Percentage Of Fast-twitch Fibers Research Articles

Effects of dietary l-theanine supplementation on pork quality and muscle fiber type transformation in finishing pigs.

This experiment aimed to investigate effects of dietary l-theanine supplementation on pork quality and muscle fiber type transformation in finishing pigs. In a 30-day experiment, 18 healthy Duroc × Landrace × Yorkshire (DLY) pigs with an average body weight of 86.03 ± 0.83 kg were randomly divided into three groups (a basal diet or a basal diet supplemented with 500 and 1000 ppml-theanine, respectively), with six duplicates and one pig per replicate. The results showed that dietary 1000 ppml-theanine supplementation significantly reduced (P < 0.05) b*24 h and drip loss. Dietary 1000 ppml-theanine supplementation significantly increased (P < 0.05) slow myosin heavy chain (MyHC) protein expression and the percentage of slow-twitch fibers, as well as significantly decreased (P < 0.05) fast MyHC protein expression and the percentage of fast-twitch fibers, accompanied by an increase in succinate dehydrogenase (SDH) and malate dehydrogenase (MDH) activities and a decrease in lactate dehydrogenase (LDH) activity. In addition, the adenosine monophosphate (AMP)-activated protein kinase (AMPK) signaling pathway was activated by l-theanine. Together, this study demonstrated for the first time that dietary supplementation of 1000 ppml-theanine can improve pork color and drip loss and promote muscle fiber type transformation from fast-twitch to slow-twitch in finishing pigs. © 2022 Society of Chemical Industry.

Dog Faces Are Faster than Wolf Faces

Humans domesticated dogs with attention to the facial expressions that dogs produce, selecting for a suite of facial movement. Domestic dogs and humans are adept at accurately understanding one another’s facial expressions and movement around the eyes in dogs is highly valued by humans. This unique, mutual ability to accurately process facial expressions is part of the dog‐human bond. An accurate understanding of how dog and wolf faces differ from one another is fundamental to understanding the processes of animal domestication, evolution of the dog, and the origins of human behavior since the Upper Paleolithic. While we know that dogs have facial musculature that wolves do not regularly possess, our understanding of evolutionary muscle physiology between these two species is incomplete. The present study was designed to assess differential distribution of slow‐twitch (type I) and fast‐twitch (type II) myosin fibers in select facial muscles of dogs and wolves. We tested the hypothesis that dog facial muscle physiology would be more similar to humans than to wolves in terms of myosin fiber type distribution. We used samples from the orbicularis oris muscle (OOM) and the zygomaticus major muscle (ZM) from a range of domestic dog breeds (Canis familiaris) and gray wolves (Canis lupus) that were part of previous studies. Samples were processed for myosin immunohistochemistry (IHC) and we selected 3‐5 sections from each specimen stained for identification of each fiber type in both the OOM and ZM muscles. We created composite images of stained sections and assessed fiber type percentages using ImageJ. Percentage of fast‐twitch fibers in dog samples ranged from 66%‐95% and wolves averaged around 25%. Slow‐twitch fiber percentages in dogs averaged only 10% while wolves averaged 29%. The dog range of type II fibers mirrors the range found in human facial muscles, indicating that the domestication process in dogs also involved selection for rapid facial movement, similar to the movement seen in human faces.

Prediction of muscle fiber composition using multiple repetition testing.

Direct determination of muscle fiber composition is invasive and expensive, with indirect methods also requiring specialist resources and expertise. Performing resistance exercises at 80% 1RM is suggested as a means of indirectly estimating muscle fiber composition, though this hypothesis has never been validated against a direct method. The aim of the study was to investigate the relationship between the number of completed repetitions at 80% 1RM of back squat exercise and muscle fiber composition. Thirty recreationally active participants’ (10 females, 20 males) 1RM back squat load was determined, before the number of consecutive repetitions at 80% 1RM was recorded. The relationship between the number of repetitions and the percentage of fast-twitch fibers from vastus lateralis was investigated. The number of completed repetitions ranged from 5 to 15 and was independent of sex, age, 1RM, training frequency, training type, training experience, BMI or muscle fiber cross-sectional area. The percentage of fast-twitch muscle fibers was inversely correlated with the number of repetitions completed (r = –0.38, P = 0.039). Participants achieving 5 to 8 repetitions (n = 10) had significantly more fast-twitch muscle fibers (57.5 ± 9.5 vs 44.4 ± 11.9%, P = 0.013) than those achieving 11–15 repetitions (n = 11). The remaining participants achieved 9 or 10 repetitions (n = 9) and on average had equal proportion of fast- and slow-twitch muscle fibers. In conclusion, the number of completed repetitions at 80% of 1RM is moderately correlated with muscle fiber composition.

