The effects of muscle architectural change with a pre-motion silent period on the subsequent muscular output during rapid voluntary movement

This study aimed to determine the characteristics of the in vivo behaviour of human muscle architecture during a pre-motion silent period (PMSP) using ultrasonography. Subjects were requested to perform rapid knee extension with vertical jumping. Electromyographic signals were recorded from the vastus lateralis (VL), vastus medialis, and biceps femoris muscles. Ultrasonic images were recorded from the VL. We found that the cross point between the fascicle and deep aponeurosis in the VL moved to the distal side before the rapid vertical jumps with PMSP. This cross point movement with PMSP was of low amplitude (mean: 1.0 ± 0.3 mm) and velocity (22.2 ± 6.1 mm/s). The amplitude and velocity of the cross point movement were significantly positively related to the angular peak velocity of knee extensor during rapid vertical jumping in trials with PMSP. These results suggest that although low levels of pre-movement muscle architectural change with PMSP may be the result of muscle relaxation behaviour rather than the result of muscle stretching behaviour, this pre-movement effect can influence subsequent muscular performance during a rapid voluntary movement. PMSP may allow pre-movement muscle architectural change to generate a better muscular condition to increase neural activation during the subsequent rapid voluntary contraction.     

An estimation of power output and work done by the human triceps surae muscle-tendon complex in jumping

In explosive movements involving the lower extremity elastic recoil and transportation of power from knee to ankle via m. gastrocnemius allow power output about the ankle to reach values over and above the maximum power output of the plantar flexors. The object of this study was to estimate the relative power and work contributions of these two mechanisms for the push-off phase in one-legged jumping. During jumps of ten subjects ground reaction forces and cinematographic data were recorded. The data were used for a kinematic and kinetic analysis of the jumps yielding, among other variables, the velocity with which origins of m. soleus and m. gastrocnemius approach insertion (V OI), and net power output about the ankle (P A). V OI of m. soleus and m. gastrocnemius were imposed upon a model of the muscle-tendon complex of m. triceps surae, and power contributions of muscle fibers (P fibers), tendinous structures (P tendon), and transportation (P transported) were calculated. During the last 150 ms before toe-off, P A was found to increase rapidly and to attain an average peak value of 1790 W. The curve obtained by summation of P fibers, P tendon and P transported closely resembled that of P A. On the instant that the latter peaked (50 ms before toe-off) P fibers and P tendon of m. triceps surae contributed 27 and 53% respectively, and P transported contributed 20%.(ABSTRACT TRUNCATED AT 250 WORDS)     

Muscle performance during maximal isometric and dynamic contractions is influenced by the stiffness of the tendinous structures.

Contractile force is transmitted to the skeleton through tendons and aponeuroses, and, although it is appreciated that the mechanocharacteristics of these tissues play an important role for movement performance with respect to energy storage, the association between tendon mechanical properties and the contractile muscle output during high-force movement tasks remains elusive. The purpose of the study was to investigate the relation between the mechanical properties of the connective tissue and muscle performance in maximal isometric and dynamic muscle actions. Sixteen trained men participated in the study. The mechanical properties of the vastus lateralis tendon-aponeurosis complex were assessed by ultrasonography. Maximal isometric knee extensor force and rate of torque development (RTD) were determined. Dynamic performance was assessed by maximal squat jumps and countermovement jumps on a force plate. From the vertical ground reaction force, maximal jump height, jump power, and force-/velocity-related determinants of jump performance were obtained. RTD was positively related to the stiffness of the tendinous structures (r = 0.55, P < 0.05), indicating that tendon mechanical properties may account for up to 30% of the variance in RTD. A correlation was observed between stiffness and maximal jump height in squat jumps and countermovement jumps (r = 0.64, P < 0.05 and r = 0.55, P < 0.05). Power, force, and velocity parameters obtained during the jumps were significantly correlated to tendon stiffness. These data indicate that muscle output in high-force isometric and dynamic muscle actions is positively related to the stiffness of the tendinous structures, possibly by means of a more effective force transmission from the contractile elements to the bone.     

