applied at the hotspot, but not at the C3 site. Our findings demonstrate that HD-tACS significantly modulates motor unit activities and force variability. The results indicate that cortical-level entrainment by tACS can lead to the modulation of spinal motor neuron activities. Additionally, this study provides further evidence that the C3 site may not be the optimal target for tACS intervention for hand muscles, highlighting the need for personalized neuromodulation strategies.

Acute Effect of Oral N-Acetylcysteine Supplementation on Fatigue Effect and Isometric Force Production in Physically Active People

The main objective of this study was to assess the ergogenic effect of N-acetylcysteine (NAC) on myotendinous isometric force production in physically active people after being subjected to a fatigue protocol. Twenty-three physically active people were randomly divided into the following two groups: NAC (n = 12; age = 26.8 ± 4.5 years, height = 173.1 ± 7.2 cm, and weight = 75.5 ± 7.5 kg), who received 2400 mg oral NAC, and control (n = 11; age = 23.4 ± 5.8 years, height = 175.9 ± 4.5 cm, and weight = 72.3 ± 9.9 kg), who received a placebo, for eight days. The isometric force production was assessed pre- and post-NAC supplementation during a maximal voluntary contraction test (MVC) and also during a fatigue protocol composed of seven sets of ten maximal isometric contraction repetitions of 5 s, with 5 s of rest between repetitions and 20 s between sets. No differences were observed between the groups in the force production values at any moment, and no side effects were found after NAC supplementation. After supplementation, a significant decrease in force was observed in both groups, but this significant loss of force started one set later in the NAC group compared to the control group (4th set vs. 5th set), which could be an ergogenic effect of the treatment. Therefore, oral daily supplementation with 2400 mg of NAC for eight days, could delay the decrease in force production during an isometric exercise protocol and without adverse side effects.

Skeletal muscle reprogramming enhances reinnervation after peripheral nerve injury

Peripheral Nerve Injuries (PNI) affect more than 20 million Americans and severely impact quality of life by causing long-term disability. PNI is characterized by nerve degeneration distal to the site of nerve injury resulting in long periods of skeletal muscle denervation. During this period, muscle fibers atrophy and frequently become incapable of “accepting” innervation because of the slow speed of axon regeneration post injury. We hypothesize that reprogramming the skeletal muscle to an embryonic-like state may preserve its reinnervation capability following PNI. To this end, we generate a mouse model in which NANOG, a pluripotency-associated transcription factor is expressed locally upon delivery of doxycycline (Dox) in a polymeric vehicle. NANOG expression in the muscle upregulates the percentage of Pax7+ nuclei and expression of eMYHC along with other genes that are involved in muscle development. In a sciatic nerve transection model, NANOG expression leads to upregulation of key genes associated with myogenesis, neurogenesis and neuromuscular junction (NMJ) formation. Further, NANOG mice demonstrate extensive overlap between synaptic vesicles and NMJ acetylcholine receptors (AChRs) indicating restored innervation. Indeed, NANOG mice show greater improvement in motor function as compared to wild-type (WT) animals, as evidenced by improved toe-spread reflex, EMG responses and isometric force production. In conclusion, we demonstrate that reprogramming muscle can be an effective strategy to improve reinnervation and functional outcomes after PNI.

Reliability of a Flight Helmet-Attached Force Gauge in Measuring Isometric Neck Muscle Strength

INTRODUCTION: Strong neck muscles may decrease the risk of flight-induced neck pain and possible disability among fast jet pilots. The purpose of this study was to examine the intra- and interrater reliability of a commercial force gauge attached to a pilot’s helmet for measuring isometric force production of the neck muscles. METHODS: A total of 41 subjects performed maximal isometric cervical flexion, extension, and lateral flexion in two measurement sessions for intrarater reliability, and 31 of these subjects participated in a third session for measuring interrater reliability. Delayed muscle soreness and neck pain were assessed using the Visual Analog Scale before and after each measurement session. The intraclass correlation coefficient (ICC) was used to compare values between the test and retest assessments. RESULTS: The overall interrater reliability was good (ICC 0.79–0.90), whereas the intrarater reliability varied from moderate to good (ICC 0.58–0.84). In both intra- and interrater reliability, the flexion test had good (ICC 0.84–0.89) reliability, while the lateral flexion test results had moderate to good (ICC 0.73–0.90) reliability. The extension test had the lowest reliability in both intra- (ICC 0.58) and interrater (ICC 0.79) tests. The average visual analog scale score (from 1–100 scale) prior to the second measurement session was 16 ± 18 in delayed muscle soreness and 0 ± 0 in neck pain. DISCUSSION: The present study demonstrated that the helmet-attached force gauge is a reliable, safe, and clinically applicable method to evaluate isometric neck strength in the flexion and lateral flexion directions. Honkanen T, Mattila V, Kinnunen O, Janhunen M, Sovelius R, Vaara JP, Kyröläinen H. Reliability of a flight helmet-attached force gauge in measuring isometric neck muscle strength. Aerosp Med Hum Perform. 2024; 95(10):788–793.

