Stretch-shortening Cycle Research Articles

Evidence for Muscle Cell-Based Mechanisms of Enhanced Performance in Stretch-Shortening Cycle in Skeletal Muscle.

Force attained during concentric contraction (active shortening) is transiently enhanced following eccentric contraction (active stretch) in skeletal muscle. This phenomenon is called stretch-shortening cycle (SSC) effect. Since many human movements contain combinations of eccentric and concentric contractions, a better understanding of the mechanisms underlying the SSC effect would be useful for improving physical performance, optimizing human movement efficiency, and providing an understanding of fundamental mechanism of muscle force control. Currently, the most common mechanisms proposed for the SSC effect are (i) stretch-reflex activation and (ii) storage of energy in tendons. However, abundant SSC effects have been observed in single fiber preparations where stretch-reflex activation is eliminated and storage of energy in tendons is minimal at best. Therefore, it seems prudent to hypothesize that factor(s) other than stretch-reflex activation and energy storage in tendons contribute to the SSC effect. In this brief review, we focus on possible candidate mechanisms for the SSC effect, that is, pre-activation, cross-bridge kinetics, and residual force enhancement (RFE) obtained in experimental preparations that exclude/control the influence of stretch-reflex activation and energy storage in tendons. Recent evidence supports the contribution of these factors to the mechanism of SSCs, and suggests that the extent of their contribution varies depending on the contractile conditions. Evidence for and against alternative mechanisms are introduced and discussed, and unresolved problems are mentioned for inspiring future studies in this field of research.

The effects of ageing on functional capacity and stretch-shortening cycle muscle power.

[Purpose] To examine the effects of age and gender in an ageing population with respect to functional decline and the relationship between muscle power and functional capacity. [Participants and Methods] The cohort (N=154) was subdivided into youngest-old (65–70 years.; n=62), middle-old (71–75 years.; n=46), and oldest-old (76–81 years.; n=46). Measures of mechanical muscle function included countermovement jump height, muscle power, leg strength and grip strength. Functional performance-based measures included heel-rise, postural balance, Timed Up and Go, and gait speed. [Results] The oldest-old performed significantly worse than the middle-old, whereas the youngest-old did not outperform the middle-old to the same extent. Increased contribution of muscle power was observed with increasing age. Males had consistently higher scores in measures of mechanical muscle function, whereas no gender differences were observed for functional capacity. [Conclusion] The age-related decline in functional capacity appears to accelerate when approaching 80 years of age and lower limb muscle power seems to contribute to a greater extent to the preservation of functional balance and gait capacity at that stage. Males outperform females in measures of mechanical muscle function independent of age, while the findings give no support for the existence of gender differences in functional capacity.

Acute Effects of Muscular Fatigue on Vertical Jump Performance in Acrobatic Gymnasts, Evaluated by Instrumented Insoles: A Pilot Study

The study of fatigue during training is becoming a very useful tool to avoid possible injuries not only during the training sessions but also during recovery time. Many researches have proved that concepts such as muscular fatigue and postactivation potentiation have a close relationship. With this aim, vertical jump can provide a very important information that can help to analyze the muscular fatigue that happened during this type of activity, mainly if the monitoring system is able to measure jumping parameters during their regular training session in their natural training environment. This study was performed with instrumented insoles called ECnsole. These insoles were tested with a group of twelve volunteers. In a tumbling surface, the participants performed a jumping protocol in three conditions: rest, fatigue‐induced, and recovery. Using these validated insoles, the acrobatic gymnasts showed an inability to use the stretch‐shortening cycle for improving vertical jumping performance after fatigue condition, although no deterioration of jump performance was found.

