Eccentric Velocity Research Articles

Why Should Athletes Brake Fast? Influence of Eccentric Velocity on Concentric Performance During Countermovement Jumps at Different Loads.

The aim of the present study was to analyze the effect of different eccentric tempos on eccentric kinetics and kinematics and the subsequent concentric performance when performing countermovement jumps against different loads. After 1-repetition-maximum assessment and 2 familiarization sessions, 13 well-trained participants performed, in randomized order, 12 sets (4 tempos × 3 loads) of 4 repetitions of the loaded countermovement-jump exercise. The eccentric tempos analyzed were 5 and 2seconds, as fast as possible, and accelerated (ie,without pause between repetitions), while the loads used were 30%, 50%, and 70% of 1-repetition maximum. Several kinetic and kinematic variables during both phases were recorded by linking a linear position transducer to the barbell. The eccentric work was greater in the accelerated condition despite no changes in the eccentric depth. The peak and mean propulsive velocities were greater in the as-fast-as-possible and accelerated conditions. Correlation analysis showed that, compared with the 5-second condition, the increased concentric performance in the accelerated condition was related to the difference in eccentric work performed in the last 100milliseconds of the eccentric phase (r > .770). Contrary to current practices, the current study highlights the need for performing the eccentric phase of loaded countermovement jumps, a common exercise performed by athletes for both training and evaluation purposes, as fast as possible. This allows not only a greater eccentric work but also improved concentric performance.

Relationship between the Nordic hamstring strength test and isokinetic hamstring strength test at different eccentric contraction velocities

ABSTRACT The main purpose of the study was to analyse the relationship between the Nordic hamstring (NH) and the gold standard isokinetic eccentric strength measurements at three different eccentric velocities (30°, 60°, and 90°/s). Twenty-seven (15 male, 12 female) physical education students, experienced in strength training, performed isokinetic and NH tests. The results of the isokinetic hamstring strength test were statistically significantly higher compared to the results of the NH test, regardless of the velocity of the isokinetic test. The correlations observed ranged from moderate to large (0.419 ≤ r ≤ 0.582) and were statistically significant for the left leg, right leg, and the average results between the legs. However, no statistically significant correlations were found between the measures for inter-leg symmetry variables. We observed excellent intra-session reliability of the NH test results (ICC2.1 = 0.891–920), but only good reliability (ICC2.1 = 0.601) for the intra-leg symmetry variable. Although NH strength measurements were found to be highly reliable, sports practitioners should be aware of the discrepancy between the measures and interpret the results carefully. Eccentric hamstring strength and inter-leg symmetry cannot be evaluated interchangeably with the NH test and gold standard isokinetic test performed in the 90° hip flexion position.

Speed Matters in Nordic Hamstring Exercise: Higher Peak Knee Flexor Force during Fast Stretch-Shortening Variant Compared to Standard Slow Eccentric Execution in Elite Athletes.

Hamstring strain injuries are prevalent in many sports. Research has demonstrated that the Nordic hamstring exercise (NHE), a knee-dominant exercise addressing the posterior chain muscles, can aid in reducing the risk of hamstring injuries in athletes. However, most research on hamstring injury prevention has focused on performing the eccentric version of the NHE (NHEECC). In contrast, in sports, it is quite frequent for athletes to use an eccentric-concentric version of the NHE. Additionally, eccentric NHE is typically performed using a slow, controlled tempo. The effect of a fast stretch-shortening cycle NHE (NHESSC) compared to standard slow NHEECC on peak knee flexor force has not been investigated. The aim of the study was therefore to investigate fast NHESSC vs. standard slow NHEECC. Our hypothesis posited that peak knee flexor force would be greater for fast NHESSC compared with standard slow NHEECC. The study involved 22 elite athletes (actively competing in both national and international events) consisting of female (n = 10) and male (n = 7) track and field athletes and male football players (n = 5), aged 17-31 years. The participants performed maximum trials of slow NHEECC and fast NHESSC repetitions in which measurement of bilateral peak knee flexor force was conducted at the ankle with the use of a load cell. During the NHEs, a linear encoder was used to measure both the position where the peak knee flexor force was recorded and the average eccentric velocity. SSC contributed to an enhanced NHE performance, where bilateral absolute peak knee flexor force was 13% higher for fast NHESSC vs. standard slow NHEECC (822 vs. 726 N, p < 0.01, ES = 0.54). Participants achieved a 32% greater forward distance at the breakpoint stage during NHEECC compared to the coupling phase for NHESSC (54 vs. 41 cm, p < 0.001, ES = 1.37). Eccentric average velocity was more than three times higher for NHESSC compared with NHEECC (0.38 vs. 0.12 m/s, p < 0.001, ES = 3.25). The key findings of this study were that SSC contributed to an enhanced NHE performance, where absolute peak knee flexor force was 13% greater for fast NHESSC compared to standard slow NHEECC. The fast NHESSC could therefore be an interesting alternative to the standard slow NHEECC execution, as it may offer potential advantages for sprint performance, as well as hamstring injury prevention and rehabilitation.