Inter-Digit Low Level Force Coordination in a Complex Isometric Pinch Tracking Task

This study examined whether target pursuit tracking by a performer-controlled computer cursor around a square diamond-shaped circuit, using isometric pinch grip force production, would show a significant difference in performance metrics dependent on the clockwise sense of the target movement along the trajectory path. The target template incorporated path segments requiring all four possible combinations of directional force modulation patterns (increasing and decreasing isometric pinch forces of the thumb and index finger). Overall, it was found that cursor positional accuracy was greater during counterclockwise pursuit, that steadiness was greater during clockwise pursuit, and that the cursor bearing angle with respect to target movement was biased toward cursor positioning being within the interior of the trajectory circuit regardless of clockwise sense.

Effects of Neuromuscular Training on Muscle Architecture, Isometric Force Production, and Stretch-Shortening Cycle Function in Trained Young Female Gymnasts.

Moeskops, S, Oliver, JL, Radnor, JM, Haff, GG, Myer, GD, Ramachandran, AK, Kember, LS, Pedley, JS, and Lloyd, RS. Effects of neuromuscular training on muscle architecture, isometric force production, and stretch-shortening cycle function in trained young female gymnasts. J Strength Cond Res 38(9): 1640-1650, 2024-This study evaluated the effects of a 10-month neuromuscular training (NMT) intervention on muscle architecture, isometric force production, and stretch-shortening cycle (SSC) function. Thirty-seven girls aged 6-12 years were placed into gymnastics + NMT (gNMT; n = 15), gymnastics only (GYM; n = 10), or maturity-matched control (CON; n = 12) groups. The gNMT group followed a 10-month NMT program in addition to gymnastics training, whereas the GYM group only participated in gymnastics training. Isometric midthigh pull (IMTP) and drop jump (DJ) kinetics were measured, in addition to muscle thickness, fascicle length, and pennation angle of the gastrocnemius medialis at baseline and at 4, 7, and 10 months. A 3 × 4 (group × time) repeated-measures ANCOVA (covariate, % predicted adult height) was used to evaluate within-group changes. Significance level was set at p < 0.05. Significant interaction effects were observed in muscle thickness, absolute (PF abs ) and relative peak force (PF rel ) in the IMTP and various DJ variables. The gNMT group demonstrated improvements in muscle thickness, IMTP PF abs and PF rel , and DJ kinetics, most commonly evidenced from 7 months onward. The GYM group's muscle thickness also significantly improved, accompanied by improvements in some DJ kinetics. The CON group did not experience any desirable changes. Overall, NMT elicited positive changes in muscle thickness, PF abs and PF rel , and SSC function to a greater extent than gymnastics training alone or growth and maturation. As most adaptations took 7 months, longer-term NMT programs should be implemented with youth female gymnasts.

Changes in corticospinal excitability during motor imagery by physical practice of a force production task: Effect of the rate of force development during practice

