In vivo human triceps surae and quadriceps femoris muscle function in a squat jump and counter movement jump

An optic fibre method was used to measure in humans in vivo Achilles (ATF) and patellar tendon forces (PTF) during submaximal squat jumps (SJ) and counter movement jumps (CMJ). Normal two-legged jumps on a force plate and one-legged jumps on a sledge apparatus were made by four volunteers. Kinetics, kinematics, and muscle activity from seven muscles were recorded. The loading patterns of the tendomuscular system differed among the jumping conditions, but were similar when the jumping height was varied. Peak PTF were greater than ATF in each condition. In contrast to earlier simulation studies it was observed that tendomuscular force could continue to increase during the shortening of muscle-tendon unit in CMJ. The concentric tendomuscular output was related to the force at the end of the stretching phase while the enhancement of the output in CMJ compared to SJ could not be explained by increases in muscle activity. The stretching phase in CMJ was characterised by little or no electromyogram activity. Therefore, the role of active stretch in creating beneficial conditions for the utilisation of elastic energy in muscle was only minor in these submaximal performances. The modelling, as used in the present study, showed, however, that tendon underwent a stretch-shortening cycle, thus having potential for elastic energy storage and utilisation. In general, the interaction between muscle and tendon components may be organised in a manner that takes advantage of the basic properties of muscle at given submaximal and variable activity levels of normal human locomotion. Accepted: 28 June 2000     

In vivo human triceps surae and quadriceps femoris muscle function in a squat jump and counter movement jump

An optic fibre method was used to measure in humans in vivo Achilles (ATF) and patellar tendon forces (PTF) during submaximal squat jumps (SJ) and counter movement jumps (CMJ). Normal two-legged jumps on a force plate and one-legged jumps on a sledge apparatus were made by four volunteers. Kinetics, kinematics, and muscle activity from seven muscles were recorded. The loading patterns of the tendomuscular system differed among the jumping conditions, but were similar when the jumping height was varied. Peak PTF were greater than ATF in each condition. In contrast to earlier simulation studies it was observed that tendomuscular force could continue to increase during the shortening of muscle-tendon unit in CMJ. The concentric tendomuscular output was related to the force at the end of the stretching phase while the enhancement of the output in CMJ compared to SJ could not be explained by increases in muscle activity. The stretching phase in CMJ was characterised by little or no electromyogram activity. Therefore, the role of active stretch in creating beneficial conditions for the utilisation of elastic energy in muscle was only minor in these submaximal performances. The modelling, as used in the present study, showed, however, that tendon underwent a stretch-shortening cycle, thus having potential for elastic energy storage and utilisation. In general, the interaction between muscle and tendon components may be organised in a manner that takes advantage of the basic properties of muscle at given submaximal and variable activity levels of normal human locomotion.     

In vivo muscle mechanics during locomotion depend on movement amplitude and contraction intensity

The effects of movement amplitude and contraction intensity on triceps surae and quadriceps femoris muscle function were studied during repetitive hopping. In vivo forces from Achilles and patellar tendons were recorded with the optic fibre technique from eight volunteers. The performances were filmed (200 Hz) to determine changes in muscle-tendon unit length and velocity. When hopping with a small amplitude (23° knee flexion during the ground contact phase), the Achilles tendon was primarily loaded whereas patellar tendon forces were greater in large-amplitude hopping (56° knee flexion). In spite of the different magnitudes of stretch in the quadriceps femoris muscle, the stretching velocity and activity patterns of the quadriceps muscle were similar in both conditions. Simultaneously performed electromyographic (EMG) recordings revealed that preferential preactivation of the gastrocnemius muscle was evident in both jumping conditions. The triceps surae muscle was strongly active in the eccentric phase of small-amplitude hopping. Results from hopping with small knee-joint displacement suggest that there may be a particular frequency and jumping height at which the elastic bouncing is best utilized and at the same time the concentric phase is most economical. Results also support earlier observations that the economy of the shortening phase must be compromised at some point in order to produce more power and improve the jumping height. Electronic Publication     

Influences of tendon stiffness,joint stiffness,and electromyographic activity on jump performances using single joint

