Propulsive Phase Research Articles

Comparison of Countermovement Jump Strategy With and Without An Arm Swing

The countermovement jump (CMJ) is commonly used to assess both acute neuromuscular performance as well as adaptions to periods of training. Two methodologies are typically employed when performing the CMJ assessment. The first allows for the use of an arm swing (AS) to add a level of sport-specificity to the testing. The second restricts the movement of the arms (NAS) to allow for an assessment of the musculature of only the lower body. Thus, the purpose of this investigation was to examine differences in jump strategy between the two methodologies. Twenty-five female Division I collegiate athletes (volleyball = 13, beach volleyball = 12) participated in this investigation. Participants performed two CMJ in both the AS and NAS conditions. A paired samples t-test was used to evaluate differences in jump performance and jump strategy variables. During the braking phase of the CMJ statistical higher force values (p < 0.01) were seen in the NAS condition while longer phase durations were present in the AS condition (p < 0.001). No difference was seen in braking net impulse. During the propulsive phase statistically greater duration was seen in the AS condition (p < 0.001) leading to a greater propulsive net impulse (p < 0.001). The AS condition also displayed greater jump heights, countermovement depth and time to take off durations (p < 0.001) with no differences in reactive strength index modified. When performing CMJ assessments practitioners should consider which methodology they use carefully as the NAS assessment used a more force driven strategy while the AS used a time driven strategy.

Mechanical power distribution of the lower limbs changed during intermittent 300 countermovement jumps.

The aim of this study was to investigate the effect of 300 intermittent countermovement jumps (CMJs) on the mechanical power distribution at the joints of the lower limbs and the influence of the upper body to explain vertical jump performance. Fifteen male sport students (age 24.5 ± 2.3years; body height 1.85 ± 0.06m; body mass 84.8 ± 8.5kg) performed a set of intermittent 300 CMJs at maximal effort. An inverse-dynamic approach was used to calculate the mechanical power at the hip, knee, and ankle joint for each jump. Jump height and mechanical power in the knee and ankle joints decreased significantly (p < .010), while remained the same in the hip joint. In contrast, a significant increased vertical velocity was observed for the upper body segment. In addition, a significant higher angular momentum at the center of mass was detected during the braking and propulsion phase. The findings highlight a fatigue-related decrease in lower limb power, particularly in the knee and ankle joints, which changed the mechanical power distribution at the joints of the lower limbs. The trunk extensor muscles were probably able to counteract the fatigue-related decrease in lower limb power by increased vertical velocity of the upper body segment and higher angular momentum at the center of mass during the braking and propulsion phase. Accordingly, the most effective way to maintain jumping performance in fatigued state would be to improve the fatigue resistance of the knee extensors, ankle plantar flexors, and trunk extensor muscles.

Changing the Mandibular Position in Rowing: A Brief Report of a World-Class Rower.

We investigated the acute biophysical responses of changing the mandibular position during a rowing incremental protocol. A World-class 37-year-old male rower performed two 7 × 3 min ergometer rowing trials, once with no intraoral splint (control) and the other with a mandibular forward repositioning splint (splint condition). Ventilatory, kinematics and body electromyography were evaluated and compared between trials (paired samples t-test, p ≤ 0.05). Under the splint condition, oxygen uptake was lower, particularly at higher exercise intensities (67.3 ± 2.3 vs. 70.9 ± 1.5 mL·kg-1·min-1), and ventilation increased during specific rowing protocol steps (1st-4th and 6th). Wearing the splint condition led to changes in rowing technique, including a slower rowing frequency ([18-30] vs. [19-32] cycles·min-1) and a longer propulsive movement ([1.58-1.52] vs. [1.56-1.50] m) than the control condition. The splint condition also had a faster propulsive phase and a prolonged recovery period than the control condition. The splint reduced peak and mean upper body muscle activation, contrasting with an increase in lower body muscle activity, and generated an energetic benefit by reducing exercise cost and increasing rowing economy compared to the control condition. Changing the mandibular position benefited a World-class rower, supporting the potential of wearing an intraoral splint in high-level sports, particularly in rowing.

