Force Production Capabilities Research Articles

Specific tension of human muscle in vivo: a systematic review.

The intrinsic force production capability of human muscle can be expressed as "Specific Tension," or, the maximum force generated per cross-sectional area of muscle fibers. This value can be used to determine, for example, whether muscle quality changes during exercise, atrophy, disease, or hypertrophy. A value of 22.5 N/cm2 for mammalian muscle has generally become accepted based on detailed studies of small mammals. Determining the specific tension of human muscle is much more challenging as almost all determinations are indirect. Calculation of human muscle specific tension requires an understanding of that muscle's contribution to joint torque, its activation magnitude, tendon compliance, and joint moment arm. Determining any of these parameters is technically challenging in humans and thus, it is no surprise that human specific tension values reported vary from 2 to 73 N/cm2. In this systematic review, we screened 1,506 published papers and identified the 30 studies published between 1983 and 2023 that used appropriate methods and which reported 96 human specific tension values. We weighted each parameter based on whether it was directly measured, estimated, or calculated based on the literature, with decreasing weighting used, the more indirect the methods. Based on this exhaustive review of the relevant human literature, we suggest that the most accurate value that should be used for human muscle specific tension is 26.8 N/cm2.

Relationships and Within-Group Differences in Physical Attributes and Golf Performance in Elite Amateur Female Players.

The aim of the present study was to examine the association between a comprehensive physical testing battery and measures of golf performance in elite female amateur players. Nineteen category one (handicap ≤ 5) or better golfers (age: 16.26 ± 1.28 years, height: 166.26 ± 3.62 cm, mass: 64.04 ± 11.27 kg, wingspan: 146.53 ± 15.59 cm, handicap: +1.45 ± 0.7) volunteered to participate in this investigation. All golfers attended a single 90 min testing session where golf shot data (clubhead speed [CHS], ball speed, carry distance, and smash factor) were measured with a Trackman 4 launch monitor and a battery of physical assessments were carried out. These included anthropometric data and assessments for seated thoracic rotation, the isometric mid-thigh pull (IMTP), isometric bench press, countermovement jump (CMJ), and seated medicine ball throws for distance. Pearson's r correlations showed CHS was the golf metric that most commonly demonstrated large associations with physical testing data, most notably with force at 100 ms during the isometric bench press (r = 0.70). Median split analysis was also conducted for the IMTP (force at 200 ms), isometric bench press (force at 100 ms), and CMJ (positive impulse). The results showed that players who produced more force at 200 ms during the IMTP exhibited a greater CHS (g = 1.13), ball speed (g = 0.90), and carry distance (g = 1.01). In addition, players with a greater positive impulse during the CMJ showed a greater ball speed (g = 0.93), carry distance (g = 1.29), and smash factor (g = 1.27). Collectively, these results highlight the relevance of explosive force production capabilities in both the lower and upper body for female golfers. This information can be used by practitioners to better target key physical attributes during testing and training of female players.

Association of multi-phase rates of force development during an isometric leg press with vertical jump performances.

This study aimed to elucidate characteristics of explosive force-production capabilities represented by multi-phase rate of force developments (IRFDs) during isometric single-leg press (ISLP) through investigating relationships with countermovement (CMJ) and rebound continuous jump (RJ) performances. Two-hundred-and-thirty male athletes performed ISLP, CMJ with an arm swing (CMJAS), and RJ with an arm swing (RJAS). IRFDs were measured during ISLP using a custom-built dynamometer, while CMJAS and RJAS were measured on force platforms. The IRFDs were obtained as rates of increase in force across 50 ms in the interval from the onset to 250 ms. Jump height (JH) was obtained from CMJAS, while RJAS provided JH, contact time (CT), and reactive strength index (RSI) values. All IRFDs were correlated with CMJAS-JH (ρ = 0.20-0.45, p ≤ 0.003), RJAS-JH (ρ = 0.22-0.46, p ≤ 0.001), RJAS-RSI (ρ = 0.29-0.48, p < 0.001) and RJAS-CT (ρ = -0.29 to -0.25, p ≤ 0.025). When an influence of peak force was considered using partial rank correlation analysis, IRFDs during onset to 150 ms were correlated with CMJAS-JH (ρxy/z = 0.19-0.36, p ≤ 0.004), IRFDs during onset to 100 ms were correlated with RJAS-JH and RJAS-RSI (ρxy/z = 0.33-0.36, p < 0.001), and IRFD during onset to 50 ms was only correlated with RJAS-CT (ρxy/z = -0.23, p < 0.001). The early phase (onset to 150 ms) IRFDs measured using ISLP enabled the assessment of multiple aspects of leg-extension strength characteristics that differ from maximal strength; these insights might be useful in the assessment of the athletes' leg-extension strength capabilities.