The present study aimed to examine the influences of tendon stiffness, joint stiffness, and electromyographic activity on jump performances consisting of a single-joint movement. Twenty-four men performed three kinds of unilateral maximal jump using only the ankle joint (squat jump: SJ; countermovement jump: CMJ; drop jump: DJ) on the sledge apparatus. The relative differences in the jump height of CMJ and DJ compared to SJ were defined as pre-stretch augmentation. During jumping exercises, electromyographic activities (mEMG) were recorded from the plantar flexors. Ankle joint stiffness was calculated as the change in the joint torque divided by the change in ankle joint angle during the eccentric phase of DJ. Achilles tendon stiffness was measured using ultrasonography during isometric plantar flexion. No significant correlations were found between joint stiffness and pre-stretch augmentation in both CMJ and DJ. On the contrary, tendon stiffness was significantly correlated with pre-stretch augmentation in both CMJ (r = −0.471) and DJ (r = −0.502). The relative mEMG value of CMJ (to that of SJ) during the concentric phase was significantly correlated with pre-stretch augmentation (r = 0.481), although this relationship was not found in DJ. These results suggested that (1) the greater jump height in CMJ could be explained by both the tendon elasticity and the increased activation level of muscle, (2) tendon elasticity played a more significant role in the enhancement of jump height during DJ, and (3) joint stiffness was not related to either pre-stretch augmentation or tendon stiffness.     

Comparison of force–velocity relationships of vastus lateralis muscle in isokinetic and in stretch‐shortening cycle exercises

Aim: This study investigated the force–velocity characteristics of the vastus lateralis (VL) muscle fascicle and muscle–tendon unit (MTU) in isolated lengthening and shortening actions, and during natural movement. Methods: Four subjects performed maximal eccentric and concentric knee extensions (60, 120 and 180° s^?1). Unilateral counter movement jumps and drop jumps in the sledge apparatus served as natural movements. Vastus lateralis fascicle lengths were determined from ultrasonography. In vivo patellar tendon forces (PTF) were measured with an optic fibre technique. Patellar tendon force was derived to VL force according to the cross‐sectional area of the muscle. Force in the direction of fascicle was calculated by dividing the VL force value by cosine of the fascicle angle. Force–velocity curves were constructed using angle specific values from isokinetic knee extensions (classical curve) and using instantaneous values from jumping exercises. Results: In the fascicle level, we did not find an enhanced muscle force in the jumping performances as compared with the classical force–velocity curve. In the muscle–tendon level, the instantaneous force at high muscle–tendon shortening speeds exceeded that extrapolated according to Hill's equation. Conclusion: This difference between fascicle and muscle–tendon behaviour suggests that the neural input in fast stretch‐shortening cycle exercises minimizes the length changes in muscle fascicle and enables storage and recoil of energy from elastic components that contributes to the enhanced mechanical output of the MTU during the push‐off phase.     

In vivo muscle fibre behaviour during counter-movement exercise in humans reveals a significant role for tendon elasticity

Six men performed a single ankle plantar flexion exercise in the supine position with the maximal effort with counter movement (CM, plantar flexion preceded by dorsiflexion) and without counter movement (NoCM, plantar flexion only) produced by a sliding table that controlled applied load to the ankle (40 % of the maximal voluntary force). The reaction force at the foot and ankle joint angle were measured using a force plate and a goniometer, respectively. From real-time ultrasonography of the gastrocnemius medialis muscle during the movement, the fascicle length was determined. The estimated peak force, average power, and work at the Achilles' tendon during the plantar flexion phase in CM were significantly greater than those in NoCM. In CM, in the dorsiflexion phase, fascicle length initially increased with little electromyographic activity, then remained constant while the whole muscle-tendon unit lengthened, before decreasing in the final plantar flexion phase. In NoCM, fascicle length decreased throughout the movement and the fascicle length at the onset of movement was longer than that of the corresponding phase in CM. It was concluded that during CM muscle fibres optimally work almost isometrically, by leaving the task of storing and releasing elastic energy for enhancing exercise performance to the tendon.     

Comparison of force-velocity relationships of vastus lateralis muscle in isokinetic and in stretch-shortening cycle exercises

AIM: This study investigated the force-velocity characteristics of the vastus lateralis (VL) muscle fascicle and muscle-tendon unit (MTU) in isolated lengthening and shortening actions, and during natural movement. METHODS: Four subjects performed maximal eccentric and concentric knee extensions (60, 120 and 180 degrees s-1). Unilateral counter movement jumps and drop jumps in the sledge apparatus served as natural movements. Vastus lateralis fascicle lengths were determined from ultrasonography. In vivo patellar tendon forces (PTF) were measured with an optic fibre technique. Patellar tendon force was derived to VL force according to the cross-sectional area of the muscle. Force in the direction of fascicle was calculated by dividing the VL force value by cosine of the fascicle angle. Force-velocity curves were constructed using angle specific values from isokinetic knee extensions (classical curve) and using instantaneous values from jumping exercises. RESULTS: In the fascicle level, we did not find an enhanced muscle force in the jumping performances as compared with the classical force-velocity curve. In the muscle-tendon level, the instantaneous force at high muscle-tendon shortening speeds exceeded that extrapolated according to Hill's equation. CONCLUSION: This difference between fascicle and muscle-tendon behaviour suggests that the neural input in fast stretch-shortening cycle exercises minimizes the length changes in muscle fascicle and enables storage and recoil of energy from elastic components that contributes to the enhanced mechanical output of the MTU during the push-off phase.     