An evaluation of a bespoke modified UCBL foot orthosis on subjects with flat foot using kinetic measurements and user comfort scores: A randomized controlled trial

AimThe purpose of this study was to assess and evaluate the effect of a bespoke Modified UCBL Foot Orthosis (MUFO) using both kinetic parameters (Centre of Pressure (CoP) and the Ground Reaction Force (GRF) pattern) and comfort scores in subjects diagnosed with flat foot. MethodThis study included thirty-four young adults with symptomatic flatfeet. Two Kistler force plates (100 Hz) were used to record the CoP sway and GRF pattern during four conditions; 1) an MUFO and standard-fit shoe; 2) the University of California-Berkley Lab (UCBL) insole and standard-fit shoe; 3) barefoot and 4) standard-fit shoe only. The magnitude of subject comfort with UCBL and MUFO also was measured by a 10 cm Visual Analogue Scale (VAS) during walking. ResultsThe MUFO decreased mean lateral displacement in the initial phase and midstance of gait compared to barefoot walking. During the propulsion phase use of the new MUFO produced more lateral excursion with a mean difference of 3 mm) P < 0.001(compared to barefoot walking and standard shoe wear. No significant difference in comfort rate was found between the MUFO and UCBL (P = 0.165). ConclusionThe MUFO produced effective pronation control and decreased the CoP displacement in all of stance phase.

Relationship between Countermovement Jump and Sprint Performance in Professional Football Players.

Objectives: The aim of this study was to assess the relationship between the countermovement jump (CMJ) and sprint performance of professional football players, and to determine which strength and speed elements assessed by the CMJ translate into effective running. Methods: The research sample comprised 87 male professional football players (age 23.7 ± 4.20 years; body mass 82.33 ± 6.56 kg; body height 1.86 ± 0.05 m) who performed the CMJ on a dual-force platform, as well as the 30 m sprint test. The time and velocity of the run were recorded by photocells at 0, 5, 10, and 30 m of the distance. Results: No significant differences were noted in the time or velocity of the sprint over the initial 5 m between the groups of football players with a higher and lower braking rate of force development (RFD) in the CMJ (p > 0.05). However, at subsequent intervals (5-10 m and 10-30 m), players with a higher braking RFD achieved significantly better time and velocity than those with a lower RFD. Significant correlations in the group with a lower braking RFD between the CMJ and sprint variables occurred in the propulsion phase of the CMJ and most of them were in the first interval (0-5 m). In the group with a higher braking RFD, significant relationships were visible in both the propulsion (concentric) and braking (eccentric) phases of the CMJ, mainly during the second and third intervals of the sprint test. Conclusions: The noted observations may suggest that the relationship between strength and running performance is more complex than previously indicated, and that higher strength in the CMJ does not fully correlate with better sprinting. Therefore, it has been hypothesized that training aimed at generally increasing strength may not always be fully beneficial for running performance in football players and hence specific training guidelines are suggested for targeted strengthening of the required muscle performance characteristics. This may possibly contribute to reducing the unnecessary muscle overload during both training and matches, thereby preventing sports-related injuries.

Using Barbell Acceleration to Determine the 1 Repetition Maximum of the Jump Shrug.

