Ca2+ and force during dynamic contractions in mouse intact skeletal muscle fibers.

Muscle fiber force production is determined by the excitation frequency of motor nerves, which induce transient increases in cytoplasmic free Ca2+ concentration ([Ca2+]i) and the force-generating capacity of the actomyosin cross-bridges. Previous studies suggest that, in addition to altered cross-bridge properties, force changes during dynamic (concentric or eccentric) contraction might be affected by Ca2+-dependent components. Here we investigated this by measuring [Ca2+]i and force in mouse muscle fibers undergoing isometric, concentric, and eccentric contractions. Intact single muscle fibers were dissected from the flexor digitorum brevis muscle of mice. Fibers were electrically activated isometrically at 30-100 Hz and after reaching the isometric force plateau, they were actively shortened or stretched. We calculated the ratio (relative changes) in force and [Ca2+]i attained in submaximal (30 Hz) and near-maximal (100 Hz) contractions under isometric or dynamic conditions. Tetanic [Ca2+]i was similar during isometric, concentric and eccentric phases of contraction at given stimulation frequencies while the forces were clearly different depending on the contraction types. The 30/100 Hz force ratio was significantly lower in the concentric (44.1 ± 20.3%) than in the isometric (50.3 ± 20.4%) condition (p = 0.005), whereas this ratio did not differ between eccentric and isometric conditions (p = 0.186). We conclude that the larger force decrease by decreasing the stimulation frequency during concentric than during isometric contraction is caused by decreased myofibrillar Ca2+ sensitivity, not by the decreased [Ca2+]i.

Measurements of tendon length changes during stretch-shortening cycles in rat soleus.

The muscle force attained during concentric contractions is augmented by a preceding eccentric contraction (the stretch-shortening cycle (SSC) effect). At present, tendon elongation is considered the primary mechanism. However, we recently found that the magnitude of the SSC effect was not different, even after removing the Achilles tendon. To resolve these discrepant results, direct measurement of changes in Achille tendon length is required. Therefore, this study aimed to elucidate the influence of tendon elongation on the SSC effect by directly measuring the changes in Achilles tendon length. The rat soleus was subjected to pure concentric contractions (pure shortening trials) and concentric contractions with a preceding eccentric contraction (SSC trials). During these contractions, the Achilles tendon length was visualized using a video camera. The muscle force attained during the concentric contraction phase in the SSC trial was significantly larger than that in the pure shortening trial (p = 0.022), indicating the existence of the SSC effect. However, the changes in Achilles tendon length were not different between trials (i.e., the magnitude of tendon shortening attained during the shortening phase was 0.20 ± 0.14 mm for the SSC trial vs. 0.17 ± 0.09 mm for the pure shortening trial), indicating that the observed SSC effect is difficult to be explained by the elastic energy stored in tendons or muscle-tendon interaction. In conclusion, the effect of tendon elongation on the SSC effect should be reconsidered, and other factors may contribute to the SSC effect.

Force loss induced by inhibiting cross-bridge cycling is mitigated in eccentric contraction.

We examined whether the force loss induced by 2,3-butanedione monoxime affects isometric and eccentric forces differently. Single skinned muscle fibers were activated at an average sarcomere length of 2.4 µm and then stretched to 3.0 µm. This trial was performed with and without 2,3-butanedione monoxime to calculate the magnitude of force loss attained at several time points: pre-stretch phase at 2.4 µm, eccentric phase, end of eccentric contraction, and post-stretch phase at 3.0 µm. The magnitude of force loss was significantly larger in the pre-stretch phase than at the other time points. Further, the mitigated force loss in the eccentric contraction was more prominent in the long condition than in the short condition. We suggest that the eccentric force is relatively preserved compared with the reference isometric force (pre-stretch) when cross-bridge cycling is inhibited, possibly because of the contribution of the elastic force produced by titin.

Influence of caffeine on the maximal isometric and concentric force produced by skinned fibers.