Sex Comparisons for Relative Peak Torque and Electromyographic Mean Frequency During Fatigue

Purpose: This study compared the relative peak torque and normalized electromyographic (EMG) mean frequency (MNF) responses during fatiguing isokinetic muscle actions for men versus women. Method: Twenty men (M age ± SD = 22 ± 2 years) and 20 women (M age ± SD = 22 ± 1 years) performed 50 maximal concentric isokinetic muscle actions of the leg extensors at a velocity of 180°/s while surface EMG signals were detected from the vastus lateralis, rectus femoris, and vastus medialis. The dependent variables were initial, final, and average peak torque; percent decline; the estimated percentage of fast-twitch fibers for the vastus lateralis; and the linear slope coefficients and y-intercepts for normalized EMG MNF versus repetition number. The data were analyzed with independent-samples t tests and 2-way mixed-factorial analyses of variance. Results: The mean initial, final, and average peak torque values for men were greater than those for women. There were no mean differences for percent decline and the estimated percentage of fast-twitch fibers for the vastus lateralis. There were also no sex differences for the linear slope coefficients, but there were differences among the muscles (vastus medialis>vastus lateralis>rectus femoris). The mean y-intercept for the vastus lateralis for men was greater than that for women. Conclusions: Men demonstrated greater peak torque values than those for women, but the declines in peak torque and normalized EMG MNF were similar between the sexes. The vastus medialis was more fatigue-resistant than both the vastus lateralis and rectus femoris.

Noninvasive Monitoring of Training Induced Muscle Adaptation with -MRS: Fibre Type Shifts Correlate with Metabolic Changes

Purpose. To evaluate training induced metabolic changes noninvasively with 31P magnetic resonance spectroscopy (31P-MRS) for measuring muscle fibre type adaptation. Methods. Eleven volunteers underwent a 24-week training, consisting of speed-strength, endurance, and detraining (each 8 weeks). Prior to and following each training period, needle biopsies and 31P-MRS of the resting gastrocnemius muscle were performed. Fibre type distribution was analyzed histologically and tested for correlation with the ratios of high energy phosphates ([PCr]/[Pi], [PCr]/[βATP] and [PCr + Pi]/[βATP]). The correlation between the changes of the 31P-MRS parameters during training and the resulting changes in fibre composition were also analysed. Results. We observed an increased type-II-fibre proportion after speed-strength and detraining. After endurance training the percentage of fast-twitch fibres was reduced. The progression of the [PCr]/[Pi]-ratio was similar to that of the fast-twitch fibres during the training. We found a correlation between the type-II-fibre proportion and [PCr]/[Pi] (r = 0.70, P < 0.01) or [PCr]/[βATP] (r = 0.69, P < 0.01); the correlations between its changes (delta) and the fibre-shift were significant as well (delta[PCr]/[Pi] r = 0.66, delta[PCr]/[βATP] r = 0.55, P < 0.01). Conclusion. Shifts in fibre type composition and high energy phosphate metabolite content covary in human gastrocnemius muscle. Therefore 31P-MRS might be a feasible method for noninvasive monitoring of exercise-induced fibre type transformation.

Sensitivity analysis of an energetic muscle model applied at whole body level in recumbent pedalling

Musculoskeletal models are used in order to describe and analyse the mechanics of human movement. In order to get a complete evaluation of the human movement, energetic muscle models were developed and were shown to be promising. The aim of this work is to determine the sensitivity of muscle mechanical and energetic model estimates to changes in parameters during recumbent pedalling. Inputs of the model were electromyography and joint angles, collected experimentally on one participant. The sensitivity analysis was performed on muscle-specific tension, physiological cross-sectional area, muscle maximal force, tendon rest length and percentage of fast-twitch fibres using an integrated sensitivity ratio. Soleus, gastrocnemius, vasti, gluteus and medial hamstrings were selected for the analyses. The energetic model was found to be always less sensitive to parameter changes than the mechanical model. Tendon slack length was found to be the most critical parameter for both energetic and mechanical models even if the effect on the energetic output was smaller than on muscle force and joint moments.

