The basic criterion of efficient sprinting velocity is developing the highest possible ground reaction force in the shortest time possible during the contact phase of single running stride (22,28,29). Contact time in the elite sprinters equals 80–95 milliseconds with ground reaction force exceeding 3–4 times the body weight of the athletes. Movement structures in sprinting and plyometric exercises are very similar in relation to muscular contractions. Development of force is a result of connection between eccentric and concentric muscular contractions. Sprinting in its natural movement structure comprises of the series of jumps from one leg to another. In turn, plyometric exercises are very specific for sprinting action in terms of both external movement structure and time of execution. Transition from eccentric to concentric contraction, stretch-shortening cycle (SSC), needs to be as short as possible (19). Therefore, plyometric exercises exaggerate the SSC.
In SSC, the eccentric-concentric cycle consists of muscle stretching because of external force and muscle shortening in the second phase (18). If the concentric phase of contraction follows an eccentric phase quickly enough, the elastic components release the accumulated energy in kinetic and mechanical work at the beginning of the concentric phase, thereby increasing the muscular force (34) and their benefits include increased recruitment of the fast-twitch fibers and greater elastic energy return from the tendons. In addition, the tendons are able to store and release more elastic energy so that the Achilles and knee tendons can contribute greater mechanical power to sprinting speed. Muscles and tendons are 2 springs in series (13), and in the translation of energy expenditure to ground forces during running more energy is stored in the more compliant spring (11). This is particularly valid in sprint running, where the time of contact phase in sprinter's stride is very limited. In turn, Arampatzis et al. (1) found that leg stiffness increased with increasing running speed and Farley et al. (12) found that stiffness of the leg spring can change as much as 2-fold to accommodate different hopping frequencies.
Recently, we reported increased interest in muscle and tendons stiffness and how it relates to power production or, more precisely, how stiffness related to some variables of vertical jumps as a classical representation of plyometric exercises. According to Dumke et al. (11), both tendon and muscles properties may be very important in transfer of elastic energy in running. As stiffness increases, less muscle activation is required, and therefore energy expenditure is spared. The contribution of the elastic properties of the muscle-tendon complex depends on the velocity of the transition. The transition must be as fast as possible and should not exceed an interval about 260 milliseconds. From a mechanical standpoint, the stiffness of musculotendinous system is closely related to the ground contact time during the landing and the knee angle during the amortization phase. Lower ground contact time means more stiffness during amortization and lower angle change in the knee.
The findings in the literature regarding the effect of vertical jumps on the sprinting are quite substantial but are not consistent. The classical plyometrics exercises such as countermovement (CMJ) and drop jump (DJ) recently have been well researched and shown to increase performance in jumping and short sprinting (10, 20, or 40 m). Namely, revealed high correlations between these 2 types of jumps with the results of starting acceleration (4,7,17,24,27,28,30,32,39) and 100-m sprinting speed (16,17,38). However, little scientific information is available, which directly compares the effectiveness of CMJ with DJ and their impact on the diversity of the results in men's 60- and 100-m sprint. Therefore, the purpose of this investigation was to examine the relationship between selected variables of lower extremities explosive power measure via CMJ and DJ and the sprinting ability over 60- and 100-m dash in elite and subelite sprinters. It was hypothesized that better results in 60- and 100-m dash are accomplished due a greater ability to use the impact energy produced because elite sprinters eccentrically absorb the force of landing during CMJ and DJ, and therefore the stiffness of the musculotendinous system may better determine the ability to store and use such energy.