Techmanski, BS, Kissick, CR, Loturco, I, and Suchomel, TJ. Using barbell acceleration to determine the 1 repetition maximum of the jump shrug. J Strength Cond Res 38(8): 1486-1493, 2024-The purpose of this study was to determine the 1 repetition maximum (1RM) of the jump shrug (JS) using the barbell acceleration characteristics of repetitions performed with relative percentages of the hang power clean (HPC). Fifteen resistance-trained men (age = 25.5 ± 4.5 years, body mass = 88.5 ± 15.7 kg, height = 176.1 ± 8.5 cm, relative 1RM HPC = 1.3 ± 0.2 kg·kg-1) completed 2 testing sessions that included performing a 1RM HPC and JS repetitions with 20, 40, 60, 80, and 100% of their 1RM HPC. A linear position transducer was used to determine concentric duration and the percentage of the propulsive phase (P%) where barbell acceleration was greater than gravitational acceleration (i.e., a>-9.81 m·s-2). Two 1 way repeated measures ANOVA were used to compare each variable across loads, whereas Hedge's g effect sizes were used to examine the magnitude of the differences. Concentric duration ranged from 449.7 to 469.8 milliseconds and did not vary significantly between loads (p = 0.253; g = 0.20-0.39). The P% was 57.4 ± 7.2%, 64.8 ± 5.9%, 73.2 ± 4.3%, 78.7 ± 4.0%, and 80.3 ± 3.5% when using 20, 40, 60, 80, and 100% 1RM HPC, respectively. P% produced during the 80 and 100% 1RM loads were significantly greater than those at 20, 40, and 60% 1RM (p < 0.01, g = 1.30-3.90). In addition, P% was significantly greater during 60% 1RM compared with both 20 and 40% 1RM (p < 0.01, g = 1.58-2.58) and 40% was greater than 20% 1RM (p = 0.003, g = 1.09). A braking phase was present during each load and, thus, a 1RM JS load was not established. Heavier loads may be needed to achieve a 100% propulsive phase when using this method.

Pool-Based Surfboard Elicits Activation of Posterior Shoulder Muscles During a Surfing Stroke.

Pexa, BS, Johnston, CD, Elder, EE, Ford, KR, Patterson, MQ, and Myers, JB. Pool-based surfboard elicits activation of posterior shoulder muscles during a surfing stroke. J Strength Cond Res 38(7): 1300-1304, 2024-Surfboard paddling may activate posterior shoulder muscles, which are critical to baseball pitchers' injury risk and performance. The purpose of this study was to measure posterior shoulder muscle activation during different phases of the surf stroke (propulsion vs. recovery) on a pool-based surfboard. Twenty healthy active adult subjects completed a familiarization and testing session with the pool-based surfboard. During the testing session, electromyography (EMG) sensors were placed on 6 posterior shoulder muscles: latissimus dorsi, infraspinatus, posterior deltoid, upper trapezius, middle trapezius, and lower trapezius. Subjects completed 4 laps in a pool at 3 separate resistances (low, moderate, and heavy) in a randomized order. The peak EMG signal during each phase (propulsion and recovery) was recorded. A 2-way within subject ANOVA (resistance-by-phase) with post hoc Bonferroni's corrections was used to identify differences in EMG activation. There was a significant main effect of phase for the latissimus dorsi (F = 91.3, p < 0.001), upper trapezius (F = 36.5, p < 0.001), middle trapezius (F = 33.8, p < 0.001), and lower trapezius (F = 21.6, p < 0.001). The latissimus dorsi demonstrated higher activation during the propulsion phase (p < 0.001), and all trapezius muscles demonstrated higher activation during the recovery phase (p < 0.001). There was a significant main effect of resistance for the posterior deltoid (F = 3.4, p = 0.043), with higher muscle activation in the low resistance trials compared with the heavy resistance trials (p = 0.036). Recreationally active individuals demonstrate activation of the posterior shoulder when using a pool-based surfboard. This pool-based surfboard may be beneficial to activate the posterior musculature and may be more accessible than standard surfing to baseball athletes.

Spatiotemporal parameters and kinematics differ between race stages in trail running-a field study.

Trail running is an emerging discipline with relatively few studies performed in ecological conditions. The aim of this work was to investigate if and how spatiotemporal parameters (STP) and kinematics differ between initial and final stage of a field trial. Twenty trail runners (10 F, 10 M) were recruited and ran a solo 9.1 km trial. During the test, participants wore a GPS watch and an IMU-based motion capture system. Running speed, elapsed time, STP and kinematics were compared between initial and final stage, separately for uphill (UH) and downhill (DH) sections. Running speed decreased in the final stage ( ). Total test time was more correlated to the time elapsed in UH sections. In the final stage and in both UH and DH sections, contact time and duty factor increased, whilst stride length and flight time decreased ( ). In the final stage, ankle joint was more dorsiflexed in stance and swing phases in UH sections and stance phase only in DH sections ( ). In the final stage, knee joint was less extended in swing phase in UH and DH sections, as well as less extended in stance in UH sections ( ). In the final stage, hip joint was less flexed in the swing phase in UH and DH sections ( ). In the final stage, forward trunk lean was higher across the entire gait cycle in in UH sections ( ). Trunk contralateral axial rotation was lower, in DH sections ( ). During the final stage, results indicate a less efficient propulsion phase, in both UH and DH sections. In UH sections, results suggest lower energy generation at the ankle joint. In DH sections, results suggest that the kinematics of swing leg may play a role in sub-optimizing propulsion phase. This study demonstrates how, in UH and DH sections, similar changes in spatiotemporal parameters can be elicited by dissimilar changes in running kinematics. To optimize performance in trail running, coaches and practitioners are advised to work on different (incline-specific) aspects of running technique.