Hereditary Dominance of Fast-Twitch Fibers in Skeletal Muscles and Relation of Thyroid Hormone under Physiological Conditions in Rats

To investigate whether muscle fiber compositions of various skeletal muscles were correlated with each other and whether muscle fiber type composition was related to the serum thyroid hormone level, fast-twitch fiber dominant rats (FFDR), which had a higher percentage of fast-twitch (FT) fibers in the deep portion of the lateral head of the gastrocnemius muscle (GAS), were compared to a control group. The FFDR group was obtained by the selective breeding for high-percentage FT fibers in the GAS, and the control group was obtained by random breeding from the same base population. The percentages of FT fibers of the GAS, soleus (SOL), vastus intermedius (VI), biceps brachii (BB), rectus abdominis (RA) and extensor digitorum longus (EDL) muscles were analyzed. The percentages of FT fibers of the GAS, SOL, VI and BB in the FFDR group were significantly higher than in the control group (p<0.05), and percent FT fibers of the GAS were significantly correlated with those of the SOL (r = 0.62, p<0.01), VI (r = 0.45, p <0.05) and BB (r = 0.43, p<0.05). On the other hand, no significant differences between FFDR and the control group and no significant correlations with the GAS were observed in percent FT fibers of the RA and EDL. The total 3,3′,5-triiodo-L-thyronine (T₃) level in FFDR was not significantly different from the control group and was not significantly correlated with the percentage of FT fibers. These data suggest that the percentages of FT fibers of various skeletal muscles are genetically correlated with each other, but some FT muscles are not because they might be close to the upper limits of the genetic adaptive range. In addition, percentages of FT fibers are independent of the total T₃ level under physiological conditions.

Heredity of muscle fiber composition and correlated response of the synergistic muscle in rats.

The extent to which muscle fiber composition was determined by genes transmitted from parents and the correlated response of the synergistic muscle were analysed by using successive selection of rats. The foundation population (G0) was prepared by random choice from heterogeneous stock produced by random mating of three strains, Wistar-Imamichi, Fischer 344, and Donryu. Selective mating for a high percentage of fast-twitch fibers (%FT) in the deep portion of the lateral head of the gastrocnemius muscle and random mating were made from G0 to the seventh generation (G7). Successive selection caused a significant increase (P < 0.05) of %FT in the gastrocnemius muscle. The realized heritability for seven generations was 0.29. %FT in the soleus muscle of the selected line also increased (P < 0.05). In addition, significantly positive correlations (P < 0.05) were found between %FT in the gastrocnemius and soleus muscles in G1-G7. We conclude that approximately 29% of the variation of %FT was determined by genes transmitted from parents, and %FT in the gastrocnemius muscle was positively correlated with that of the soleus muscle.

Changes in stiffness induced by hindlimb suspension in rat soleus muscle.

Soleus muscle atrophy was induced by hind-limb suspension of rats for 3 weeks with the intention of inducing a relative increase in the percentage of fast-twitch fibres and assessing modifications in muscle stiffness. A method of dual controlled releases was used to obtain tension/extension curves and force/velocity relationships characterizing the mechanical behaviour of the soleus. Fibre typing was achieved by myofibrillar adenosine 5'-triphosphatase staining. Results showed that hindlimb suspension decreased the percentage of slow-twitch fibres (-31%) to the profit of fast-twitch fibres (+370%) and intermediate fibres (+255%). This led to an increase in maximal shortening velocity. Tension/extension curves indicated a decrease in soleus stiffness after 3 weeks of unloading. Changes in elastic properties are interpreted in terms of modifications occurring in the active part and the passive part of the so-called series elastic component. These changes also suggest that the parameters derived from a twitch are inappropriate to account for modifications in speed-related properties of muscle.

Human skeletal muscle fibre types and force: velocity properties.