Resistance training modalities: comparative analysis of effects on physical fitness, isokinetic muscle functions, and core muscle biomechanics.

This study aimed to evaluate the effects of varied resistance training modalities on physical fitness components, body composition, maximal strength assessed by one-repetition maximum (1RM), isokinetic muscle functions of the shoulder and knee joints, and biomechanical properties of core muscles. Forty participants were randomly assigned to four groups: control group (CG, n = 10), compound set training group (CSG, n = 10), pyramid set training group (PSG, n = 10), and superset training group (SSG, n = 10). Excluding the CG, the other three groups underwent an 8-week resistance training program, three sessions per week, at 60%-80% of 1RM intensity for 60-90min per session. Assessments included body composition, physical fitness components, 1RM, isokinetic muscle functions, and biomechanical properties (muscle frequency, stiffness, etc.) of the rectus abdominis and external oblique muscles. The PSG demonstrated the most significant improvement in relative peak torque during isokinetic testing of the shoulder and knee joints. Compared to the CG, all exercise groups exhibited positive effects on back strength, sprint performance, 1RM, and core muscle biomechanics. Notably, the PSG showed superior enhancement in external oblique stiffness. However, no significant differences were observed among the exercise groups for rectus abdominis biomechanical properties. Structured resistance training effectively improved maximal strength, functional performance, and core muscle biomechanics. The pyramidal training modality conferred specific benefits for isokinetic muscle functions and external oblique stiffness, suggesting its efficacy in enhancing force production capabilities and core stability.

A three-dimensional musculoskeletal model of the pelvis and lower limb of Australopithecus afarensis.

Musculoskeletal modeling is a powerful approach for studying the biomechanics and energetics of locomotion. Australopithecus (A.) afarensis is among the best represented fossil hominins and provides critical information about the evolution of musculoskeletal design and locomotion in the hominin lineage. Here, we develop and evaluate a three-dimensional (3-D) musculoskeletal model of the pelvis and lower limb of A. afarensis for predicting muscle-tendon moment arms and moment-generating capacities across lower limb joint positions encompassing a range of locomotor behaviors. A 3-D musculoskeletal model of an adult A. afarensis pelvis and lower limb was developed based primarily on the A.L. 288-1 partial skeleton. The model includes geometric representations of bones, joints and 35 muscle-tendon units represented using 43 Hill-type muscle models. Two muscle parameter datasets were created from human and chimpanzee sources. 3-D muscle-tendon moment arms and isometric joint moments were predicted over a wide range of joint positions. Predicted muscle-tendon moment arms generally agreed with skeletal metrics, and corresponded with human and chimpanzee models. Human and chimpanzee-based muscle parameterizations were similar, with some differences in maximum isometric force-producing capabilities. The model is amenable to size scaling from A.L. 288-1 to the larger KSD-VP-1/1, which subsumes a wide range of size variation in A. afarensis. This model represents an important tool for studying the integrated function of the neuromusculoskeletal systems in A. afarensis. It is similar to current human and chimpanzee models in musculoskeletal detail, and will permit direct, comparative 3-D simulation studies.

