Cross-Bridges and Sarcomeric Non-cross-bridge Structures Contribute to Increased Work in Stretch-Shortening Cycles

Unlocking the benefit of active stretch: the eccentric muscle action, not the preload, maximizes muscle-tendon unit stretch-shortening cycle performance

Stretch-shortening cycles (SSCs) outperform shortening contractions preceded by isometric contractions in terms of enhanced force/torque, work, and power production during shortening. This so-called SSC effect is presumably related to the active muscle stretch before shortening in SSCs. However, it remains unclear whether the effects of stretch-induced higher preload level or stretch-induced history dependence maximize the SSC effect. Therefore, we analyzed fascicle behavior, muscle-tendon unit (MTU) shortening work, and torque/force (n = 12 participants) via ultrasound and dynamometry during electrically stimulated submaximal plantar flexion contractions from 10° plantarflexion to 15° dorsiflexion. To elucidate the effects of preload level and preload modality (i.e., contraction type) on shortening performance, muscle-tendon unit shortening was preceded by fixed-end (SHO), active stretch (SSC), and preload-matched fixed-end (MATCHED) contractions. Before shortening, MATCHED and SCC had the same preload level (1% torque difference), similar joint position, and muscle fascicle lengths. Compared with SHO, shortening work was significantly (P < 0.001, partial η2 = 0.749) increased by 85% and 55% for SSC and MATCHED, respectively, with SSC shortening work being significantly higher than MATCHED (P = 0.016). This indicates that preload contributes by 65% to the overall SSC effect so that 35% needs to be referred to stretched-induced history-dependent mechanisms. In addition, SSC showed larger fascicle forces at the end of shortening (P < 0.001) and 20% less depressed isometric torque following shortening compared with MATCHED (P < 0.001). As potential decoupling effects by the series elastic element were controlled by matching the preload levels, we conclude that the difference between SSC and MATCHED is related to stretch-induced long-lasting history-dependent effects.NEW & NOTEWORTHY Using a torque-matched preload protocol, we found that 2/3 of the performance enhancement in muscle-tendon unit stretch-shortening cycles (SSCs) is caused by the increased preload level. The remaining 1/3 is owed to the long-lasting history-dependent effects triggered during the stretch in SSCs. This increased performance output is attributed to passive elastic structures within the contractile element that do not require additional muscle activation, therefore contributing to the higher efficiency of the neuromuscular system in SSCs.

Impact of lengthening velocity on the generation of eccentric force by slow-twitch muscle fibers in long stretches

After an initial increase, isovelocity elongation of a muscle fiber can lead to diminishing (referred to as Give in the literature) and subsequently increasing force. How the stretch velocity affects this behavior in slow-twitch fibers remains largely unexplored. Here, we stretched fully activated individual rat soleus muscle fibers from 0.85 to 1.3 optimal fiber length at stretch velocities of 0.01, 0.1, and 1 maximum shortening velocity, vmax, and compared the results with those of rat EDL fast-twitch fibers obtained in similar experimental conditions. In soleus muscle fibers, Give was 7%, 18%, and 44% of maximum isometric force for 0.01, 0.1, and 1 vmax, respectively. As in EDL fibers, the force increased nearly linearly in the second half of the stretch, although the number of crossbridges decreased, and its slope increased with stretch velocity. Our findings are consistent with the concept of a forceful detachment and subsequent crossbridge reattachment in the stretch's first phase and a strong viscoelastic titin contribution to fiber force in the second phase of the stretch. Interestingly, we found interaction effects of stretch velocity and fiber type on force parameters in both stretch phases, hinting at fiber type-specific differences in crossbridge and titin contributions to eccentric force. Whether fiber type-specific combined XB and non-XB models can explain these effects or if they hint at some not fully understood properties of muscle contraction remains to be shown. These results may stimulate new optimization perspectives in sports training and provide a better understanding of structure-function relations of muscle proteins.

Jump Performance and Its Relationship with Lower Body Joint Kinetics and Kinematics in Children with Cerebral Palsy

PURPOSE

The aim was to quantify jump performance in children with cerebral palsy (CP) and determine if the expected deficit is related to their lower body joint kinetics and kinematics.

METHODS

Twenty-four ambulatory ( n = 17 level I and 7 level II in the Gross Motor Function Classification System) children with spastic CP ( n = 13 unilateral and 11 bilateral) and 24 age-, sex-, and race-matched typically developing controls were studied. Jump height and peak power and range of motion at the hip, knee, and ankle of the more affected limb in children with CP and the nondominant limb in controls were assessed during a countermovement jump using three-dimensional motion capture and a force platform.

