Both the elongation of attached crossbridges and residual force enhancement contribute to joint torque enhancement by the stretch-shortening cycle

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.

Passive force enhancement is not abolished by shortening of single rabbit psoas fibres

Passive force enhancement is defined as the increase in steady-state passive force following deactivation of an actively stretched muscle compared to the corresponding passive force following passive stretching of the muscle. Passive force enhancement has been associated with contributing to the residual force enhancement property, providing stability to sarcomeres, and preventing sarcomeres from over-stretching during eccentric muscle action. Despite its functional importance, the molecular mechanisms underlying passive force enhancement remain unknown. Specifically, it remains unknown how passive force enhancement develops and how it is abolished. Incidental observations on cat soleus muscles led to the speculation that passive force enhancement is abolished when the actively stretched muscle is deactivated and then passively shortened to its pre-stretched length. Here, we tested this hypothesis using skinned fibres from rabbit psoas and rejected it. Rather, we found that passive force enhancement increased following shortening of the fibres to their pre-stretched length (2.4 µm), and furthermore, that the passive force enhancement increased by 70-106% when the shortening and subsequent stretch to the original length (3.6 µm) increased in duration (200 ms, 6 s, and 14 s). These results indicate that passive force enhancement increases during a shortening-stretch cycle, and that this increase is time-dependent. We propose that this increase in passive force enhancement is caused by titin; specifically, with a refolding of titin's immunoglobulin domains that were unfolded during the active fibre stretching that produced the residual and passive force enhancement. Molecular level experiments are required to test this proposal.

Relationship Between Isokinetic Lower-Limb Joint Strength, Isometric Time Force Characteristics, and Leg-Spring Stiffness in Recreational Runners

Neuromuscular characteristics, such as lower-limb joint strength and the ability to rapidly generate force, may play an important role in leg-spring stiffness regulation. This study aimed to investigate the relationship between isokinetic knee and ankle joint peak torque (PT), the force-time characteristics of isometric mid-thigh pull (IMTP), and leg stiffness (K_leg)/vertical stiffness (K_vert) in recreationally trained runners. Thirty-one male runners were recruited and underwent three separate tests. In the first session, the body composition, K_leg, and K_vert at running speeds of 12 and 14 km⋅h^–1 were measured. In the second session, isokinetic knee and ankle joint PT at 60°⋅s^–1 were tested. The force-time characteristics of the IMTP were evaluated in the final session. Pearson’s product-moment correlations, with the Benjamini–Hochberg correction procedure, showed that the knee flexor concentric and eccentric and extensor concentric PT (r = 0.473–0.654, p < 0.05) were moderate to largely correlated with K_leg and K_vert at 12 and 14 km⋅h^–1. The knee extensor eccentric PT (r = 0.440, p = 0.050) was moderately correlated with the 14 km⋅h^–1K_vert. The ankle plantar flexor concentric and dorsiflexor eccentric PT (r = 0.506–0.571, p < 0.05) were largely correlated with K_leg at 12 km⋅h^–1. The ankle plantar flexor concentric and eccentric and dorsiflexor eccentric PT (r = 0.436–0.561, p < 0.05) were moderate to largely correlated with K_vert at 12 and 14 km⋅h^–1. For IMTP testing, high correlation was only found between the IMPT peak force (PF) and K_vert at 14 km⋅h^–1 (r = 0.510, p = 0.014). Thus, superior leg-spring stiffness in recreational runners may be related to increased knee and ankle joint strength, eccentric muscular capacity, and maximal force production.

Effect of Active Lengthening and Shortening on Small-Angle X-ray Reflections in Skinned Skeletal Muscle Fibres

Our purpose was to use small-angle X-ray diffraction to investigate the structural changes within sarcomeres at steady-state isometric contraction following active lengthening and shortening, compared to purely isometric contractions performed at the same final lengths. We examined force, stiffness, and the 1,0 and 1,1 equatorial and M3 and M6 meridional reflections in skinned rabbit psoas bundles, at steady-state isometric contraction following active lengthening to a sarcomere length of 3.0 µm (15.4% initial bundle length at 7.7% bundle length/s), and active shortening to a sarcomere length of 2.6 µm (15.4% bundle length at 7.7% bundle length/s), and during purely isometric reference contractions at the corresponding sarcomere lengths. Compared to the reference contraction, the isometric contraction after active lengthening was associated with an increase in force (i.e., residual force enhancement) and M3 spacing, no change in stiffness and the intensity ratio I_1,1/I_1,0, and decreased lattice spacing and M3 intensity. Compared to the reference contraction, the isometric contraction after active shortening resulted in decreased force, stiffness, I_1,1/I_1,0, M3 and M6 spacings, and M3 intensity. This suggests that residual force enhancement is achieved without an increase in the proportion of attached cross-bridges, and that force depression is accompanied by a decrease in the proportion of attached cross-bridges. Furthermore, the steady-state isometric contraction following active lengthening and shortening is accompanied by an increase in cross-bridge dispersion and/or a change in the cross-bridge conformation compared to the reference contractions.