Transcranial magnetic stimulation studies have indicated that the physical practice of a force production task increases corticospinal excitability during motor imagery (MI) of that task. However, it is unclear whether this practice-induced facilitation of corticospinal excitability during MI depends on a repeatedly practiced rate of force development (RFD). We aimed to investigate whether corticospinal excitability during MI of an isometric force production task is facilitated only when imagining the motor task with the same RFD as the physically practiced RFD. Furthermore, we aimed to examine whether corticospinal excitability during MI only occurs immediately after physical practice or is maintained. Twenty-eight right-handed young adults practiced isometric ramp force production using right index finger abduction. Half of the participants (high group) practiced the force production with high RFD, and the other half (low group) practiced the force production with low RFD. Questionnaire scores indicating MI ability were similar in the two groups. We examined the force error relative to the target force during the force production task without visual feedback, and motor evoked potential (MEP) amplitudes of the first dorsal interosseous (FDI) and abductor pollicis brevis (APB) muscles during the MI of the force production task under practiced and unpracticed RFD conditions before, immediately after, and 20 min after physical practice. Our results demonstrated that the force error in both RFD conditions significantly decreased immediately after physical practice, irrespective of the RFD condition practiced. In the high group, the MEP amplitude of the FDI muscle during MI in the high RFD condition significantly increased immediately after practice compared to that before, whereas the MEP amplitude 20 min after practice was not significantly different from that before practice. Conversely, the MEP amplitude during MI in the high RFD condition did not change significantly in the low group, and neither group had significant changes in MEP amplitude during MI in the low RFD condition. The facilitatory effect of corticospinal excitability during MI with high RFD observed only immediately after physical practice in the high RFD condition may reflect short-term functional changes in the primary motor cortex induced by physical practice.

Longitudinal changes in anthropometric, physiological, and physical qualities of international women's rugby league players.

This is the first study to assess longitudinal changes in anthropometric, physiological, and physical qualities of international women's rugby league players. Thirteen forwards and 11 backs were tested three times over a 10-month period. Assessments included: standing height and body mass, body composition measured by dual x-ray absorptiometry (DXA), a blood panel, resting metabolic rate (RMR) assessed by indirect calorimetry, aerobic capacity (i.e.,[Formula: see text]) evaluated by an incremental treadmill test, and isometric force production measured by a force plate. During the pre-season phase, lean mass increased significantly by ~2% for backs (testing point 1: 47 kg; testing point 2: 48 kg) and forwards (testing point 1: 50 kg; testing point 2: 51 kg) (p = ≤ 0.05). Backs significantly increased their [Formula: see text] by 22% from testing point 1 (40 ml kg-1 min-1) to testing point 3 (49 ml kg-1 min-1) (p = ≤ 0.04). The [Formula: see text] of forwards increased by 10% from testing point 1 (41 ml kg-1 min-1) to testing point 3 (45 ml kg-1 min-1), however this change was not significant (p = ≥ 0.05). Body mass (values represent the range of means across the three testing points) (backs: 68 kg; forwards: 77-78 kg), fat mass percentage (backs: 25-26%; forwards: 30-31%), resting metabolic rate (backs: 7 MJ day-1; forwards: 7 MJ day-1), isometric mid-thigh pull (backs: 2106-2180 N; forwards: 2155-2241 N), isometric bench press (backs: 799-822 N; forwards: 999-1024 N), isometric prone row (backs: 625-628 N; forwards: 667-678 N) and bloods (backs: ferritin 21-29 ug/L, haemoglobin 137-140 g/L, iron 17-21 umol/L, transferrin 3 g/L, transferring saturation 23-28%; forwards: ferritin 31-33 ug/L, haemoglobin 141-145 g/L, iron 20-23 umol/L, transferrin 3 g/L, transferrin saturation 26-31%) did not change (p = ≥ 0.05). This study provides novel longitudinal data which can be used to better prepare women rugby league players for the unique demands of their sport, underpinning female athlete health.

Danicamtiv affected isometric force and cross-bridge kinetics similarly in skinned myocardial strips from male and female rats.

Myotropes are pharmaceuticals that have recently been developed or are under investigation for the treatment of heart diseases. Myotropes have had varied success in clinical trials. Initial research into myotropes have widely focused on animal models of cardiac dysfunction in comparison with normal animal cardiac physiology-primarily using males. In this study we examined the effect of danicamtiv, which is one type of myotrope within the class of myosin activators, on contractile function in permeabilized (skinned) myocardial strips from male and female Sprague-Dawley rats. We found that danicamtiv increased steady-state isometric force production at sub-maximal calcium levels, leading to greater Ca2+-sensitivity of contraction for both sexes. Danicamtiv did not affect maximal Ca2+-activated force for either sex. Sinusoidal length-perturbation analysis was used to assess viscoelastic myocardial stiffness and cross-bridge cycling kinetics. Data from these measurements did not vary with sex, and the data suggest that danicamtiv slows cross-bridge cycling kinetics. These findings imply that danicamtiv increases force production via increasing cross-bridge contributions to activation of contraction, especially at sub-maximal Ca2+-activation. The inclusion of both sexes in animal models during the formative stages of drug development could be helpful for understanding the efficacy or limitation of a drug's therapeutic impact on cardiac function.