RESULTS

Compared with controls, children with CP had lower jump height (33%, Cohen's d ( d ) = 1.217), peak power at the knee (39%, d = 1.013) and ankle (46%, d = 1.687), and range of motion at the hip (32%, d = 1.180), knee (39%, d = 2.067), and ankle (46%, d = 3.195; all P < 0.001). Jump height was positively related to hip, knee, and ankle power and range of motion in children with CP ( rs range = 0.474-0.613, P < 0.05), and hip and ankle power and knee and ankle range of motion in controls ( rs range = 0.458-0.630, P < 0.05). The group difference in jump height was no longer detected when ankle joint power, ankle range of motion, or knee range of motion was statistically controlled ( P > 0.15).

CONCLUSIONS

Jump performance is compromised in children with CP and is associated with low power generation and range of motion in the lower limb, especially at the ankle.

Differential leg and trunk operation during skipping without and with hurdles in bipedal Japanese macaque

When locomoting bipedally at higher speeds, macaques preferred unilateral skipping (galloping). The same skipping pattern was maintained while hurdling across two low obstacles at the distance of a stride within our experimental track. The present study investigated leg and trunk joint rotations and leg joint moments, with the aim of clarifying the differential leg and trunk operation during skipping in bipedal macaques. Especially at the hip, the range of joint rotation and extension at lift off was larger in the leading than in the trailing leg. The flexing knee absorbed energy and the extending ankle generated work during each step. The trunk showed only minor deviations from symmetry. Hurdling amplified the differences and notably resulted in a quasi-elastic use of the leading knee and in an asymmetric operation of the trunk.

Conserved mammalian muscle mechanics during eccentric contractions

Skeletal muscle has a broad range of biomechanical functions, including power generation and energy absorption. These roles are underpinned by the force-velocity relationship, which comprises two distinct components: a concentric and an eccentric force-velocity relationship. The concentric component has been extensively studied across a wide range of muscles with different muscle properties. However, to date, little progress has been made in accurately characterising the eccentric force-velocity relationship in mammalian muscle with varying muscle properties. Consequently, mathematical models of this muscle behaviour are based on a poorly understood phenomenon. Here, we present a comprehensive assessment of the concentric force-velocity and eccentric force-velocity relationships of four mammalian muscles (soleus, extensor digitorum longus, diaphragm and digastric) with varying biomechanical functions, spanning three orders of magnitude in body mass (mouse, rat and rabbits). The force-velocity relationship was characterised using a hyperbolic-linear equation for the concentric component a hyperbolic equation for the eccentric component, at the same time as measuring the rate of force development in the two phases of force development in relation to eccentric lengthening velocity. We demonstrate that, despite differences in the curvature and plateau height of the eccentric force-velocity relationship, the rates of relative force development were consistent for the two phases of the force-time response during isovelocity lengthening ramps, in relation to lengthening velocity, in the four muscles studied. Our data support the hypothesis that this relationship depends on cross-bridge and titin activation. Hill-type musculoskeletal models of the eccentric force-velocity relationship for mammalian muscles should incorporate this biphasic force response. KEY POINTS: The capacity of skeletal muscle to generate mechanical work and absorb energy is underpinned by the force-velocity relationship. Despite identification of the lengthening (eccentric) force-velocity relationship over 80 years ago, no comprehensive study has been undertaken to characterise this relationship in skeletal muscle. We show that the biphasic force response seen during active muscle lengthening is conserved over three orders of magnitude of mammalian skeletal muscle mass. Using mice with a small deletion in titin, we show that part of this biphasic force profile in response to muscle lengthening is reliant on normal titin activation. The rate of force development during muscle stretch may be a more reliable way to describe the forces experienced during eccentric muscle contractions compared to the traditional hyperbolic curve fitting, and functions as a novel predictor of force-velocity characteristics that may be used to better inform hill-type musculoskeletal models and assess pathophysiological remodelling.

