Combined effect of elastic energy and myoelectrical potentiation during stretch-shortening cycle exercise

In addition to the utilization of muscle's elastic energy enhancement of performance in exercise involving stretch-shortening cycle might be also due to simultaneous increase of myoelectrical activity. This hypothesis was tested by examining three athletes during jumping exercise on force-platform. Vertical jumps were performed with and without preliminary counter-movement, and the jumps were called counter-movement jump (CMJ) and squatting jump (SJ), respectively. In both conditions several jumps were performed also with extra loads on the shoulders (15–220% of b. wt.). Additional droppingjumps (DJ) were executed from different heights (20–100 cm). During jumping exercise myoelectrical activity of selected muscles from the quadriceps femoris was monitored with surface electrodes. The results obtained were similar to those reported in isolated muscle and as expected, the prestretch in CMJ shifted the force-velocity curve of concentric work to the right. In two cases enhancement of performance was attributed primarily to restitution of elastic energy because myoelectrical activity was similar to that observed in SJ. In one subject increased myoelectrical activity was observed during the concentric phase of CMJ. In DJ condition the EMG activity during eccentric phase was much higher than in SJ. Therefore the high performance in this condition was attributed to both elastic energy and reflex potentiation. In eccentric work of CMJ the average force decreased with the increase of stretching speed. This phenomenon was associated with a light increase of EMG activity. The observed results emphasize that both elastic energy and reflex potentiation may operate effectively during stretch-shortening cycle activity.     

Store and recoil of elastic energy in slow and fast types of human skeletal muscles

Stretch-shortening cycle refers to the mechanical condition in which store and recoil of elastic energy occur in the skeletal muscle. This leads to a greater work output when compared to a simple shortening contraction. The subjects performed vertical jumps with and without preliminary counter-movement and with small and large knee angular displacement. The results indicated that those subjects who had more fast twitch (FT) fibers benefited more from the stretching phase performed with high speed and short angular displacement. The amounts of elastic energy stored in this phase were 30 and 26 N × kgBW^-1, respectively, for FT and slow twitch (ST) type subjects. The recoil of elastic energy was proportional to the amount of energy storage. In large amplitude jumps where transient period between stretch and shortening is long the both types of subjects demonstrated similar amount of storage of elastic energy (17 and 16 N × kgBW^-1, respectively). However, the re-use of this elastic energy was greater in ST group (24%) as compared to the FT group (17%). The results can be interpreted through differences in sarcomere cross-bridge life times between fast and slow muscle fibers. The slow type muscle may be able to retain the cross-bridge attachment for a longer period of time and therefore it may utilize elastic energy better in a slow type ballistic motion.     

Electromechanical delay in human skeletal muscle under concentric and eccentric contractions

Summary In contraction of skeletal muscle a delay exists between the onset of electrical activity and measurable tension. This delay in electromechanical coupling has been stated to be between 30 and 100 ms. Thus, in rapid movements it may be possible for electromyographic (EMG) activity to have terminated before force can be detected. This study was designed to determine the dependence of the EMG-tension delay upon selected initial conditions at the time of muscle activation. The rigth forearms of 14 subjects were passively oscillated by a motor-driven dynamometer through flexion-extension cycles of 135 deg at an angular velocity of 0.5 rad/s. Upon presentation of a visual stimulus the subjects maximally contracted the relaxed elbow flexors during flexion, extension, and under isometric conditions. The muscle length at the time of the stimulus was the same in all three conditions. An on-line computer monitoring surface EMG (Biceps and Brachioradialis) and force calculated the electromechanical delay. The mean value for the delay under eccentric condition, 49.5 ms, was significantly different (p<0.05) from the delays during isometric (53.9 ms) and concentric activity (55.5 ms). It is suggested that the time required to stretch the series elastic component (SEC) represents the major portion of the measured delay and that during eccentric muscle activity the SEC is in a more favorable condition for rapid force development.     

Neuromuscular function and mechanical efficiency of human leg extensor muscles during jumping exercises

The influence of prestretch amplitude on the mechanical efficiency was examined with 5 subjects, who performed 5 different series of vertical jumps, each of which differed with respect to the mechanics of the knee joint action during the prestretch (eccentric) phase of the contact on the floor. Electromyographic activity was recorded from the major extensor muscles during the entire work period of 1 min per series. In addition, expired air was collected during the test and recovery for determination of energy expenditure. Mechanical work was calculated from the vertical displacement of the body during the jumps. The results indicated that high net efficiency of 38.7% was observed in condition where amplitude of knee bending in eccentric phase was small. In large range motion the corresponding net efficiency was 30.1%. In jumps where no prestretching of extensor muscles ocurred the net efficiency was 19.7%. The high efficiency of small amplitude jumps was characterized by low myoelectrical activity of the leg extensor muscles during the positive (concentric) work phase. In addition, the small amplitude jumps had shorter transition time in the stretch-shortening cycle, high average eccentric force and high stretching speed. Therefore the results suggest that the restitution of elastic energy, which was also related to the length change and stiffness of the muscles during stretch, plays an important role in regulating the mechanical efficiency of work.     