Caffeine is one of the most famous and widely used ergogenic drugs, especially by athletes to improve sports performance. Caffeine is known to enhance muscle contraction by facilitating Ca2+ release from the sarcoplasmic reticulum. While the effect of caffeine on the cross-bridge dynamics has also investigated, the results is controversial. Therefore, the purpose of this study was to examine the influence of caffeine on cross-bridge dynamics using skinned fiber preparations from rabbit soleus (N = 19 in total). We performed isometric contractions at an average sarcomere length of 2.4 µm; thereafter, skinned fibers were shortened by 20% of the fiber length at a velocity of 0.1 mm/s (slow shortening) or 0.5 mm/s (fast shortening). The contractions were performed under both normal and caffeine-containing activating solution conditions to compare the isometric, slow concentric, and fast concentric forces between conditions. The isometric force did not differ between normal and caffeine-containing activating solution conditions. Similarly, the concentric forces obtained during the slow and fast shortening trials did not differ between conditions. We also measured the stiffness and the rate of force redevelopment (kTR) during the isometric contraction phase and found that these values were not different between normal and caffeine conditions. Based on these results, we conclude that the influence of caffeine on cross-bridge dynamics is negligible, and the ergogenic effect of caffeine, from the view of muscle contractility, is by facilitating Ca2+ release, as suggested in previous studies, and not by modulating the cross-bridge dynamics.

Residual force enhancement is attenuated for quick stretch conditions.

It is generally accepted that the magnitude of residual force enhancement is not affected by stretching velocity. However, we recently found that when the stretching velocity was too large, the magnitude of residual force enhancement was attenuated or vanished in cat soleus muscle in situ. Therefore, the purpose of this study was to extend the knowledge by conducting very quick stretches in skinned rabbit psoas and soleus muscle fibers. Skinned psoas (N = 17) or soleus (N = 13) muscle fibers were activated isometrically at an average sarcomere length of 2.7 µm, and then, actively stretched to an average sarcomere length of 3.0 µm in 0.5 or 500 ms, followed by an isometric contraction at that length. In addition, a purely isometric reference contraction was conducted at an average sarcomere length of 3.0 µm to calculate the magnitude of residual force enhancement for both stretch velocities. The magnitude of residual force enhancement was significantly different between 0.5 ms stretch (101.5 ± 5.6% for psoas, and 101.5 ± 2.3% for soleus) and 500 ms stretch (106.2 ± 5.9% for psoas, and 106.8 ± 5.1% for soleus) while the magnitude of residual force enhancement was not significantly different between muscles. In line with our previous finding, the magnitude of residual force enhancement can vary when the stretch velocity is very large. This finding can contribute to clarify the key mechanism for inducing residual force enhancement and to explain contradicting results obtained in previous studies.

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 I1,1/I1,0, and decreased lattice spacing and M3 intensity. Compared to the reference contraction, the isometric contraction after active shortening resulted in decreased force, stiffness, I1,1/I1,0, 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 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.

Comparison of the relative muscle volume of triceps surae among sprinters, runners, and untrained participants.

Muscle hypertrophy is considered more prominent in fast-twitch than in slow-twitch muscles. This leads to the hypothesis that the relative muscle volume of the medial gastrocnemius (MG) and lateral gastrocnemius (LG) becomes larger than that of the soleus (SOL) in highly trained participants because MG and LG include more fast-twitch muscles than SOL. Thus, we compared relative muscle volume among highly trained sprinters, long-distance runners, and untrained participants to examine whether the above hypothesis is correct. Magnetic resonance imaging was used to calculate the muscle volume of MG, LG, and SOL from 126 participants. The total muscle volume of the three muscles and the relative muscle volume of each muscle with respect to the total muscle volume were calculated. The total muscle volume was significantly larger in the sprinters than in the long-distance runners and untrained participants. The relative muscle volume of MG was significantly larger in the sprinters than in the long-distance runners and untrained participants and that of SOL was significantly smaller in the sprinters than in the long-distance runners and untrained participants. These results indicate that the relative muscle volume can vary among participants, possibly due to fiber type-dependent muscle hypertrophy.

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.

Muscle Recruitment Pattern of the Hamstring Muscles in Hip Extension and Knee Flexion Exercises.