Do lower limb kinetic and kinematic differences in drop jump landings persist when players have returned to sport following knee ligamentous injuries?

Knee injuries (KI) are identified as one of the most burdensome lower limb injuries in football with long and complicated rehabilitation and high re-injury risk. Current research often describes technique deficits being present upon RTS, often being a result of poor return to sport (RTS) testing procedures. More recently, research has advocated the use of single leg vertical jump assessments, due to the specificity of the movement to injury mechanisms and the ability to consider different phases of the task to inform practitioners of joint contributions to the propulsion and landing phases. The aim of this research is to identify whether these functional deficits exist 12-24 months post RTS authorisation, to help inform future RTS in applied practice. Twenty-four semi-professional male footballers (mean ± SD: age 25 ± 4 years; stature 182 ± 7 cm; body mass 80 ± 8 kg) were recruited into a previously injured and a control group. Participants completed 5 repetitions of single leg drop jump (SLDJ). Jump height, ground contact time, reactive strength index, peak moments, angles and power contributions for the hip, knee and ankle were measured using Qualisys Track Manager then exported to Visual 3D. This study was approved by the Edge Hill University Ethics Committee. No significant findings were identified for landing one however, during the propulsion phase there was a difference between groups (P = 0.026, d=1.14) for knee flexion at take-off. Most differences were identified during landing two bilaterally in both groups. Within the injured group, differences were identified for peak hip (P = 0.011, d = 0.028) and knee powers (P &lt; 0.001, d = 1.269), hip (P &lt; 0.001, d = 1.924) and knee power contribution % (P &lt; 0.001, d = 1.269). No differences were identified for the performance measures or through performing statistical parametric mapping for hip and knee powers for all three phases. Joint coupling analysis was conducted for the total ground contact time amalgamating the landing and propulsion phase, this was analysed as Hip-Knee, Hip-Ankle and Knee-Ankle, with significant findings in Hip-Knee and Hip-ankle. Both groups elicited bilateral differences however, no differences between the injured limb and dominant control limb were identified. To better apply this form of analysis to RTS testing, functional movements such should be undertaken when athletes are injury free to provide baseline measurements to refer to if undertaking rehabilitation from KI. Likewise, the current study identified that joint contributions could be identified in a more applied setting using the range of motion data from each joint.

Unrestrained Versus Vertically Restrained Loaded Countermovement Jumps: Are There Any Differences in the Components of Force Application?

The objective of this study was to compare a number of variables derived from the vertical and horizontal force components between loaded countermovement jumps performed in a Smith machine (SM modality; vertically restrained jumps) and with free weights (FW modality; unrestrained jumps). Twenty-three recreationally trained individuals, 6 women and 17 men, performed on a 3D force platform 5 maximal countermovement jump trials against 3 external loads (30%, 50%, and 70% of the SM 1-repetition maximum) using the SM and FW jumping modalities on separate sessions. The SM modality promoted greater values for virtually all the variables derived from the vertical force component (maximal force, maximal and minimum velocity, and impulse) and also shorter durations of the braking and propulsive phases. Regardless of the countermovement jump phase (braking or propulsive), the impulse directed toward the backward direction was always considerably greater for the SM compared with the FW modality. These results evidence that for recreationally trained individuals, the SM modality could be more effective to increase the general force capacity of the leg muscles due to increased external stability, while the FW modality is preferable when the orientation of force application is a crucial consideration, as it reduces the horizontal force component.