The Effect of Varying Eccentric Velocity on Muscle Hypertrophy

Resistance training is essential to muscle hypertrophy as it fatigues fibres through time-under-tension (TUT). As myocyte energy depletes, metabolites accrete, leading to inflammation to increase cell size, so it is adapted for future stimuli. TUT can be measured by varying eccentric velocities: i.e., the rate at which a muscle lengthens under load. A longer period of lengthening will lead to greater metabolite accretion and inflammation. However, it is unknown whether TUT is a threshold or if it can gradually increase and lead to more muscle growth. Through a literature review and experiment, this project investigates the effect of varying eccentric velocity on muscle hypertrophy. Previous research in the field of muscle physiology and metabolism were explored, with an emphasis on eccentric training. The supplementary experiment measured shoulder growth in response to the medial deltoid exercise called lateral raises, where different eccentric velocities were assigned to groups. Individualistic daily calorie and protein intake were controlled to ensure that sufficient nutrients were available for recovery and performance. Surprisingly, post-experimental research suggested that high-velocity eccentric training was best for hypertrophy due to greater levels of force production. This was consistent with the experiment, which found that the group with a fast-velocity eccentric, a lower TUT, experienced greater growth. They also exhibited greater strength gain due to neuromuscular junction adaptation. These findings related to TUT are significant for designing exercise regimens that are optimal for the prevention and rehabilitation of musculoskeletal injuries and disorders. The review’s findings suggest that fast-velocity eccentric contractions are ideal for increasing muscle size and strength.

Kinematic and Kinetic Characteristics of Repetitive Countermovement Jumps with Accentuated Eccentric Loading.

Two methods for challenging the musculoskeletal and nervous systems to better exploit the stretch-shortening cycle (SSC) mechanism during plyometric training are reactive strength exercises and accentuated eccentric loading (AEL). Combining repetitive, reactive jumping with AEL poses a novel approach, in which the effects of both methods may be combined to elicit a unique stimulus. This study compared kinematic, kinetic, and electromyographic variables between a control (CON1) and two AEL conditions (AEL2 and AEL3). Additionally, non-reactive and reactive jumps performed within these sets were compared. Participants performed two sets of six countermovement jumps (CMJ) under each loading condition. AEL3 had moderate to large positive effects (es) on peak and mean eccentric force (es = 1.1, 0.8, respectively; both p < 0.01), and eccentric loading rate (es = 0.8, p < 0.01), but no effect on concentric variables or muscle activation intensity. The effects of AEL2 were similar but smaller. With or without AEL, there were moderate to large positive effects associated with reactive CMJ (second jump in a set, compared to the first) on peak and mean eccentric velocity (es = 1.7, 0.8, respectively; both p < 0.01), peak and mean eccentric force (es = 1.3, 1.2, p < 0.01), eccentric loading rate (es = 1.3, p < 0.01) and muscle activity (es = 1.8–1.9, p < 0.01). Concentric variables did not differ. Thus, the flight phase and act of landing during reactive jumps elicited greater increases in eccentric forces, loading rates, and muscle activity than AEL. Nonetheless, kinetic variables were greatest when AEL was combined with reactive jumping. Considering the limitations or complexity associated with most AEL protocols, sets of repetitive (reactive) CMJ may be more pragmatic for augmenting eccentric kinetic variables and neuromuscular stimuli during training.

Agreement Between Kinovea Video Analysis and The Open Barbell System for Resistance Training Movement Outcomes.