We aimed to compare dynamic exercise performance between hip extension exercises with different knee angles and between knee flexion exercises with different hip angles, and to investigate the recruitment pattern of the hamstrings in each exercise. Seven men performed 4 isokinetic exercises (3 maximal concentric contractions at 30°/s (peak torque) and 30 maximal concentric contractions at 180°/s (total work)): hip extension with the knee fully extended (HEke) and with the knee flexed at 90° (HEkf) and knee flexion with the hip fully extended (KFhe) and with the hip flexed at 90° (KFhf). The recruitment pattern of the hamstrings was evaluated in each exercise using magnetic resonance imaging (T2 calculation). The HEke condition showed significantly greater peak torque than the HEkf condition (p < 0.05). The KFhf condition had significantly greater peak torque and total work values than the KFhe condition (p < 0.05). Although the biceps femoris long head, semitendinosus, and semimembranosus had significantly increased post-exercise T2 values in the HEke (p < 0.05), KFhe, and KFhf conditions (p < 0.01), the T2 increase values were significantly greater under the KFhf than the HEke condition (p < 0.05). The semitendinosus showed a significantly greater T2 increase value than other muscles under both KFhe and KFhf conditions (p < 0.05). Performance of hip extension and knee flexion exercises increases when the hamstring muscles are in a lengthened condition. The hamstring muscles (particularly the semitendinosus) are more involved in knee flexion than in hip extension.

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.

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.

Influence of muscle length on the stretch-shortening cycle in skinned rabbit soleus.

Muscle force generated during shortening is instantaneously increased after active stretch. This phenomenon is called as stretch-shortening cycle (SSC) effect. It has been suggested that residual force enhancement contributes to the SSC effect. If so, the magnitude of SSC effect should be larger in the longer muscle length condition, because the residual force enhancement is prominent in the long muscle length condition. This hypothesis was examined by performing the SSC in the short and long muscle length conditions. Skinned fibers obtained from rabbit soleus (N = 20) were used in this study. To calculate the magnitude of SSC effect, the SSC trial (isometric-eccentric-concentric-isometric) and the control trial (isometric-concentric-isometric) were conducted in the short (within the range of 2.4 to 2.7 µm) and long muscle (within the range of 3.0 to 3.3 µm). The magnitude of SSC effect was calculated as the relative increase in the mechanical work attained during the shortening phase between control and SSC trials. As a result, the magnitude of SSC effect was significantly larger in the long (176.8 ± 18.1%) than in the short muscle length condition (157.4 ± 8.5%) (p < 0.001). This result supports our hypothesis that the magnitude of SSC effect is larger in the longer muscle length condition, possibly due to the larger magnitude of residual force enhancement.

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.

Current Understanding of Residual Force Enhancement: Cross-Bridge Component and Non-Cross-Bridge Component.

Muscle contraction is initiated by the interaction between actin and myosin filaments. The sliding of actin filaments relative to myosin filaments is produced by cross-bridge cycling, which is governed by the theoretical framework of the cross-bridge theory. The cross-bridge theory explains well a number of mechanical responses, such as isometric and concentric contractions. However, some experimental observations cannot be explained with the cross-bridge theory; for example, the increased isometric force after eccentric contractions. The steady-state, isometric force after an eccentric contraction is greater than that attained in a purely isometric contraction at the same muscle length and same activation level. This well-acknowledged and universally observed property is referred to as residual force enhancement (rFE). Since rFE cannot be explained by the cross-bridge theory, alternative mechanisms for explaining this force response have been proposed. In this review, we introduce the basic concepts of sarcomere length non-uniformity and titin elasticity, which are the primary candidates that have been used for explaining rFE, and discuss unresolved problems regarding these mechanisms, and how to proceed with future experiments in this exciting area of research.

Contribution of the Achilles tendon to force potentiation in a stretch-shortening cycle.

Muscle force during concentric contractions is potentiated by a preceding eccentric contraction: a phenomenon known as the stretch-shortening cycle (SSC) effect. Tendon elongation is often considered to be the primary factor for this force potentiation. However, direct examination of the influence of tendon elongation on the SSC effect has not been made. The aim of this study was to evaluate the contribution of tendon elongation to the SSC effect by comparing the magnitude of the SSC effect in the rat soleus with and without the Achilles tendon. The rat soleus was subjected to concentric contractions without pre-activation (CON) and concentric contractions with an eccentric pre-activation (ECC). For the 'with-tendon' condition, the calcaneus was rigidly fixed to a force transducer, while for the 'without-tendon' condition, the soleus was fixed at the muscle-tendon junction. The SSC effect was calculated as the ratio of the mechanical work done during the concentric phase for the ECC and the CON conditions. Substantial and similar (P=0.167) SSC effects were identified for the with-tendon (318±86%) and the without-tendon conditions (271±70%). The contribution of tendon elongation to the SSC effect was negligible for the rat soleus. Other factors, such as pre-activation and residual force enhancement, may cause the large SSC effects and need to be evaluated.