Effects of a vigorous play activity intervention program for preschoolers to improve the stretch-shortening cycle in lower limbs

Effects of a vigorous play activity intervention program for preschoolers to improve the stretch-shortening cycle in lower limbs

The Influence of Verbal Cues on Drop Jump Landing Strategies in NCAA Division I Soccer Players

Jump testing to monitor athletic performance and development has become commonplace in the field of strength and conditioning. Verbal cues of jump tests, such as the drop jump, has significant effects on performance metrics and movement strategy. The purpose of this study was to use a force platform analysis to investigate landing depth and global flexion in the lower extremity joints at initial impact between cues to maximize jump height or minimize ground contact time. Another purpose of this study was to repeat previous investigations on the influence of cueing DVJs to maximize height or minimize ground contact time on the Reactive Strength Index (RSI). Thirty-nine Division I soccer players performed one DVJ (DVJ_Height) for maximum height and another DVJ (DVJ_Quick) for quickness. Differences in dependent variables were analyzed by Bayesian paired samples t-tests. RSI was greatest in the DVJ_Quick condition (Mean_diff = 0.36 ± 104 (95% HDI: .232, .492). Ground contact time was shorter with the DVJ_Quick (Mean_diff = -0.19 ± 0.028, 95% HDI: -0.224, -0.155). Jump height was less with the DVJ_Quick (Mean_diff = -0.033 ± 0.007, 95% HDI: -0.068,.0035). Landing depth decreased during the DVJ_Quick (Mean_diff = -0.093 ± 0.018, 95% HDI: -0.118, -0.066). Global flexion in the lower extremities at impact decreased with a DVJ_Quick (Mean_diff = -0.057 ± 0.025, 95% HDI: -0.084, -0.031). DVJ_Quick resulted in a large increase in RSI due to disproportionate decreases in ground contact time (large decrease) compared to jump height (small decrease). The increase in RSI with DVJ_Quick coincided with a decrease in landing depth, suggesting a stiff landing strategy and increased stretch-shortening cycle intensity.

Spring-loaded body mass equivalent horizontal reactive countermovement jump ground contact and flight times, but not peak forces, are comparable to vertical jumping

Horizontal (cylinder-based) sledge jumping has been shown to ameliorate multi-system deconditioning induced by long-term bed-rest. However, biomechanics differ from 1 g vertical jumping, in particular prolongation of ground contact times (GCT), reduction of peak force, rate of force development (RFD) (and presumably stretch shortening cycle [SSC] efficacy) and stiffness, whilst also requiring relatively complex equipment. Thus, we sought to determine if horizontal spring-loaded countermovement jumps were more analogous to vertical jumping.9 healthy (5 female) subjects (27 ± 7yrs; 169.0 ± 5.3 cm; 63.6 ± 2.6 kg) performed 10 reactive countermovement jumps vertically, and horizontally (randomized) when lay on a spring-loaded carriage performed against loading (at lift-off) equivalent (±6%) to their body weight. Jump kinetics, kinematics and lower limb/trunk electromyographic activity were compared between conditions (paired t-tests).Mean flight and GCTs did not differ, however, peak jump height (p = 0.003; d = −0.961) was greater when jumping horizontally. In contrast, ground reaction forces (zGRF) during take-off (p < 0.001; d = 1.645) and landing (p = 0.002; d = 1.309), peak acceleration (p = 0.001; d = 1.988), leg stiffness (p = 0.001; d = 2.371) and RFD (p = 0.023; d = 1.255) were lower horizontally. Mean rectus femoris activity was lower during landing (p = 0.033; d = 0.691) when horizontal, but did not differ during either take-off or land-lift. Mean medial gastrocnemius activity was significantly (p = 0.018; d = 0.317) lower during horizontal take-off.Spring-loading (1 g at take-off) maintained short GCTs and flight times presumably maintaining muscle SSC efficacy in a manner that appears intuitive (in young active subjects), simple, robust and potentially compatible with spaceflight. Whether appropriate jump characteristics can be achieved in older subjects and in μg/hypogravity needs to be determined. However, greater jump height, lower peak zGRF, RFD and leg stiffness along with reduced lower limb and trunk muscle activity suggests that 1 g at take-off is insufficient to replicate vertical jump biomechanics. Thus, further investigation is warranted to optimize, and evaluate spring-loaded jumping as a gravity-independent multi-systems countermeasure on Earth, and in Space.