Abstract 2030: Endothelial Deletion Of Bag3 Does Not Impair Ischemic Limb Recovery In A Murine Model Of Peripheral Artery Disease

Background: Peripheral artery disease (PAD) is a highly prevalent vascular occlusive disease which mostly affects the lower limb. Emerging studies proved the critical functions of endothelial cells in arteriogenesis and ischemic limb blood flow recovery. However, very few targets with endothelial cell exclusivity have been identified. Here, we explored the role of endothelial BCL-2 associated athanogene 3 (Bag3) in ischemic limb recovery. Hypothesis: We hypothesized that the loss of endothelial specific BAG3 would attenuate blood flow and muscle recovery following HLI. Methods: Hind limb ischemia (HLI) was induced in Bag3 Cdh5(PAC)-CreERT2 male and female mice after tamoxifen (2mg/day x 5 days) or vehicle treatment ( n ≥12/group). Limb blood flow was measured by laser Doppler perfusion imaging (LDPI) and isometric muscle force production, vascular density and muscle morphology were assessed after HLI. Results: LDPI analysis revealed no differences in the blood flow between groups at HLI d7 ( n=9 ; day 7, 21.86% ischemic/non-ischemic ± 3.80% vs 18.05% ± 1.96% SEM; p=0.05). There was no deficit in isometric force recovery of the EDL ( n ≥12; 57.76 ± 15.92 vs 83.05 ± 27.12 SEM; p =0.05) in KO animals at HLI d7. Similar rates of CD31+ vessel disorganization was observed in both groups at HLI d7, ( n =7; 4626.0 um 2 CD31+ area ± 869 vs 4870.9 ± 1056 SEM) consistent with recovering ischemic tissue. There were also no differences in blood flow recovery assessed by LDPI at HLI d28 ( n =7; 50.74% ischemic/non-ischemic ± 7.4% vs 59.17 ± 8.21; n=9). Conclusions: Endothelial cell specific Bag3 knockout did not impair vascular morphology or perfusion recovery at baseline or following HLI in a murine model of peripheral artery disease. No differences in muscle morphology or contractile function at baseline or after HLI at either timepoint were observed. In summary, we reveal an unexpectedly minor role of endothelial cell BAG3 in tissue recovery following ischemic injury.

Abstract 3026: Genetic Deletion Of Skeletal Muscle Bag3 Exacerbates Ischemic Myopathy

Background: Bcl2-associated athanogene 3 (BAG3) is a multifunctional adaptor protein that regulates diverse cellular functions. Mutations in BAG3 are causally linked to myofibrillar myopathy and dilated cardiomyopathy in humans and now appear to be one of the most common to cause human disease. We previously identified a murine coding variant in Bag3 that contributes to hindlimb ischemia (HLI) pathology, however, no cell specific data on Bag3’s role in PAD pathology exists. Hypothesis: We hypothesized that muscle specific BAG3 is required for both the restoration of blood flow and muscle recovery following HLI. Methods: We generated a genetic model of lifelong muscle cell (mature fiber and progenitor) Bag3 deletion (Pax7-Cre; Bag3 -/- ) on a BL6 parental background. Male and Female mice (12-20 weeks; WT: n=45; KO: n=32) were evaluated at baseline and HLI d7 and d28 for blood flow via LDPI, isometric muscle force production, vascular density and muscle morphology. Bag3 deletion was validated by recombination, qRT-PCR and Western Blotting in whole tissue, isolated myofibers, and FACS muscle progenitor cells. Results: KO reduced muscle force production (n=18; 145.8mN Force ± 28.6 vs 56.9 ± 16.8 SEM; p=0.0208) and capillary perfusion (n=7; 0.17 CD31+/Lectin+ vessels ± 0.02 vs 0.09 ± 0.01 SEM p=0.0491) indicative of the multi-compartmental effect of BAG3 loss in skeletal muscle at HLI d7. Extended HLI (d28) revealed a KO phenotype characterized by reduced blood flow recovery (LDPI; n= 7; .68 ischemic/non limb ± 0.5 vs .38 ± 0.6 SEM p=0.0011) and sustained myopathy including reduced myofiber size ( n =7; 1585um 2 ± 161 vs 1093 ± 169 SEM; p =0.05) and increased tissue lesion size ( n =7; 2.5% muscle area ± 2.0 vs 48.3% ± 21.6; p =0.03). Conclusion: Loss of skeletal muscle Bag3 exacerbates myopathy and reduces capillary perfusion and blood flow recovery following HLI and demonstrate a requirement for BAG3 in recovery from limb ischemia.