It has been reported that there is a relationship between power output and fibre type distribution in mixed muscle. The strength of this relationship is greater in the range of 3-8 rad.s-1 during knee extension compared to slower or faster angular knee extensor speeds. A mathematical model of the force: velocity properties of muscle with various combinations of fast- and slow-twitch fibres may provide insight into why specific velocities may give better predictions of fibre type distribution. In this paper, a mathematical model of the force:velocity relationship for mixed muscle is presented. This model demonstrates that peak power and optimal velocity should be predictive of fibre distribution and that the greatest fibre type discrimination in human knee extensor muscles should occur with measurement of power output at an angular velocity just greater than 7 rad.s-1. Measurements of torque:angular velocity relationships for knee extension on an isokinetic dynamometer and fibre type distribution in biopsies of vastus lateralis muscles were made on 31 subjects. Peak power and optimal velocity were determined in three ways: (1) direct measurement, (2) linear regression, and (3) fitting to the Hill equation. Estimation of peak power and optimal velocity using the Hill equation gave the best correlation with fibre type distribution (r < 0.5 for peak power or optimal velocity and percentage of fast-twitch fibres). The results of this study confirm that prediction of fibre type distribution is facilitated by measurement of peak power at optimal velocity and that fitting of the data to the Hill equation is a suitable method for evaluation of these parameters.

Heterogeneity of Muscle Energetics in Human Muscle Fiber Type Using 31P Nuclear Magnetic Resonance

To evaluate the association between muscular phosphate metabolites or intracellular pH at rest and muscle fiber composition of musculus vastus lateralis in human, we measured phosphorus nuclear magnetic resonance (31P NMR) spectra and performed muscle biopsy in 30 subjects. The intracellular pH tended to be higher in cases with a higher percentage of fast-twitch fibers (% FT fibers). On the other hand, no relationship was observed between phosphocreatine (PCr)/inorganic phosphate (Pi) and percent FT fibers. This is inconsistent with previously reported results of 31P NMR studies in animals. These results suggest that PCr concentration is the same for the two major fiber types (FT and slow-twitch [ST] fibers) of human skeletal muscle. Knowledge of which relationship between muscle fiber composition and intracellular pH accounts for our results awaits further investigation.

Comparison of the Histochemical and Contractile Properties of Human Gastrocnemius Muscle

Contractile and histochemical properties of the human gastrocnemius muscle were compared in 21 male athletes ranging from 20 to 29 years of age. Surface electrical stimulation was used to determine muscle twitch parameters. The contractile variables of the muscle twitch were: latency (L), time to peak force (TPF1 and TPF2), peak force (Pf1 and Pf2), half-contraction time (1/2 CT), and half-relaxation time (1/2 RT). Muscle samples from the belly of the medial gastrocnemius muscle were obtained using the needle biopsy technique. The samples were treated histochemically for myosin ATPase in order to classify the fibers as slow twitch (ST) or fast twitch (FT) and to determine fiber areas. Multiple correlations were performed between the contractile and histochemical variables. The results demonstrated significant (p < 0.05) positive relationships between the calculated twitch recordings and percentage of FT fibers (R = 0.85), percentage of ST fibers (R = 0.85), relative percent area of FT fibers (R = 0.84), and relative percent area of ST fibers (R = 0.84). The results suggest that fiber type percentage and relative percent area determined using myosin ATPase are related to electrically stimulated isometric contractile properties. J Orthop Sports Phys Ther 1991;13(6):322-328.

Muscle Fibre Types, Ubiquinone Content and Exercise Capacity in Hypertension and Effort Angina

The composition of skeletal muscle fibre expressed as a percentage of slow twitch (ST), type I or "red" and fast twitch (FT), type II or "white" were determined in patients with hypertension (HT) or with severe ischaemic heart disease (IHD) and compared to age matched controls. Similarly, exercise capacity expressed as the cycle intensity eliciting a blood lactate concentration corresponding to 2.0 mmol x 1-1 were compared with healthy controls. Both patient groups had a higher percentage of FT fibres with relatively lower exercise capacities than their controls. The exercise capacities were reduced even when the relationship of decreased capacity with the percentage of increased FT was considered. There was an increase IHD but not in HT in patients with fibre subgroup FTc, which most probably reflected fibre trauma. Both patient groups were low in the skeletal muscle mitochondrial electron carrier and unspecific antioxidant ubiquinone, coenzyme Q10 or CoQ10. Patients with IHD but not HT showed, however, a faster fall in the ratio CoQ10 over ST% the higher the percentage value of ST. The ratio reflects the antioxidant activity related to CoQ10 in the fibre hosting most of the oxidative metabolism. A low ratio indicates a risk of metabolic lesion and cell trauma. This could explain fibre plasticity and offer an alternative cause to heredity in elucidating in deviating muscle fibre composition in patients with HT and IHD.