Indirect evidence for elastic energy playing a role in limb recovery during toad hopping

Elastic energy is critical for amplifying muscle power during the propulsive phase of anuran jumping. In this study, we use toads (Bufo marinus) to address whether elastic recoil is also involved after take-off to help flex the limbs before landing. The potential for such spring-like behaviour stems from the unusually flexed configuration of a toad''s hindlimbs in a relaxed state. Manual extension of the knee beyond approximately 90° leads to the rapid development of passive tension in the limb as underlying elastic tissues become stretched. We hypothesized that during take-off, the knee regularly extends beyond this, allowing passive recoil to help drive limb flexion in mid-air. To test this, we used high-speed video and electromyography to record hindlimb kinematics and electrical activity in a hindlimb extensor (semimembranosus) and flexor (iliofibularis). We predicted that hops in which the knees extended further during take-off would require less knee flexor recruitment during recovery. Knees extended beyond 90° in over 80% of hops, and longer hops involved greater degrees of knee extension during take-off and more intense semimembranosus activity. However, knee flexion velocities during recovery were maintained despite a significant decrease in iliofibularis intensity in longer hops, results consistent with elastic recoil playing a role.     

Gear ratios at the limb joints of jumping dogs

An increase in gear ratio of the limb extensor muscles during joint extension has been suggested to be a mechanism that facilitates optimal power production by skeletal muscles. The objectives of this study were to: (1) measure gear ratios at the wrist, elbow, shoulder, ankle, knee, and hip joints of jumping dogs, (2) compute the work performed by each of these joints, and (3) measure muscle shortening velocity for a joint exhibiting an increasing gear ratio during joint extension. The gear ratio out-lever was computed by dividing the ground reaction force (GRF) moment by the GRF, whereas the in-lever was directly measured as the perpendicular distance from the joint center to the line of action of the extensor muscle. In addition, changes in fascicle length were measured from the vastus lateralis muscle using sonomicrometry. Of the joints examined, only the gear ratios at the shoulder and knee joints increased during jumping in a manner that could facilitate peak power production of actively shortening muscles. The vastus lateralis was found to shorten at an average velocity of 3.20 muscle lengths per second. This is similar to estimates of shortening velocity that produce peak muscular power in mammals the size of dogs. Additionally, the knee extensors were found to produce a large proportion (26.6%) of the positive external work of the limbs. These observations suggest that dynamic gearing in jumping dogs may allow the extensor muscles of the knee joint to shorten in a way that maximizes their power production.     

Reliability of leg muscle electromyography in vertical jumping

In this study we aimed to determine the reliability of the surface electromyography (EMG) of leg muscles during vertical jumping between two test sessions, held 2 weeks apart. Fifteen females performed three maximal vertical jumps with countermovement. The displacement of the body centre of mass (BCM), duration of propulsion phase (time), range of motion (ROM) and angular velocity of the knee and surface EMG of four leg muscles (rectus femoris, vastus medialis. biceps femoris and gastrocnemius) were recorded during the jumps. All variables were analysed throughout the propulsion and mid-propulsion phases. Intraclass correlation coefficients (ICC) for the rectus femoris, vastus medialis, biceps femoris and gastrocnemius were calculated to be 0.88, 0.70, 0.24 and 0.01, respectively. BCM, ROM and time values all indicated ICC values greater than 0.90, and the mean knee angular velocity was slightly lower, at 0.75. ICCs between displacement of the BCM and integrated EMG (IEMG) of the muscles studied were less than 0.50. The angular velocity of the knee did not correlate well with muscle activity. Factors that may have affected reliability were variations in the position of electrode replacement, skin resistance, cross-talk between muscles and jump mechanics. The results of this study suggest that while kinematic variables are reproducible over successive vertical jumps, the degree of repeatability of an IEMG signal is dependent upon the muscle studied.     