Volume loss during muscle reinnervation surgery is correlated with reduced CMAP amplitude but not reduced force output in a rat hindlimb model.

Introduction: Muscle reinnervation (MR) surgery offers rehabilitative benefits to amputees by taking severely damaged nerves and providing them with new denervated muscle targets (DMTs). However, the influence of physical changes to muscle tissue during MR surgery on long-term functional outcomes remains understudied. Methods: Our rat hindlimb model of MR surgery utilizes vascularized, directly neurotized DMTs made from the lateral gastrocnemius (LG), which we employed to assess the impact of muscle tissue size on reinnervation outcomes, specifically pairing the DMT with the transected peroneal nerve. We conducted MR surgery with both DMTs at full volume and DMTs with partial volume loss of 500mg at the time of surgery (n = 6 per group) and measured functional outcomes after 100days of reinnervation. Compound motor action potentials (CMAPs) and isometric tetanic force production was recorded from reinnervated DMTs and compared to contralateral naïve LG muscles as positive controls. Results: Reinnervated DMTs consistently exhibited lower mass than positive controls, while DMTs with partial volume loss showed no significant mass reduction compared to full volume DMTs (p = 0.872). CMAP amplitudes were lower on average in reinnervated DMTs, but a broad linear correlation also exists between muscle mass and maximum CMAP amplitude irrespective of surgical group (R2 = 0.495). Surprisingly, neither MR group, with or without volume loss, demonstrated decreased force compared to positive controls. The average force output of reinnervated DMTs, as a fraction of the contralateral LG's force output, approached 100% for both MR groups, a notable deviation from the 9.6% (±6.3%) force output observed in our negative control group at 7days post-surgery. Tissue histology analysis revealed few significant differences except for a marked decrease in average muscle fiber area of reinnervated DMTs with volume loss compared to positive controls (p = 0.001). Discussion: The results from our rat model of MR suggests that tissue electrophysiology (CMAPs) and kinesiology (force production) may recover on different time scales, with volumetric muscle loss at the time of MR surgery not significantly reducing functional outcome measurements for the DMTs after 100days of reinnervation.

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.

5-Hydroxymethylfurfural reduces skeletal muscle superoxide production and modifies force production in rats exposed to hypobaric hypoxia.

Decreased blood-tissue oxygenation at high altitude (HA) increases mitochondrial oxidant production and reduces exercise capacity. 5-Hydroxymethylfurfural (5-HMF) is an antioxidant that increases hemoglobin's binding affinity for oxygen. For these reasons, we hypothesized that 5-HMF would improve muscle performance in rats exposed to a simulated HA of ~5500 m. A secondary objective was to measure mitochondrial activity and dynamic regulation of fission and fusion because they are linked processes impacted by HA. Fisher 344 rats received 5-HMF (40 mg/kg/day) or vehicle during exposure to sea level or HA for 72 h. Right ankle plantarflexor muscle function was measured pre- and post-exposure. Post-exposure measurements included arterial blood gas and complete blood count, flexor digitorum brevis myofiber superoxide production and mitochondrial membrane potential (ΔΨm), and mitochondrial dynamic regulation in the soleus muscle. HA reduced blood oxygenation, increased superoxide levels and lowered ΔΨm, responses that were accompanied by decreased peak isometric torque and force production at frequencies >75 Hz. 5-HMF increased isometric force production and lowered oxidant production at sea level. In HA exposed animals, 5-HMF prevented a decline in isometric force production at 75-125 Hz, prevented an increase in superoxide levels, further decreased ΔΨm, and increased mitochondrial fusion 2 protein expression. These results suggest that 5-HMF may prevent a decrease in hypoxic force production during submaximal isometric contractions by an antioxidant mechanism.

Acute Effect of L-Citrulline Supplementation on Resistance Exercise Performance and Muscle Oxygenation in Recreationally Resistance Trained Men and Women.

