Peak Force Output Research Articles

Comparing the Effects of Static Stretching Alone and in Combination with Post-Activation Performance Enhancement on Squat Jump Performance at Different Knee Starting Angles.

We aimed to investigate the impact of isolated static stretching (4 sets of 30 seconds) and its combined form with 10 repetitive drop jumps on lower limb performance during squat jumps at different knee joint starting angles (60°, 90°, and 120°). Thirteen participants completed three randomly ordered experimental visits, each including a standardized warm-up and squat jumps at three angles, apart from the intervention or control. Information was gathered through a three-dimensional movement tracking system, electromyography system, and force platform. The electromyography data underwent wavelet analysis to compute the energy values across the four wavelet frequency bands. The average power (Pavg), peak power (Ppeak), peak ground reaction force (GRFpeak), peak center of mass velocity (Vpeak), and force-velocity relationship at peak power (SFv) were extracted from the force and velocity-time data. The results revealed no significant influence of isolated static stretching, or its combined form with drop jumps, on the energy values across the frequency bands of the gastrocnemius, biceps femoris and rectus femoris, or the Pavg or Ppeak (P > 0.05). However, at 120°, static stretching reduced the GRFpeak (P = 0.001, d = 0.86) and SFv (P < 0.001, d = 1.12), and increased the Vpeak (P = 0.001, d = 0.5). The GRFpeak, Pavg, Ppeak, and SFv increased with an increase in the joint angle (P < 0.05), whereas the Vpeak decreased (P < 0.05). These findings suggest that static stretching does not diminish power output during squat jumps at the three angles; however, it alters GRFpeak, Vpeak, and the relative contributions of force and velocity to peak power at 120°, which can be eliminated by post-activation performance enhancement. Moreover, compared to 60° and 90°, 120° was more favorable for power and peak force output.

Half-Squat and Jump Squat Exercises Performed Across a Range of Loads: Differences in Mechanical Outputs and Strength Deficits.

Loturco, I, McGuigan, MR, Freitas, TT, Bishop, C, Zabaloy, S, Mercer, VP, Moura, TBMA, Arruda, AFS, Ramos, MS, Pereira, LA, and Pareja-Blanco, F. Half-squat and jump squat exercises performed across a range of loads: differences in mechanical outputs and strength deficits. J Strength Cond Res 37(5): 1052-1056, 2023-The aim of this study was to compare the peak force (PF), peak power (PP), and peak velocity (PV) outputs produced during half-squat (HS) and jump squat (JS) exercises executed at 20, 40, 60, and 80% of 1 repetition maximum (1RM) in the HS (HS 1RM) and to compute and compare the strength deficit (SDef) achieved in these exercises across these loads. Twenty-four national rugby union players (age: 25.7 ± 3.6 years) performed HS 1RM and a progressive loading test in the HS and JS exercises. The PF, PP, and PV values were obtained in all loads for both exercises, and the SDef was calculated as the percentage difference between the PF at distinct relative intensities and the PF at HS 1RM. The differences in HS and JS variables were determined using an analysis of variance with repeated measures. Higher PF, PP, and PV outputs were generated in the JS in comparison with the HS exercise ( p < 0.05); moreover, the SDef magnitudes were significantly lower in the JS ( p < 0.01), for all loading conditions. Importantly, the differences in SDef, and as a consequence, PF, PP, and PV decreased progressively with increasing load. Overall, the loaded JS exhibited increased levels of PF, PP, and PV and reduced levels of SDef when compared to the traditional HS performed across a range of loads. The JS is indicated to reduce the SDef and improve the athletes' ability to apply force at higher velocities. Nevertheless, with heavier loads (i.e., ≥80% HS 1RM), its potential advantages and effectiveness may be seriously compromised.

The Influence of Maximal Strength and Knee Angle on the Reliability of Peak Force in the Isometric Squat.