Muscular strength and jumping performance relationships in young women athletes

The relationships between muscular strength and vertical jumping performance were examined in young women (14-19 years) track and field jumpers (n = 20) and volleyball players (n = 21). The knee extensor muscular strength measured at 9 knee angles was correlated with jumping height and peak power at the squat (SJ) and the countermovement (CMJ) vertical jump tests. Pearson product coefficient of correlation was used to test the significance of these relationships (p 0.80). Specifically, in the volleyball players, the strong relationships were noted for muscular strength at the knee angle range of 40 degrees to 90 degrees and CMJ jumping height as well as SJ peak power. Results indicate the dissimilarity in the relationships between the knee extensor muscular strength and jumping performance in the young female track and field jumpers and volleyball players. In addition, it appears that the measure selected to evaluate jumping performance alters the correlational results.     

Influence of eccentric strength of knee extensor muscles on biomechanical factors of a vertical drop jump

During a vertical drop jump (VDJ), the human neuromuscular system absorbs and reuses external loads applied to the lower extremity by coordinating the musculoskeletal system. This study aims to investigate the influence of the eccentric strength of the knee extensor muscles on the biomechanical factors of a VDJ. Participants were divided into two groups based on the eccentric strength of their knee extension muscles: low eccentric (L_ECC) and high eccentric (H_ECC) strength groups. The VDJ joint kinematics and kinetics of the lower extremity, the fascicle behavior of the vastus lateralis, and the muscle activation of the knee extensor muscles were simultaneously recorded during maximum-effort VDJ. Compared with the L_ECC group, the H_ECC group showed a higher jump, greater knee and ankle joint stiffness, and smaller fascicle length change. These findings suggest that the eccentric strength capacity of the knee extensor muscles accounts for the different biomechanical strategies (bouncing-type for H_ECC and absorbing-type for L_ECC) observed between the groups. Consequently, the eccentric strength of the knee extensor muscle may be an essential factor in determining the biomechanical strategy for VDJ and should be considered in the jumping performance enhancement training paradigm.     

Differential serial sarcomere number adaptations in knee extensor muscles of rats is contraction type dependent.

Sarcomerogenesis, or the addition of sarcomeres in series within a fiber, has a profound impact on the performance of a muscle by increasing its contractile velocity and power. Sarcomerogenesis may provide a beneficial adaptation to prevent injury when a muscle consistently works at long lengths, accounting for the repeated-bout effect. The association between eccentric exercise, sarcomerogenesis and the repeated-bout effect has been proposed to depend on damage, where regeneration allows sarcomeres to work at shorter lengths for a given muscle-tendon unit length. To gain additional insight into this phenomenon, we measured fiber dynamics directly in the vastus lateralis (VL) muscle of rats during uphill and downhill walking, and we measured serial sarcomere number in the VL and vastus intermedius (VI) after chronic training on either a decline or incline grade. We found that the knee extensor muscles of uphill walking rats undergo repeated active concentric contractions, and therefore they suffer no contraction-induced injury. Conversely, the knee extensor muscles during downhill walking undergo repeated active eccentric contractions. Serial sarcomere numbers change differently for the uphill and downhill exercise groups, and for the VL and VI muscles. Short muscle lengths for uphill concentric-biased contractions result in a loss of serial sarcomeres, and long muscle lengths for downhill eccentric-biased contractions result in a gain of serial sarcomeres.     

A multivariate approach to predicting knee extensor performance

The impact of two predictor variables (estimated knee extensor fast-twitch fiber percentage, body mass) on performance measures (vertical jump power output, leg press peak angular velocity) were examined. Subjects (25 men, 27 women) performed 5 workouts involving 2 vertical jump, leg press, and 50-repetition isokinetic tests (to estimate knee extensor fast-twitch fiber percentage). Multivariate regression determined the following significant (p < 0.05) vertical jump equations: predicted male power output = -59.3464 + 1.566 (estimated knee extensor fast-twitch muscle fiber percent) + 15.7884 (body mass), predicted female power output = 36.1574 + 3.4248 (estimated knee extensor fast-twitch muscle fiber percent) + 9.8633 (body mass). Leg press peak angular velocity equations were insignificant by gender; thus, pooled data yielded the following: predicted leg press peak angular velocity = 18.6187 + 0.235 (estimated knee extensor fast-twitch muscle fiber percent) + 0.3801 (body mass). Body mass explained more variance for each performance measure.     