Ultrasound and surface electromyography analyses reveal an intensity dependent active stretch-shortening cycle of the vastus lateralis muscle during ergometer rowing

A rowing cycle is characterised by a stretch-shortening cycle (SSC) at the quadriceps femoris muscle-tendon unit (MTU) level. However, due to the associated decoupling between MTU and muscle fascicle length changes, it remains unclear whether a rowing cycle causes active stretch at the muscle level. Fifteen young, sub-elite, male rowers (19.5 ± 1.6 yr; 1.94 ± 0.06 m; 91.9 ± 5.4 kg; rowing experience: 7.5 ± 2.8 yr) performed randomised 60-s rowing intervals using a traditional style at a low (LiR) and high intensity (HiR) and a micro-pause style at a low intensity (MpR). Muscle activity, knee joint angles, and muscle fascicle length changes from the left-sided vastus lateralis (VL) muscle were quantified using surface electromyography, inertial measurement units, and B-mode ultrasound imaging, respectively. All rowing conditions showed active fascicle stretch during late knee flexion (p≤0.001, standardised mean difference (SMD) ≥0.72) and subsequent active fascicle shortening throughout knee extension. Active fascicle stretch duration, amplitude and velocity (rANOVA: p≤0.001, ηp2 = 0.49) were not significantly different (p≥0.174; SMD≤0.26) between LiR and MpR, but were significantly increased during HiR (p≤0.001; SMD≥0.70). The percentage of rowing cycles that involved active fascicle stretch (rANOVA: p≤0.001, ηp2 = 0.95; post-hoc: p≤0.001, SMD≥0.87) was also significantly higher for HiR (98.3 ±12.9%) compared with both LiR (65.0 ± 48.1%) and MpR (68.3 ± 46.9%). In conclusion, rowing involves SSC at the VL muscle fascicle level, but the amount of active stretch differs between rowing intensities, with the longest, largest, and fastest active stretch occurring during HiR. SSC-based mechanisms may therefore contribute more to rowing performance during HiR than LiR or MpR.HighlightsSurface electromyography and ultrasound imaging revealed stretch-shortening cycles (SSCs) of the vastus lateralis muscle fascicles during rowingIncreased active fascicle stretch duration, amplitude and velocity from low- to high-intensity rowing indicate that SSC-based mechanisms likely contribute more to performance during high-intensity rowingThe SSC within the vastus lateralis muscle was independent of the rowing style at the same low rowing intensity.

Joint Coordination and Muscle-Tendon Interaction Differ Depending on The Level of Jumping Performance

The countermovement jump is a popular measurement modality to evaluate muscle power in sports and exercise. Muscle power is essential to achieve a high jump, yet the well-coordinated movement of the body segments, which optimizes the stretch-shortening cycle (SSC) effects, is also required. Among the proposed explanations of SSC effects, this study investigated whether the ankle joint kinematics, kinetics, and muscle-tendon interaction depend on the level of jump skill and the jump task. Sixteen healthy males were grouped as a function of their jump height (High jumpers; greater than 50 cm, Low jumpers; less than 50 cm). They were instructed to jump with two intensities; light effort (20 % of their height) and maximal effort. Joint kinematics and kinetics of the lower limbs were analyzed using a 3-dimensional motion analysis system. The muscle-tendon interaction was investigated using B-mode real-time ultrasonography. As the jump intensity increased, all participants jumped with increased joint velocity and power. However, the high jumper shows less fascicle shortening velocity (-0.2 ± 0.1 m/s) than the low jumper group (-0.3 ± 0.1 m/s) and greater tendon velocity, which indicated the capability of elastic energy recoil. In addition, the delayed onset time of ankle extension in the high jumper implies better use of the catapult mechanism. The findings of this study showed that the muscle-tendon interaction differs depending on the jump skill level, suggesting a more efficient neuromuscular control in skilled jumpers.

Lower Limb Unilateral and Bilateral Strength Asymmetry in High-Level Male Senior and Professional Football Players

This study sought to assess the relationship between different jumping asymmetries and associated performance variables in high-level male senior and professional football players. Nineteen football players with at least 12 years of training experience (23.2 ± 3.1 years of age; 75.2 ± 4.8 kg of body mass and 181 ± 0.06 cm of height) participated in this study performing countermovement jump (CMJ), squat jump (SJ), single-leg CMJ and drop jump (DJ), associated performance variable eccentric utilization ratio (EUR), stretch-shortening cycle (SSC), bilateral deficit (BLD), and limb symmetry index (LSI) were determined. High correlations were observed between different methodologies of jump tests and associated performance indicators (SSC, BLD, EUR), except LSI. Moreover, CMJ and SJ results were different (p < 0.05), but no differences were found between interlimb in CMJ (p = 0.19) and DJ (p = 0.14). Between the same limbs and different jumps differences were detected in CMJ and DJ (p < 0.01), and it has also been found that the laterality effect size on strength was small in CMJ (ES = 0.30) and DJ (ES = 0.35). LSI between CMJ and DJ was not different despite higher mean values in CMJ, and although mean BLD was positive (>100%), the results highlight the need for individual evaluation since eight players scored negatively. An in-depth and accurate analysis of performance in preseason screening jump tests should be considered, aiming to detect injury risk, specifically evaluating different jumping test methodologies, and determining jumping associated performance variables for each test, namely EUR, SSC, BLD, and LSI. Specific muscle-strengthening exercises could be implemented based on this study results and outcomes, aiming to reduce injury risks and lower extremity asymmetries and to enhance individual football performance in high-level male senior and professional football players. Sports institutions should pay special attention regarding potential health problems in athletes exposed to daily high training loads.