This study aimed to investigate the test-retest reliability of peak force in the isometric squat across the strength spectrum using coefficient of variation (CV) and intra-class correlation coefficient (ICC). On two separate days, 59 healthy men (mean (SD) age 23.0 (4.1) years; height 1.79 (0.7) m; body mass 84.0 (15.2) kg) performed three maximal effort isometric squats in two positions (at a 120° and a 90° knee angle). Acceptable reliability was observed at both the 120° (CV = 7.5 (6.7), ICC = 0.960 [0.933, 0.977]) and 90° positions (CV = 9.2 (8.8), ICC = 0.920 [0.865, 0.953]). There was no relationship between peak force in the isometric squat and the test-retest reliability at either the 120° (r = 0.052, p = 0.327) or 90° (r = 0.014, p = 0.613) positions. A subgroup of subjects (n = 17) also completed the isometric squat test at a 65° knee angle. Acceptable reliability was observed in this position (CV = 9.6 (9.3), ICC = 0.916 [0.766, 0.970]) and reliability was comparable to the 120° and 90° positions. Therefore, we deem isometric squat peak force output to be a valid and reliable measure across the strength spectrum and in different isometric squat positions.

Dynamic Correspondence of the Hang Power Clean to Skating Starts in Men's Ice Hockey

ABSTRACT The transfer of training of the hang power clean to skating starts in men's ice hockey was evaluated applying Verkhoshansky and Siff's proposed criteria of dynamic correspondence to the existing research. A relatively high degree of dynamic correspondence was evidenced throughout the criteria, although with a consideration of the sagittal plane alone. Based on this evaluation, the use of the hang power clean in training increases ice hockey players' capacity to express their skill through an improvement in peak force output and an increase in rate of force development in lower limb extension and is therefore recommended to practitioners training ice hockey players.

Acute Effects of Elastic Bands as Resistance or Assistance on EMG, Kinetics, and Kinematics During Deadlift in Resistance-Trained Men.

The aim of the study was to compare neuromuscular activation, kinetics and kinematics in three variations of the deadlift: (1) free weights, (2) free weights with elastic bands as resistance (bands anchored to the ground) and (3) free weights with elastic bands as assistance (bands attached above the bar). Sixteen resistance-trained men performed one repetition of the three variations as fast as possible using a 2-repetition maximum load in randomized and counterbalanced order. Muscle activation (gluteus maximus, semitendinosus, biceps femoris, erector spinae, vastus lateralis, and vastus medialis), kinematics (average-, peak-, and time to peak velocity), and kinetics (average-, peak,-and time to peak force) were measured during the ascending movement. Resisted and assisted deadlifts led to higher average and peak force outputs (p < 0.001–0.037, ES = 0.29–0.58), and time to peak velocity was shorter when compared to the free weights deadlift (p = 0.005–0.010, ES = 0.83–1.01). However, peak force was achieved faster when using free weights (p < 0.001, ES = 1.58–2.10) and assisted deadlifts had a lower peak velocity compared to resisted and free weights deadlift (p = 0.004–0.046, ES = 0.43–0.60). There were no significant differences in muscle activation between the different conditions (p = 0.082–1.000). In conclusion, the assisted and resisted deadlift produced higher force when compared to free weights. However, free weight and resisted deadlift seem more favorable for the barbell velocity. These findings are of importance for athletes and coaches which should select exercise depending on the goal of the session.

The effects of inorganic phosphate on contractile function of slow skeletal muscle fibres are length-dependent