Dynamics of Changes in Muscle Architecture, Force, Strength-Velocity Properties of the Muscles of Lower Limbs in Humans under the Influence of a Three Week Unloading

The present study examined the hypothesis that significant alterations in antigravity muscle architecture would occur with dry water immersion (DI) and that some structural changes may be seen in postural muscles because of the overall decrease in physical activity. Ten men (age (mean ± SE) – 24.5 ± 3.9 years, height – 176.1 ± 1.2 m, mass – 71.1 ± 3.4 kg) volunteered for the study. The healthy men underwent of DI for 21 days. All subjects did not use physical training during the exposure. The contractile properties of the muscle ankle extensors and flexors (maximal voluntary contraction (MVC) and strength-velocity relationship) were evaluated using an isokinetic dynamometer. The internal architecture of the triceps surae muscle of its two heads (medial (MG) and lateral (LG) gastrocnemius muscles) was determined by in vivo ultrasound (US) at ankle angles of –15° (dorsiflexion), 0° (neutral anatomical position), and +30° (plantarflexion) with an angle in the knee joint of 0°. In each position, longitudinal US of MG and LG were obtained at the proximal levels 30% (MG and LG) of the distance between the popliteal crease and the center of the lateral malleolus. US images were detected at rest for each ankle position, and the fiber length (Lf) and pennation angle (Θf) relative to the aponeurosis were determined. After DI the MVC muscle ankle extensors decreased from 122.6 ± 43.1 to 99.5 ± 22.7 N (19%). Although there was a significant no change in dorsiflexion. After DI with the ankle angle increasing from –15° to +30°, Lf changes from 43 ± 1 to 32 ± 2 mm (25.6%, p 0.01) for MG and from 45 ± 2 to 34 ± 1 mm (24.4%, p 0.01) for LG, and Θf increased from 21° ± 1° to 26° ± 2° (23.8%) for MG and from 14° ± 1° to 18° ± 2° (28.6%) for LG. Collectively, the present data suggest that the architecture and contractile capacity of human pennate muscle are interrelated, in vivo. The finding that amongst the antigravity muscles, the MG deteriorated to a greater extent than the LG is possibly related to the differences in relative load that this muscle possibly experiences during daily loading. Different Lf and Θf, and their changes by after unloading, might be related to differences in force-producing capabilities of the muscles and elastic characteristics of tendons and aponeuroses. Structural muscle changes can be considered as an adaptive process, in response to disuse.

The role of alpha actinin paralogs in cardiac hypertrophy and contractile force production.

Sarcomeres are the basic contractile unit within heart muscle cells (cardiac myocytes) that drive each heartbeat. New sarcomere formation (sarcomerogenesis) is concomitant with cardiac myocytes enlargement and is associated with physiological heart growth and cardiomyopathies. A detailed understanding of sarcomerogenesis is of high interest as it would inform our knowledge of both normal and pathological heart biology. Each sarcomere is bordered by Z-lines which mechanically link adjacent sarcomeres within myofibrils to transmit the forces of contraction. A key structural protein at Z-lines is the muscle-specific actin filament bundler, α-actinin 2. Depletion of α-actinin 2 from cardiac myocytes disrupts sarcomerogenesis, but nothing is known about the roles of other α-actinin paralogs. α-actinin 4—sometimes called a “non-muscle” paralog—is expressed in cardiac myocytes ∼30 fold lower than α-actinin 2. Using an assay we developed using cultured iPSC-derived cardiac myocytes, we show that α-actinin 4 colocalizes with α-actinin 2 both in the precursors of sarcomeres as well as within the Z lines of sarcomeres. Unlike the loss of sarcomeres due to the depletion of α-actinin 2, depletion of α-actinin 4 resulted in larger cardiac myocytes with more sarcomeres. Our data indicates that this is due to a stabilization of α-actinin 2. We hypothesized that depletion of α-actinin 4 in vivo would result in an enlargement of the heart. Therefore, we depleted α-actinin 4 from zebrafish embryos. Zebrafish have a two chambered heart, which consists of an atrium and ventricle. To our surprise, only the atrium was significantly enlarged. Ongoing work is suggestive of a mechanism in which the presence of α-actinin 4 within cardiac myocytes of the atrium reduces the number, size, and force producing capabilities of sarcomeres. This mechanism could explain why the atrium is physically weaker than the ventricle.