Stretching the story of titin and muscle function

The discovery of the giant protein titin, also known as connectin, dates almost half a century back. In this review, I recapitulate major advances in the discovery of the titin filaments and the recognition of their properties and function until today. I briefly discuss how our understanding of the layout and interactions of titin in muscle sarcomeres has evolved and review key facts about the titin sequence at the gene (TTN) and protein levels. I also touch upon properties of titin important for the stability of the contractile units and the assembly and maintenance of sarcomeric proteins. The greater part of my discussion centers around the mechanical function of titin in skeletal muscle. I cover milestones of research on titin's role in stretch-dependent passive tension development, recollect the reasons behind the enormous elastic diversity of titin, and provide an update on the molecular mechanisms of titin elasticity, details of which are emerging even now. I reflect on current knowledge of how muscle fibers behave mechanically if titin stiffness is removed and how titin stiffness can be dynamically regulated, such as by posttranslational modifications or calcium binding. Finally, I highlight novel and exciting, but still controversially discussed, insight into the role titin plays in active tension development, such as length-dependent activation and contraction from longer muscle lengths.

Muscle preflex response to perturbations in locomotion: In vitro experiments and simulations with realistic boundary conditions

Neuromuscular control loops feature substantial communication delays, but mammals run robustly even in the most adverse conditions. In vivo experiments and computer simulation results suggest that muscles' preflex-an immediate mechanical response to a perturbation-could be the critical contributor. Muscle preflexes act within a few milliseconds, an order of magnitude faster than neural reflexes. Their short-lasting action makes mechanical preflexes hard to quantify in vivo. Muscle models, on the other hand, require further improvement of their prediction accuracy during the non-standard conditions of perturbed locomotion. Our study aims to quantify the mechanical work done by muscles during the preflex phase (preflex work) and test their mechanical force modulation. We performed in vitro experiments with biological muscle fibers under physiological boundary conditions, which we determined in computer simulations of perturbed hopping. Our findings show that muscles initially resist impacts with a stereotypical stiffness response-identified as short-range stiffness-regardless of the exact perturbation condition. We then observe a velocity adaptation to the force related to the amount of perturbation similar to a damping response. The main contributor to the preflex work modulation is not the change in force due to a change in fiber stretch velocity (fiber damping characteristics) but the change in magnitude of the stretch due to the leg dynamics in the perturbed conditions. Our results confirm previous findings that muscle stiffness is activity-dependent and show that also damping characteristics are activity-dependent. These results indicate that neural control could tune the preflex properties of muscles in expectation of ground conditions leading to previously inexplicable neuromuscular adaptation speeds.

The history-dependent features of muscle force production: A challenge to the cross-bridge theory and their functional implications

The cross-bridge theory predicts that muscle force is determined by muscle length and the velocity of active muscle length changes. However, before the formulation of the cross-bridge theory, it had been observed that the isometric force at a given muscle length is enhanced or depressed depending on active muscle length changes before that given length is reached. These enhanced and depressed force states are termed residual force enhancement (rFE) and residual force depression (rFD), respectively, and together they are known as the history-dependent features of muscle force production. In this review, we introduce early attempts in explaining rFE and rFD before we discuss more recent research from the past 25 years which has contributed to a better understanding of the mechanisms underpinning rFE and rFD. Specifically, we discuss the increasing number of findings on rFE and rFD which challenge the cross-bridge theory and propose that the elastic element titin plays a role in explaining muscle history-dependence. Accordingly, new three-filament models of force production including titin seem to provide better insight into the mechanism of muscle contraction. Complementary to the mechanisms behind muscle history-dependence, we also show various implications for muscle history-dependence on in-vivo human muscle function such as during stretch-shortening cycles. We conclude that titin function needs to be better understood if a new three-filament muscle model which includes titin, is to be established. From an applied perspective, it remains to be elucidated how muscle history-dependence affects locomotion and motor control, and whether history-dependent features can be changed by training.