Effects of isometric squat training on the tendon stiffness and jump performance

The present study aimed to investigate the effect of isometric squat training on human tendon stiffness and jump performances. Eight subjects completed 12 weeks (4 days/week) of isometric squat training, which consisted of bilateral leg extension at 70% of maximum voluntary contraction (MVC) for 15 s per set (10 sets/day). Before and after training, the elongations of the tendon–aponeurosis complex in the vastus lateralis muscle and patella tendon were directly measured using ultrasonography while the subjects performed ramp isometric knee extension up to MVC. The relationship between the estimated muscle force and tendon elongation was fitted to a linear regression, the slope of which was defined as stiffness. In addition, performances in two kinds of maximal vertical jumps, i.e. squatting (SJ) and counter-movement jumps (CMJ), were measured. The training significantly increased the volume (P<0.01) and MVC torque (P<0.01) of the quadriceps femoris muscle. The stiffness of the tendon–aponeurosis complex increased significantly from 51±22 (mean ± SD) to 59±24 N/mm (P=0.04), although that of the patella tendon did not change (P=0.48). The SJ height increased significantly after training (P=0.03), although the CMJ height did not (P=0.45). In addition, the relative difference in jump height between SJ and CMJ decreased significantly after training (P=0.02). These results suggest that isometric squat training changes the stiffness of human tendon–aponeurosis complex in knee extensors to act negatively on the effects of pre-stretch during stretch-shortening cycle exercises.     

Muscle–tendon interaction and EMG profiles of world class endurance runners during hopping

The present study examined the muscle–tendon interaction of ten international level Kenyan runners. Ultrasonography and kinematics were applied together with EMG recordings of lower limb muscles during repetitive hopping performed at maximal level. The ten Kenyans had longer gastro Achilles tendon at rest (p < 0.01) as compared with ten control subjects matched in height. Conversely, the stretching and shortening amplitudes of the tendinous tissues of the medial gastrocnemius (MG) muscle were significantly smaller in the Kenyans than in controls during the contact phase of hopping. This applied also to the fascicle length changes, which were smaller and more homogeneous among Kenyans. These limited musculo-tendinous changes resulted in higher maximal hopping height and in larger power despite their reduced body weight. The associated finding of a greater shortening to stretching ratio of the MG tendinous tissues during contact could imply that the Kenyan MG muscle–tendon unit is optimized to favor efficient storage and recoil of elastic energy, while operating at optimal muscle fascicle working range (plateau region).     

Mechano-elastic properties of human muscles at different temperatures.

The effect of changes in the muscle temperature on their ability to store elastic energy was studied by having 5 trained subjects perform maximal vertical jumps on a force platform, with and without counter movement, at muscle temperatures between about 32 degrees C and 37 degrees C. The results showed that the heights of vertical jumps were considerably reduced at lowered temperature, but the gain in height after a counter movement in the form of a jump down from a height of 0.4 m over the force platform, was significantly higher in the cold condition. T o test whether this was due to an increased stiffness of the muscles, experiments with imposed sinusoidal length variations at 14 Hz were performed. Delta force XDelta length-1 (i.e.stiffness) increased with isometric tension independent of muscle temperature. Experiments in which the rate of tension development and relaxation in voluntary maximal isometric contractions were measured at different muscle temperatures showed that maximal isometric tension changed by less than 1% per degree but the rate of tension development and relaxation by 3-5% and 5% per degree, respectively, in the temperature range studied (30 degrees to 40 degrees). These data may be explained by the hypothesis that the series elastic components of the active muscle are located in the cross-bridges between myosin and actin filaments. The storage of elastic energy would be enhanced if the rate of breaking of these bridges were decreased at lower temperatures.     