Alterations of mechanical characteristics of human skeletal muscle during strength training

Summary To investigate the influence of strength training on the mechanical characteristics of human skeletal muscle, 14 male subjects went through training of combined heavy concentric and eccentric contractions three times a week for 16 weeks. The strength training program consisted mainly of dynamic exercises for leg extensors with loads of 80 to 120% of one maximum repetition. The force-time curves produced during various vertical jumps were the basis for calculation of various mechanical parameters. In addition to a great increase (p<0.001) in maximal isometric force, heavy resistance strength training also caused significant (p<0.05–0.01) increases in heights and in various mechanical parameters in positive work phases of vertical and drop jumps. The increase in positive force during a fast dynamic contraction was correlated (p<0.01) with the reduced time to produce a certain submaximal force level in isometric condition. No changes in the elastic properties of the muscle were observed as judged from the difference between the counter-movement and squat jumps. When the training was followed by the 8-week detraining period a great decrease (p<0.001) in maximal force took place, but only minor changes (ns) were observed in fast force production.Supported in part by the grants from The Finnish Olympic Committee and Central Sport Federation     

Potentiation of the mechanical behavior of the human skeletal muscle through prestretching

Force–velocity and power–velocity curves in a vertical jump involving movements around several joints were derived from vertical ground reaction forces and knee angular velocities. The jumps were performed with weights from 10 to 160 kg added on the shoulders. The obtained curves from a semi–squatting static starting position resembled those reported for isolated muscles or single muscle groups. Vertical jumps were also performed in the conditions where the shortening of the leg extensors was preceded by prestretching of the active muscles either through a preparatory counter–movement or dropping down on the force–platform from the various heights ranging from 20 to 100 cm. Prestretching modified through a range of velocities the force–velocity and power–velocity curves by increasing both the ground reaction forces and the calculated mechanical power. Thus the results are similar to those reported in isolated muscles. In studies with isolated muscle preparation the nervous connections have not been intact and therefore it is suggested that increase in the performance of the skeletal muscles through prestretching, in the conditions of the present study, was attributed to the combined effects of the utilization of stored elastic energy and the reflex potentiation of muscle activation.     

Gradual potentiation of isometric muscle force during constant electrical stimulation

An investigation was carried out into how stimulation frequency and stimulation history affect the potentiation of muscle force during 20s of constant stimulation of the two knee extensors in isometric conditions. Stimulation frequency significantly affected the potentiation pattern: low-frequency (2.5–10 Hz) stimulation showed a reduction and subsequent enhancement of force, and high-frequency (14.3–25 Hz) stimulation showed only enhancement of force. The degree of enhancement in force and time-to-peak decreased with the stimulation frequency. Whereas conditioning stimulation (both 40 Hz and 14.3 Hz) significantly enhanced the muscle force above 85%, following main stimulation (14.3 Hz) after short rest (10 s and 50 s, respectively) induced little force enhancement (below 8%). In particular, when the frequency of the conditioning stimulation was 14.3 Hz, the initial force at the main stimulation showed a very similar value to the final force value of the conditioning stimulation (above 90% similarity). The potentiated twitch force slowly decayed during rest, with an average time constant of 2.4 min. These observations indicate that muscle potentiation depends on the stimulation frequency and stimulation history, and therefore a computer model of potentiation can play an important role in predicting muscle force and body movement induced by electrical stimulation.     

Effects of previous muscle contractions on cyclic movement dynamics

Summary In addition to muscle elastic energy, enhancement of movement performance in a stretch-shortening cycle could also be due to an increase in initial muscle force during the stretching phase. This hypothesis was tested by examining 9 male physical education students during maximum voluntary knee extensions performed with and without previous knee flexion. In both conditions movements were performed with various external loads. In addition, the force-velocity curve (FVC) parameters of the knee extensor muscles were also determined. As simple model of a muscle impulse was constructed in order to select independent biomechanical variables relevant to movement dynamics. The experimental results demonstrated that previous knee flexion enhanced the maximum angular velocity of knee extension. This effect decreased with increasing movement duration (i.e. increased external load), as well as giving positive correlation coefficients between the magnitude of this effect and the rate of development of knee extensor tension. These results are discussed in relation to a model of the dynamics. It is shown that previous muscle contractions performed during braking in the negative movement phase might play an important role in enhancing performance in cyclic movements. This role would be especially important in transient contractions of primarily slow twitch fiber muscles.     