Eccentric muscle contractions: from single muscle fibre to whole muscle mechanics

Eccentric muscle loading encompasses several unique features compared to other types of contractions. These features include increased force, work, and performance at decreased oxygen consumption, reduced metabolic cost, improved energy efficiency, as well as decreased muscle activity. This review summarises explanatory approaches to long-standing questions in terms of muscular contraction dynamics and molecular and cellular mechanisms underlying eccentric muscle loading. Moreover, this article intends to underscore the functional link between sarcomeric components, emphasising the fundamental role of titin in skeletal muscle. The giant filament titin reveals versatile functions ranging from sarcomere organisation and maintenance, providing passive tension and elasticity, and operates as a mechanosensory and signalling platform. Structurally, titin consists of a viscoelastic spring segment that allows activation-dependent coupling to actin. This titin-actin interaction can explain linear force increases in active lengthening experiments in biological systems. A three-filament model of skeletal muscle force production (mediated by titin) is supposed to overcome significant deviations between experimental observations and predictions by the classic sliding-filament and cross-bridge theories. Taken together, this review intends to contribute to a more detailed understanding of overall muscle behaviour and force generation-from a microscopic sarcomere level to a macroscopic multi-joint muscle level-impacting muscle modelling, the understanding of muscle function, and disease.

Enhancement of Muscle Shortening Torque Preloaded with Muscle Lengthening is Joint-Specific

Our cross-sectional study aimed to investigate joint specificity of concentric muscle torque enhancement after a maximum eccentric contraction for the knee versus ankle joints across two different movement velocities (120°/s and 180°/s). After a familiarization session, 22 healthy young adults randomly performed concentric (CONC) and maximum eccentric preloaded concentric (EccCONC) muscle strength tests of the knee extensors and ankle plantar flexors of the non-dominant leg on an isokinetic strength testing device. We calculated the ratio between EccCONC and CONC (EccCONC/CONC) for all the conditions as the marker of concentric muscle torque enhancement. Separate two-way (joints x velocity) within repeated measures ANOVAs were used to determine joint-specific torque differences at 120°/s and 180°/s. CONC and EccCONC were greater for the knee extensors versus ankle plantar flexors at 120°/s and 180°/s (32.86%-102%; p < 0.001 for both); however, EccCONC/CONC was greater for the ankle plantar flexors than knee extensors at 120°/s (52.4%; p < 0.001) and 180°/s (41.9%; p < 0.001). There was a trend of greater EccCONC/CONC for the knee extensors at 180°/s than 120°/s (6.6%; p = 0.07). Our results show that greater concentric muscle torque enhancement after a maximal eccentric contraction occurs for the ankle plantar flexors versus knee extensors. Whether the joint- specificity of concentric muscle torque enhancement after a maximal eccentric contraction differentially affects sports performance is unknown. Our data provide a reference framework to investigate joint-specific concentric muscle torque enhancement for general and clinical athletic populations.

Eccentric exercise-induced muscle weakness amplifies the history dependence of force

INTRODUCTION

Following active lengthening or shortening contractions, isometric steady-state torque is increased (residual force enhancement; rFE) or decreased (residual force depression; rFD), respectively, compared to fixed-end isometric contractions at the same muscle length and level of activation. Though the mechanisms underlying this history dependence of force have been investigated extensively, little is known about the influence of exercise-induced muscle weakness on rFE and rFD.

PURPOSE

Assess rFE and rFD in the dorsiflexors at 20%, 60%, and 100% maximal voluntary torque (MVC) and activation matching, and electrically stimulated at 20% MVC, prior to, 1 h following, and 24 h following 150 maximal eccentric dorsiflexion contractions.

METHODS

Twenty-six participants (13 male, 24.7 ± 2.0y; 13 female, 22.5 ± 3.6y) were seated in a dynamometer with their right hip and knee angle set to 110° and 140°, respectively, with an ankle excursion set between 0° and 40° plantar flexion (PF). MVC torque, peak twitch torque, and prolonged low frequency force depression were used to assess eccentric exercise-induced neuromuscular impairments. History-dependent contractions consisted of a 1 s isometric (40°PF or 0°PF) phase, a 1 s shortening or lengthening phase (40°/s), and an 8 s isometric (0°PF or 40°PF) phase.

RESULTS

Following eccentric exercise; MVC torque was decreased, prolonged low frequency force depression was present, and both rFE and rFD increased for all maximal and submaximal conditions.

CONCLUSIONS

The history dependence of force during voluntary torque and activation matching, and electrically stimulated contractions is amplified following eccentric exercise. It appears that a weakened neuromuscular system amplifies the magnitude of the history-dependence of force.