The purpose of this study was to examine the agreement between Kinovea video analysis software and the previously validated Open Barbell System (OBS) for assessing average concentric and eccentric velocity, eccentric duration, peak concentric velocity, height of peak concentric velocity, and concentric and eccentric range of motion. Sixteen resistance-trained males performed 11 repetitions at various intensities (between 60-100% of one-repetition maximum) on both the back squat and the bench press. For each repetition, all outcome measures were assessed with Kinovea and the OBS. Agreement between devices for each outcome measure was analyzed with paired t-tests, intraclass correlation coefficients with 95% confidence intervals, Bland-Altman plots, and folded empirical cumulative distribution plots (Mountain plots). All outcome variables were found to be statistically different (i.e. they did not agree) between the two devices (p < 0.001), except for average eccentric velocity (p = 0.315), as determined by the paired t-tests. Mountain plots had long tails for every outcome measure and Bland-Altman plots showed large limits of agreement for all outcome measures except concentric range of motion and the height of peak concentric velocity, demonstrating low agreement between devices. Based upon these findings and the previous validation of the OBS, these devices should not be used interchangeably.

Determining concentric and eccentric force\u2013velocity profiles during squatting

PurposeThe force–velocity relationship of muscular contraction has been extensively studied. However, previous research has focussed either on isolated muscle or single-joint movements, whereas human movement consists of multi-joint movements (e.g. squatting). Therefore, the purpose of this study was to investigate the force–velocity relationship of isovelocity squatting.MethodsFifteen male participants (24 ± 2 years, 79.8 ± 9.1 kg, 177.5 ± 6 cm) performed isovelocity squats on a novel motorised isovelocity device (Kineo Training System) at three concentric (0.25, 0.5, and 0.75 m s−1) and three eccentric velocities (− 0.25, − 0.5, and − 0.75 m s−1). Peak vertical ground reaction forces, that occurred during the isovelocity phase, were collected using dual force plates (2000 Hz) (Kistler, Switzerland).ResultsThe group mean squat force–velocity profile conformed to the typical in vivo profile, with peak vertical ground reaction forces during eccentric squatting being 9.5 ± 19% greater than isometric (P = 0.037), and occurring between − 0.5 and − 0.75 m s−1. However, large inter-participant variability was identified (0.84–1.62 × isometric force), with some participants being unable to produce eccentric forces greater than isometric. Sub-group analyses could not identify differences between individuals who could/could not produce eccentric forces above isometric, although those who could not tended to be taller.ConclusionsThese finding suggest that variability exists between participants in the ability to generate maximum eccentric forces during squatting, and the magnitude of eccentric increase above isometric cannot be predicted solely based on a concentric assessment. Therefore, an assessment of eccentric capabilities may be required prior to prescribing eccentric-specific resistance training.

Influence of Muscle Architecture on Maximal Rebounding in Young Boys.

Radnor, JM, Oliver, JL, Waugh, CM, Myer, GD, and Lloyd, RS. Influence of muscle architecture on maximal rebounding in young boys. J Strength Cond Res 35(12): 3378-3385, 2021-The aims of the current study were to (a) investigate differences in maximal rebound jump kinetics in boys at different stages of maturity and (b) determine the relationship of muscle architecture characteristics to maximal rebound jump kinetics. One hundred twenty-seven male, secondary school children were categorized into maturity groups (pre-, circa-, and post-peak height velocity) based on their maturity offset value. Muscle architecture of the gastrocnemius medialis (GM) and vastus lateralis (VL) was evaluated at rest using B-mode ultrasonography. Subjects then performed maximal rebound jumps quantified on a force platform. There were moderate to large differences across all maturity groups for peak ground reaction force, impulse measures, and average power variables (d = 0.73-2.67; p < 0.05). Gastrocnemius medialis and VL muscle thickness explained between 38.5 and 55.8% of the variance in peak force, impulse, and power variables; however, muscle architecture was less important determinant of contact time, jump height, reactive strength index, rate of force development, eccentric velocity, concentric velocity, and allometrically scaled measures (3.3-17.2%). The current results indicate that most kinetics used during maximal vertical rebounding are greater in more mature boys. Furthermore, maturational increases in GM muscle architecture seem important for maximal vertical jumping and are specifically associated with increased force, power, and impulse measures. Practically, these findings may underline benefits in targeting resistance training activities that are focused to increase lower limb muscle mass to positively influence maximal rebounding kinetics in young boys.

Importance of Joint Angle-Specific Hip Strength for Skating Performance in Semiprofessional Ice Hockey Athletes.