Biomechanical behavior of muscle-tendon complex during dynamic human movements

This paper reviews the research findings regarding the force and length changes of the muscle-tendon complex during dynamic human movements, especially those using ultrasonography and computer simulation. The use of ultrasonography demonstrated that the tendinous structures of the muscle-tendon complex are compliant enough to influence the biomechanical behavior (length change, shortening velocity, and so on) of fascicles substantially. It was discussed that the fascicles are a force generator rather than a work generator; the tendinous structures function not only as an energy re-distributor but also as a power amplifier, and the interaction between fascicles and tendinous structures is essential for generating higher joint power outputs during the late pushoff phase in human vertical jumping. This phenomenon could be explained based on the force-length/velocity relationships of each element (contractile and series elastic elements) in the muscle-tendon complex during movements. Through computer simulation using a Hill-type muscle-tendon complex model, the benefit of making a countermovement was examined in relation to the compliance of the muscle-tendon complex and the length ratio between the contractile and series elastic elements. Also, the integral roles of the series elastic element were simulated in a cyclic human heel-raise exercise. It was suggested that the storage and reutilization of elastic energy by the tendinous structures play an important role in enhancing work output and movement efficiency in many sorts of human movements.     

Rectus femoris and vastus intermedius fiber excursions predicted by three-dimensional muscle models

Computer models of the musculoskeletal system frequently represent the force-length behavior of muscle with a lumped-parameter model. Lumped-parameter models use simple geometric shapes to characterize the arrangement of muscle fibers and tendon; this may inaccurately represent changes in fiber length and the resulting force-length behavior, especially for muscles with complex architecture. The purpose of this study was to determine the extent to which the complex features of the rectus femoris and vastus intermedius architectures affect the fiber changes in length ("fiber excursions"). We created three-dimensional finite-element models of the rectus femoris and vastus intermedius muscles based on magnetic resonance (MR) images, and compared the fiber excursions predicted by the finite-element models with fiber excursions predicted by lumped-parameter models of these muscles. The finite-element models predicted rectus femoris fiber excursions (over a 100 degrees range of knee flexion) that varied from 55% to 70% of the excursion of the muscle-tendon unit and vastus intermedius fiber excursions that varied from 55% to 98% of the excursion muscle-tendon unit. In contrast, the lumped-parameter model of the rectus femoris predicted fiber excursions that were 86% of the excursion of the muscle-tendon unit and vastus intermedius fiber excursions that were 97% of the excursion of the muscle-tendon unit. These results suggest that fiber excursions of many fibers are overestimated in lumped-parameter models of these muscles. These new representations of muscle architecture can improve the accuracy of computer simulations of movement and provide insight into muscle design.     

Effects of electromyostimulation training and volleyball practice on jumping ability

The aim of this study was to investigate the influence of a 4-week electromyostimulation (EMS) training program on the vertical jump performance of 12 volleyball players. EMS sessions were incorporated into volleyball sessions 3 times weekly. EMS consisted of 20-22 concomitant stimulations of the knee extensor and plantar flexor muscles and lasted approximately 12 minutes. No significant changes were observed after EMS training for squat jump (SJ) and counter movement jump (CMJ) performance, while the mean height and the mean power maintained during 15 seconds of consecutive CMJs significantly increased by approximately 4% (p < 0.05). Ten days after the end of EMS training, the jumping height significantly (p < 0.05) increased compared with baseline also for single jumps (SJ +6.5%, CMJ +5.4%). When the aim of EMS resistance training is to enhance vertical jump ability, sport-specific workouts following EMS would enable the central nervous system to optimize the control to neuromuscular properties.     

Interaction between fascicle and tendinous tissues in short-contact stretch-shortening cycle exercise with varying eccentric intensities.