The Effects of High-Intensity Interval Exercise on Skeletal Muscle and Cerebral Oxygenation during Cycling and Isokinetic Concentric and Eccentric Exercise

The aim of the present study was to study the effects of cycling and pure concentric and pure eccentric high-intensity interval exercise (HIIE) on skeletal muscle (i.e., vastus lateralis) and cerebral oxygenation. Twelve healthy males (n = 12, age 26 ± 1 yr, body mass 78 ± 2 kg, height 176 ± 2 cm, body fat 17 ± 1% of body mass) performed, in a random order, cycling exercise and isokinetic concentric and eccentric exercise. The isokinetic exercises were performed on each randomly selected leg. The muscle and the cerebral oxygenation were assessed by measuring oxyhemoglobin, deoxyhemoglobin, total hemoglobin, and tissue saturation index. During the cycling exercise, participants performed seven sets of seven seconds maximal intensity using a load equal to 7.5% of their body mass while, during isokinetic concentric and eccentric exercise, they were performed seven sets of five maximal muscle contractions. In all conditions, a 15 s rest was adopted between sets. The cycling HIIE caused greater fatigue (i.e., greater decline in fatigue index) compared to pure concentric and pure eccentric isokinetic exercise. Muscle oxygenation was significantly reduced during HIIE in the three exercise modes, with no difference between them. Cerebral oxygenation was affected only marginally during cycling exercise, while no difference was observed between conditions. It is concluded that a greater volume of either concentric or eccentric isokinetic maximal intensity exercise is needed to cause exhaustion which, in turn, may cause greater alterations in skeletal muscle and cerebral oxygenation.

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.

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 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.

Pre-activation affects the effect of stretch-shortening cycle by modulating fascicle behavior

The torque attained during active shortening is enhanced after an active stretch (stretch-shortening cycle, SSC). This study examined the influence of pre-activation on fascicle behavior and the SSC effect. Subjects exhibited the following three conditions by electrically induced plantar flexions. In the isometric-concentric (ISO-CON) condition, subjects exhibited active shortening from dorsiflexion of 15° to 0° after isometric pre-activation. In the eccentric-concentric (ECC-CON) condition, subjects exhibited the above active shortening immediately after the eccentric pre-activation. In the isometric-eccentric-concentric (ISO-ECC-CON) condition, isometric pre-activation was conducted before exhibiting the ECC-CON maneuver. Joint torque and fascicle length of the medial gastrocnemius were compared. The joint torque at the onset and end of shortening was larger in the ISO-ECC-CON than in the ISO-CON or ECC-CON conditions, while no differences were found between ISO-CON and ECC-CON conditions. The magnitude of fascicle elongation attained during the active stretch was larger in the ISO-ECC-CON than in the ECC-CON condition. This could be caused by the shorter fascicle length at the onset of active stretch due to isometric pre-activation. This shorter fascicle length could lead to larger fascicle elongation during the subsequent active stretch, which should emphasize the effect of active stretch-induced force enhancement mechanism.

Summary: Due to the larger fascicle elongation induced by a pre-activation, the effect of the stretch-shortening cycle is enhanced.

The Contraction Modalities in a Stretch-Shortening Cycle in Animals and Single Joint Movements in Humans: A Systematic Review

A systematic literature search was conducted to review the force-enhancing mechanisms caused by a stretch-shortening cycle (SSC). The review aims to yield an overview of the contraction modalities influencing the SSC performance in animals and single joint movements in humans. The search was executed in common with the PRISMA statement. CINAHL, MEDLINE (via ProQuest), PubMed, ScienceDirect, Scopus and Web of Science databases were used for the systematic search from its inception until February 2019. A quality assessment was conducted with a modified Downs and Black checklist. Twenty-five studies were included. SSC effects, leading to increased force/work during a SSC and a reduced force depression (FD) compared to a pure shortening contraction, are existent on different levels of the muscle, from single fiber experiments to the level of in vivo muscle-tendon complex. Muscle performance is dependent on shortening velocity, shortening distance, stretch distance, the time (transition phase) between stretch and shortening and the active prephase duration. Concerning stretch velocity we found conflicting results. The findings from this systematic review indicate that the mechanisms in the early phase of shortening are associated with pre-activation effects, elastic recoil and stretch reflex. Furthermore, we speculate that residual force enhancement (RFE) is mainly responsible for an increased steady-state force compared to a pure shortening contraction.