Secomb, JL, Dascombe, BJ, and Nimphius, S. Importance of joint angle-specific hip strength for skating performance in semiprofessional ice hockey athletes. J Strength Cond Res 35(9): 2599-2603, 2021-Ice hockey athletes with faster sprint acceleration and change-of-direction (COD) skating performance possess a competitive advantage. However, it is unknown whether joint angle-specific hip strength, in combination with measures known to relate to skating performance (e.g., countermovement jump [CMJ]), better explains skating performance. The purpose of this research was to determine whether hip strength in joint angles specific to skating positions and CMJ performance explains sprint skating acceleration and COD performance. Thirteen semiprofessional male hockey (26.7 ± 6.7 years; 88.4 ± 18.1 kg; 181.9 ± 5.4 cm) athletes were assessed for CMJ performance, hip abduction and adduction strength, 10-m sprint skating acceleration, and COD (505) performance. Linear multiple regressions to predict skating acceleration and COD performance were calculated with variables chosen based on functional justification and magnitude of correlation. Hip abductor relative peak force (rPF) at 25° of hip abduction and CMJ rPF explained 46.0% (adjusted) of variance in sprint acceleration performance (F[2,12] = 6.18, p = 0.02). Countermovement jump peak eccentric velocity, adductor rPF at 50° of hip abduction, and difference in abductor rPF between 50° and 25° of hip abduction explained 85.0% (adjusted) of the variance in 505 time (F[3,12] = 22.8, p < 0.001). Hip strength at joint angles functionally relevant to skating (e.g., at 25° and 50°), in combination with relevant CMJ variables, explained large and very large amounts of variance in sprint skating acceleration and COD performance in this cohort. The inclusion of joint angle-specific hip strength profiling to a physical performance testing battery may provide practitioners with more specific knowledge on the strength of the hip through abduction range of motion, which may affect skating performance.

The Influence of Active Hamstring Stiffness on Markers of Isotonic Muscle Performance.

Background: Previous research demonstrates hamstring muscle-tendon stiffness (HMTS) influences isometric strength, landing biomechanics and architectural tissue properties. However, the influence on kinetics & kinematics during other modes of strength testing (isotonic dynamometry) has yet to be established. Purpose: Investigate how HMTS influences kinetics and kinematics during a novel isotonic muscle performance test which has never been done for the hamstrings. Previous work using dynamometry has been limited to isometric or isokinetic contractions, so the novelty arises from our custom isotonic protocol which allows quantitative assessment of the stretch-shortening cycle. Methods: Twenty-six recreationally active individuals (15 males, 11 females, 23.8 ± 2.5 years) completed baseline testing for anthropometry and maximum isometric hamstring strength (MVIC). At least 48 h later, subjects completed a measure of HMTS (damped oscillation technique) followed by an isotonic knee flexion test (eccentric velocity 180°/s; concentric torque 25% of MVIC). Separate linear regression models with examination of residuals were conducted between HMTS and each muscle performance variable. Standardized coefficients determined the magnitude of the relationships. Results: Significance was found for all outcome variables tested. HMTS and rate of torque development demonstrated the strongest relationship followed by isotonic concentric peak torque. The weakest relationship observed was with isometric peak torque. Conclusions: These findings build off previous work quantifying HMTS by showing HMTS more strongly relates to dynamic versus static muscle testing and identifies the potential clinical utility of isotonic dynamometry.

Potentiating Effects of Accentuated Eccentric Loading Are Dependent Upon Relative Strength.