Experimental Approach to the Problem
Testing was carried out in a biomechanical laboratory of the Polyclinic for Physical Medicine and Rehabilitation (PEHAREC) in Pula, Croatia, on a special constructed spot on which 2 force plates (Kistler 9286A; Kistler Instrumente AG, Winterthur, Switzerland) were installed. All subjects were familiar with the testing procedures and jumping exercises, which they were performing as a part of their sprint training routine. The test was carried out at the completion of preseason training program in the weekly microcycle to correspond with the explosive strength and jumping training. The experiment was conducted in the morning, which corresponds with regular sprinters' training routine. The sprinters were instructed to be adequately hydrated, fed, and first of all well rested, before the testing day. Before the experiment, subjects were allowed to perform a 15-minute warm-up, including light jogging, stretching, and light form of jumping exercises. During the experiment, each subject performed 5 maximum effort trials of 2 different jumps: CMJ and DJ with rebound. Sixty to 90 seconds of rest time was allowed between each jump, thus allowing appropriate regeneration. The order in which the 2 conditions of jump were executed was standardized. First, athletes performed a CMJ: subjects started from an upright standing position then dynamically reached semisquat position with approximately 90° (countermovement) knee angle and executed an immediate vertical take off, with the back as straight as possible. The subjects executed the jumps as high as possible. Jumps were executed without arm movement, as the arms were held on the hips (Figure 1).
The second protocol consisted of the execution of DJ with the rebound. From a 45-cm high box, the athlete dropped down and landed on a force plate with flexed knees to semisquatted position. Next, as quickly as possible, the athlete rebounded upward in a maximal vertical jump with hands on the hips (Figure 2). The importance of landing on both the feet and a simultaneous takeoff from 2 legs during vertical rebound was emphasized to the subjects. Vertical jump height was measured using the vertical jump protocols developed by Bosco, which have 0.93 and 0.97, for the reliability and specificity, respectively.
This experiment involved 12 national-level Slovenian sprinters (age 22.4 ± 3.4 years, body height 177.6 ± 6.9 cm, and body weight 74.9 ± 5.2 kg). Average of the best results in 60-m sprint was 6.93 ± 0.12 seconds (best result 6.65 seconds); average of the best results in 100-m sprint was 10.82 ± 0.25 seconds (best result 10.39 seconds). The subjects have 5 years of active training and competition in sprinting (60, 100, and 200 m) before the experiment. According to the goal of this study, sprinters were divided into 2 groups: elite and subelite sprinters. Criteria for grouping all sprinters were the results from official competition in 60- and 100-m sprint. The basic characteristics of the 2 groups are shown in Table 1. Before study, approval by the Human Ethics Committee of the University of Ljubljana was obtained for this experiment. All subjects were notified about the risks associated with the participation in this experiment, the purpose of experiment, and then measuring procedures, and all signed an informed consent document before any testing.
Measurement of Jumping Kinematics and Kinetics Variables
A system of 9 CCD cameras (BTS Smart-D; BTS Bioengineering, Padua, Italy) with a frequency of 200 Hz and resolution 768 × 576 pixels was used to carry out a three-dimentsional kinematic analysis of vertical jumps. The BTS SMART Suite (Bioengineering) program was used to analyze kinematic parameters. The kinematic model was defined with a system of 17 markers, sensitive to the infrared light (head, shoulders, forearm, upper arm, torso, hips, thigh, calf, and foot—36). Validity of the model was tested with a walking sequence in sagittal and frontal planes. On the basis of kinematic analysis, the following parameters of vertical and depth jumps were calculated: height of takeoff, flight time, duration of take off phase, duration of eccentric phase, duration of concentric phase, velocity of takeoff, and angle in ankle, knee, and hip joints.
Vertical ground reaction force and other dynamic variables, impulse of force in eccentric and concentric phases, during 2 vertical jumps were recorded using 2 separate force platforms (600 × 400, Type 9286A; Kistler Instrumente AG) at a sampling rate of 800 Hz. Ground reaction force was measured unilaterally and bilaterally. Force was further normalized according to the body weight of the measured subjects (N·kg−1). The same force plate was mounted on the floor below the box with the subject's feet on it during the execution of DJ.
Mean values and SDs were determined for all dependent and independent variables. The differences between the 2 groups of sprinters regarding CMJ and DJ test variables were examined with a repeated-measures analysis of variance. Statistical significance was set at p < 0.05. Data were statistically analyzed with the use of SPSS for Windows 15.0 program (Chicago, IL, USA).