The Relationship of Lower-Body, Multijoint, Isometric and Dynamic Neuromuscular Assessment Variables With Snatch, and Clean and Jerk Performance in Competitive Weightlifters: A Meta-Analysis

ABSTRACT The purpose of this meta-analysis was to synthesize the literature and provide a robust estimate of the correlations between lower-body, multijoint, isometric and dynamic neuromuscular assessment variables and snatch (SN) and clean and jerk (C&amp;J) performance in competitive weightlifters. A comprehensive search through 3 electronic databases (PubMed, SPORTDiscus, and Web of Science) returned 12 studies that met the inclusion criteria. Meta-analyses were performed on Pearson's correlations between SN and C&amp;J performance and 15 variables from 5 neuromuscular assessments—countermovement jump (CMJ), squat jump (SJ), isometric midthigh pull (IMTP), back squat (BS), and front squat (FS) one repetition maximum (1RM). The FS and BS 1RM exhibited nearly perfect correlations (r = 0.93 to 0.94), whereas the IMTP peak force exhibited very large correlations (r = 0.83 to 0.85). The IMTP force at 250 ms exhibited very large correlations (r = 0.77 to 0.78), and the CMJ and SJ peak power exhibited very large to nearly perfect correlations (r = 0.88 to 0.92). These findings illustrate the importance of lower-body maximal and time-limited force-producing capabilities in weightlifters. Moreover, each assessment offers at least one variable that exhibits a correlation of &gt;0.70. Therefore, these assessments may be used to gauge weightlifting performance potential.

Does External Load Reflect Acute Neuromuscular Fatigue and Rating of Perceived Exertion in Elite Young Soccer Players?

Martínez-Serrano, A, Freitas, TT, Franquesa, X, Enrich, E, Mallol, M, and Alcaraz, PE. Does external load reflect acute neuromuscular fatigue and rating of perceived exertion in elite young soccer players? J Strength Cond Res 37(3): e1-e7, 2023-This study aimed to analyze the acute and residual effects of increased high-speed running (HSR) demands during an in-season training microcycle in young elite soccer players on localized neuromuscular fatigue (NMF) of the knee extensors (KE), posterior chain muscles, and rating of perceived exertion (RPE). Thirty-four elite young soccer players (age = 17.1 ± 0.8 years) were assessed in 2 consecutive days at different time points (baseline, POST-activation gym-based session, POST-small-sided game [SSG], POST-training 1 [TR1], POST-6H, POST-24H, POST-preventive gym-based session, and POST-training 2 [TR2]). Neuromuscular fatigue of the KE and posterior chain muscles was measured with a maximum voluntary isometric contraction (MVIC). External (total distance, number of accelerations or decelerations, and HSR distance) and internal (RPE) load was assessed during the SSG, TR1, and TR2 sessions. Players were divided through a median split, into "HIGH" or "LOW" group according to the training demands. The alpha level was set at p ≤ 0.05. A 2-way mixed effects model ANOVA showed a significant decreased in 90:20 MVIC after TR1 in the "HIGH" HSR group ( p = 0.037; effect size [ES] = 0.45). No significant differences in RPE were found after TR1 ( p = 0.637; ES = 0.58) and TR2 ( p = 0.109; ES = 0.62) when comparing the "HIGH" HSR group with the "LOW" HSR group. Assessing player's force production capabilities can be an effective strategy to detect NMF when HSR demands are acutely increased. Special caution should be taken when prescribing the training load of the training session based solely on RPE, as NMF might be present.