Muscle operates across a wide range of sarcomere lengths. Inorganic phosphate (Pi) diminishes force output of striated muscle, with greater influence at short relative to long sarcomere lengths in fast skeletal and cardiac muscle fibres. The purpose of this study was to fill a gap in the literature regarding the length-dependent effects of Pi on contractile function of slow skeletal muscle fibres. Permeabilized slow skeletal muscle fibres from rabbit soleus were assessed at average sarcomere lengths of 2.0, 2.4, or 2.8 μm, with and without 20 mM Pi added to activating solutions (22±1 °C). The magnitude of Pi-induced reductions in peak force (43 ± 7% at 2.0 μm, 38 ± 7% at 2.4 μm, and 31 ± 8% at 2.8 μm) and peak stiffness (41 ± 9% at 2.0 μm, 36 ± 8% at 2.4 μm, and 26 ± 9% at 2.8 μm) were length dependent. Peak stiffness was less affected by Pi than peak force. Pi diminished the Ca2+-sensitivity of the force-pCa and stiffness-pCa relationships to a greater extent at 2.8 μm than 2.0 μm. Comparable results were obtained from a cooperative model of Ca2+ and myosin binding to regulated actin. In conclusion, Pi is more detrimental to the peak force output of slow skeletal muscle fibres held at short relative to long sarcomere lengths, whereas Pi has a greater effect on the Ca2+-sensitivity of force production at long relative to short sarcomere lengths. Stiffness data suggest that Pi-induced reductions in force are primarily due to fewer bound cross-bridges, with a lesser contribution attributable to lower average force per cross-bridge.

Mouthguard Efficacy In Baseball Pitching Velocity

Successful pitching in baseball may be due to a number of factors including the mechanics of the motion, the strength, power, flexibility of the athlete, as well as their intent and fatigue levels. The pitching motion is a very powerful, violent, complex and abnormal range of motion of the body. In recent studies, it has been widely evidenced that the ability to produce instantaneous high peak force outputs is related to success in sport. Therefore, the ability to produce higher peak force may be related to the ability to pitching in baseball. Mouthguards have been shown to significantly increase power production in a number of dynamic exercise movements. PURPOSE: The purpose of this study was to determine if maximal and average pitching velocity could be increased when wearing a mouthguard. METHODS: Twenty-two male collegiate baseball pitchers participated in this study (age: 19.9 years old ± 1.4 years, body mass: 87.1 Kg ± 11.6 Kg, body height: 182.5 cm ± 6.1 cm). All study participants were competitive athletes at the NCAA Division 1, Division 3, or University Varsity Club level. RESULTS: Pitching velocity changes resulted in a mean increase of 0.732 km/h for all groups. Velocity change for each level tested resulted in mean increases of 1.652, 0.402, and 0.370 km/h for the university club, Division 3 and Division 1 levels, respectively. The results of a paired samples t-test analysis showed that there was a statistically significant improvement when using a mouthguard in pitching velocity across all groups combined; t(109) = 2.958, p = 0.004. Further, university club level pitchers experienced a statistically significant improvement; t(29) = 5.972, p = 0.000; while Division 3; t(39) = 0.772, p = 0.445; and Division 1; t(39) = 1.014, p = 0.317; players did not show a statistically significant improvement with the mouthguard. CONCLUSION: The authors found that a mouthguard may improve throwing velocity in male collegiate baseball athletes. These findings could be useful to both coaches and sport performance specialists that are working with pitchers to bring about increases in power output and subsequent increases in pitching velocity, simply by implementing the use of a mouthguard.

Intra-Trial Reliability and Usefulness of Isometric Mid-Thigh Pull Testing on Portable Force Plates.

The aim of this study was to assess the intra-trial reliability and usefulness of portable force plates and a customised Isometric Mid-Thigh Pull rig. Twenty males (age: 24.1 ± 2.5 years, body height: 177.7 ± 0.09 cm, body mass: 88.4 ± 17.9 kg) with weightlifting experience ± 12 months attended 1 familiarisation session and 1 testing session where 4 isometric mid-thigh pulls were performed. Maximum force, absolute peak force (PF), relative PF, allometrically scaled PF, and force (150, 200, 250 ms) were deemed reliable (ICC ≥ 0.91 and CV ≤ 9.8%) based on predetermined criteria (ICC ≥ 0.8 and CV ≤ 10%). The impulse and the rate of force development (RFD) were deemed unreliable (ICC ≤ 0.91 and CV ≥ 10 %) at all time points. Maximum force, absolute PF, relative PF to body weight and body mass, rand allometrically scaled PF, had a typical error (TE) lower than the smallest worthwhile change small effect (SWC0.2) and moderate effect (SWC0.5) and were rated as good with regard to usefulness. The TE for force at selected time points (150, 200, 250 ms) was also higher than the SWC0.2, achieving a rating of marginal, but TE was higher than SWC0.5, achieving a rating of good with regard to usefulness. Portable force plates and customised rigs can reliably determine peak force and force output at different time points and for detecting the SWC in maximum and absolute force measures, greater familiarisation may be required to establish reliability of other variables such as the impulse and the RFD.