Effects of an Auditory Versus Visual Stimulus on Reaction and Response Time During Countermovement Jumps.

Reacting and responding to an external stimulus is an important component of human performance, and they inform us about a participant's neurophysiological capabilities. Our purpose in this study was to determine whether reaction times (REACT), response times (RT), and countermovement jump (CMJ) performance differ when responding to an auditory (AUD) versus visual (VIS) stimulus. Participants were 17 college-aged volunteers (6 females and 11 males; M age = 23.0, SD = 3.4 years; M height = 174.57, SD = 10.37cm; M body mass = 73.37, SD = 13.48kg). Participants performed CMJs on force plates immediately upon receiving an AUD or a VIS stimulus. The AUD stimulus was a beep noise, while the VIS stimulus was a light on a screen in front of the participants. We determined REACT for the tibialis anterior (TA), medial gastrocnemius (GM), vastus lateralis (VL), and biceps femoris (BF) muscles to be the amount of time between stimulus onset and the initiation of the muscle's electromyographic (EMG) signal. We determined RT to be the amount of time between stimulus onset and the beginning of the participant's force production. We assessed CMJ performance via ground reaction forces during the unweighting, braking, and propulsive phases of the jump. We quantified EMG amplitude and frequency during each CMJ phase. We found RT to be faster to the AUD versus the VIS stimulus (p = .007). VL and BF muscles had faster REACT than TA and GM muscles (p ≤ .007). The AUD stimulus was associated with faster CMJ unweighting phase metrics (p ≤ .005). Thus, individuals may react and respond faster to an AUD versus VIS stimulus, with limited improvements in their subsequent physical performance.

Pectoral Fin Propulsion Performance Analysis of Robotic Fish with Multiple Degrees of Freedom Based on Burst-and-Coast Swimming Behavior Stroke Ratio

The pectoral fin propulsion of a bionic robotic fish always consists of two phases: propulsion and recovery. The robotic fish moves in a burst-and-coast swimming manner. This study aims to analyze a pair of bionic robotic fish with rigid pectoral fin propulsion with three degrees of freedom and optimize the elliptical propulsion curve with the minimum recovery stroke resistance using computational fluid dynamics methods. Then, the time allocated to the propulsion and recovery phases is investigated to maximize the propulsion performance of the bionic robotic fish. The numerical simulation results show that when the time ratio of the propulsion and recovery phases is 0.5:1, the resistance during the movement of the robotic fish is effectively reduced, and the drag-reducing effect is pronounced. According to a further analysis of pressure clouds and vortex structures, the pressure difference between the upstream and downstream fins of the pectoral fin varies with different stroke ratios. The increase in recovery phase time helps to prevent premature damage to the vortex ring structure generated during the propulsion process and improves propulsion efficiency.

Relationship between jump height and lower limb joint kinetics and kinematics during countermovement jump in elite male athletes

ABSTRACT This study aims to identify the relationship between jump height and the kinetic and kinematic parameters of the hip, knee, and ankle joints during countermovement jump (CMJ) in elite male athletes. Sixty-six elite male athletes from various sports (strength and power, winter downhill, combat, ball game, and aquatic) performed maximal effort CMJs with hands and arms crossed against their chests on force platforms. Jumping motion in the sagittal plane was recorded using video analysis and the peak torque, power, and angular velocity of the right hip, knee, and ankle joints were calculated during the propulsive phase. Correlations between the CMJ height and kinetic and kinematic parameters were investigated using Pearson’s product-moment coefficient (r) and Spearman’s rank correlation coefficient (ρ). CMJ height was highly correlated with peak hip power (ρ = 0.686, p < 0.001) and peak knee angular velocity (r = 0.517, p < 0.001), and moderately correlated with peak hip angular velocity (r = 0.438, p < 0.001) and peak hip torque (r = 0.398, p = 0.001). These results indicate that notable hip torque and power can contribute to increased angular velocity in both the knee and hip joints, ultimately increasing the CMJ height in elite male athletes.