Hovering Locomotion for UAVs With Thrust-Vectoring Control Surfaces

This letter presents a working prototype of a quad-rotor with control surfaces positioned in the propulsive slipstreams. The control surfaces provide a unique potential to redirect the air flow from the propulsion systems and produce lateral forces at a level attitude. This approach to locomotion has many useful applications in different areas of aerial sensing. Furthermore, the mechanical approach of using control surfaces for this purpose is unique, to the best of the authors’ knowledge; until this point, most aerial robot designs that have this locomotion capability make exclusive use of motors and propellers to exert lateral forces. The use of control surfaces in a propeller slipstream poses a potentially useful actuation mode for aerial vehicle designs that have control surfaces. The letter investigates lateral forces from redirected slipstreams as part of a larger development of the Multi-Section (MIST) Unmanned Aerial Vehicle at the University of Minnesota. The paper connects theoretical models with experiments on a testbed and a working prototype that demonstrate feasibility and characterize the force-production capability of slipstream-washed control surfaces.

Strength Training Guide for Personal Training Practitioners

Resistance exercise is the performance of physical exercises designed to improve strength, muscular, endurance, hypertrophy, and neuromuscular efficiency with the use of weights (Braith &amp; Stewart, 2006)[1]. Resistance exercise has long been utilized for its beneficial health qualities and propensity to elicit certain desired physiological changes (Fry, 2004)[2]. There has been a recent, and significant, increase in resistance exercise activity in American adults (NCHS, 2018)[3] attributable to factors such as autonomous compulsion and self fulfilment to extrinsic factors like health and physical appearance (Fisher et al., 2017; Heinrich et al., 2014; Ingledew &amp; Markland, 2008)[4,5,6]. As such, there is an ever-increasing need for educational material regarding resistance exercise, its benefits, purpose, and manner in which it should be conducted. Purpose- to (a) provide resistance exercise-based educational material regarding the background and rationale behind resistance training; (b) to provide a specific resistance-based exercise program to elicit strength gain; (c) to provide individuals with the knowledge to safely and effectively engage in said program; and (d) to provide the participant with expected physiological adaptations to completing the program. Methods- Two 6-week, 5-day per week resistance exercise programs – with a brief nutritional guide accompaniment – are outlined for a hypothetical participant, age 25-40, of moderate experience with fitness training, and with the goal of strength gain and moderate fat loss as a secondary goal. Results- Anticipated benefits of the program include: increased maximal strength caused by training above 85% 1RM for 2-6 sets of 1-6 reps; increased synergistic muscle groups strength which will contribute to improved prime mover strength; hypertrophy of skeletal muscles throughout the body, induced by lifts of 67-85% 1 rep max (RM) for 3-6 sets of 6-12 reps and increased resting energy expenditure (basal metabolic rate) accompanied by improved body composition. Conclusion- Continued progression though this protocol with modifications to resistance include potential improved running speed, explosive power potential, and other anaerobic sport performance factors, as well as enhanced neuromuscular efficiency associated with increased prime mover force production capabilities.

Influence of muscle fatigue on motor task performance of the hand and wrist: A systematic review.

Muscle fatigue is represented as a reduction in force production capability; however, fatigue does not necessarily result in performance impairments. As the distal upper limb serves as the end effector when interacting or manipulating objects, it is important to understand how muscle fatigue may impact motor functionality. The aim of this study was to systematically review the literature to identify how various aspects of motor performance of the distal upper limb are impaired following muscle fatigue. Four databases were searched using 23 search terms describing the distal upper limb, muscle fatigue, and various performance metrics. A total of 4561 articles were screened with a total of 28 articles extracted and critically appraised. Evidence extracted indicates that muscle fatigue results in unique impairments based on the type of motor performance being evaluated. Furthermore, much data suggests that muscle fatigue does not result in consistent, predictable performance impairments, particularly while performing submaximal tasks. Additionally, magnitude of fatigue does not directly correlate with reductions in performance outcomes at the hand and wrist. Fatiguing protocols used highlighted the importance of fatigue specificity. When fatiguing and performance tasks are similar, performance impairment is likely to be observed. The numerous muscles found in the hand and wrist, often considered redundant, play a critical role in maintaining task performance in the presence of muscle fatigue. The presence of motor abundance (e.g. multiple muscles with similar function) is shown to reduce the impairment in multiple performance metrics by compensating for reduced function of fatigued muscles. Continued exploration into various fatiguing protocols (i.e. maximal or submaximal) will provide greater insights into performance impairments in the distal upper limb.