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

Summary The running economy of seventeen athletes was studied during running at a low speed (3.3 m · s^–1) on a motor-driven treadmill. The net energetic cost during running expressed in kJ·kg^–1·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<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<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.     

Ultrasonographic assessment of human skeletal muscle size

The measurement of human muscle size is essential when assessing the effects of training, disuse and ageing. The considered gold standard for cross-sectional area measurements of muscle size is magnetic resonance imaging (MRI). However, MRI is costly and often inaccessible. The aim of the present study was to test the reproducibility and validity of a more accessible alternative method using ultrasonography (ULT). We examined the cross-sectional areas in the vastus lateralis muscle of six individuals. Axial-plane ULT scans were taken at given levels along the entire muscle length. The ULT scanning was repeated on different days (reliability) and validated against MRI-based measurements. Mean intraclass correlation coefficients were 0.998 for the reliability of ULT and 0.999 for the validity of ULT against MRI. The coefficient of variation values for cross-sectional area measurements assessed by six different experimenters were 2.1% and 0.8% for images obtained with ULT and MRI, respectively. The ULT method is a valid and reliable alternative tool for assessing cross-sectional areas of large individual human muscles. The present findings justify the application of the ULT method for the detection of changes throughout large muscles in response to training, disuse or as a consequence of sarcopenia.     

Mechanical properties and myosin heavy chain isoform composition of skinned skeletal muscle fibres from a human biopsy sample

 Experiments were conducted to investigate the mechanics of contraction of chemically skinned muscle fibre segments of a biopsied sample of single human quadriceps muscle. Subsequently, the isoforms of the myosin heavy chain (MHC) were analysed by sodium dodecyl sulphate (SDS) gel electrophoresis.Of the 41 fibres, 26 contained MHCI (type I), 11 of the fibres contained MHCIIa (type IIA), and 4 of the fibres contained both MHCI and MHCIIa (of which MHCIIa was always slightly predominant (type IIC)). Distinct differences between fibre types were found in terms of the kinetics of force responses following stepwise length changes (order of velocity: IIA > IIC > I). The differences in maximal shortening velocity and in the kinetics of Ca²⁺-dependent activation were of the same order, but much less pronounced. Type I fibres had significantly greater fibre diameters than type IIA fibres. No significant differences were found among different fibre types in terms of isometric tension, resting sarcomere length or the length change needed to discharge the elasticity of maximally Ca²⁺-activated fibres (y _o value). The distribution of shortening velocity and kinetics of stretch activation values suggest that two muscle fibre subtypes may exist in human type I fibres. Received: 15 April 1997 / Received after revision 19 May 1997 / Accepted: 20 May 1997     

Stimulation frequency and force potentiation in the human adductor pollicis muscle

Summary The effect of stimulation frequency on twitch force potentiation was examined in the adductor pollicis muscle of ten normal subjects. The ulnar nerve was supramaximally stimulated at the wrist and isometric twitch force was measured from a 3-Hz train lasting 1 s. Test stimulation frequencies of 5, 10, 20, 25, 30, 40, 50 and 100 Hz were applied for 5 s each in random order (5 min apart) and the twitches (3 Hz) were applied immediately before and after (1 s) the test freuqency and at intervals up to 5 min afterwards (10 s, and 1, 2 and 5 min). Poststimulation twitches were expressed as a percentage of the prestimulation twitch. Low frequency fatigue was not induced by the protocol since the 20: 50 Hz ratio did not alter within each session. The degree of twitch potentiation was frequency dependent, with potentiation increasing up to 50 Hz [mean 173 (SD 16)%] but the effect was markedly less at 100 Hz [mean 133 (SD 25)%,P<0.011 for all subjects. The reduced potentiation at 100 Hz may have occurred due to high frequency fatigue produced by the 100-Hz test stimulation train. The optimal frequency of those examined in the experimental group was 50 Hz but this only produced maximal potentiation in six of the ten subjects and 100 Hz always produced less potentiation. These findings have implications for electrical stimulation of muscle in the clinical setting.     