The Isometric Horizontal Push Test: Test-Retest Reliability and Validation Study.

To investigate the test-retest reliability and criterion validity of the isometric horizontal push test (IHPT), a newly designed test that selectively measures the horizontal component of maximal isometric force. Twenty-four active males with ≥3 years of resistance training experience performed 2 testing sessions of the IHPT, separated by 3 to 4 days of rest. In each session, subjects performed 3 maximal trials of the IHPT with 3 minutes of rest between them. The peak force outputs were collected simultaneously using a strain gauge and the criterion equipment consisting of a floor-embedded force plate. The test-retest reliability of peak force values was nearly perfect (intraclass correlation coefficient = ∼.99). Bland-Altman analysis showed excellent agreement between days with nearly no bias for strain gauge 1.2N (95% confidence interval [CI], -3 to 6N) and force plate 0.8N (95% CI, -4 to 6N). A nearly perfect correlation was observed between the strain gauge and force plate (r = .98, P < .001), with a small bias of 8N (95% CI, 1.2 to 15N) in favor of the force plate. The sensitivity of the IHPT was also good, with smallest worthwhile change greater than standard error of measurement for both the strain gauge (smallest worthwhile change: 29N; standard error of measurement: 17N; 95% CI, 14 to 20N) and the force plate (smallest worthwhile change: 29N; standard error of measurement: 18N; 95% CI, 14 to 19N) devices. The high degree of validity, reliability, and sensitivity of the IHPT, coupled with its affordability, portability, ease of use, and time efficacy, point to the potential of the test for assessment and monitoring purposes.

Effect of acute augmented feedback on between limb asymmetries and eccentric knee flexor strength during the Nordic hamstring exercise.

BackgroundHamstring strain injuries (HSI) are one of the most prevalent and serious injuries affecting athletes, particularly those in team ball sports or track and field. Recent evidence demonstrates that eccentric knee flexor weakness and between limb asymmetries are possible risk factors for HSIs. While eccentric hamstring resistance training, e.g. the Nordic hamstring exercise (NHE) significantly increases eccentric hamstring strength and reduces HSI risk, little research has examined whether between limb asymmetries can be reduced with training. As augmented feedback (AF) can produce significant acute and chronic increases in muscular strength and reduce injury risk, one way to address the limitation in the eccentric hamstring training literature may be to provide athletes real-time visual AF of their NHE force outputs with the goal to minimise the between limb asymmetry.MethodsUsing a cross over study design, 44 injury free, male cricket players from two skill levels performed two NHE sessions on a testing device. The two NHE sessions were identical with the exception of AF, with the two groups randomised to perform the sessions with and without visual feedback of each limb’s force production in real-time. When performing the NHE with visual AF, the participants were provided with the following instructions to ‘reduce limb asymmetries as much as possible using the real-time visual force outputs displayed in front them’. Between limb asymmetries and mean peak force outputs were compared between the two feedback conditions (FB1 and FB2) using independent t-tests to ensure there was no carryover effect, and to determine any period and treatment effects. The magnitude of the differences in the force outputs were also examined using Cohen d effect size.ResultsThere was a significant increase in mean peak force production when feedback was provided (mean difference, 21.7 N; 95% CI [0.2–42.3 N]; P = 0.048; d = 0.61) and no significant difference in between limb asymmetry for feedback or no feedback (mean difference, 5.7%; 95% CI [−2.8% to 14.3%]; P = 0.184; d = 0.41). Increases in force production under feedback were a result of increased weak limb (mean difference, 15.0 N; 95% CI [1.6–28.5 N]; P = 0.029; d = 0.22) force contribution compared to the strong limb.DiscussionThe results of this study further support the potential utility of AF in improving force production and reducing risk in athletic populations. While there are currently some financial limitations to the application of this training approach, even in high-performance sport, such an approach may improve outcomes for HSI prevention programs. Further research with more homogenous populations over greater periods of time that assess the chronic effect of such training practices on injury risk factors and injury rates are also recommended.