Active muscle stiffness is reduced during rapid unloading in muscles from TtnD112-158 mice with a large deletion to PEVK titin

Evidence suggests that the giant muscle protein, titin functions as a tunable spring in active muscle. However, the mechanisms for increasing titin stiffness with activation are not well understood. Previous studies have suggested that during muscle activation, titin binds to actin which engages the PEVK region of titin thereby increasing titin stiffness. In this study, we investigated the role of PEVK titin in active muscle stiffness during rapid unloading. We measured elastic recoil of active and passive soleus muscles from TtnD112-158 mice characterized by a 75% deletion of PEVK titin and increased passive stiffness. We hypothesized that activated TtnD112-158 muscles are more stiff than wild type muscles due to the increased stiffness of PEVK titin. Using a servomotor force lever, we compared the stress–strain relationships of elastic elements in active and passive muscles during rapid unloading and quantified the change in stiffness upon activation. Results show that the elastic modulus of TtnD112-158 muscles increased with activation. However, elastic elements developed force at 7% longer lengths and exhibited 50% lower active stiffness in TtnD112-158 soleus muscles than wild type muscles. Thus, despite having a shorter, stiffer PEVK segment, during rapid unloading, TtnD112-158 soleus muscles exhibited reduced active stiffness compared to wild type soleus muscles. These results are consistent with the idea that PEVK titin contributes to active muscle stiffness, however, the reduction in active stiffness of TtnD112-158 muscles suggests that other mechanisms compensate for the increased PEVK stiffness.

How velocity impacts eccentric force generation of fully activated skinned skeletal muscle fibers in long stretches

Eccentric muscle contractions are fundamental to everyday life. They occur markedly in jumping, running, and accidents. Following an initial force rise, stretching of a fully activated muscle can result in a phase of decreasing force ('Give') followed by force redevelopment. However, how the stretch velocity affects 'Give' and force redevelopment remains largely unknown. We investigated the force produced by fully activated single skinned fibers of rat extensor digitorum longus muscles during long stretches. Fibers were pulled from length .85 to 1.3 optimal fiber length at a rate of 1, 10 and 100% of the estimated maximum shortening velocity. 'Give' was absent in slow stretches. Medium and fast stretches yielded a clear 'Give'. After the initial force peak, forces decreased by 11.2% and 27.8% relative to the initial peak force before rising again. During the last half of the stretch (from 1.07 to 1.3 optimal fiber length, which is within the range of the expected descending limb of the force-length relationship), the linear force slope tripled from slow to medium stretch and increased further by 60% from medium to fast stretch. These results are compatible with forcible cross-bridge detachment and re-development of a cross-bridge distribution, and a viscoelastic titin contribution to fiber force. Accounting for these results can improve muscle models and predictions of multi-body simulations.

Reliable Peak Power Assessment During Concentric and Flexion-Extension-Cycle Based Rowing Strokes using a Non-Modified Rowing Ergometer

Accurate assessment of peak rowing power is crucial for rowing-specific performance testing. Therefore, within and between day reliability of a non-modified rowing ergometer was examined. 52 trained male rowers (21.0 ± 2.9 years; 1.89 ± 0.05 m; 83.2 ± 8.2 kg; 2,000-m ergometer Time Trial mean power: 369 ± 57 W) performed (two times 4) isolated concentric rowing strokes (DRIVE) and single flexion-extension cycle (FEC-type) rowing strokes (SLIDE-DRIVE) on two separate days (1 week apart). Good to excellent intraclass correlation coefficients (0.94 ≤ ICC ≤ 1.00), low standard error of measurement (≤ 2.7%), low coefficient of variation (≤ 4.9%), and suitable level of agreements (≤ 30W) for DRIVE and SLIDE-DRIVE indicated a high level of (within and between day) reliability. In addition, SLIDE-DRIVE (423 ± 157 W) revealed remarkably higher rowing power (p ≤ 0.001; η_p² = 0.601; SMD = 0.34) compared to DRIVE (370 ± 154 W). The non-modified rowing ergometer is considered to be a reliable tool for the peak power assessment during isolated concentric contraction and FEC-type rowing strokes. Notably higher power outputs (compared to an isolated concentric contraction) during FEC rowing may refer to an underlying stretch shortening cycle.