Effect Of Improving Lumbar Mobility, Spinal Stability ; With Core Stability Training To Achieve Per-sonal Best Runner

Running consists of simple basic movements, and to achieve these simple basic movements requires good mobility and stability, because the weakening of mobility and stability of the body causes misalignment or weakness of the legs so as to interfere with the absorption or propulsion phase in leg movement while running. Apply appropriate exercise methods to improve lumbar mobility and spinal stability. One training method that can be used is the core stability training method. Core stability is an exercise model to improve the ability to control the position of trunk movement through the pelvis and legs to allow optimal motion production. This study aims to ascertain whether core stability training can improve lumbar mobility and spinal stability in running athletes. The study used an experimental method with a research pattern using a group pretest-posttest design. 10 running athletes were sampled according to the criteria to be treated in the form of core stability training for 6 weeks. The athletes were assessed before and after training using the Functional Reach Test and Unilateral Hip Bridge Endurance. This study showed that there was a significant effect between core stability training and lumbar mobility (P = 0.000) and spinal stability (P = 0.000) in athletes. The results of this study show that the application of core stability exercises can increase the ability of lumbar mobility and spinal stability, so if the core stability exercises are done properly, they can increase stability in the spine and mobility in the lumbar, which causes dynamic limb movement to be more efficient. Key Words: athletics; Core Stability; Lumbar Mobility; spine stability.

Braking and Propulsion Phase Characteristics of Traditional and Accentuated Eccentric Loaded Back Squats.

The purpose of this study was to examine the differences in braking and propulsion force-time characteristics and barbell velocity between traditional (TRAD) and accentuated eccentric loaded (AEL) back squats using various load combinations. Sixteen resistance-trained men participated in four separate testing sessions which included a one repetition maximum (1RM) back squat during the first session and three squat testing sessions. During the squat testing sessions, participants either performed sets of three repetitions of TRAD back squats each with 50, 60, 70, and 80% 1RM or performed the same loads with the addition of weight releasers that increased the total eccentric weight of the first repetition of each set to either 100 (AEL-MAX) or 110% 1RM (AEL-SUPRA). Braking and propulsion mean force, duration, and impulse as well as mean and peak barbell velocity were compared between each condition and load. Significantly greater braking impulses were produced during the AEL-MAX and AEL-SUPRA conditions compared to TRAD (p < 0.03) with small-moderate effect sizes favoring AEL-SUPRA. No other significant differences existed among conditions for other braking, propulsion, or barbell velocity variables. AEL-MAX and AEL-SUPRA back squats may provide a greater braking stimulus compared to TRAD squats; however, the propulsion phase of the movement does not appear to be impacted. From a loading standpoint, larger and smaller load spreads may favor rapid and maximal force production characteristics, respectively. Further research on this topic is needed as a large portion of the braking stimulus experienced during AEL back squats may be influenced by relative strength.

Effects of Stationary Bikes and Elliptical Machines on Knee Joint Kinematics during Exercise.

Background and Objectives: This study examined the influence of stationary bikes and elliptical machines on knee movement and joint load during exercise. Materials and Methods: Twelve healthy male participants engaged in pedaling exercises on stationary bikes and elliptical machines at speeds of 50 and 70 revolutions per minute (rpm). Knee movement and joint load were assessed using a motion analysis system. Results: The results indicated that elliptical machines induced higher knee joint torque compared to stationary bikes. Notably, peak torque occurred at different joint angles, with stationary bikes reaching an earlier peak at 70°-110° and elliptical machines showing a later peak at 135°-180°. Increased pedaling speed correlated with higher peak knee joint torque on both machines. With the elliptical machine, a higher pedaling frequency correlated with increased peak forces on the knee and ankle joints, as well as vertically. Interestingly, both types of equipment were associated with enhanced peak knee joint torques during high-speed pedaling. Conversely, constant pedaling on elliptical machines limited the ankle angle and could induce inward rotation. Conclusions: This study focused on knee joint torque variations during pedaling on indoor stationary bicycles and elliptical machines. Elliptical machines showed higher peak values of forces and torque, particularly during the propulsive and recovery phases, indicating potential challenges to the knee joint. Notably, peak pedal angles occurred earlier on indoor stationary bicycles, emphasizing the impact of equipment choice on joint kinetics.