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.

Does stretching velocity affect residual force enhancement?

It is thought that the magnitude of residual force enhancement (RFE) is not affected by stretch velocity. However, the range of stretch velocities studied in previous investigations has been limited to slow and moderate velocities. High velocities of muscle stretching are associated with a loss of force and incomplete cross-bridge attachment to actin, thus creating a unique set of eccentric conditions referred to as slippage. The purpose of this study was to extend the relationship between stretch velocity and RFE to high velocities. We hypothesized that slippage at high velocities might affect RFE. We stretched cat soleus muscles for 4 mm to the plateau of the force-length relationship at speeds of 2, 4, 8, 16, 32, 64 mm/s to induce RFE, and slippage for the fastest condition. For each RFE test, a corresponding isometric reference test was conducted. Residual force enhancement was quantified as the relative increase in isometric steady state force between the experimental stretch and the isometric reference tests. Residual force enhancement was similar for all stretch speeds, as expected, with the exception of the fastest speed (64 mm/s), which was associated with slippage and no significant RFE. These results suggest that if stretch speeds are too fast, and are associated with slippage, RFE is abolished. We conclude from these findings that proper cross-bridge engagement is required during eccentric muscle action to produce RFE.

Isometric preactivation before active lengthening increases residual force enhancement.

INTRODUCTION: The isometric force attained after active stretch is greater than that attained in a purely isometric contraction. This phenomenon is called residual force enhancement (RFE). The purpose was to examine the influence of isometric preactivation conducted just before active stretch on the magnitude of RFE in plantar flexors. METHODS: In the control condition, subjects conducted isometric contraction at 15° of dorsiflexion. In the no preactivation condition, the isometric contraction at 15° of dorsiflexion was conducted after an eccentric contraction from 0° to 15° of dorsiflexion. In the isometric preactivation condition, an isometric contraction was conducted for preactivation before conducting the same routine as in the no preactivation condition. Isometric torque at the end of isometric contraction at 15° of dorsiflexion was compared among conditions to examine whether isometric preactivation affects the magnitude of RFE. The fascicle behaviors of the medial gastrocnemius were recorded by ultrasonography. RESULTS: The isometric torque attained in the preactivation condition was greater than that in the control condition (P = 0.017). There was no significant difference between no preactivation and control conditions (P = 0.744). The magnitude of fascicle elongation during active stretch was greater in the preactivation than in the no preactivation condition (P = 0.002). CONCLUSION: Compared to the control condition, greater isometric torque was observed only in the preactivation condition, indicating that substantial RFE was induced only in the preactivation condition. This difference would be explained by the lesser degree of fascicle elongation during active stretch in the no preactivation condition.

Residual Force Enhancement Is Attenuated in a Shortening Magnitude-dependent Manner.

INTRODUCTION: The isometric force attained after active stretch is greater than that attained in a purely isometric contraction. This property is referred to as residual force enhancement (RFE). Although RFE is thought to contribute to the enhanced force and power in stretch-shortening cycles (SSCs), it is unclear whether shortening that occurs after active stretch eliminates the RFE induced by active stretch. Therefore, we evaluated the influence of shortening on RFE. METHODS: Skinned rabbit soleus fibers (N = 43) were used for all tests. Residual force enhancement was compared between the stretch-only condition and the SSC condition. In the SSC conditions, shortening magnitudes were either 1% or 12.5% of fiber length. The final muscle length where RFE was evaluated was 3 µm for all trials. In the SSCs with 12.5% shortening, the isometric force before and after the SSC was also compared. RESULTS: Residual force enhancement was similar between the stretch only (7.9% ± 2.7%) and the SSC with 1% shortening condition (7.1% ± 2.9%) (P = 0.316), whereas RFE was smaller in the SSC with 12.5% shortening (3.5% ± 2.4%) than the stretch-only condition (8.4% ± 2.5%) (P < 0.001). The isometric forces after SSCs (0.437 ± 0.103 mN) were greater than those measured before the SSC (0.422 ± 0.104 mN) (P = 0.016). CONCLUSIONS: Residual force enhancement was preserved when the shortening magnitude was small and was reduced when the shortening magnitude was large. Although RFE was attenuated by the 12.5% shortening, RFE was still observed, suggesting that RFE can contribute to SSCs.