Factors influencing optimum countermovement jump performance and movement strategy in Championship professional football players: implications for player profiling

ABSTRACT Single leg countermovement jump (CMJ) is a common profiling test influenced by sport, age, sex and playing level. Controlling for these confounding variables, outfield players from an English Championship squad (n = 36) were retrospectively categorized as best (n = 10) or worst (n = 10), based on mean single leg CMJ height and flight time:contraction time ratio. Movement strategy was quantified as force-time history metrics differentiating eccentric and concentric phases. Jump height revealed that best performers elicited greater rate of force development in both phases (P ≤ 0.033), with concentric impulse the strongest predictor of performance. Time ratio also differentiated best performers as utilizing a shallower (P = 0.002) countermovement, with concentric rate of force development the strongest predictor of good performance. Successful jump height performance can mask ineffectual eccentric and stretch shortening cycle neuromuscular characteristics. Time ratio is therefore advocated as the key performance indicator, with movement strategy prioritized over gross outcome measures.

Transmission-Mode Ultrasound for Monitoring the Instantaneous Elastic Modulus of the Achilles Tendon During Unilateral Submaximal Vertical Hopping.

Submaximal vertical hopping capitalizes on the strain energy storage-recovery mechanism associated with the stretch-shortening cycle and is emerging as an important component of progressive rehabilitation protocols in Achilles tendon injury and a determinant of readiness to return to sport. This study explored the reliability of transmission mode ultrasound in quantifying the instantaneous modulus of elasticity of human Achilles tendon during repetitive submaximal hopping. A custom-built ultrasound transmission device, consisting of a 1 MHz broadband emitter and four regularly spaced receivers, was used to measure the axial velocity of ultrasound in the Achilles tendon of six healthy young adults (mean ± SD; age 26 ± 5 years; height 1.78 ± 0.11 m; weight 79.8 ± 13.6 kg) during steady-state unilateral hopping (2.5 Hz) on a piezoelectric force plate. Vertical ground reaction force and lower limb joint kinematics were simultaneously recorded. The potential sensitivity of the technique was further explored in subset of healthy participants (n = 3) that hopped at a slower rate (1.8 Hz) and a patient who had undergone Achilles tendon rupture-repair (2.5 Hz). Reliability was estimated using the mean-within subject coefficient of variation calculated at each point during the ground-contact phase of hopping, while cross-correlations were used to explore the coordination between lower limb kinematics ground reaction forces and ultrasound velocity in the Achilles tendon. Axial velocity of ultrasound in the Achilles tendon was highly reproducible during hopping, with the mean within-subject coefficient of variation ranging between 0.1 and 2.0% across participants. Ultrasound velocity decreased immediately following touch down (−19 ± 13 ms–1), before increasing by 197 ± 81 ms–1, on average, to peak at 2230 ± 87 ms–1 at 67 ± 3% of ground contact phase in healthy participants. Cross-correlation analysis revealed that ultrasound velocity in the Achilles tendon during hopping was strongly associated with knee (mean r = 0.98, range 0.95–1.00) rather than ankle (mean r = 0.67, range 0.35–0.79) joint motion. Ultrasound velocity was sensitive to changes in hopping frequency in healthy adults and in the surgically repaired Achilles tendon was characterized by a similar peak velocity (2283 ± 13 ms–1) but the change in ultrasound velocity (447 ± 21 ms–1) was approximately two fold that of healthy participants (197 ± 81 ms–1). Although further research is required, the technique can be used to reliably monitor ultrasound velocity in the Achilles tendon during hopping, can detect changes in the instantaneous elastic modulus of tendon with variation in hopping frequency and tendon pathology and ultimately may provide further insights into the stretch-shortening cycle and aid clinical decision concerning tendon rehabilitation protocols and readiness to return to sport.

A Novel Method to Categorize Stretch-Shortening Cycle Performance Across Maturity in Youth Soccer Players.