Acute Effects of Countermovement Jumps with Post-activation Potentiation Utilizing Additional Elastic Resistance during Eccentric Stretching Phase

&lt;p&gt;目的：本研究欲探究下蹲階段輔以彈性阻力之反向跳預先活動，對於6分鐘恢復時間後之跳躍表現的活化後增益效應。方法：本研究招募11名國立大學體育相關科系男性學生為受試對象，施以2循環、每循環5次的反向跳預先活動，且於下蹲制動階段介入外加彈性阻力，阻力強度分為0% BW、15% BW、30% BW三種情境，並於預先活動介入前與介入後第6分鐘後進行前測與後測。以兩塊測力板擷取前測與後測之反向跳過程中垂直地面反作用力值與時間，並計算發力率、跳躍高度、修正式反應力量指標以及下肢勁度。以相依樣本 t-test 進行統計檢驗，統計水準訂為 &amp;alpha; = .05，比較前測與後測的下肢動態力量特徵之參數是否有差異。結果：本研究結果顯示，三種介入模式皆顯著提升反向跳制動末點力值 (p &lt; .05)。15% BW與30% BW的介入模式顯著提升制動期與推蹬期發力率 (p &lt; .05)。30 % BW的介入模式顯著提升反向跳下肢勁度、修正式反應力量指標，並縮短制動期時間 (p &lt; .05)。結論：本研究之介入應有助於增進牽張反射機制之效益，尤其是在離心制動階段的肌力特徵表現上，因此，建議在進行具備牽張反射機制之動作執行前，可透過反向跳下蹲階段輔以30% BW彈性阻力進行下肢動態肌力活化之預先活動，以助於後續之運動表現。&lt;/p&gt; &lt;p&gt;&amp;nbsp;&lt;/p&gt;&lt;p&gt;Purpose: The current study aimed at exploring the immediate effect of pre-activities of countermovement jumps, with elastic resistance and 6 minutes of recovery time, on post-activation potentiation during squatting phase. Method: Eleven male college students recruited from the Physical Education department at National Dong Hwa University were asked to perform pre-activities of countermovement jumps (2 cycles, 5 repetitions for each cycle), with elastic resistances (0% BW, 15% BW, and 30% BW) during the squatting phase, followed by pre-tests and post-tests before and after six minutes of recovery time. Two AMTI force plates were used to collect data on vertical ground reaction force and time during countermovement jumps for pre- and post-tests. Rate of force development, jump heights, reactive strength index modified, and lower extremity stiffness were calculated accordingly. Dependent sample t-test was conducted (&amp;alpha;=.05) to determine if there were significant differences in the parameters related to lower extremity dynamic strength between the pre- and post-tests. Results: Pre-activities with all three interventions significantly increased the ground reaction forces during the breaking phase (p &lt; .05). Pre-activities with elastic resistances of 15% and 30% BW significantly elevated the rates of force development during the breaking and propulsive phases (p &lt; .05). Pre-activities with 30% BW elastic resistance significantly led to increased lower extremity stiffness, reactive strength index modified, and decreased breaking time (p &lt; .05). Conclusion: The added elastic resistance enhanced the benefits of stretch reflex mechanism, especially the muscle strength characteristics during the eccentric breaking phase. Based on our finding, countermovement jumps with pre-activity with elastic resistance of 30% BW is recommended prior to performing movements concerning stretch reflex mechanism.&lt;/p&gt; &lt;p&gt;&amp;nbsp;&lt;/p&gt;

Abnormal Lower Limb Biomechanics During a Bilateral Vertical Jump Despite the Symmetry in Single-Leg Vertical Hop Height in Athletes After ACL Reconstruction.