Merrigan, JJ, Tufano, JJ, and Jones, MT. Potentiating effects of accentuated eccentric loading are dependent upon relative strength. J Strength Cond Res 35(5): 1208-1216, 2021-The purpose was to evaluate the acute effects of accentuated eccentric loading (AEL) on bench press velocity and subsequent perceived effort (ratings of perceived exertion [RPE]) and soreness. Resistance-trained men (n = 8) and women (n = 2) completed 4 sets of 5 bench press repetitions with AEL and traditional loading (TL) using concentric loads of 50% (AEL50, TL50) and 65% (AEL65, TL65) 1-repetition maximum (1RM). Throughout each TL set, the eccentric load remained identical to the concentric. Variable resistance during the first repetition of AEL equaled 120% 1RM. Hierarchical Linear Modeling was used to evaluate differences between AEL and TL (p < 0.05). For the first repetition, AEL50 and AEL65 resulted in slower eccentric and concentric velocities. The increasing slope of eccentric and concentric velocity across repetitions was greater during AEL50 and AEL65 compared with TL50 and TL65, respectively (p < 0.05). As an individual's strength increased, AEL50 resulted in slower eccentric velocity and faster concentric velocity than TL50. The AEL65 resulted in faster concentric velocity than TL65 (p < 0.05). Mean protocol comparisons revealed trivial to small effects between AEL and TL. There were no differences in RPE or soreness between protocols with soreness ratings remaining unchanged from baseline (1.80 ± 0.20 AU; p < 0.05). Overall, AEL was not effective for increasing concentric velocity during the bench press with current loading protocols. Yet, stronger individuals may exhibit increases in concentric velocity from AEL, which may be a result of different pacing strategies employed during the eccentric phase. Furthermore, when using the current AEL protocols, eccentric intensities were increased with no greater RPE or soreness.

Concentric and eccentric inertia–velocity and inertia–power relationships in the flywheel squat

ABSTRACT The aim of this study was to evaluate the effects of varying flywheel inertia on velocity and power during flywheel squats. Fifteen healthy physically active males performed 6 maximal effort flywheel half-squats at each of 0.029, 0.061, 0.089 and 0.121 kg·m2, with velocity recorded via 3D motion capture and power recorded via inbuilt transducer. Peak concentric velocity (χ2 = 37.9; p < 0.001), peak eccentric velocity (χ2 = 24.9; p < 0.001), mean concentric velocity (F(3) = 52.7; p < 0.001) and mean eccentric velocity (χ2 = 16.8; p < 0.001) all tended to decrease with increases in flywheel inertia, whereas the ratio of peak eccentric to peak concentric power (F(3) = 4.26; p = 0.010) tended to increase. Flywheel inertia had no significant effect on peak concentric or eccentric power, or the ratio of eccentric to concentric peak or mean velocities. The best fit subject-specific inertia–velocity relationships were reported for peak concentric velocity (median linear R2 = 0.95, median logarithmic R2 = 0.97). The results suggest that velocity, rather than power, should be used to prescribe and monitor flywheel squat exercise intensities, and that individualized linear relationships between inertia and peak concentric velocity can be used for this purpose.

Effectiveness of Accentuated Eccentric Loading: Contingent on Concentric Load.

To identify acute effects of a single accentuated eccentric loading (AEL) repetition on subsequent back-squat kinetics and kinematics with different concentric loads. Resistance-trained men (N = 21) participated in a counterbalanced crossover design and completed 4 protocols (sets × repetitions at eccentric/concentric) as follows: AEL65, 3 × 5 at 120%/65% 1-repetition maximum (1-RM); AEL80, 3 × 3 at 120%/80% 1-RM; TRA65, 3 × 5 at 65%/65% 1-RM; and TRA80, 3 × 3 at 80%/80% 1-RM. During AEL, weight releasers disengaged from the barbell after the eccentric phase of the first repetition and remained off for the remaining repetitions. All repetitions were performed on a force plate with linear position transducers attached to the barbell, from which eccentric and concentric peak and mean velocity, force, and power were derived. Eccentric peak velocity (-0.076 [0.124]m·s-1; P = .01), concentric peak force (187.8 [284.4]N; P = .01), eccentric mean power (-145.2 [62.0]W; P = .03), and eccentric peak power (-328.6 [93.7]W; P < .01) during AEL65 were significantly greater than TRA65. When collapsed across repetitions, AEL65 resulted in slower eccentric velocity and power during repetition 1 but faster eccentric and concentric velocity and power in subsequent repetitions (P ≤ .04). When comparing AEL80 with TRA80, concentric peak force (133.8 [56.9]N; P = .03), eccentric mean power (-83.57 [38.0]W; P = .04), and eccentric peak power (-242.84 [67.3]W; P < .01) were enhanced. Including a single supramaximal eccentric phase of 120% 1-RM increased subsequent velocity and power with concentric loads of 65% 1-RM, but not 80% 1-RM. Therefore, AEL is sensitive to the magnitude of concentric loads, which requires a large relative difference to the eccentric load, and weight releasers may not need to be reloaded to induce performance enhancement.