Pedley, JS, Lloyd, RS, Read, PJ, Moore, IS, Myer, GD, and Oliver, JL. A novel method to categorize stretch-shortening cycle performance across maturity in youth soccer players. J Strength Cond Res 36(9): 2573-2580, 2022-This study used a novel method to categorize stretch-shortening cycle (SSC) function during a drop jump (DJ) using the force-time curve. This method was then used to determine the effect of maturity status upon SSC function and effect of SSC function on DJ performance. Prepeak, circa-peak, and postpeak height velocity male youth soccer players completed a preseason 30-cm DJ onto a force plate. Stretch-shortening cycle function was categorized as poor (impact peak and not spring-like), moderate (impact peak and spring-like), or good (no impact peak and spring-like). Interactions between SSC function and maturity status, and SSC function and kinetic variables were explored. Youth soccer players displaying good SSC function were older and more mature than those with poor SSC function; however, 9.9% of post peak height velocity still displayed poor SSC function. Players with good SSC function recorded significantly shorter ground contact times, reduced time between peak landing and takeoff force, reduced center of mass displacement, and significantly greater takeoff forces than players with moderate and poor SSC function (all p < 0.05). SSC function during a standardized DJ improves with maturation, but a portion of mature players still demonstrate poor SSC function. Good SSC function was associated with improved DJ outcome measures except jump height. Tailored training interventions based on SSC competency may be required to optimally enhance SSC function.

The stretch-shortening cycle effect is prominent in the inhibited force state

It has been suggested that residual force enhancement (RFE) contributes to the work enhancement observed in stretch-shortening cycles (SSC). Based on recent findings that RFE was preserved in the reduced force state, one may speculate that the SSC effect may be preserved in the reduced force state as well. The purpose of this study was to examine the magnitude of the SSC effect in inhibited skeletal muscle force states. Normal and inhibited force conditions were analyzed using skinned rabbit soleus fibres (N = 18). The inhibited force condition was achieved by adding 2,3-Butanedione monoxime into the activating solution. For both conditions, a SSC test and a pure shortening test were performed. In the SSC tests, fibres were activated at an average sarcomere length of 2.4 μm, and then stretched to 3.0 μm. Immediately after the end of the stretch, fibres were shortened to 2.4 μm. In the pure shortening tests, fibres were activated at an average sarcomere length of 3.0 μm and then shortened to 2.4 μm. The relative increase in mechanical work in the shortening phase of the SSC compared to the pure shortening condition was defined as the SSC effect index, and the magnitude of the SSC effect was compared between the normal and the inhibited force condition. The SSC effect was greater in the inhibited compared to the normal force condition (p < 0.001). We conclude that the SSC effect is at least in part preserved in the reduced force state.

Contribution of Stretch-Induced Force Enhancement to Increased Performance in Maximal Voluntary and Submaximal Artificially Activated Stretch-Shortening Muscle Action.

In everyday muscle action or exercises, a stretch-shortening cycle (SSC) is performed under different levels of intensity. Thereby, compared to a pure shortening contraction, the shortening phase in a SSC shows increased force, work, and power. One mechanism to explain this performance enhancement in the SSC shortening phase is, besides others, referred to the phenomenon of stretch-induced increase in muscle force (known as residual force enhancement; rFE). It is unclear to what extent the intensity of muscle action influences the contribution of rFE to the SSC performance enhancement. Therefore, we examined the knee torque, knee kinematics, m. vastus lateralis fascicle length, and pennation angle changes of 30 healthy adults during isometric, shortening (CON) and stretch-shortening (SSC) conditions of the quadriceps femoris. We conducted maximal voluntary contractions (MVC) and submaximal electrically stimulated contractions at 20%, 35%, and 50% of MVC. Isometric trials were performed at 20° knee flexion (straight leg: 0°), and dynamic trials followed dynamometer-driven ramp profiles of 80°–20° (CON) and 20°–80°–20° (SSC), at an angular velocity set to 60°/s. Joint mechanical work during shortening was significantly (p < 0.05) enhanced by up to 21% for all SSC conditions compared to pure CON contractions at the same intensity. Regarding the steady-state torque after the dynamic phase, we found significant torque depression for all submaximal SSCs compared to the isometric reference contractions. There was no difference in the steady-state torque after the shortening phases between CON and SSC conditions at all submaximal intensities, indicating no stretch-induced rFE that persisted throughout the shortening. In contrast, during MVC efforts, the steady-state torque after SSC was significantly less depressed compared to the steady-state torque after the CON condition (p = 0.034), without significant differences in the m. vastus lateralis fascicle length and pennation angle. From these results, we concluded that the contribution of the potential enhancing factors in SSCs of the m. quadriceps femoris is dependent on the contraction intensity and the type of activation.