A limb symmetry index (LSI) of >90% for single-leg horizontal hop distance is recommended as a cutoff point for safe return to sports after anterior cruciate ligament reconstruction (ACLR). Despite achieving this threshold, abnormal lower limb biomechanics continue to persist in athletes after ACLR. Symmetry in single-leg vertical hop height appears to be more difficult to achieve and can be a better representation of knee function than single-leg horizontal hop distance. To explore whether an LSI of >90% for single-leg vertical hop height can represent normal lower limb biomechanics in athletes during a bilateral vertical jump after ACLR. Controlled laboratory study. According to the LSI for single-leg vertical hop height, 46 athletes who had undergone ACLR with an autologous ipsilateral bone-patellar tendon-bone or hamstring tendon graft were divided into a low symmetry group (LSI <90%; n = 23) and a high symmetry group (LSI >90%; n = 23), and 24 noninjured athletes were selected as the control group. The kinematic and kinetic characteristics during a bilateral vertical jump were compared between the low symmetry, high symmetry, and control groups. During the propulsion phase of the bilateral vertical jump, the operated side in the high symmetry group showed a lower knee extension moment than the nonoperated side (P = .001). At peak vertical ground-reaction force, the operated side in the high symmetry group showed a lower knee internal rotation moment compared with the control group (P = .016). Compared with the nonoperated side, the operated side in the high symmetry group showed a higher hip extension moment (P = .002), lower knee extension moment (P < .001), lower ankle plantarflexion moment (P < .001), and lower vertical ground-reaction force (P = .023). Despite achieving an LSI of >90% for single-leg vertical hop height, athletes after ACLR showed abnormal lower limb biomechanical characteristics during the bilateral vertical jump. Symmetrical single-leg vertical hop height may not signify ideal biomechanical or return-to-sports readiness in this population.

Sex differences in the determination of prescribed load in ballistic bench press.

Introduction: The objectives of the present study were twofold: first, to identify the specific relative load at which the concentric motion transforms into a purely propulsive action among women, and second, to compare the load-velocity relationships between men and women during the bench press throw. Methods: Fourteen men and fourteen women participated in a test where they progressively increased the load until reaching their one-repetition maximum (1RM) in the bench press exercise. Linear regression models were employed to elucidate the relationships between load and velocity, as well as load and the propulsive phase (% of total concentric time). Additionally, ANCOVA was utilized to compare the linear regression models between men and women. Results: The results revealed strong and linear associations between load and mean propulsive velocity (MPV) for both men and women, as well as between load and the propulsive phase. Notably, there were significant differences in MPV and the propulsive phase concerning load between men and women. Women transitioned into a fully propulsive concentric phase at approximately 80% of their 1RM, while men achieved this entirely propulsive phase at around 85% of their 1RM. Furthermore, women exhibited reduced velocities when handling lighter relative loads compared to men. Conversely, women demonstrated higher velocities when dealing with loads exceeding 85% of their 1RM in contrast to their male counterparts. Discussion: These findings hold notable implications for prescribing bench press throw loads for women, which should differ from those recommended for men. Further studies are necessary to validate the efficacy of the proposed load recommendations.

The effect of proposed changes for the rotation and throwing phases on some kinematics and performance variables for U20 weightlifters

Weightlifters from the youth division of five clubs in the Najaf Governorate. The researcher divided the research sample into two groups (experimental and control), and the proposed changes in technical performance were applied to the members of the experimental group while traditional technical performance was applied to the members of the control group.The educational program, which consists of three educational units for each week for a period of four weeks, was put into practice after the pre-tests, which included the option of (shooting from full performance), and the results showed that there were significant differences between the two groups and in favor of the experimental group in the post-test of throwing from the full movement, which came in line with the researchers' hypothesis .In order to give the shooter the most explosive power possible and to achieve the best mechanical position by managing the pause period between the rotation and propulsion phases, he attributed the reason to the significance and impact of the proposed changes on the throwing distance for this competition.The traditional technical performance, which is in the form of bending at the start of rotation, rotation, then extension, and pushing, results in difficulty controlling the stopping period between the two phases (rotation and pushing), as well as less explosive power. This is achieved by using the instantaneous rapid bending and stretching method and without a long pause. In contrast to the suggested modification, the researcher suggested performing research on examples of discus effectiveness.