Concurrent complex and endurance training for recreational marathon runners: Effects on neuromuscular and running performance

Marathon performance is influenced by factors such as aerobic capacity and those related to neuromuscular function. Complex training (CPX) is a multicomponent training method, wherein heavy strength and plyometric exercises alternate within a single session and is an effective method to improve neuromuscular adaptations. This study compared the effects of CPX, heavy strength training (HST) and endurance-strength (EST) combined with running endurance training on neuromuscular adaptations and running performance in 38 recreational marathoners (age:31.4 ± 3.8 years, VO2max:57.6 ± 6.8 ml·kg−1·min−1). Athletes were allocated in 3 groups: CPX, HST and EST and were tested for one maximum repetition strength (1RM), squat jump and countermovement jumps (SJ, CMJ), leg press (LP) concentric and eccentric strength, running economy (RE) and velocity at VO2max (vVO2max) before and after the 6-week intervention. CPX and HST were performed 2 times per week in conjunction with the running endurance training. RE and vVO2max improved in CPX and HST groups (p < 0.01, RE: −5% to −6.4%, vVO2max: 5.7% and 4.2%, respectively) with no change in EST. Similarly, all neuromuscular performance indicators improved in CPX and HST (p < 0.0167, 1RM strength:19.7% to 25.1%, SJ and CMJ: 5.3% to 11.6%, LP concentric and eccentric strength: 5.5% to 18.0%). In summary, 6-week of concurrent CPX or HST and endurance training resulted in similar improvement in maximum strength, RE, and vVO2max. Importantly, both CPX and HST training resulted in greater improvements in eccentric strength and RE compared to EST that performed concurrent endurance-strength and endurance training.

Effects of Westbound Trans-meridian Travel on Countermovement Jump Performance in International-Level Rowers.

Everett, KLA, Chapman, DW, Mitchell, JA, and Ball, N. Effects of westbound trans-meridian travel on countermovement jump performance in international-level rowers. J Strength Cond Res 36(7): 1961-1965, 2022-International long-haul air travel is a necessity for many athletes as part of their training or competition schedules. However, long-haul travel is also associated with impaired neuromuscular function. An athlete's lower body neuromuscular status is frequently assessed and monitored using countermovement jump (CMJ). This study assessed the impact of westbound trans-meridian travel on changes in CMJ performance in highly trained aerobic athletes. Highly trained male rowers (n = 21, 23.7 ± 1.4 years, 1.91 ± 0.07 m, 86.9 ± 9.9 kg) undertook a westbound flight incurring 22 hours of flight time with 30 hours of total travel time across 9 time zones. Athletes completed a single set of 6 loaded CMJ repetitions before and after travel with performance measured using a digital optical encoder attached to a 20 kg barbell. Each CMJ repetition was assessed for mean concentric velocity, jump height (JH), eccentric displacement, JH:dip ratio, mean power, and mean eccentric velocity, with comparisons made between mean set changes before and after travel. Changes were compared using a one tail paired t-test and characterized using Cohen's d (95% confidence interval) effect sizes. Significance was accepted at p ≤ 0.05. Small-to-moderate changes were observed in all variables following westbound travel. These changes may be because of retention of high training loads before travel resulting in the athletes being in a fatigued state, whereas travel time worked as a forced recovery period. Reductions in CMJ performance are likely following westbound travel, however, may be affected by training fatigue before travel that may confound CMJ performance as a monitoring metric.

The Influence Of Hamstring Muscle-tendon Stiffness On Isolated And Dynamic Velocity Based Parameters