L-citrulline serves as a nitric oxide precursor with the potential to increase blood flow and improve resistance exercise performance, yet more research is needed to examine its ergogenic potential. To examine the effect of L-citrulline supplementation on resistance exercise performance, muscle oxygenation, and the subjective perception of effort, energy, focus, fatigue, and muscle pump, eighteen resistance-trained men (n = 11) and women (n = 7) (21.4 ± 1.8 years; 172.3 ± 7.5 cm; 76.9 ± 10.8 kg) were randomly assigned for supplementation with 8 g of L-citrulline (CIT) or a placebo (PL) in a cross-over fashion one hour prior to testing. Participants completed an isometric mid-thigh pull test (IMTP), a ballistic bench press protocol [two sets of two repetitions at 75% 1-repetition maximum (1 RM) with maximum ballistic intent], and a strength-endurance bench press protocol [five repetition-maximum sets at 75% 1RM]. Barbell velocity and power were measured via a linear position transducer during the ballistic protocol, while the repetitions completed, volume load and muscle oxygenation were quantified during the strength-endurance protocol. Subjective measures were assessed at the baseline and immediately pre- and post-exercise. Repeated measures of the analysis of variance and Bayesian equivalents revealed no significant interactions, providing evidence favoring the null hypothesis (BF10 < 1) for IMTP (PL 497.5 ± 133.6 vs. CIT 492.5 ± 129.4 N), barbell velocity, and power, and repetitions completed (PL 36.7 ± 7.2 vs. CIT 36.9 ± 8.1 repetitions). There were also no significant interactions for muscle oxygenation parameters or subjective measures except perceived fatigue. Women reported greater fatigue across all time points compared to men (~1.88 au, p = 0.045, BF10 = 0.2). The results indicate that a single 8 g dose of L-citrulline did not enhance isometric force production, muscle endurance, or muscle oxygenation parameters during the protocol implemented in this study.

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.

Motor unit-based synergies in a non-compartmentalized muscle.

The concept of synergies has been used to address the grouping of motor elements contributing to a task with the covariation of these elements reflecting task stability. This concept has recently been extended to groups of motor units with parallel scaling of the firing frequencies with possible contributions of intermittent recruitment (MU-modes) in compartmentalized flexor and extensor muscles of the forearm stabilizing force magnitude in finger pressing tasks. Here, we directly test for the presence and behavior of MU-modes in the tibialis anterior, a non-compartmentalized muscle. Ten participants performed an isometric cyclical dorsiflexion force production task at 1Hz between 20 and 40% of maximal voluntary contraction and electromyographic (EMG) data were collected from two high-density wireless sensors placed on the skin over the right tibialis anterior. EMG data were decomposed into individual motor unit frequencies and resolved into sets of MU-modes. Inter-cycle analysis of MU-mode magnitudes within the framework of the uncontrolled manifold (UCM) hypothesis was used to quantify force-stabilizing synergies. Two or three MU-modes were identified in all participants and trials accounting, on average, for 69% of variance and were robust to cross-validation measurements. Strong dorsiflexion force-stabilizing synergies in the space of MU-modes were present in all participants and for both electrode locations as reflected in variance within the UCM (median 954, IQR 511-1924) exceeding variance orthogonal to the UCM (median 5.82, IQR 2.9-17.4) by two orders of magnitude. In contrast, MU-mode-stabilizing synergies in the space of motor unit frequencies were not present. This study offers strong evidence for the existence of synergic control mechanisms at the level of motor units independent of muscle compartmentalization, likely organized within spinal cord circuitry.

The Effects Of Differing Density Of Swim-Training Sessions On Shoulder Range Of Motion and Isometric Force Production In National and University Level Swimmers.