Results in the Table 1 reveal that the elite sprinters are slightly older with a higher body weight and height, and with statistically significantly better results in 60- and 100-m sprint running. Tables 2 and 3 present mean values and SDs of variables for the CMJ and 45-cm DJ. In both jumps, the 2 groups of sprinters statistically significantly differentiated into 6 parameters. The height of the CMJ was 65.4 cm in the elite sprinters and only 57.5 in the subelite sprinters, with a statistically significant difference of 7.8 cm. In DJ, the difference in the height of jump between the 2 groups amounted to 8.7 cm. Important differences between the groups have also been noticed in the vertical takeoff velocity for both CMJs and DJs. Furthermore, the velocity of body center of gravity (BCG) in the eccentric phase of the DJ importantly discriminated elite sprinters from subelite sprinters.
The purpose of this investigation was to examine the relationship between the selected variables of lower-extremity explosive power measure via CMJ and DJ and sprinting ability over 60- and 100-m dash in elite and subelite sprinters.
Some studies indicated that the CMJ revealed a high correlation with some phases of sprint running (9,16,21,25,28,30,38). Natural movements, such as vertical jump and sprint running, consist of a combination of eccentric-concentric contraction with the only difference being the duration of contractions, which in sprinting lasts between 85 and 100 milliseconds, whereas in the CMJ, it lasts approximately 400 milliseconds (2,3). In other classifications, the jumps have been further divided into slow and fast SSC performance: the CMJ, a measure of slow (>250 milliseconds) SSC performance, and the DJ, a measure of fast (<250 milliseconds) performance (16). Better sprinters have slightly longer cumulative contact time and longer time of both eccentric and concentric phases. The answer can be found in the ratio between the force and speed, where at higher speed, a smaller ground reaction force can be produced. Efficiency of vertical jump does not only depend on the production of force but also on the time of force production. A product of these 2 parameters defines an impulse of force, which is one of the most important factors for achieving maximal height of jumps (19). Looking at the force impulse, higher values can be noticed in the category of better sprinters in both absolute impulse of force and force impulse in eccentric and concentric phases of the CMJ. Bobbert et al. (4) have found that the time of production of force is an important factor in defining a mechanical effect in complex multijoint movements, such as vertical jump. Similar conclusions can also be found in Voigt et al. (37), stating that the optimal strategy of CMJ is based on a slower eccentric phase (pre-stretch phase).
The efficiency of vertical jump from an anatomically functional aspect depends on the transformation of rotational into translatory movement. In the transfer of rotational kinetic energy into translatory movement, the 2 joint (biarticular) muscles of the thigh (hamstring or ischiocrural muscles) play an important role (5). This group of muscles consists of m. semimembranosus, m. semitendinosus, and m. biceps femoris. These muscles primarily perform extension of the hip joint in a closed kinetic chain and the flexion of knee joint. Length of the 2 joint thigh muscles varies significantly and depends mainly on the position of knee and hip joints. The efficiency of these muscles transpires mostly in high angular velocities of joints in lower extremities. The activity of thigh muscles is also important in vertical jumps, in conditions of concentric or eccentric-concentric muscular effort, because of the intersegmentary transfer of energy and the optimization of takeoff action (5,6). In vertical jumps, muscles join in the takeoff action according to the proximal-distal principle of muscular activation. In the first phase of the jump, when vertical velocity of BCG increases, mostly the extensors of torso and hips are active. Key factors in this phase of takeoff action is m. gluteus maximus, which develops high force because of relatively low angular velocity of hips. Thigh muscles achieve the highest level of activation at the beginning of hip extension (8). In continuance of takeoff action, the knee extensors join in resulting in a transfer of energy from hip onto the knee via m. rectus femoris. The last phase of takeoff is concluded with the 2 joint calf muscles (m. gastrocnemius).