Reactive strength index-modified: reliability, between group comparison, and relationship between its associated variables.

To investigate and compare the reliability of reactive strength index-modified (RSImod) and its associated variables (jump height [JH] and [time to take-off]) 20 combat fighters and 18 physically active men participated in this study. They visited the laboratory three times; firstly, for jump familiarization and two sessions for test-retest (2–7 days apart). For both groups, the between-day changes in performance were trivial to small (≤ 1.1%). The coefficient of variation (CV) comparisons (i.e. CV ratio) demonstrated that combat athletes had a lower test-retest variation for RSImod (0.87) and JH (0.80) than non-athletes. Combat athletes demonstrated a greater JH than physically active men (0.43 vs 0.37; p = 0.03, g = 0.73), but small and non-significant differences were observed for RSImod (0.60 vs 0.55; p = 0.24, g = 0.38) and TTT (0.70 vs 0.72; p = 0.32, g = 0.33). RSImod was more positively correlated with JH (r = 0.75–0.87; p < 0.001) than negatively correlated with TTT (r = 0.45–0.54; p < 0.001). This study suggests that RSImod is a reliable variable obtained during CMJ testing in combat athletes and physically active men, with scores being slightly better for combat athletes. In terms of performance, combat athletes jumped higher than physically active men, but no differences in RSImod or TTT were observed. Lastly, RSImod was more strongly related to JH than TTT, and this was more evident in athletes than nonathletes. This indicates that the combat athletes were able to better utilize their (equal) time spent jumping (higher), possibly via greater utilization of the stretch shortening cycle, faster or more optimal motor unit recruitment, or an array of other factors.

Stretch-Shortening Cycle Function of Lower Limbs After Cycling in Triathletes.

Takahashi, K, Shirai, Y, and Nabekura, Y. Stretch-shortening cycle function of lower limbs after cycling in triathletes. J Strength Cond Res 36(9): 2610-2614, 2022-Impaired cardiorespiratory response and changes in biomechanical variables occur when running after cycling relative to isolated running. Nevertheless, little is known about the causes of these changes or the training to prevent them. This study aimed (a) to determine whether stretch-shortening cycle (SSC) function decreases after cycling exercise and (b) to determine whether the decreases in SSC function are related to brick training. Eleven male university triathletes performed hopping tests to measure SSC function before and after cycling (30 minutes of cycling at 110% ventilatory threshold). Stretch-shortening cycle function was calculated as the ratio of the jump height to the time spent in contact with the ground (reactive strength index [RSI]). Brick training was evaluated by the total experience of brick training. The RSI significantly decreased after the cycling exercise (-10.7%; p < 0.01), but changes in RSI after cycling did not significantly correlate with the total experience of brick training, despite a large effect size ( p < 0.10; r = 0.62). These results suggest that SSC function decreases after cycling and that brick training is potentially useful for inhibiting decreases in SSC function after cycling.

Effects of exercise intensity on the stretch-shortening cycle function of the lower limbs after cycling

Effects of exercise intensity on the stretch-shortening cycle function of the lower limbs after cycling

Long-term effects of school barefoot running program on sprinting biomechanics in children: A case-control study

BackgroundThe acute changes of running biomechanics in habitually shod children when running barefoot have been demonstrated. However, the long-term effects of barefoot running on sprinting biomechanics in children is not well understood. Research questionHow does four years of participation in a daily school barefoot running program influence sprint biomechanics and stretch-shortening cycle jump ability in children? MethodsOne hundred and one children from barefoot education school (age, 11.2 ± 0.7 years-old) and 93 children from a control school (age, 11.1 ± 0.7 years-old) performed 50 m maximal shod and barefoot sprints and counter movement jump and five repeated-rebound jumping. To analyse sprint kinematics, a high-speed camera (240 fps) was used. In addition, foot strike patterns were evaluated by using three high-speed cameras (300 fps). Jump heights for both jump types and the contact times for the rebound jump were measured using a contact mat system. Two-way mixed ANOVA was used to examine the effect of school factor (barefoot education school vs control school) and footwear factor (barefoot vs shod) on the sprinting biomechanics. ResultsSprinting biomechanics in barefoot education school children was characterised by significantly shorter contact times (p = 0.003) and longer flight times (p = 0.005) compared to control school children regardless of footwear condition. In shod sprinting, a greater proportion of barefoot education school children sprinted with a fore-foot or mid-foot strike compared to control school children (p < 0.001). Barefoot education school children also had a significantly higher rebound jump height (p = 0.002) and shorter contact time than control school children (p = 0.001). SignificanceThe results suggest that school-based barefoot running programs may improve aspects of sprint biomechanics and develop the fast stretch-shortening cycle ability in children. In order to confirm this viewpoint, adequately powered randomised controlled trials should be conducted.