Previous research has demonstrated sex differences in active hamstring muscle-tendon stiffness as well as associations between stiffness and landing biomechanics and architectural tissue properties. Little is known whether active stiffness relates to other measures of muscle function (i.e., maximum voluntary isometric contraction [MVIC], isotonic dynamometry) and performance (i.e., sprinting). PURPOSE: To conduct a pilot study examining the relationship between active hamstring muscle-tendon stiffness and isometric torque production, isotonic velocity development, and sprint velocity. A secondary research interest was to examine the relationship between the isotonic velocity parameters and sprint performance. METHODS: Nine recreationally active and healthy subjects (7 males, 2 females) (24 ± 3 years) completed an MVIC for the hamstrings (30° knee flexion) followed by three 40m sprint trials using an infrared timing gate system (Brower, Draper, UT). At least 48 hours later, subjects completed a measure of hamstring muscle-tendon stiffness (damped oscillatory technique) and an isotonic knee flexion test on a fixed dynamometer (Biodex, Shirley, NY). The isotonic test began at 90° of knee flexion and was eccentric/concentric through a 90° arc of motion with subjects lying prone. Eccentric velocity was set at 180°/s and concentric torque was set at 25% of MVIC. Bivariate correlations were conducted between stiffness, MVIC (normalized to body mass), sprint times, time to peak velocity, rate of velocity development (RVD, 0-100ms), and rebound time (-50°/s to +50°/s). RESULTS: Stiffness (24.3 ± 11.7 Nm/kg) was only significantly related (r= .681, P= .043) to MVIC (1.48 ± 0.23 Nm/kg). Stiffness was not significantly related to rebound time (0.076 ± 0.026s) (r= -.354, P= .350), time to peak velocity (0.214 ± 0.034s) (r= -.285, P= .457), RVD0-100 (1314.64 ± 222.30°/s2) (r= -.336, P= .377), or sprint velocities (5.90 ± 0.56 m/s) (r= .248, P= .521). Rebound time (r= -.535, P= .138) and RVD0-100 (r= -.436, P= .241) were not significantly related to sprint times. CONCLUSIONS: Although only the association between stiffness and normalized MVIC reached statistical significance, the magnitude of the other observed relationships provides impetus for further study with a larger sample size.

TheEffects of Eccentric Duration on Squat and Bench Press Concentric Performance

Many resistance training exercises incorporate both eccentric and concentric phases of muscle contraction. Through the stretch shortening cycle, the velocity and magnitude in which the eccentric phase is completed directly affects performance during the concentric phase. PURPOSE: Therefore, the purpose of this research was to investigate the effects of eccentric phase duration on concentric outcomes at 60% and 80% of one-repetition maximum (1RM) in the back squat and bench press. METHODS: Sixteen resistance-trained males (Age: 23.25±2.57yrs, Height: 171.82±7.48cm, Body Mass: 81.96±12.16 kg) completed four laboratory visits as follows: Day 1- 1RM testing; Day 2- establishment of normative eccentric durations; Days 3 and 4- randomized fast (0.75 times) or slow (2.0 times) eccentric duration conditions, which were controlled by visual and auditory metronomes. Outcome measures assessed during the concentric phase were: average concentric velocity (ACV), peak concentric velocity (PCV), rating of perceived exertion (RPE), range of motion (ROM), and barbell path. A one-way ANOVA and Pearson’s Product Moment correlations were used for analysis with significance set at p≤0.05. RESULTS: Eccentric duration was significantly and inversely correlated with average concentric velocity (ACV) at 60% (r = -0.408) and 80% (r = -0.477) of 1RM squat and at 100% (r = -0.604) of 1RMM bench press. At 60% of 1RM squat, both fast and slow eccentric conditions produced greater (p<0.001) peak concentric velocity (PCV) than normative duration with fast also producing greater PCV than slow (p=0.044). Eccentric duration had no impact on RPE, ROM, or barbell path. CONCLUSIONS: Therefore, our results show that well-trained athletes performing a deliberately faster eccentric phase may enhance squat and bench press performance. However, caution should be used when interpreting these results as athletes who already perform a fast eccentric duration may not benefit from deliberately increasing eccentric velocity.

Catching drifting pebbles

Context.Coagulation theory predicts that micron-sized dust grains grow into pebbles, which drift inward towards the star when they reach sizes of mm−cm. When they cross the orbit of a planet, a fraction of these drifting pebbles will be accreted. In the pebble accretion mechanism, the combined effects of the planet’s gravitational attraction and gas drag greatly increase the accretion rate.Aims.We calculate the pebble accretion efficiencyε2D– the probability that a pebble is accreted by the planet – in the 2D limit (pebbles reside in the midplane). In particular, we investigate the dependence ofε2Don the planet eccentricity and its implications for planet formation models.Methods.We conduct N-body simulations to calculate the pebble accretion efficiency in both the local frame and the global frame. With the global method we investigate the pebble accretion efficiency when the planet is on an eccentric orbit.Results.We find that the local and the global methods generally give consistent results. However, the global method becomes more accurate when the planet is more massive than a few Earth masses or when the aerodynamic size (Stokes number) of the pebble is larger than 1. The efficiency increases with the planet’s eccentricity once the relative velocity between the pebble and the planet is determined by the planet’s eccentric velocity. At high eccentricities, however, the relative velocity becomes too high for pebble accretion. The efficiency then drops significantly and the accretion enters the ballistic regime. We present general expressions forε2D. Applying the obtained formula to the formation of a secondary planet, in resonance with an already-formed giant planet, we find that the embryo grows quickly due to its higher eccentricity.Conclusions.The maximumε2Dfor a planet on an eccentric orbit is several times higher than for a planet on a circular orbit, but this increase gives the planet an important headstart and boosts its following mass growth. The recipe forε2Dthat we have obtained is designed to be implemented into N-body codes to simulate the growth and evolution of planetary systems.