A key criterion of an efficient transformation of rotational movement into translatory movement is a vertical velocity of BCG at the time of takeoff. In this parameter, statistically significant differences have been noticed between the 2 groups of sprinters. Difference in vertical velocity was 0.29 m·s−1, leading to the conclusion that better sprinters use a proximal-distal principle of muscular chain in vertical jumps in a more optimal way. The same principle is also important in sprint running in the phase of running stride, where the force is generated by the extensors of torso, hips, knees, and the plantar flexors of ankle joint. In a 45-cm DJ, statistically significant differences between the groups of elite and subelite sprinters were revealed in 3 parameters: height of jump, velocity of BCG in eccentric phase, and velocity of BCG in concentric phases. Previous studies indicated a high correlation between the DJ and sprinting speed (23,33). A high correlation between DJs and starting acceleration over 10 m have been found by Mero et al. (28), Rimmer and Sleivert (32), Marković (23), and Maulder et al. (25). Neuromuscular mechanisms in the execution of DJ and sprinting strides are very similar. Faster stretching of muscular-tendon complex, shorter time, and the amplitude of movement all result in higher amount of elastic energy. It is known that muscular-tendon complex (Achilles tendon, m. gastrocnemius medialis, gastrocnemius lateralis, and m. soleus), in conditions of a higher velocity of eccentric-concentric cycle, stores higher amount of kinetic energy in a form of elastic energy (6,17). Dalleau et al. (10) claimed that the energy cost of running was significantly related to the stiffness of the propulsive leg and that when apparent stiffness of the subject decreased, the energy cost of running subsequently decreased. Therefore, overall leg and musculotendinous unit stiffness may be considered an important component of running economy (10,20,35).
Generation of elastic energy also means shorter contact times, which is a decisive factor in sprinting. If the time of contact with the surface is longer, a part of absorbed kinetic energy is transformed into chemical energy—heat (12). In comparison with the group of subelite sprinters, sprinters from the elite group have shorter cumulative duration of contact phase (elites = 160.4 milliseconds and subelites = 171.2 milliseconds) and shorter duration of eccentric phase in the 45-cm DJ; however, the difference is not statistically significant. According to some study, the key mechanism to shorten the contact time in conditions of eccentric-concentric cycle, SSC (12), is an efficient preactivation of agonists and synergists of ankle joint (m. gastrocnemius lateralis, m. gastrocnemius medialis, m. soleus, and m. tibialis). Preactivation starts 100 milliseconds before the contact of foot with the ground (15). Agonists and synergists provide increased stiffness of ankle joint, regulated by the central motor program (joint stiffness regulation), which controls and synchronizes the work of flexors and extensors in ankle before contact with the ground (15,31). Young (38) has found that in sprinters, the training of DJs significantly shortens contact times and improves the height of jumps. A short contact phase is one of the most important factors in sprint running, both from the point of view of higher frequency and the velocity of takeoff in sprinting stride. In powerful motor structures, such as sprint running, the time available for generation of force is one of the most important limiting factors.
There is 1 significant difference between the sprint running and DJ test evaluating elastic strength. From the biomechanical point of view, sprinting represents alternate activity of left and right leg, i.e., a unilateral activity. According to Mero et al. (28), realization of strength in sprint running considerably depends on intramuscular and intermuscular coordination. Vertical jumps are a typical example of bilateral activity. Nevertheless, similarity between these 2 activities exists particularly from the aspect of ground reaction force. In the phase of maximal sprinting velocity, the vertical ground reaction force amounts to 1,300–1,600 N (20) on each leg. The sum of the ground reaction force on both legs is thus between 2,600 and 3,200 N. In DJ, elite sprinters achieve in average a bilateral ground reaction force 2,984 and subelite even 3,132 N. Unilateral ground reaction force amounts in elite sprinters is 1,492 N and in subelite sprinters is up to 1,566 N. Similarly, the impulse of force in eccentric phase of jump is in average higher in the group of subelite sprinters compared with the group of elite sprinters (elite 149.18 N·s and subelite 156.03 N·s). Apparently, subelite sprinters, despite a higher ground reaction force, are not capable of realizing higher jumps than the elite sprinters. Elite sprinters in average achieved 8.7 cm higher vertical jumps after the 45-cm DJ than the subelite sprinters. According to the kinematic parameters (duration of takeoff and duration of the eccentric and concentric phase) and dynamic parameters (maximal force reaction and impulse of force in the eccentric and concentric phase), it can be concluded that elite sprinters use a strategy of jumping with a fast eccentric-concentric cycle, whereas the subelite sprinters use a strategy of slow eccentric-concentric cycle.