When You Become a Superman: Subliminal Exposure to Death-Related Stimuli Enhances Men’s Physical Force

Research based on terror management theory (TMT) has consistently found that reminders to individuals about their mortality engender responses aimed at shoring up faith in their cultural belief system. Previous studies have focused on the critical role that the accessibility of death-related thought plays in these effects. Moreover, it has been shown that these effects occur even when death-related stimuli are presented without awareness, suggesting the unconscious effects of mortality salience. Because one pervasive cultural ideal for men is to be strong, we hypothesized that priming death-related stimuli would lead to increasing physical force for men, but not for women. Building on self-escape mechanisms from TMT, we propose that the mechanism that turns priming of death-related stimuli into physical exertion relies on the co-activation of the self with death-related concepts. To test this hypothesis, we subjected 123 participants to a priming task that enabled us to combine the subliminal priming of death-related words with briefly presented self-related words. Accordingly, three different conditions were created: a (control) condition in which only self-related stimuli were presented, a (priming) condition in which death-related words were subliminally primed but not directly paired with self-related stimuli, and a (priming-plus-self) condition in which death-related words were subliminally primed and immediately linked to self-related stimuli. We recorded handgrip force before and after the manipulations. Results showed that male participants in the priming-plus-self condition had a higher peak force output than the priming and control conditions, while this effect was absent among female participants. These results support the hypothesis that unconscious mortality salience, which is accompanied with self-related stimuli, increases physical force for men but not for women. The gender difference may reflect the cultural belief system, in which individuals are taught that men should be strong. Thus, the unconscious mortality salience produced by exposure to the death-related stimuli motivates need to conform to this internalized cultural standard.

Effects of Cluster Sets and Rest-Redistribution on Mechanical Responses to Back Squats in Trained Men.

Eight resistance-trained men completed three protocols separated by 48-96 hours. Each protocol included 36 repetitions with the same rest duration, but the frequency and length of rest periods differed. The cluster sets of four (CS4) protocol included 30 s of rest after the 4th, 8th, 16th, 20th, 28th, and 32nd repetition in addition to 120 s of rest after the 12th and 24th repetition. For the other two protocols, the total 420 s rest time of CS4 was redistributed to include nine sets of four repetitions (RR4) with 52.5 s of rest after every four repetitions, or 36 sets of single repetitions (RR1) with 12 s of rest after every repetition. Mean (MF) and peak (PF) force, velocity (MV and PV), and power output (MP and PP) were measured during 36 repetitions and were collapsed into 12 repetitions for analysis. Repeated measures ANOVA 3 (protocol) x 12 (repetition) showed a protocol x repetition interaction for PF, MV, PV, MP, and PP (p-values from <0.001 to 0.012). No interaction or main effect was present for MF. During RR1, MV, PV, MP, and PP were maintained, but decreased throughout every 4-repetition sequence during CS4 and RR4. During CS4 and RR4, PF was less following a rest period compared to subsequent repetitions, whereas PF was maintained during RR1. These data indicate that rest redistribution results in similar average kinetics and kinematics, but if total rest time is redistributed to create shorter but more frequent sets, kinetics and kinematics may remain more constant.

Muscle peak force output effected by 14 week of karate training in young female karatekas.

Muscle peak force output effected by 14 week of karate training in young female karatekas.

Force Outputs during Squats Performed Using a Rotational Inertia Device under Stable versus Unstable Conditions with Different Loads.