The relationship between muscle kinetic parameters and kinematic variables in a complex movement

Summary Kinematic variables of the vertical jump (jumping height, jump phase durations and joint angles) were measured on 39 male physical education students. In addition, kinetic parameters of the hip and knee extensors, and of the plantar flexors (maxima voluntary force and its rate of development) were recorded on the same subjects, in isometric conditions. The results demonstrated significant positive correlations between kinetic parameters of the active muscle groups and jumping height (r=0.217−0.464). The dominant effect on these correlations was due to the knee extensors. Correlations between these parameters and the duration of the jump phases were much weaker. Correlation coefficients between kinetic parameters and limb angles in the lowest body position showed that fast force production in one muscle group was related to a significant decrease in the joint angles of distant body segments. Multiple correlation coefficients between leg extensor parameters and kinematic variables (ranging between 0.256 for the duration of the counter-movement phase and 0.616 for jump height) suggested that kinetic parameters could explain more than a quarter of the variability of this complex human movement. Therefore, the conclusion was drawn that an extended set of measurements of the relevant musculo-skeletal system parameters could predict a considerable amount of the variability of human movement. However, high correlation coefficients between the same kinetic parameters of different muscle groups suggest that not all active muscle groups have to be included in the measurements.     

基于生物力学有限元法的足跟痛发生与康复机制

目的 研究足跟痛病症的发生与康复机制,为临床上足跟痛治疗方法的有效性提供理论依据。方法 对足跟痛患者足膝部CT、MRI影像数据进行三维重建,建立患者足膝部的骨骼-肌肉复合有限元模型。基于所建模型采用有限元方法,仿真模拟小腿肌肉挛缩对足踝部生物力学性能的影响。结果 在小腿肌肉提升力的作用下,足底压力从足跟区向足掌区转移,且不同肌肉作用力组合方案对压力分布不产生明显差异。足底筋膜张力升高,跟骨表面产生应力集中。在240 N作用力下,跟腱附着位置和跟骨结节处产生应力峰值,分别高达10.82、11.2 MPa。结论 小腿部肌肉和跟腱中产生应力集中,会导致足踝部生物力学特性发生变化,引发足跟部疼痛。释放集中应力恢复踝关节中各骨骼和关节的位置,从而改善整体生物力学环境的方法是治疗足跟痛的康复机制。     

Mechano-Elastic Properties of Human Muscles at Different Temperatures

The effect of changes in the muscle temperature on their ability to store elastic energy was studied by having 5 trained subjects perform maximal vertical jumps on a force platform, with and without counter movement, at muscle temperatures between about 32°C and 37°C. The results showed that the heights of vertical jumps were considerably reduced at lowered temperature, but the gain in height after a counter movement in the form of a jump down from a height of 0.4 m over the force platform, was significantly higher in the cold condition. To test whether this was due to an increased stiffness of the muscles, experiments with imposed sinusoidal length variations at 14 Hz were performed. Δforce ×Δlength^-1 (i.e. stiffness) increased with isometric tension independent of muscle temperature. Experiments in which the rate of tension development and relaxation in voluntary maximal isometric contractions were measured at different muscle temperatures showed that maximal isometric tension changed by less than 1 % per degree but the rate of tension development and relaxation by 3–5 % and 5 % per degree, respectively, in the temperature range studied (30° to 40°). These data may be explained by the hypothesis that the series elastic components of the active muscle are located in the cross-bridges between myosin and actin filaments. The storage of elastic energy would be enhanced if the rate of breaking of these bridges were decreased at lower temperatures.     

Jump training with different loads: effects on jumping performance and power output

Purpose

To investigate the selective effects of different types of external loads applied in vertical jump training on both the performance and muscle power output of the squat (SJ) and countermovement jump (CMJ).

Methods

Physically active males practiced maximum unconstrained vertical jumps over an 8-week period with no load, with either a negative or positive load exerted by a nearly constant external force that altered their body weight, and with a loaded vest that increased both the body weight and inertia. The magnitude of all applied loads corresponded to 30 % of body weight.

Results

A similar training-associated increase in jump height was observed in all experimental groups in both CMJ (7.4–11.8 %) and SJ (6.4–14.1 %). The relative increase in power output was comparable to the increase in jump height in SJ (7.4–11.5 %), while the power increase in CMJ was relatively small and load-specific (0.5–9.5 %). The observed differences could originate from the changes in the CMJ pattern, reflected through the depth of the counter movement that particularly increased after the training with negative load (42 %) and no load (21 %). The same participants also revealed increased CMJ duration, reduced ground reaction forces, as well as reduced maximum and average power output when compared with other training groups.