Effects of resistance training using known vs unknown loads on eccentric-phase adaptations and concentric velocity.

The aims of this study were to compare both eccentric- and concentric-phase adaptations in highly trained handball players to 4weeks of twice-weekly rebound bench press throw training with varying loads (30%, 50% and 70% of one-repetition maximum [1-RM]) using either known (KL) or unknown (UL) loads and to examine the relationship between changes in eccentric- and concentric-phase performance. Twenty-eight junior team handball players were divided into two experimental groups (KL or UL) and a control group. KL subjects were told the load prior each repetition, while UL were blinded. For each repetition, the load was dropped and then a rebound bench press at maximum velocity was immediately performed. Both concentric and eccentric velocity as well as eccentric kinetic energy and musculo-articular stiffness prior to the eccentric-concentric transition were measured. Results showed similar increases in both eccentric velocity and kinetic energy under the 30% 1-RM but greater improvements under 50% and 70% 1-RM loads for UL than KL. UL increased stiffness under all loads (with greater magnitude of changes). KL improved concentric velocity only under the 30% 1-RM load while UL also improved under 50% and 70% 1-RM loads. Improvements in concentric movement velocity were moderately explained by changes in eccentric velocity (R2 =.23-.62). Thus, UL led to greater improvements in concentric velocity, and the improvement is potentially explained by increases in the speed (as well as stiffness and kinetic energy) of the eccentric phase. Unknown load training appears to have significant practical use for the improvement of multijoint stretch-shortening cycle movements.

Do Training Induced Changes in Quadriceps Tendon Stiffness Result in Reflective Changes in Jump Kinematics?

Both high-load strength training and high-velocity power training are effective means of increasing vertical jump height. However, only strength training is able to elicit increases in tendon stiffness. Because a stiffer tendon will require more force to adequately lengthen it and will recoil at a greater velocity when lengthened changes in tendon stiffness may alter joint kinematics differently than power training. PURPOSE: To determine if training induced changes in tendon stiffness will alter jump kinematics. HYPOTHESIS: Strength training will increase tendon stiffness that will elicit subsequent changes in jump kinematics that differ from those experienced by power training. METHODS: 36 physically active but non-strength trained males were randomly assigned to one of four groups: a strength training group that performed high-resistance parallel-depth squats (SQ-P, n = 8), one of two power training groups that performed high-speed jump squats from either a self selected depth (JS, n = 10) or parallel depth (JS-P, n =9) or a control group (C). Each intervention was 8-weeks in duration before and after which subjects had their quadriceps tendon stiffness measured during a ramp isometric contraction with tandem ultrasonography and their knee kinematics measured using high-speed cinematography during a maximal vertical jump. Comparisons were made between groups and time using a 4x2 mixed model MANOVA with Bonferoni post-hoc (p < 0.05). RESULTS: Quadriceps tendon stiffness increased in SQ-P (p = 0.001) but not JS, JS-P or C. Vertical jump knee kinematics showed a main effect of time on peak knee flexion (p = 0.001) and eccentric knee velocity (p = 0.005), but not in peak concentric knee velocity or acceleration or movement duration. Post-hoc analysis revealed JS and JS-P increased eccentric velocity and knee flexion while SQ-P and C had no significant changes to jump kinematics. CONCLUSION: While SQ-P resulted in an increase in quadriceps tendon stiffness it did not result in any alterations in vertical jump knee kinematics. In contrast, power training, which did not influence tendon properties, resulted a faster and shallower countermovement. These results do not support our primary hypothesis and suggest that factors outside of quadriceps tendon stiffness will have a greater effect on joint kinematics.