The purpose of the study was to compare the force outputs achieved during a squat exercise using a rotational inertia device in stable versus unstable conditions with different loads and in concentric and eccentric phases. Thirteen male athletes (mean ± SD: age 23.7 ± 3.0 years, height 1.80 ± 0.08 m, body mass 77.4 ± 7.9 kg) were assessed while squatting, performing one set of three repetitions with four different loads under stable and unstable conditions at maximum concentric effort. Overall, there were no significant differences between the stable and unstable conditions at each of the loads for any of the dependent variables. Mean force showed significant differences between some of the loads in stable and unstable conditions (P < 0.010) and peak force output differed between all loads for each condition (P < 0.045). Mean force outputs were greater in the concentric than in the eccentric phase under both conditions and with all loads (P < 0.001). There were no significant differences in peak force between concentric and eccentric phases at any load in either stable or unstable conditions. In conclusion, squatting with a rotational inertia device allowed the generation of similar force outputs under stable and unstable conditions at each of the four loads. The study also provides empirical evidence of the different force outputs achieved by adjusting load conditions on the rotational inertia device when performing squats, especially in the case of peak force. Concentric force outputs were significantly higher than eccentric outputs, except for peak force under both conditions. These findings support the use of the rotational inertia device to train the squatting exercise under unstable conditions for strength and conditioning trainers. The device could also be included in injury prevention programs for muscle lesions and ankle and knee joint injuries.

A new method for sudden mechanical perturbation with axial load, to assess postural control in sitting and standing

Sudden application of load along a sagittal or coronal axis has been used to study trunk stiffness, but not axial (vertical) load. This study introduces a new method for sudden-release axial load perturbation. Prima facie validity was supported by comparison with standard mechanical systems. We report the response of the human body to axial perturbation in sitting and standing and within-day repeatability of measures. Load of 20% of body weight was released from light contact onto the shoulders of 22 healthy participants (10 males). Force input was measured via force transducers at shoulders, output via a force plate below the participant, and kinematics via 3-D motion capture. System identification was used to fit data from the time of load release to time of peak load–displacement, fitting with a 2nd-order mass–spring–damper system with a delay term. At peak load–displacement, the mean (SD) effective stiffness measured with this device for participants in sitting was 12.0(3.4)N/mm, and in standing was 13.3(4.2)N/mm. Peak force output exceeded input by 44.8 (10.0)% in sitting and by 30.4(7.9)% in standing. Intra-class correlation coefficients for within-day repeatability of axial stiffness were 0.58 (CI: −0.03 to 0.83) in sitting and 0.82(0.57–0.93) in standing. Despite greater degrees of freedom in standing than sitting, standing involved lesser time, downward displacement, peak output force and was more repeatable in defending upright postural control against the same axial loads. This method provides a foundation for future studies of neuromuscular control with axial perturbation.

Comparisons of eccentric knee flexor strength and asymmetries across elite, sub-elite and school level cricket players.

Background. There has been a continual increase in injury rates in cricket, with hamstring strain injuries (HSIs) being the most prominent. Eccentric knee flexor weakness and bilateral asymmetries are major modifiable risk factors for future HSIs. However, there is a lack of data relating to eccentric hamstring strength in cricket at any skill level. The objective of this study was to compare eccentric knee flexor strength and bilateral asymmetries in elite, sub-elite and school level cricket players; and to determine if playing position and limb role influenced these eccentric knee flexor strength indices.Methods. Seventy four male cricket players of three distinct skill levels performed three repetitions of the Nordic hamstring exercise on the experimental device. Strength was assessed as the absolute and relative mean peak force output for both limbs, with bilateral asymmetries. Differences in mean peak force outputs between skill level and playing positions were measured.Results. There were no significant differences between elite, sub-elite and school level athletes for mean peak force and bilateral asymmetries of the knee flexors. There were no significant differences observed between bowler’s and batter’s mean peak force and bilateral asymmetries. There were no significant differences between front and back limb mean peak force outputs.Discussion. Skill level, playing position and limb role appeared to have no significant effect on eccentric knee flexor strength and bilateral asymmetries. Future research should seek to determine whether eccentric knee flexor strength thresholds are predictive of HSIs in cricket and if specific eccentric knee flexor strengthening can reduce these injuries.