Reactive and eccentric strength contribute to stiffness regulation during maximum velocity sprinting in team sport athletes and highly trained sprinters

This study investigated the role of reactive and eccentric strength in stiffness regulation during maximum velocity sprinting (Vmax) in team sport athletes compared with highly trained sprinters. Thirteen team sport athletes and eleven highly trained sprinters were recruited. Vmax was measured using radar, and stiffness regulation was inferred from modelled vertical and leg spring stiffness. Reactive strength (RSI) was determined from a 0.50 m drop jump, and an eccentric back squat was used to assess maximum isoinertial eccentric force. Trained sprinters attained a higher Vmax than team sport athletes, partly due to a briefer contact time and higher vertical stiffness. Trained sprinters exhibited a moderately higher RSI via the attainment of a briefer and more forceful ground contact phase, while RSI also demonstrated large to very large associations with vertical stiffness and Vmax, respectively. Isoinertial eccentric force was largely correlated with Vmax, but only moderately correlated with vertical stiffness. Reactive and eccentric strength contribute to the ability to regulate leg spring stiffness at Vmax, and subsequently, the attainment of faster sprinting speeds in highly trained sprinters versus team sport athletes. However, stiffness regulation appears to be a task-specific neuromuscular skill, reinforcing the importance of specificity in the development of sprint performance.

Force depression following a stretch-shortening cycle depends on the amount of residual force enhancement established in the initial stretch phase

Studies on residual force enhancement (rFE) and residual force depression (rFD) of the muscle-tendon unit (MTU) have typically been conducted independent of each other, with little information available on how stretch-induced rFE affects the shortening phase and the steady-state MTU isometric force at the end of stretch-shortening cycles (SSCs). We showed previously that when rFE is kept constant, but the force at the end of the stretch is varied by changing the stretch speed, the steady-state forces at the end of SSCs were the same. These results led to the hypothesis that the amount of rFE of the MTU established in the initial stretch phase of SSCs determines the steady-state force following the shortening phase of SSCs. This study was aimed at testing this hypothesis. Steady-state MTU isometric thumb adduction forces were measured for pure isometric contractions, following pure shortening contractions, following pure stretch contractions, and following SSCs with constant shortening speed and magnitude. However, two stretch magnitudes (30° and 10° thumb abduction) and stretch speeds (15°/sec and ~ 60°/sec, respectively) were chosen such that forces at the end of the stretch phase of the SSCs were the same, while rFE differed substantially. As hypothesized, the steady-state isometric MTU forces following SSCs were positively related to the stretch-magnitude dependent amount of rFE established in the stretch phase and were independent of the force reached at the end of the stretch phase in SSCs. Among many competing theories, these results can potentially be explained with the idea that there is a length-specific engagement of a passive structural element at the initial length of muscle activation.

Influence of stretch magnitude on the stretch-shortening cycle in skinned muscle fibres

The mechanical work attained during muscle fibre shortening is increased by prior stretching. Recently, we suggested that residual force enhancement (RFE) may contribute to this enhanced work. RFE can be changed reliably by changing the stretch magnitude. Therefore, the purpose of this study was to examine the effect of stretch magnitude, and by association RFE, on the mechanics of the stretch-shortening cycle (SSC) in skinned skeletal muscle fibres. Three tests were performed using skinned rabbit soleus fibres (N=18). The first test was a pure shortening contraction in which fibres were activated and then shortened from an average sarcomere length of 3.3 μm to 3.0 μm. The second test was a SSC in which fibres were activated and stretched from 3.0 μm to 3.3 μm, and then shortened to 3.0 μm. The third test was a SSC in which fibres were activated and stretched from 2.4 μm to 3.3 μm, and then shortened to 3.0 μm. The mechanical work during shortening and the force maintained 15 s after the end of shortening were determined. The relative increase in mechanical work with respect to the pure shortening condition was greater for the large than for the small stretch condition (P<0.001). Similarly, the relative increase in force 15 s after the end of shortening was greater for the large than for the small stretch condition (P=0.043). We conclude that increasing the magnitude of stretch results in an increase in mechanical work and increased force at steady state following the stretch, probably because of the greater RFE.

Influence of residual force enhancement and elongation of attached cross-bridges on stretch-shortening cycle in skinned muscle fibers

Increased muscle force during stretch‐shortening cycles (SSCs) has been widely examined. However, the mechanisms causing increased muscle force in SSCs remain unknown. The purpose of this study was to determine the influence of residual force enhancement and elongation of attached cross‐bridges on the work enhancement in SSCs. For the Control condition, skinned rabbit soleus fibers were elongated passively from an average sarcomere length of 2.4 to 3.0 μm, activated and then actively shortened to 2.4 μm. For the Transition condition, fibers were elongated actively from an average sarcomere length of 2.4 to 3.0 μm. Two seconds after the end of active lengthening, fibers were actively shortened to 2.4 μm. In the SSC condition, fibers were lengthened actively from an average sarcomere length of 2.4 to 3.0 μm, and then immediately shortened actively to 2.4 μm. Increased muscle force in the SSCs was quantified by the increase in mechanical work during active shortening compared to the mechanical work measured during the purely active shortening contractions. Work enhancement was significantly greater in the SSC compared to the Transition conditions. This difference was associated with the pause given between the active lengthening and shortening phase in the Transition test, which likely resulted in a reduction of the average elongation of the attached cross‐bridges caused by active stretching. Since some work enhancement was still observed in the Transition condition, another factor, for example the stretch‐induced residual force enhancement, must also have contributed to the work enhancement in SSCs.