Effect of plyometric training on dynamic leg strength and jumping performance in rhythmic gymnastics: A preliminary study

BACKGROUND: Reactive performance is an important component of rhythmic gymnastics. So far, it is unclear whether additional plyometric training in female gymnasts shows an increase in performance. OBJECTIVE: The aim of the study was to examine the effect of additional plyometric training in rhythmic gymnastics on the reactive jumping performance and strength of the lower leg muscles. METHODS: Fifteen rhythmic gymnasts (age: 12.3 ± 2.6 years, height: 1.47 ± 0.12 m, body weight: 37.3 ± 9.3 kg, BMI: 16.7 ± 2.1 kg*m-2; competition level: national and international championships, Tanner stages I–III) participated in the study. The athletes were assigned to an experimental (EG) and a control group (CG). The EG performed plyometric exercises three times per week in addition to the regular training. Before and after six weeks of training the reactive jump performance, the work of dorsi flexors and plantar flexors performed during isokinetic plantarflexion, as well as the performance in two sport-specific tests were measured. RESULTS: In contrast to the CG, in the EG the jump height (pre: 24.8; post: 27.25 cm; p< 0.05) and the reactive-strength-index (pre: 1.01; post: 1.19; p< 0.01) increased significantly. The EG achieved significant improvements in the counter movement jump test (pre: 27.0 cm; post: 31.5 cm; p< 0.01) and in the sport specific double rope jump test (jumps per minute, pre: 18.0; post: 23.0; p< 0.01). Furthermore, a significant increase in work performed during plantarflexion was found in the EG for the right leg (pre: 24.9 J; post: 29.7 J; p< 0.01) and a tendency to increase for the left leg (pre: 26.4 J; post: 37.7 J; p= 0.05). CONCLUSION: Both reactive strength and dynamic force can be efficiently increased by plyometric training. It may be recommended to include plyometric exercises in the training regime of rhythmic gymnasts.

Muscle Actuators, Not Springs, Drive Maximal Effort Human Locomotor Performance

The current investigation examined muscle-tendon unit kinematics and kinetics in human participants asked to perform a hopping task for maximal performance with variational preceding milieu. Twenty-four participants were allocated post-data collection into those participants with an average hop height of higher (HH) or lower (LH) than 0.1 m. Participants were placed on a customized sled at a 20º angle while standing on a force plate. Participants used their dominant ankle for all testing and their knee was immobilized and thus all movement involved only the ankle joint and corresponding propulsive unit (triceps surae muscle complex). Participants were asked to perform a maximal effort during a single dynamic countermovement hop (CMH) and drop hops from 10 cm (DH10) and 50 cm (DH50). Three-dimensional motion analysis was performed by utilizing an infrared camera VICON motion analysis system and a corresponding force plate. An ultrasound probe was placed on the triceps surae muscle complex for muscle fascicle imaging. HH hopped significantly higher in all hopping tasks in comparison to LH. In addition, the HH group concentric ankle work was significantly higher in comparison to LH during all of the hopping tasks. Active muscle work was significantly higher in HH in comparison to LH as well. Tendon work was not significantly different between HH and LH. Active muscle work was significantly correlated with hopping height (r = 0.97) across both groups and hopping tasks and contributed more than 50% of the total work. The data indicates that humans primarily use a motor-driven system and thus it is concluded that muscle actuators and not springs maximize performance in hopping locomotor tasks in humans.

Residual force enhancement in humans: Is there a true non-responder?

When an active muscle is stretched and kept isometrically active, the resulting force is enhanced compared to a purely isometric reference contraction at the same muscle length and activity; a generally accepted muscle property called residual force enhancement (rFE). Interestingly, studies on voluntary muscle action regularly identify a significant number of participants not showing rFE. Therefore, the aim was to unmask possible confounders for this non-responsive behavior. Ten participants performed maximum voluntary isometric plantarflexion contractions with and without preceding stretch. Contractions were accompanied by the assessment of voluntary activation using the twitch-interpolation technique. The same test protocol was repeated four additional times with a least on day rest in-between. Additionally, at the first and fifth sessions, a submaximal tetanic muscle-stimulation condition was added. At both muscle-stimulation sessions mean rFE higher 10% (p < 0.028) was found. In contrast, during voluntary muscle action, individual participants showed inconsistent rFE across sessions and only one session (#3) had significant rFE (5%; p = 0.023) in group means. As all participants clearly had rFE in electrical stimulation conditions, structural deficits cannot explain the missing rFE in voluntary muscle action. However, we also did not find variability in voluntary activation levels or muscle activity as the confounding characteristics of "non-responders."