Short-Term Effects of “Composite” Training on Strength, Jump, and Sprint Performance in Hurling Players

Byrne, PJ, Moody, JA, Cooper, S-M, Farrell, E, and Kinsella, S. Short-term effects of "composite training" on strength, jump, and sprint performance in hurling players. J Strength Cond Res 36(8): 2253-2261, 2022-The purpose of this study was to compare the short-term effects of "composite" training to sprint training on strength, jump, and sprint acceleration performance in hurling players. A randomized counterbalanced group design with baseline test, pretest and post-test measures was used. Twenty-five hurling players volunteered to participate and 21 completed the study. Subjects were divided into a "composite" (COMP group, n = 10) or a sprint training (SPRINT group, n = 11) group. Both groups trained twice per week for 7 weeks with the SPRINT group performing 6 repetitions of 20 m sprints and the COMP group completing 6 repetitions (1 repetition = 3 bounce drop jumps [BDJs] with a 20 m sprint after 15 seconds recovery). Significant differences existed pretraining to post-training for the COMP group for BDJ contact time (-7.25%; p = 0.05) and countermovement jump (CMJ) variables (height: 7.43%, p = 0.006; force: 5.24%, p = 0.05; power: 15.11%, p = 0.001). No significant differences were found between groups at baseline and for group by time interactions. Significant improvements were observed pretraining to post-training in both groups for the following: absolute 3 repetition maximum (3RM) back squat strength (12.73-17.62%, p = 0.01), 5 m (5.74-9.49%, p = 0.006-0.04), 10 m (4.27-5.59%, p = 0.007-0.02), and 20 m (3.35-3.98%, p = 0.003-0.01). In conclusion, "composite" training is effective in enhancing fast stretch-shortening cycle efficiency inducing CMJ force and power augmentation. However, "composite" and sprint training are effective training approaches for enhancing maximal strength and sprint performance in a time efficient manner in hurling players.

Stiffness in Running: A Narrative Integrative Review

ABSTRACT Researching running spatiotemporal parameters has deepened knowledge of the spring-mass model and its capacity to estimate and predict kinetic and kinematic variables. However, although the impact of lower-limb stiffness on athletic performance seems considerable, recognition of lower-limb stiffness among coaches and practitioners remains sparse. This review highlights factors influencing lower-limb stiffness during running and their relationship to injury prevention and performance. Current knowledge of lower-body stiffness during running is integrated and new scientific scope offered. Because the different variables are correlational, it is strongly recommended not to measure their effects on lower-limb stiffness individually. The spring-mass model behavior when altering variables such as footwear or foot-strike pattern needs cautious examination. Although both stretch-shortening cycle and lower-limb stiffness are key parameters in elastic strain energy storage and release during sport activities, hormonal fluctuations (particularly associated with the menstrual cycle) should draw special attention because they affect musculotendinous stiffness properties and therefore may influence athletic performance and injury prevalence. Introducing lower-limb strength training would improve the characteristics of stiffness in running, contributing to maximized performance and reduced injury risk.