Feature Investigation of a Segmented Pole Quasi- Halbach Tubular-Linear Synchronous Machine

The paper is aimed at an investigation of the features of a tubular-linear synchronous quasi-Halbach machine (T-LSM) where the radially-magnetized PMs are substituted by four equal segments with parallel magnetization. This substitution is done in an attempt to improve the machine cost-effectiveness which makes it a viable candidate to equip free-piston engine-based series hybrid propulsion systems. The study is initiated by an analytical formulation of the air gap flux density considering (i) the case of the conventional quasi-Halbach T-LSM and (ii) the case of the segmented pole one. The comparison between the two T-LSMs is extended to a 3D finite element analysis (FEA)-based investigation of their no- and on-load features. Following the prototyping of the quasi-Halbach T-LSM with segmented poles, selected features predicted by 3D FEA are experimentally-validated. It is shown that while the PM segmentation slightly affects the machine back-EMF and cogging force, it has a remarkable fallout on its force production capability which makes it necessary the assessment of the cost-performance trade-off.

Development, evaluation and application of a novel markerless motion analysis system to understand push-start technique in elite skeleton athletes.

This study describes the development, evaluation and application of a computer vision and deep learning system capable of capturing sprinting and skeleton push start step characteristics and mass centre velocities (sled and athlete). Movement data were captured concurrently by a marker-based motion capture system and a custom markerless system. High levels of agreement were found between systems, particularly for spatial based variables (step length error 0.001 ± 0.012 m) while errors for temporal variables (ground contact time and flight time) were on average within ± 1.5 frames of the criterion measures. Comparisons of sprinting and pushing revealed decreased mass centre velocities as a result of pushing the sled but step characteristics were comparable to sprinting when aligned as a function of step velocity. There were large asymmetries between the inside and outside leg during pushing (e.g. 0.22 m mean step length asymmetry) which were not present during sprinting (0.01 m step length asymmetry). The observed asymmetries suggested that force production capabilities during ground contact were compromised for the outside leg. The computer vision based methods tested in this research provide a viable alternative to marker-based motion capture systems. Furthermore, they can be deployed into challenging, real world environments to non-invasively capture data where traditional approaches are infeasible.

The Influence of Growth, Maturation and Resistance Training on Muscle-Tendon and Neuromuscular Adaptations: A Narrative Review.

The purpose of this article is to provide an overview of the growth, maturation and resistance training-related changes in muscle-tendon and neuromuscular mechanisms in youth, and the subsequent effect on performance. Sprinting, jumping, kicking, and throwing are common movements in sport that have been shown to develop naturally with age, with improvements in performance being attributed to growth and maturity-related changes in neuromuscular mechanisms. These changes include moderate to very large increases in muscle physiological cross-sectional area (CSA), muscle volume and thickness, tendon CSA and stiffness, fascicle length, muscle activation, pre-activation, stretch reflex control accompanied by large reductions in electro-mechanical delay and co-contraction. Furthermore, a limited number of training studies examining neuromuscular changes following four to 20 weeks of resistance training have reported trivial to moderate differences in tendon stiffness, muscle CSA, muscle thickness, and motor unit activation accompanied by reductions in electromechanical delay (EMD) in pre-pubertal children. However, the interaction of maturity- and training-related neuromuscular adaptions remains unclear. An understanding of how different neuromuscular mechanisms adapt in response to growth, maturation and training is important in order to optimise training responsiveness in youth populations. Additionally, the impact that these muscle-tendon and neuromuscular changes have on force producing capabilities underpinning performance is unclear.