Effect of brief ultrasound exposure on post-tetanic potentiation in cardiac muscle

The effect of a short 4 second ultrasound application (1.0 W/cm² SATA at 963 KHz) on the post-tetanic-potentiation of isolated isometrically contracting rat papillary muscle has been evaluated. Post tetanic-potentiation was produced in hypoxic isolated papillary muscle by interrupting the control stimulation rate of 6/minute with 10 stimulating pulses at 0.2 sec intervals for one cycle. Ultrasound application varied from one second prior to the stimulating train to a period covering the entire train. Ultrasound application just preceding and impinging upon the pulse train, enhanced post-tetanic-potentiation contractions. However, delay in ultrasound application and covering the stimulating pulse train, reduced post-tetanic-potentiation contractions. These data suggest that a window exists for the effects of ultrasound on contracting myocardium which may be used to probe critical events in the cardiac cycle.     

Myosin light chain phosphorylation and isometric twitch potentiation in intact human muscle

The effects of a single voluntary contraction of the quadriceps muscle group on phosphate incorporation into the phosphorylatable light chains (P-light chains) of fast and slow myosin isolated from the vastus lateralis muscle and potentiation of the electrically stimulated twitch tension was studied in intact human muscle. Twitch potentiation was maximal 20 s after the voluntary contraction. Thereafter, twitch potentiation declined, but was still significantly higher than pre-contraction values 2 min after the voluntary contraction. Phosphate incorporation into the P-light chain of fast myosin followed a similar time course to twitch potentiation, but no phosphate was incorporated into slow myosin P-light chains. These observations suggest that myosin light chain kinase activity is mainly associated with fast-twitch muscle fibers and, in agreement with previous studies, suggests that twitch potentiation associated with P-light chain phosphorylation is confined to the fast-twitch fibers of human muscle.     

Influence of aging on the mechanical behavior of leg extensor muscles

Summary Age dependence of the mechanical behavior of leg extensor muscle was investigated using vertical jumps with and without a stretch-shortening cycle on the force-platform. A total 226 subjects (113 females and 113 males) ranging in age from 4–73 years were examined. The results indicated in general that performance in males was better than that in females. This difference was reduced when body weight was taken into consideration. The peak performance of the various parameters, such as average force, height of rise of center of gravity, net impulse, and also the average power output, was reached in both sexes between the ages of 20 and 30 years. For example, the average vertical force in squatting had the following mean values in the various age groups of the male subjects: 114 N (4–6 years), 402 N (13–17), 618 N (18–28), 508 N (29–40), 435 N (41–49), 320 N (54–65), 315 N (71–73 years). When the jumps were performed using the stretch-shortening cycle, the potential of the mechanical performance after prestretching was also sensitive to aging in a similar manner. The results suggest that it is not only the performance of pure concentric contraction that is influenced by the maturation and aging processes but, the that elastic behavior of muscle and reflex potentiation are also affected by the same processes.Supported in part by a grant No. 8318/78/78 from the Ministry of Education, Finland     

Denervated frog skeletal muscle

The effects of denervation on several mechanical and electrical parameters of frog sartorius muscle have been investigated. In denervated muscles, there is no change in the resting potential and a relatively small change in the action potential. The first alteration in the action potential is a reduction of about 30% in the maximum rate of repolarization in muscles that have been denervated for 40 days or longer. Later, the overshoot and maximum rate of depolarization also decline. No tetrodotoxin resistant action potentials could be detected. Fibrillatory potentials were observed infrequently and in most cases in depolarized fibers. Twitch tension is significantly reduced by denervation while the tetanus tension is practically unaffected by denervation. The experiments suggest that the decline in twitch tension produced by denervation reflect a defect in some step of the excitation contraction coupling sequence. On the other hand, post-tetanic potentiation of the twitch is much larger in denervated than in control muscles. This potentiation in denervated muscles is paralleled by an increased action potential duration which returns to its pretetanic duration with a time course indistinguishable from that of the twitch potentiation.     

Short-range elasticity after tetanic stimulation in single muscle fibres of the frog

The time course of the stiffness during the relaxation period following tetanic stimulation was studied in isolated single muscle fibres of the frog. The stiffness was measured by subjecting the fibres to a sudden stretch at constant velocity from an initial sarcomere length of 2.2 μm, and related to the tension carried by the muscle fibre immediately before the stretch. Both stiffness and tension fell close to the resting level during the first second (4°C) after the end of stimulation. While stiffness and tension fell proportionally during the phase of almost exponential decline in tension following the shoulder in the tension recording, the relative drop in stiffness was less than the relative drop in tension during the preceding phase of almost linear decline in tension corresponding to the first 300–400 ms after the end of stimulation.