Surface electromyography and force production ofa novel strength training method suitable for microgravity

Objectives: Current protocols for resistance training in space flight are hindered by size, mass, noise, vibration and cost, and potentially still lack efficacy. The purpose of the present study was to examine the muscle activation and force output for con tralateral limb resisted training compared to traditional resistance training with a view towards the practicality of use in microgravity environments. Design: Following ethical approval, employing a within-subject design 12 healthy, resistance-trained males performed a 1-repetition maximum bench press (BP) and three isometric tests at differing elbow joint angles (ISO45˚, ISO90˚, ISO135˚) using a Micro-Gym device. Methods: Surface electromyography (sEMG) was used to assess peak amplitude of the pectoralis major (PM), anterior deltoid (AD) and triceps brachii (TB) muscles. Peak force output for each condition was also measured. Results: Significant effects by condition were found with planned comparisons revealing statistically significant differences for peak sEMG amplitude for TB in addition to peak force between BP and ISO45, ISO90, and ISO135 (p < 0.05). Analyses revealed similar peak sEMG amplitude for PM and AD for BP and isometric conditions (p > 0.05). Conclusions: The present study suggests that a contralateral limb resisted training method could be an efficacious method of recruiting motor units and thus may catalyse muscle fibre adaptations in strength and hypertrophy. This novel method might have considerable application to coaches or trainers not wishing to transport large and heavy equipment or in microgravity environments.

Exploiting elasticity: Modeling the influence of neural control on mechanics and energetics of ankle muscle–tendons during human hopping

We present a simplified Hill-type model of the human triceps surae-Achilles tendon complex working on a gravitational–inertial load during cyclic contractions (i.e. vertical hopping). Our goal was to determine the role that neural control plays in governing muscle, or contractile element (CE), and tendon, or series elastic element (SEE), mechanics and energetics within a compliant muscle–tendon unit (MTU). We constructed a 2D parameter space consisting of many combinations of stimulation frequency and magnitude (i.e. neural control strategies). We compared the performance of each control strategy by evaluating peak force and average positive mechanical power output for the system (MTU) and its respective components (CE, SEE), force–length (F–L) and –velocity (F–V) operating point of the CE during active force production, average metabolic rate for the CE, and both MTU and CE apparent efficiency. Our results suggest that frequency of stimulation plays a primary role in governing whole-MTU mechanics. These include the phasing of both activation and peak force relative to minimum MTU length, average positive power, and apparent efficiency. Stimulation amplitude was primarily responsible for governing average metabolic rate and within MTU mechanics, including peak force generation and elastic energy storage and return in the SEE. Frequency and amplitude of stimulation both played integral roles in determining CE F–L operating point, with both higher frequency and amplitude generally corresponding to lower CE strains, reduced injury risk, and elimination of the need for passive force generation in the CE parallel elastic element (PEE).

Vitamin D sufficiency associates with an increase in anti-inflammatory cytokines after intense exercise in humans

The purpose of this study was to identify the influence of vitamin D status (insufficient vs. sufficient) on circulating cytokines and skeletal muscle strength after muscular injury. To induce muscular injury, one randomly selected leg (SSC) performed exercise consisting of repetitive eccentric–concentric contractions. The other leg served as the control. An averaged serum 25(OH)D concentration from two blood samples collected before exercise and on separate occasions was used to establish vitamin D insufficiency (<30ng/mL, n=6) and sufficiency (>30ng/mL, n=7) in young, adult males. Serum cytokine concentrations, single-leg peak isometric force, and single-leg peak power output were measured before and during the days following the exercise protocol. The serum IL-10 and IL-13 responses to muscular injury were significantly (both p<0.05) increased in the vitamin D sufficient group. The immediate and persistent (days) peak isometric force (p<0.05) and peak power output (p<0.05) deficits in the SSC leg after the exercise protocol were not ameliorated with vitamin D sufficiency. We conclude that vitamin D sufficiency increases the anti-inflammatory cytokine response to muscular injury.