The interaction between fascicle and tendinous tissues (TT) in short-contact drop jumps (DJ) with three different drop heights [low (Low), optimal (OP), and high (High)] was examined with 11 subjects. The ground reaction force (F(z)) and ankle and knee joint angles were measured together with real-time ultrasonography (fascicle length) and electromyographic activities of the medial gastrocnemius (MG) and vastus lateralis (VL) muscles during the movement. With increasing drop height, the braking force and flight time increased from Low to OP (P < 0.05). In High, the braking force increased but the flight time decreased compared with OP (P < 0.05). During contact of Low and OP conditions, the length of muscle-tendon unit and TT underwent lengthening before shortening in both MG and VL muscles. However, the two muscles differed in the fascicle behaviors. The MG fascicles behaved isometrically or shortened, and the VL fascicles underwent lengthening before shortening during contact. In High, the TT lengthening in both muscles decreased compared with OP (P < 0.05). The rapid stretch occurred in the MG fascicles but not in VL fascicles during the braking phase. The elastic recoil ratio decreased in both muscles with increasing the intensity during DJ. These findings demonstrated that TT underwent lengthening before shortening during DJ. However, the efficacy of elastic recoil decreased with increasing the drop intensity. The effective catapult action in TT can be limited by the drop intensity. In addition, the measured muscles behaved differently during DJ, providing evidence that each muscle may have a specific means of fascicle-TT interaction.     

Effect of forward trunk inclination on joint power output in vertical jumping

It is commonly accepted that vertical jump performance is a good indicator of maximal joint power. Some studies, however, have indicated that knee joint power output in the vertical jump is limited due to forward trunk inclination early in the push-off. The aim of this experimental study was to investigate the effect of forward trunk inclination on joint power output in vertical jumping. A group of 20 male subjects performed maximal vertical countermovement jumps from stance while minimizing the contribution of arm swing by holding their hands on their hips (arms akimbo). They also performed maximal jumps while holding the trunk as upright as possible throughout the jump, still holding the arms akimbo. Jump height, joint kinematics (angles), and joint kinetics (torque, power) were calculated. Jump height of vertical jumps while holding the trunk upright was 10% less than in normal jumps. Hip joint power was decreased by 37% while knee joint power was increased by 13%. Ankle joint power did not change. These results demonstrated that maximal jump performance does not necessarily represent maximal power of each individual joint. The implication is that jump performance may well be a good representation of overall joint power; it is, however, not an accurate measure to evaluate maximal individual joint power as part of contemporary training and rehabilitation methods.     

Evidence for a vertebrate catapult: elastic energy storage in the plantaris tendon during frog jumping

Anuran jumping is one of the most powerful accelerations in vertebrate locomotion. Several species are hypothesized to use a catapult-like mechanism to store and rapidly release elastic energy, producing power outputs far beyond the capability of muscle. Most evidence for this mechanism comes from measurements of whole-body power output; the decoupling of joint motion and muscle shortening expected in a catapult-like mechanism has not been demonstrated. We used high-speed marker-based biplanar X-ray cinefluoroscopy to quantify plantaris muscle fascicle strain and ankle joint motion in frogs in order to test for two hallmarks of a catapult mechanism: (i) shortening of fascicles prior to joint movement (during tendon stretch), and (ii) rapid joint movement during the jump without rapid muscle-shortening (during tendon recoil). During all jumps, muscle fascicles shortened by an average of 7.8 per cent (54% of total strain) prior to joint movement, stretching the tendon. The subsequent period of initial joint movement and high joint angular acceleration occurred with minimal muscle fascicle length change, consistent with the recoil of the elastic tendon. These data support the plantaris longus tendon as a site of elastic energy storage during frog jumping, and demonstrate that catapult mechanisms may be employed even in sub-maximal jumps.     

Muscle fiber and tendon length changes in the human vastus lateralis during slow pedaling.