Only a quick transformation of eccentric into a concentric contraction while using a stretch reflex enables an efficient transfer of elastic energy from the first into the second phase of takeoff action. In the pre-stretch phase of elongation of muscles and tendons, the larger part of elastic energy is stored in serial elastic muscle elements (aponeurosis, tendon, cross-bridges) and smaller part in parallel elastic elements (muscular fascia, connective tissue, sarcolemma). This energy is released in concentric phase together with a chemical energy of a muscle. A part of elastic energy is available only for 15–120 milliseconds, which is a lifetime of cross-bridges (19). The speed of eccentric-concentric cycle in elite sprinters is mostly a result of statistically significantly higher speed of BCG in the amortization of jump phase and the extension of jump phase. At a time of leaving the ground, the average vertical velocity of elite sprinters is 0.31 m·s−1 higher in comparison to subelite sprinters. Arampatzis et al. (1) found that leg stiffness increased with increasing running speed, and Farley et al. (12) found that stiffness of the leg spring can change as much as twofold to accommodate different hopping frequencies. Spurrs et al. (35) proposed that because of the increases in stiffness derived from plyometric training, subject would achieve greater propulsion for the same or less energy cost, thereby improving economy and consequently running performance.
The DJ is a complex multijoint movement, where intermuscular coordination particularly of agonists and synergists is of great importance and has been revealed as an important diagnostic instrument of result prediction for sprint running. Bissas and Havenetidis (3) claimed that the sprint ability is linked with DJ performance, especially the DJ from a height of 30 cm. It is suggested that the above tests may prove useful in preparing and testing the sprint ability and sprint-specific strength levels. Mehmet et al. (26) in their investigation concluded that the DJ height is demonstrated to be a more effective way to reflect Vo2max during sprint running than the other vertical and horizontal jumps tests at the beginning of the preparation phase.
In summary, the statistically significant (p < 0.05) differences between the sprinters of both groups were revealed in 6 kinematic and dynamic parameters. In CMJ, the differences between the groups of sprinters were revealed in parameters (height: elites = 65.39 cm, subelites = 57.55 cm; take off velocity: elites = 3.23 m·s−1, subelites = 2.94 m·s−1) height of the jump, vertical velocity of BCG, and the impulse of force in the concentric phase of the jump (concentric impulse: elites = 123.91 N·s; subelites = 108.06 N·s). In DJ, the elite and subelite sprinters differentiated in the realization of movement velocity in the eccentric and concentric phases (take off velocity: elites = 3.18 m·s−1, subelites = 2.87 m·s−1; eccentric velocity: elites = 3.05 m·s−1, subelites = 2.81 m·s−1). Elite sprinters better use the stretch reflex, which allows them to more efficiently transfer elastic energy from the first into the second phase of take off action.
This investigation provides evidence that vertical jumps and DJs are very important tools to meet the demands in sprint training according eccentric-concentric muscular work in lower extremities components. Therefore, the jumping ability can attribute to enhance the SSC in sprint performance by better utilization of stored elastic energy. In jumping activities, the DJ showed better quality than CMJ in the neuromuscular specificity. Furthermore, this investigation determined that jumps are a reliable and objective measuring instrument for diagnosing and planning of training processes in the area of power development. Because of the relatively small sample of measured subjects, the results of the present study need to be considered with some degree of scientific caution; however, the present investigation supports previous plyometric investigations.
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Keywords:Copyright © 2013 by the National Strength & Conditioning Association.
track and field; training; sprint running; diagnostic; plyometric; elastic energy