Leg muscle cross-sectional area measured by ultrasound is highly correlated with MRI

BackgroundThe leg muscles are important for balance, posture, and movement during static and dynamic activity. Obtaining cross-sectional area measurements (CSA) of the leg muscles helps researchers understand the health and force production capability of individual leg muscles. Therefore, having an easy to use and readily available method to assess leg muscle CSA is needed. Thus, the purpose of this study was to compare the magnitude, repeatability, and validity of CSA measurements of select leg muscles from ultrasound (US) and the current gold standard, magnetic resonance imaging (MRI).Methods20 healthy volunteers participated in this study. Each participant was imaged via US and MRI. The muscles of interest obtained on each participant consisted of the tibialis anterior at both 30 and 50% of the shank length, tibialis posterior at both 30 and 50% of the shank length, the flexor digitorum longus, the fibularis (peroneus) longus, and the fibularis (peroneus) brevis.ResultsStrong Pearson correlations were seen for all of the muscles when comparing US to MRI with a range from .7840 to .9676. For all measurements, standard error of the measurement ranged from .003 to 0.260 cm2. Minimum detectable difference for muscle measurements ranged from .008 cm2 for MRI fibularis longus and fibularis brevis to .693 cm2 for MRI of tibialis anterior at 30%. US minimum detectable difference ranged from .125 cm2 for the tibialis posterior muscle at 30% to .449 cm2 for the tibialis anterior muscle at 50%.ConclusionsBased on these results ultrasound is a valid method to obtain CSA of muscles of the leg when compared with MRI.

Isometric Mid-Thigh Pull Performance in Rugby Players: A Systematic Literature Review.

The isometric mid-thigh pull (IMTP) is a multi-joint test of whole-body force production relevant to rugby players. “Rugby AND (mid-thigh pull OR midthigh pull OR mid thigh pull” were searched in PubMed, Sportdiscus, Academic Search Premier, CINAHL Plus with Full Text, and Google Scholar; the final date of search was 24 January 2018. Data extraction from 24 articles included subject characteristics, force data, and IMTP testing procedures. Select ranges of peak forces reported were: Youth: 1162–2374 N; Academy: 1855–3104 N; Professional: 2254–3851 N. Rate of force development (RFD) at 100 and 200 ms ranged from 5521 to 11,892 N and 5403 to 8405 N, respectively, among professional rugby players. Studies’ research design were of moderate quality, but most studies lacked detailed reporting of IMTP procedures. Variability of force characteristics derived from the IMTP within similar populations (e.g., approximately 200% difference in peak force between samples of professional rugby league players) as well as large and unexpected overlaps between dissimilar populations, limit conclusions about force production capabilities relative to playing level, likely due to limitations and lack of standardization of IMTP procedures. Greater uniformity in IMTP testing procedures and reporting is needed. This manuscript provides a guide for reporting needs when presenting results from an IMTP in research.

Force enhancement after stretch of isolated myofibrils is increased by sarcomere length non-uniformities

When a muscle is stretched during a contraction, the resulting steady-state force is higher than the isometric force produced at a comparable sarcomere length. This phenomenon, also referred to as residual force enhancement, cannot be readily explained by the force-sarcomere length relation. One of the most accepted mechanisms for the residual force enhancement is the development of sarcomere length non-uniformities after an active stretch. The aim of this study was to directly investigate the effect of non-uniformities on the force-producing capabilities of isolated myofibrils after they are actively stretched. We evaluated the effect of depleting a single A-band on sarcomere length non-uniformity and residual force enhancement. We observed that sarcomere length non-uniformity was effectively increased following A-band depletion. Furthermore, isometric forces decreased, while the percent residual force enhancement increased compared to intact myofibrils (5% vs. 20%). We conclude that sarcomere length non-uniformities are partially responsible for the enhanced force production after stretch.