The Effect of Leucine Supplementation on Muscle Damage and Muscular Function Following Eccentric Exercise

PURPOSE: In vitro, the amino acid leucine has been able to reduce proteolysis and be a potent stimulus for protein synthesis. The purpose of this study was to determine the effect of leucine supplementation on indices of muscle damage and muscular function following eccentric-based resistance exercise. METHODS: Twenty-seven untrained individuals (Height: 178.62 ± 5.54 cm; Mass: 77.71 ± 13.46 kg) were randomly divided into 3 groups; leucine, placebo and control. The leucine and placebo groups performed 100 depth jumps from 60 cm and 6 sets of 10 repetitions of eccentric-only leg presses. Either leucine (250 mg/kg bm) or placebo was ingested at 3 time points during exercise and each recovery day following exercise. Each group's level of muscle damage was determined via serum levels of creatine kinase and myoglobin at pre-exercise (PRE) and 24, 48, 72 and 96 hours post-exercise. Creatine kinase was assessed using basic spectophotometric techniques and myoglobin was assessed using an ELISA. Muscle function was determined by peak force (PF) during an isometric squat and jump height (JH) and peak concentric force (PCF) during both a static jump (SJ) and countermovement jump (CMJ) at PRE and 24h, 48h, 72h and 96h post-exercise. Data was analyzed using a general linear model with repeated measures and Bonferroni post hoc tests. RESULTS: Creatine kinase was significantly elevated from PRE (354.8 ± 317.5 U/L) for the leucine group at 24h (713.4 ± 473.0 U/L) and significantly elevated from PRE (185.9 ± 117.7 U/L) for the placebo group at 72h (501.7 ± 434.3 U/L) and 96h (455.6 ± 326.0 U/L). Myoglobin was significantly elevated from PRE (24.03 ± 15.4 ng/ml) for the placebo group at 24h (66.3 ± 59.6 ng/ml) and 96h (52.9 ± 41.3 ng/ml). PF for the isometric squat significantly decreased across all time points for placebo group and 24h, 48h, and 72h for the leucine group. SJ JH significantly decreased from PRE (0.42 ± 0.05 m) for the placebo group at 24h (0.38 ± 0.04 m), 48h (0.38 ± 0.06 m) and 72h (0.39 ± 0.04 m). CMJ JH significantly decreased from PRE (0.46 ± 0.03 m) for the leucine group at 48h (0.42 ± 0.04 m), 72h (0.43 ± 0.03 m) and 96h (0.39 ± 0.14 m) and also from PRE (0.48 ± 0.05 m) for the placebo group at 24h (0.44 ± 0.08 m), 48h (0.43 ± 0.07 m) and 72h (0.43 ± 0.06 m). SJ PCF significantly increased from PRE (1652.1 ± 226.0 N) for the leucine group at 48h (1728.9 ± 264.8 N), 72h (1725.8 ± 279.5 N) and 96h (1730.0 ± 270.0 N) and was significantly higher compared to the placebo group at 24h and 48h. CMJ PCF significantly increased from PRE (1673.6 ± 308.7 N) for the placebo group at 48h (1760.6 ± 399.5 N). CONCLUSIONS: Leucine supplementation does not appear to be a viable choice for reducing immediate muscle damage following eccentric exercise; however leucine may facilitate the recovery process. Leucine may need further examination, as it was unable to shown a between group difference compared to placebo but did minimize the change in muscle damage markers compared to PRE. Leucine supplementation did not attenuate the initial decrement in PF during an isometric squat but did return PF back to baseline by the 96 hour time point. For contractions that are not accompanied by a stretch-shorten cycle, leucine supplementation minimized the decrement in muscle function and maintained PF output when compared to a placebo. Practical Applications: Supplementing with leucine by itself may not completely reduce the levels of muscle damage that follow high-intensity resistance training, but may help maintain force generating capabilities. Therefore, increasing leucine consumption, in concurrence with other nutritional recovery protocols, may help maintain muscle function during contractions that are not preceded by a stretch-shorten cycle.