The determinants of skeletal muscle force and power: Their adaptability with changes in activity pattern

A kinetic model of the cross-bridge cycle in skeletal muscle is presented with special reference to the rate limiting steps regulating the peak rate of force development ( dP dt ), peak force (P o), and the maximal shortening speed (V max). Force production in skeletal muscle is dependent on the number of cross-bridges in the strongly bound, high-force state (AM •-ADP), and during a peak isometric contraction this state is the dominant cross-bridge form. The peak force and power output of a muscle depends upon numerous factors to include: (1) muscle and fiber size and length: (2) architecture, such as the angle and physical properties of the fiber-tendon attachment, and the fiber to muscle length ratio: (3) fiber type: (4) number of cross-bridges in parallel: (5) force per cross-bridge: (6) peak dP dt : (7) force-velocity relationship: (8) fiber V max: (9) force-pCa 2+ relationship: and (10) the force-frequency (action potential Hz) relationship. In this paper, we discuss these determinants of force and power output, and consider how they adapt to both muscle unloading (induced by hindlimb suspension) and programs of regular endurance exercise. Slow- and fast-twitch fibers have similar capacities to generate specific tension (kg cm −2). However, fast fibers show a considerably higher peak dP dt , V max, and power output. The high V max of the fast-twitch fiber is likely due to the high myofibrillar ATPase activity of the fast myosin isozyme. Both hindlimb suspension and regular endurance exercise have been shown to induce fiber type specific changes in single fiber function. For example, fiber size and the peak tetanic tension of the slow oxidative (SO), fast oxidative glycolytic (FOG), and fast glycolytic (FG) fiber types were generally unaltered by endurance exercise-training. In contrast, hindlimb suspension produced cell atrophy in all fiber types and a reduced specific tension in the SO but not the FOG or FG fiber types. Both exercise-training and HS shifted the force-pCa curve to the right, and increased the V max of the SO fiber type. From the standpoint of work capacity or the ability to move a load, the important functional property is power output. Peak power is obtained at loads considerably below 50% of P o, and it is correlated with the percentage of fast-twitch fibers. Peak power can be increased by both dynamic and isometric programs of exercise-training. This adaptation should improve performance as a high correlation exists between power and maximal sprinting speed in man. Future attempts to model the contribution of a particular muscle to a given movement or specific bone torque will need to consider the percentage and distribution of each fiber type, the functional differences between fiber types, and the level of daily activity generally experienced by the muscle or muscle group.

Time-course of force production by fast isometric contraction of the knee extensor in young and elderly subjects.

Rapid force production by isometric contraction of the knee extensor was examined in a wide range of force output for 12 healthy elderly (65-86 years) and 12 young (20-35 years) subjects. Time-course of tension development and duration of first burst EMG activities (AG-1) of the vastus medialis muscle were compared between both groups. Significant increase in the elderly as compared with the young was found in duration from the onset of the EMG to the rise of tension (TLT), but not in time from the rise to peak of force (FTmax), although FTmax tended to be longer in the elderly than the young at three different levels of force output. AG-1 duration was also prolonged in the elderly but the difference was not significant. The prolongation of TLT in the elderly suggests that spatio-temporal recruitments of the motor units and/or the percentage of fast twitch fibers decreases with aging.

An approach to noninvasive fiber type determination by NMR.

In vivo 31P NMR spectroscopy was used to determine the ratios of creatine phosphate (PCr) to adenosine triphosphate (ATP) and inorganic phosphate (Pi) in leg and arm muscles of four sprinters, one marathon runner, and two sedentary subjects. Both ratios were definitely higher in the sprinters indicating that, since muscle ATP and Pi concentrations are constant, the PCr muscle content of these athletes is higher than usual. Sprinters are known to have higher percentages of fast-twitch fibers, which are richer in PCr than slow-twitch fibers. It is concluded that measurements of muscle ATP, PCr, and Pi through in vivo NMR spectroscopy could be used to determine muscle fiber composition.

Fibre conduction velocity and fibre composition in human vastus lateralis.

The relationship between muscle fibre composition and fibre conduction velocity was investigated in 19 male track athletes, 12 sprinters and 7 distance runners, aged 20-24 years, using needle biopsy samples from vastus lateralis. Cross sectional areas of the fast twitch (FT) and slow twitch (ST) fibres were determined by histochemical analysis. The percentage of FT fibre areas ranged from 22.6 to 93.6%. Sprinters had a higher percentage of FT fibres than distance runners. Muscle fibre conduction velocity was measured with a surface electrode array placed along the muscle fibres, and calculated from the time delay between 2 myoelectric signals recorded during a maximal voluntary contraction. The conduction velocity ranged from 4.13 to 5.20 m.s-1. A linear correlation between conduction velocity and the relative area of FT fibres was statistically significant (r = 0.84, p less than 0.01). This correlation indicates that muscle fibre composition can be estimated from muscle fibre conduction velocity measured noninvasively with surface electrodes.