Conclusion

Jump training with the applied loads could lead to a comparable improvement in jumping performance. However, the observed load-specific adaptations of CMJ pattern could decouple the training-associated increase in jump height from the increase in muscle power output.     

Differential strain patterns of the human gastrocnemius aponeurosis and free tendon,in vivo

AIM: The mechanical characteristics of the human free tendon and aponeurosis, in vivo, remains largely unknown. The present study evaluated the longitudinal displacement of the separate free Achilles tendon and distal (deep) aponeurosis of the medial gastrocnemius muscle during voluntary isometric contraction. METHODS: Ultrasonography-obtained displacement of the free tendon and tendon-aponeurosis complex, electromyography of the gastrocnemius, soleus, and dorsiflexor muscles, and joint angular rotation were recorded during isometric plantarflexion (n = 5). Tendon cross-sectional area, moment arm and segment lengths (L(o)) were measured using magnetic resonance imaging. Tendon force was calculated from joint moments and tendon moment arm, and stress was obtained by dividing force by cross-sectional area. The difference between the free tendon and tendon-aponeurosis complex deformation yielded separate distal aponeurosis deformation. Longitudinal aponeurosis and tendon strain were obtained from the deformations normalized to segment lengths. RESULTS: At a common tendon force of 2641 +/- 306 N, the respective deformation and Lo were 5.85 +/- 0.85 and 74 +/- 0.8 mm for the free tendon and 2.12 +/- 0.64 and 145 +/- 1.3 mm for the distal aponeurosis, P < 0.05. Longitudinal strain was 8.0 +/- 1.2% for the tendon and 1.4 +/- 0.4% for the aponeurosis, P < 0.01. Stiffness and stored energy was 759 +/- 132 N mm(-1) and 6.14 +/- 1.89 J, respectively, for the free tendon. Cross-sectional area of the Achilles tendon was 73 +/- 4 mm2, yielding a stress of 36.5 +/- 4.6 MPa and Young's modulus of 788 +/- 181 MPa. CONCLUSION: The free Achilles tendon demonstrates greater strain compared with that of the distal (deep) aponeurosis during voluntary isometric contraction, which suggests that separate functional roles may exist during in vivo force transmission.     

Strong relationships exist between muscle volume, joint power and whole-body external mechanical power in adults and children

The present study investigated whether differences between adults and children in mechanical power during single-joint knee extension tasks and the complex multijoint task of jumping could be explained by differences in the quadriceps femoris muscle volume. Peak power was calculated during squat jumps, from the integral of the vertical force measured by a force plate, and during concentric knee extensions at 30, 90, 120, 180 and 240 deg·s⁻¹, and muscle volume was measured from magnetic resonance images for 10 men, 10 women, 10 prepubertal boys and 10 prepubertal girls. Peak power during jumping and isokinetic knee extension was significantly higher in men than in women, and in both adult groups compared with children ( P < 0.01), although there were no differences between boys and girls. When power was normalized to muscle volume, the intergroup differences ceased to exist for both tasks. Peak power correlated significantly with quadriceps volume ( P < 0.01), with r ² values of 0.8, 0.86, 0.81, 0.78 and 0.81 from isokinetic knee extension at angular velocities of 30, 90, 120, 180 and 240 deg·s⁻¹, respectively, and with an r ² value of 0.9 from squat jumps. These results indicate that the quadriceps femoris muscle volume accounts largely for the increase in power that occurs with maturation in the two genders not only in kinematically constrained knee extensions but also in multijoint tasks. Future studies should examine the role of other factors relating to the generation and transmission of contractile power, such as muscle architecture, tendon stiffness and external mechanical leverage.     

Mechanics of human triceps surae muscle in walking, running and jumping

Length changes of the muscle-tendon complex (MTC) during activity are in part the result of length changes of the active muscle fibres, the contractile component (CC), and also in part the result of stretch of elastic structures [series-elastic component (SEC)]. We used a force platform and kinematic measurements to determine force and length of the human calf muscle during walking, running and squat jumping. The force-length relation of the SEC was determined in dynamometer experiments on the same four subjects. Length of the CC was calculated as total muscle-tendon length minus the force dependent length of the SEC. The measured relations between force and length or velocity were compared with the individually determined force-length and force-velocity relations of the CC. In walking or running the negative work performed in the eccentric phase was completely stored as elastic energy. This elastic energy was released in the concentric phase, at speeds well exceeding the maximum shortening speed predicted by the Hill force-velocity relation. Speed of the CC, in contrast, was positive and low, well within the range predicted by the measured force-velocity properties and compatible with a favourable muscular efficiency. These effects were also present in purely concentric contractions, like the squatted jump. Contractile component length usually started at the far end of the force-length relation. Inter-individual differences in series-elastic stiffness were reflected in the force and length recordings during natural activity.     