Muscle fascicle lengths of vastus lateralis (VL) muscle were measured in five healthy men during slow pedaling to investigate the interaction between muscle fibers and tendon. Subjects cycled at a pedaling rate of 40 rpm (98 W). During exercise, fascicle lengths changed from 91 +/- 7 (SE) to 127 +/- 5 mm. It was suggested that fascicles were on the descending limb of their force-length relationship. The average shortening velocity of fascicle was greater than that of muscle-tendon complex in the first half of the knee extension phase and was less in the second half. The maximum shortening velocity of fascicle in the knee extension phase was less than that of muscle-tendon complex by 22 +/- 9%. These discrepancies in velocities were mainly caused by the elongation of the tendinous tissue. It was suggested that the elasticity of VL tendinous tissue enabled VL fascicles to develop force at closer length to their optimal length and kept the maximum shortening velocity of VL fascicles low during slow pedaling.     

Mechanical behavior of the quadriceps femoris muscle tendon unit during low-load contractions.

We examined the relationships between morphology and muscle-tendon dynamics of the quadriceps femoris muscle of 11 men using velocity-encoded phase-contrast magnetic resonance imaging (MRI). Thigh muscle electromyography and joint range of motion were first measured outside the MRI scanner during knee extension-flexion tasks that were performed at a rate of 40 times/min with elastic bands providing peak resistance of 5.2 kp (SD 0.4) to the extension. The same movement was repeated inside the MRI scanner bore where tissue velocities and muscle morphology were recorded. The average displacement in the proximal and distal halves of the rectus femoris and vastus intermedius aponeuroses was different (P = 0.049), reflecting shortening (1.6%), but the tensile strain along the length of the aponeuroses was uniform. The aponeurosis behavior varied among individuals, and these individual patterns were best explained by the differences in relative cross-sectional area of rectus femoris to vastus muscles (r = 0.71, P = 0.014). During dynamic contraction, considerable deformation of muscles in the axial plane caused an anatomic measure such as muscle thickness to change differently (decrease or increase) in different sites of measurement. For example, when analyzed from the axial images, the vastus lateralis thickness did not change (P = 0.946) in the frontal plane through femur but increased in a 45 degrees oblique plane between the frontal and sagittal planes (P = 0.004). The present observations of the heterogeneity and individual behavior emphasize the fact that single-point measurements do not always reflect the overall behavior of muscle-tendon unit.     

Effects of muscle damage on stretch-shortening cycle function and muscle stiffness control

This experiment examined the effect of eccentric contraction-induced muscle damage on the stretch-shortening cycle and vertical leg spring stiffness during jumping activities. Ten moderately active male and female adult volunteers participated in this study (aged 23 +/- 2.3 years). Temporary muscle damage to the knee extensors was administered by a bout of eccentric contractions on an isokinetic dynamometer. Measurements were obtained of maximum voluntary force and of take-off velocities for single-leg countermovement jumps (CMJs), squat jumps (SJs), and drop jumps (DJs), performed on a specially constructed sledge and force plate apparatus. These measurements were obtained before and after the damage intervention, and the undamaged leg was used as a control. The results indicated that eccentric muscle damage significantly affected stretch-shortening cycle performance by causing relatively greater reductions in SJ performance than CMJ or DJ. The muscle damage intervention also significantly increased leg-spring stiffness, which indicates that the changes in leg stiffness may be an important adaptation resulting from eccentric exercise.     

Jumping height in former elite athletes

To evaluate lower-limb explosive strength with respect to lifetime athletic activity, we measured vertical jumping height on a contact mat in former male runners (n = 28). soccer players (n = 31), weightlifters (n = 29) and shooters (n = 29) (age range 45 68 years). There were no statistically significant age-adjusted sport-group differences in jumping height, but differences by sport were evident among the subgroup of athletes without hip or knee osteoarthritis (n = 65) (P < 0.05). Thus, sports that increased jumping height also predisposed to lower-limb osteoarthritis. After adjustment for age and sport, the subjects without osteoarthritis jumped higher than those with osteoarthritis (n = 33) (P < 0.01). In a multiple linear regression analysis, age, reported hip and knee disability, and knee pain reduced jumping height. Hours spent in team-training during the past 12 months and the hours spent during their lifetime in power training were associated with improved vertical jumping height and together explained 41% of the difference among the subjects. The ability to jump even among athletes with hip or knee osteoarthritis would suggest that former elite athletes possess advanced lower limb muscle function.