Mechanical measures during maximal velocity knee extension exercise and their relation to fibre composition of the human vastus lateralis muscle.

A method for measuring the maximal velocity of knee extension exercise is described using a very light lever arm. Instrumentation of the lever arm with a potentiometer and accelerometer also allows for the measurement of peak acceleration, time to peak acceleration, the average rate of development of acceleration (jerk) and peak torque. With this apparatus and surface electromyography, electromechanical delay (EMD) was also determined. This apparatus was tested using 17 female and 10 male subjects, and the measures obtained were related to the percentage of fast twitch fibres (% FT) and the relative area of fast twitch fibres (% FTA) in the vastus lateralis determined from duplicate muscle biopsy samples. Peak velocity of unloaded knee extension averaged 12.1 +/- 1.2 and 12.2 +/- 1.7 rad.s-1 for females and males, respectively, and were not significantly different. As well, peak acceleration, time to peak acceleration jerk and EMD values were not significantly different between the female and male subjects, but the mean peak torque for the female subjects (73.5 +/- 14.7 N.m) was significantly lower than that for the males (98.4 +/- 31.5 N.m). Peak acceleration was significantly correlated with %FT (r = 0.40, P = 0.04) for the total subject population. None of the other measures was significantly related to either %FT or %FTA for the male and female subjects or the combined population of subjects.

Relationship between the efficiency of muscular work during jumping and the energetics of running.

The running economy of seventeen athletes was studied during running at a low speed (3.3 m X s-1) on a motor-driven treadmill. The net energetic cost during running expressed in kJ X kg-1 X km-1 was on average 4.06. As expected, a positive relationship was found between the energetic cost and the percentage of fast twitch fibres (r = 0.60, n = 17, p less than 0.01). In addition, the mechanical efficiency during two different series of jumps performed with and without prestretch was measured in thirteen subjects. The effect of prestretch on muscle economy was represented by the ratio between the efficiency of muscular work performed during prestretch jumps and the corresponding value calculated in no prestretch conditions. This ratio demonstrated a statistically significant relationship with energy expenditure during running (r = -0.66, n = 13, P less than 0.01), suggesting that the elastic behaviour of leg extensor muscles is similar in running and jumping if the speeds of muscular contraction during eccentric and concentric work are of similar magnitudes.

The effect of fatigue on store and re-use of elastic energy in slow and fast types of human skeletal muscle.

Stretch-shortening exercises are characterized by enhancement of performance when compared to the work output performed in shortening conditions. There is evidence that fast subjects are unable to re-use great amounts of elastic energy during stretch-shortening cycles performed with slow stretching speed and large stretching length. In the present study, 14 subjects possessing different fibre types in m. vastus lateralis performed vertical jumps with and without preliminary countermovement and with large angular displacement and slow stretching speed The jumping tests were executed before and immediately after fatigue induced by short intense exercises (60 s of continuous rebound jumping). The results indicated that the percentage of re-use of elastic energy was more pronounced in slow subjects compared to fast ones during the test performed before fatigue (28.3% vs. 22.8%). In contrast fast subjects demonstrated a greater percentage re-use of elastic energy than slow ones after fatigue (32% vs. 22.5%). Similarly, the negative relationship observed before fatigue, between the percentage of re-use of elastic energy and percentage of fast twitch fibres (r = 0.50, n = 14, P less than 0.05), was reversed after fatigue (r = 0.55, n = 14, P less than 0.05). The results can be interpreted through differences in sarcomere cross-bridges life-times between fast and slow twitch muscle fibres. The slow twitch-type muscle fibre may be able to retain the cross-bridge attachment for a longer period of time during no fatigued conditions, and therefore it may utilize elastic energy better in slow type ballistic motion. On the other hand, fast twitch type muscle fibres are more affected by fatigue, which might have induced a remarkable decrease of the cross-bridge attachment detachment cycle. Decrease of the cross-bridge rate cycle might allow fast twitch-type muscle fibres to retain longer the elastic energy stored during the stretching phase and then re-use it during positive phase.