Seasonal changes in leg strength and vertical jump ability in internationally competing ski jumpers

The aim of the present study was to investigate the effects of the seasonal changes in heavy strength training on maximal strength and vertical jump ability in internationally competing ski jumpers. A repeated-measures design was used to follow-up the changes in strength, vertical jump capacity, and neuromuscular efficiency (expressed as the ratio between squat jump height and the relative isometric force) in the ski jumpers. Measurements were performed in November (pre), January (middle of the competition season), and in March (end of the competition season). The weekly number of strength training sessions, absolute, and relative peak isometric squat force was significantly reduced during the competition period (p < 0.05). The body mass was reduced from pre-season to the middle of the competition season and remained at this level at the end of the competition season (p < 0.05). Squat jump height remained unchanged from pre-season until the end of the competition season (p < 0.05). Neuromuscular efficiency increased from pre-season until the end of the competition season (p < 0.05). The present study shows that maximal strength and body weight is reduced from pre-season to the end of the competitive season in internationally competing ski jumpers. The vertical jump ability did not change from pre-season to the end of the competitive season, while the neuromuscular efficiency increased during the competitive season. These findings indicate that coaches and athletes should emphasize adequate nutritional strategies and to apply a larger focus on strength maintenance training during the competitive season to maximize ski jump performance.     

Changes in height of jump,maximal voluntary contraction force and low‐frequency fatigue after 100 intermittent or continuous jumps with maximal intensity

Healthy untrained males (age 25.4 ± 1.7 years, n=12) gave their informed consent to take part in all experiments within the study. After 100 intermittent (every 20 s) drop jumps from the height of 0.4 m, jumps with counter‐movement to 90° angle in the knee and immediate maximal rebound (eccentric–concentric exercise; E–C) and 100 continuous jumps (five bouts of 20 jumps with counter‐movement to 90° angle in the knee with 10 s between bouts) (maximal exercise; M) with maximal intensity, the height of vertical jump decreased in a similar way, and this decrease did not depend on the performance mode of jumps. After E–C and M jumping exercises, there was a significant (P < 0.001) decrease in maximal voluntary contraction force, as well in the force generated by electrical stimulation at all stimulation frequencies, and these values were not restored to the initial level even after 24 h. After the E–C exercise, however, the muscle contraction force generated at different stimulation frequencies and, especially, at low‐stimulation frequencies (1–20 Hz) decreased to a significantly (P < 0.05–0.001) greater extent than after M exercise. Twenty minutes after the end of M exercise, there was still a greater increase in low‐frequency fatigue (LFF) and it was no different from the LFF registered 20 min after the end of E–C exercise. Twenty‐four hours after the M exercise, however, LFF was smaller than its respective value after E–C exercise. There was no significant relationship between the values of LFF after E–C and M exercises. This may indicate that there are differences in the origin of the LFF after the E–C and M exercises.     

Prestretch potentiation of human skeletal muscle during ballistic movement

The conditions associated prior to and during the transition from prestretch to shortering may have considerable influence on the final performance of muscle. In the present study male subjects of good physical condition performed vertical jumps on the force-platform with and without preliminary counter movement. In the counter movement jump (CMJ) the amplitude of the knee bending, velocity of the prestretch and the force attained at end of prestretch were the primary parameters of interest. In addition the coupling time indicating the transition from the eccentric (prestretch) phase to the concentric phase was recorded from the angular displacement and reaction force curves. In the final calculation the mechanical performance parameters of CMJ were always compared with those of the jumps performed without counter movement. The results indicated in general first that CMJ enhanced the average concentric force and average mechanical power by 423 N (66%) and 1158 W (81%), respectively. This potentiation effect was the higher the higher was the force at end of prestretch (p<0.001). Similarly, the prestretch speed (p<0.001) and short coupling time (p<0.01) were associated with enhanced performance during the concentric phase. The average coupling time was 23 ms. The results are interpreted through changes in the prestretch conditions to modify the acto-myosin cross-bridge formation so that the storage and utilization of elastic energy is associated with high prestretch speed, high eccentric force and short coupling time. The role of the reflex potentiation is also suggested as additional enhancement of the final performance.