Well-developed physical qualities (i.e., greater load capacity) in athletes can provide protection against injuries. Although higher competitive level swimmers have more developed physical qualities, no studies have investigated how physical qualities of the shoulder respond to a swim-training session in different competitive levels. To compare baseline shoulder external rotation range of motion (ER ROM) and isometric peak torque of the shoulder internal rotators (IR) and external rotators (ER) between national and university level swimmers with differing training volumes. To compare the post-swim changes of these physical qualities between groups. Cross-sectional. Ten male swimmers (age= 18.7 ± 1.2 years) were divided into high-load (N= 5 national-level, weekly swim-volume= 37.0 ± 2.7 km) and low-load groups (N= 5 university-level, weekly swim-volume= 6.8 ± 1.8 km). For each group, shoulder active ER ROM and isometric peak torque of the shoulder IR and ER were measured before and immediately after a high-intensity swim-training session (for each group, the hardest swim-session of the week was analyzed). The results were evaluated by the level of significance (p-value), effect size, and whether changes exceeded the measurement error. University-level swimmers had lower baseline ER torque (p= 0.006; d= 2.55) and IR torque (p= 0.011; d= 2.42) than national-level swimmers. For post-swim analysis, ER ROM decreased more in university swimmers (change= -6.3° to -8.4°; d= 0.75-1.05) than national counterparts (change= -1.9° to -5.7°; d= 0.43-0.95). Greater drops in rotation torque were found in university swimmers (IR change= -15% to -21.0%; d= 0.83-1.66; ER change= -9.0% to -17.0%; d= 1.14-1.28) compared to national swimmers (IR change= -10.0% to -13.0%; d= 0.61-0.91; ER change= -3.7% to -9.1%; d= 0.50-0.96). The average change of all tests in university swimmers exceeded the minimal detectable change (MDC), whereas in national level swimmers some tests exceeded the MDC. Despite this, only post-swim ER torque in the dominant side (p= 0.003; d= 1.18) was significantly lower in university swimmers (possibly due to the small sample size). University swimmers have less baseline shoulder external and internal rotator torque and had greater drops of all shoulder physical qualities after a swim-training session, which may have implications for injury risk. However, due to the sample size, the results have to be interpreted with caution. 3.

Skin Application of Menthol Enhances Maximal Isometric Lifting Performance.

Abstract Over, DM, Arjomandkhah, N, Beaumont, JD, Goodall, S, and Barwood, MJ. Skin application of menthol enhances maximal isometric lifting performance. J Strength Cond Res 37(3): 564–573, 2023—Topical application of menthol to the skin improves perception (i.e., makes subjects feel cooler) and changes submaximal neuromuscular recruitment facilitating force generation. We explored the effect of menthol (0.2% concentration; sprayed on the legs), on perception and maximal, dynamic (DLT) and isometric (IMLT) (weight) lifting tasks. Nine resistance-trained male subjects (mean ± SD: 24 ± 5 years; 75.7 ± 8.8 kg; 174 ± 10 cm; 5 repetition maximum deadlift [5RM] 132.3 ± 28.5 kg) were tested using a repeated measures design; we hypothesized that performance would improve. Before completing the DLT (i.e., deadlift performance 75% 1RM) and a midthigh pull dynamometer IMLT, subjects were sprayed with (∼125 ml) of menthol or control spray. Performance, electromyography (root mean squared [rmsEMG], rectus femoris [RF], biceps femoris [BF], and medial gastrocnemius [MG]), perceptions (leg thermal sensation [TSlegs] and comfort [TClegs] and perceived exertion [RPE] and readiness to train), heart rate, and skin temperature were measured. Data were compared using analysis of variance (effect size ) and t test to a 0.05 alpha level supported by Bayesian analysis. Dynamic lifting task performance was unchanged, although BF rmsEMG was higher (i.e., greater muscle activation in final [10th] repetition). Isometric lifting task force production was higher in the menthol spray (148 ± 30 kgf) condition (control spray 140 ± 30 kgf; p = 0.035, = 0.444) with corresponding higher rmsEMG (BF 3.8 ± 1.46 vs. control spray 2.9 ± 0.34 V; p = 0.049, = 0.403). TSlegs was lower after menthol spray before IMLT; subjects felt slightly cool. Menthol spray enhances isometric weightlifting performance with corresponding changes in neuromuscular activity, partially supporting our hypothesis.

Second Harmonic Generation Morphometry of Muscle Cytoarchitecture in Living Cells.

The architectural structure of cells is essential for the cells' function, which becomes especially apparent in the highly "structure functionally" tuned skeletal muscle cells. Here, structural changes in the microstructure can have a direct impact on performance parameters, such as isometric or tetanic force production. The microarchitecture of the actin-myosin lattice in muscle cells can be detected noninvasively in living cells and in 3D by using second harmonic generation (SHG) microscopy, forgoing the need to alter samples by introducing fluorescent probes into them. Here, we provide tools and step-by-step protocols to guide the processes of obtaining SHG microscopy image data from samples, as well as extracting characteristic values from the image data to quantify the cellular microarchitecture using characteristic patterns of myofibrillar lattice alignments.