Characterization of the Force Production Capabilities of Paralyzed Trunk Muscles Activated With Functional Neuromuscular Stimulation in Individuals With Spinal Cord Injury.

This study characterizes the input-output properties of paralyzed trunk muscles activated by FNS, and explores co-activation of muscles. Four participants with various spinal cord injuries (C7 AIS-B, T4 AIS-B, T5 AIS-A, C5 AIS-C) were constrained so lumbar forces were transmitted to a load cell while an implanted neuroprosthesis activated otherwise paralyzed hip and paraspinal muscles. Isometric force recruitment curves in the nominal seated position were generated by inputting the level of stimulation (pulse width modulation) while measuring the resulting muscle force. Two participants returned for a second experiment where muscles were co-activated to determine if their actions combined linearly. Recruitment curves of most trunk and hip muscles fit sigmoid shaped curves with a regression coefficient above 0.75, and co-activation of the muscles combined linearly across the hip and lumbar joint. Subject specific perturbation plots showed one subject is capable of resisting up to a 300N perturbation anteriorly and 125N laterally; with some subjects falling considerably below these values. Development of a trunk stability control system can use sigmoid recruitment dynamics and assume muscle forces combine linearly. This study informs future designs of multi-muscle, and multi-dimensional FNS systems to maintain seated posture and stability.

The Kinematic and Kinetic Development of Sprinting and Countermovement Jump Performance in Boys.

BackgroundThe aim of the study was to examine the kinematics and kinetics of sprint running and countermovement jump performance between the ages of 8–9, and 11–12 years old boys in order to understand the developmental plateau in performance.Methods18 physically active boys (Age: 10.1 ± 1.6), in an under 9 years old (U9) and an under 12 years old (U12) group performed 15 m sprints and countermovement jumps. A 3D motion analysis system (200 Hz), synchronized with four force platforms (1,000 Hz), was used to collect kinematic and kinetic data during the first stance phase of the sprint run and the countermovement jump.ResultsThe U12 group had a significantly greater height (U9: 1.364 ± 0.064 m; U12: 1.548 ± 0.046 mm), larger mass (U9: 30.9 ± 3.5 kg; U12: 43.9 ± 5.0 kg), superior sprint performance over 0–5 m (U9: 1.31 ± 0.007 s; U12: 1.23 ± 0.009 s) and 0–15 m (U9: 3.20 ± 0.17 s; U12: 3.01 ± 0.20 s), and increased jump height (U9: 0.17 ± 0.06 m; U12: 0.24 ± 0.10 m) than the under nine group. During the first stance phase of the sprint the U12 group had a significantly greater vertical (U9: 0.22 ± 0.02 BW/s; U12: 0.25 ± 0.03 BW.s) and horizontal impulse (U9: 0.07 ± 0.02 BW/s; U12: 0.09 ± 0.03 BW.s) than the U9 group. When performing a countermovement jump the U12 group had a significantly greater mean average eccentric force (U9: 407.3 ± 55.0 N; U12: 542.2 ± 65.1 N) and mean average concentric force (U9: 495.8 ± 41.3 N; U12: 684.0 ± 62.1 N). Joint kinematics for the countermovement jump were significantly different between age groups for the ankle range of motion (U9: 80.6 ± 17.4°; U12: 64.1 ± 9°) and knee minimum joint angle (U9: −5.7 ± 3.9°; U12: 0.0 ± 4.4°). Conclusion: The study demonstrates for the first time that the development of physically active boys between the ages of 8–9 to 11–12 years increased the ground reaction forces and impulses during sprint running and countermovement jumps, but that sprint running technique had not developed during this period. Furthermore, countermovement jump technique was still emerging at the age of 8–9 years old. Practitioners need to implement on-going fine-grained sprint running and CMJ technique sessions to ensure that the increased force producing capabilities that come with age are appropriately utilized.