Differences in stretch-shortening cycle and residual force enhancement between muscles

It has been suggested that cross bridge kinetics and residual force enhancement (RFE) affect force in the stretch-shortening cycle (SSC). Because cross bridge kinetics and titin isoforms, which are thought to be related to RFE, differ between muscles, the SSC effect may be also muscle-dependent. Thus, we compared the SSC effect between psoas and soleus muscles, which have a distinct fiber type distribution and different titin isoforms. Four tests (SSC, SSC control, RFE, RFE control) were conducted using isolated, skinned fibers of psoas and soleus. In the SSC tests, fibers were activated at an average sarcomere length of 2.4 μm, stretched to 3.0 μm, and shortened to 2.4 μm. In the SSC control tests, fibers were activated at an average sarcomere length of 3.0 μm and then shortened to 2.4 μm. The relative increase in mechanical work obtained during shortening between tests was defined as the SSC effect. In the RFE tests, fibers were activated at an average sarcomere length of 2.4 μm and then stretched to 3.0 μm, while the RFE control tests consisted of an isometric contraction at 3.0 μm. The difference in steady-state isometric force between tests was defined as RFE. The SSC effect was greater in soleus than in psoas, while the RFE was the same for both muscles. Since the SSC effect was greater in soleus, while the RFE was the same, the observed greater SSC effect is probably not directly caused by RFE, but may be related to differences in cross bridge kinetics.

The Effect of a Stretch-Shortening Cycle on Muscle Activation and Muscle Oxygen Consumption: A Study of History-Dependence.

Caron, KE, Burr, JF, and Power, GA.. The effect of a stretch-shortening cycle on muscle activation and muscle oxygen consumption: a study of history-dependence. J Strength Cond Res 34(11): 3139-3148, 2020-Stretch-shortening cycles (SSCs) are observed in a variety of human movements and are associated with increases in performance. Few studies have considered the effects of stretch-induced residual force enhancement (rFE) and shortening-induced residual force depression (rFD) during an SSC, and none have considered these properties during voluntary contractions. With force matched via a robotically resisted Smith machine, we hypothesized that in the isometric steady-state following an SSC (a) muscle activation (electromyography) of the knee and hip extensors would be greater and (b) muscle oxygen consumption be higher than the reference isometric condition (ISO), but less than the rFD condition. Subjects (n = 20, male, 24.9 ± 3.9 year) performed a squat exercise over 100-140° knee angle and a matched ISO at the top and bottom of the squat. After active shortening, the vastus medialis (VM), vastus lateralis (VL), and gluteus maximus (GM) showed activation increase in the rFD-state compared with ISO (∼15%, ∼11%, and ∼25% respectively). During the isometric steady-state following the SSC, there was no difference in activation as compared with ISO for VM, VL, but GM showed an activation increase of ∼15%. VM and VL showed an activation increase in the rFD-state compared with the isometric steady-state following SSC (∼16 and ∼10% respectively). Muscle oxygen consumption (tissue saturation index) was not different during the isometric steady-states following rFD and SSC compared with ISO. During a voluntary SSC exercise, the activation increase expected in the FD-state was attenuated, with no change in muscle oxygen consumption. The concomitant role of rFE and rFD during a voluntary position-matched SSC seems to counteract shortening-induced activation increase and may optimize movement economy.

Gender and Age Related Differences in Leg Stiffness and Reactive Strength in Adolescent Team Sports Players.

The aim of the present study was to identify potential gender differences in leg stiffness and reactive strength during hopping tasks in 13 to16-year old team sports players. Reactive strength index (RSI) and leg stiffness were obtained in two consecutive seasons from 51 girls (U14: n = 31, U16: n = 20) and 65 boys (U14: n = 32, U16: n = 33). A significant main effect on absolute (U14: p = 0.022, η2= 0.084; U16: p < 0.001, η2= 0.224) and relative leg stiffness (U14 p<0.001; η2= 0.195; U16; p = 0.008, η2= 0.128) for gender was found in both groups with values higher in boys than in girls. For absolute and relative stiffness gender differences in the U14 group were significant in the 1st year only (p=0.027 and p=0.001), and for the U16s in the 2nd year only (p < 0.001 and p = 0.022). For RSI, a significant main effect for gender was observed in the U16 group only (p < 0.001 η2= 0.429) with values significantly higher in boys than in girls in both years of measurement (p = 0.001; p < 0.001). Results of this study support previous limited findings, mostly related to non-athletes, suggesting lower stretch-shortening cycle capability in adolescence female compared to male, however our data only partly supports the theory that quality of neuromuscular functions increases with age until post puberty.