The effect of stretch-shortening magnitude and muscle-tendon unit length on performance enhancement in a stretch-shortening cycle

Stretch-induced residual force enhancement (rFE) is associated with increased performance in a stretch–shortening cycle (SSC). Although the influence of different range of motions and muscle–tendon unit lengths has been investigated in pure stretch-hold experiments in vivo, the contribution to a SSC movement in human muscles remains unclear. In two sessions, 25 healthy participants performed isometric reference (ISO), shortening hold (SHO) and SSC contractions on an isokinetic dynamometer. We measured the net knee-joint torque, rotational mechanical work, knee kinematics and fascicle behavior (m. vastus lateralis) of the upper right leg. In session 1 the SHO- and SSC-magnitude was changed respectively (SHO: 50°–20°, 80°–20° and 110°–20°; SSC: 20°–50°–20°, 20°–80°–20° and 20°–110°–20°) and in session 2 the muscle–tendon unit length (SHO: 50°–20°, 80°–50° and 110°–80°; SSC: 20°–50°–20°, 50°–80°–50° and 80°–110°–80°; straight leg = 0°). In both sessions, rotational work was significantly (p < 0.05) increased in the SSC compared to the SHO contractions (in the range of 8.1–17.9%). No significant difference of joint torque was found in the steady-state for all SSC-magnitudes compared to the corresponding SHO contractions in session 1. In session 2, we found only significantly (p < 0.05) less depressed joint torque in the SSC at the longest muscle–tendon unit length compared to the corresponding SHO condition, without any differences in knee kinematics and fascicle behavior. Therefore, the physiological relevance of rFE might be particularly important for movements at greater muscle–tendon unit lengths.

Power Amplification Increases With Contraction Velocity During Stretch-Shortening Cycles of Skinned Muscle Fibers

Muscle force, work, and power output during concentric contractions (active muscle shortening) are increased immediately following an eccentric contraction (active muscle lengthening). This increase in performance is known as the stretch-shortening cycle (SSC)-effect. Recent findings demonstrate that the SSC-effect is present in the sarcomere itself. More recently, it has been suggested that cross-bridge (XB) kinetics and non-cross-bridge (non-XB) structures (e.g., titin and nebulin) contribute to the SSC-effect. As XBs and non-XB structures are characterized by a velocity dependence, we investigated the impact of stretch-shortening velocity on the SSC-effect. Accordingly, we performed in vitro isovelocity ramp experiments with varying ramp velocities (30, 60, and 85% of maximum contraction velocity for both stretch and shortening) and constant stretch-shortening magnitudes (17% of the optimum sarcomere length) using single skinned fibers of rat soleus muscles. The different contributions of XB and non-XB structures to force production were identified using the XB-inhibitor Blebbistatin. We show that (i) the SSC-effect is velocity-dependent-since the power output increases with increasing SSC-velocity. (ii) The energy recovery (ratio of elastic energy storage and release in the SSC) is higher in the Blebbistatin condition compared with the control condition. The stored and released energy in the Blebbistatin condition can be explained by the viscoelastic properties of the non-XB structure titin. Consequently, our experimental findings suggest that the energy stored in titin during the eccentric phase contributes to the SSC-effect in a velocity-dependent manner.

Energy Cost of Force Production After a Stretch-Shortening Cycle in Skinned Muscle Fibers: Does Muscle Efficiency Increase?

Muscle force is enhanced during shortening when shortening is preceded by an active stretch. This phenomenon is known as the stretch-shortening cycle (SSC) effect. For some stretch-shortening conditions this increase in force during shortening is maintained following SSCs when compared to the force following a pure shortening contraction. It has been suggested that the residual force enhancement property of muscles, which comes into play during the stretch phase of SSCs may contribute to the force increase after SSCs. Knowing that residual force enhancement is associated with a substantial reduction in metabolic energy per unit of force, it seems reasonable to assume that the metabolic energy cost per unit of force is also reduced following a SSC. The purpose of this study was to determine the energy cost per unit of force at steady-state following SSCs and compare it to the corresponding energy cost following pure shortening contractions of identical speed and magnitude. We hypothesized that the energy cost per unit of muscle force is reduced following SSCs compared to the pure shortening contractions. For the SSC tests, rabbit psoas fibers (n = 12) were set at an average sarcomere length (SL) of 2.4 μm, activated, actively stretched to a SL of 3.2 μm, and shortened to a SL of 2.6 or 3.0 μm. For the pure shortening contractions, the same fibers were activated at a SL of 3.2 μm and actively shortened to a SL of 2.6 or 3.0 μm. The amount of ATP consumed was measured over a 40 s steady-state total isometric force following either the SSCs or the pure active shortening contractions. Fiber stiffness was determined in an additional set of 12 fibers, at steady-state for both experimental conditions. Total force, ATP consumption, and stiffness were greater following SSCs compared to the pure shortening contractions, but ATP consumption per unit of force was the same between conditions. These results suggest that the increase in total force observed following SSCs was achieved with an increase in the proportion of attached cross-bridges and titin stiffness. We conclude that muscle efficiency is not enhanced at steady-state following SSCs.

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.

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.