Jumping capabilities are crucial during sport performance. Explosive sports such as gymnastics, baseball, football, volleyball, and basketball all require athletes to vertically jump. Thus, power production can be the difference between winning and losing. Various training methods have been researched to determine their impact on vertical jumping (4,9,10,13,16,18–21,24–29). Recently, research has investigated methods that may elicit postactivation potentiation (PAP). The underlying theory of PAP is that if an athlete performs a heavy resistance exercise and then waits a specific amount of time before performing an explosive activity that uses the same muscles, the explosive activity may be enhanced (3,5,10,15,17,20,26,30,34). To date, there are 2 main physiological mechanisms thought to explain PAP: increased regulatory light chain phosphorylation and recruitment of motor units (6,10,11,15,20,26). Increased motor neuron excitability may lead to increased motor unit activation and synchronization (11,12,23,26), leading to acute performance enhancements. Therefore, more Ca++ is released from the sarcoplasmic reticulum, allowing more cross-bridges to be formed and more force can be produced (6,10,11,15,20,26). However, the differential effects of overspeed versus overload methods of eliciting PAP are still unknown.
Research has indicated that an overload stimulus of >80% of an athlete’s 1 repetition maximum may elicit PAP (3,5,6,15,30,34). Previous research suggests that variables including training age, history (acute and chronic), rest periods, type of exercise, warm-up, fiber type quality and quantity, and an athlete’s overall strength level (8,10,15,20,26,32,34) critically influence the effectiveness of this strategy. However, there is no standardized protocol when specifically trying to induce PAP for vertical jumping.
A number of studies have investigated protocols that have resulted in significant PAP effects, whereas others have not. This may be because of the contribution of PAP responses being individualized, different methodology, subject population, and training status (7,14,16,18,34). Hence, overload intensity, volume, frequency, and rest periods have all been manipulated (1,33). Another concept, known as overspeed, is a method that has recently been investigated. Overspeed training, via assistance, has been shown to increase sprint speed and takeoff velocity during a vertical jump (2,8,24,31). By using an overspeed concept, velocity can be increased to supramaximal levels, which may acutely enhance vertical jump performance by increasing motor neuron excitability and motor unit synchronization (11,12,23,27).
To date, there is no definitive rest period after overspeed jumping to enhance bodyweight (BW) vertical jump performance. Therefore, the purpose of this study was to examine the influence of rest periods after assisted jumps on BW vertical jump performance.
Experimental Approach to the Problem
This was a within-subjects study that investigated the influence of rest intervals after assisted jumping on BW vertical jump performance in physically active men. Each subject attended 5 consecutive sessions separated by 24 hours each. The first visit was a control session, which included baseline BW jumps and familiarization with the assisted pulley system. Each subject then completed 4 additional sessions consisting of 5 assisted jumps and then 3 BW jumps after 1 of 4 random rest intervals. This allowed us to examine the effect of different rest intervals after assisted jumping on BW vertical jump.
Twenty healthy recreationally trained men (involved in a minimum of 3 hours of recreational vertical jump activities per week for the past 6 months) participated in this study (age: 22.85 ± 1.84 years; height: 179.44 ± 5.99 cm; mass: 81.73 ± 9.51 kg). Any subject with current health limitations such as lower-body orthopedic or musculoskeletal injuries was excluded. Before participating, subjects read and signed a university institutional review board–approved informed consent.
Subjects were instructed to come to the laboratory 5 consecutive days separated by 24 hours each. They were also instructed to maintain normal training status throughout the week. During session 1, subjects read and signed an informed consent and were measured for height and body mass. Subjects performed a dynamic warm-up of 3 exercises consisting of Frankenstein marches, knee hugs, and gate swings. Each exercise covered 20 m. During the warm-up and for the remainder of the session, subjects wore a full-body harness (Protecta, Seattle, WA, USA), which had a D-ring at the cervical area (C7), a nonslip chest strap buckle, and subpelvic strap buckles. The design of the harness allowed the subjects to perform the vertical jump without movement restriction and to maintain an upright position as they jumped. After the warm-up, a velocity transducer (Model HX-VPA-200-L7M; UniMeasure, Inc., Corvallis, OR, USA) was attached, via the D-ring on the harness, which was attached to the ceiling directly overhead. They then performed 3 BW maximal countermovement vertical jumps with hands on their hips while standing on an AMTI force plate (Advanced Mechanical Technology, Inc., Watertown, MA, USA) with 15 seconds of rest between each jump (base). After baseline, subjects were familiarized with the assisted pulley system in preparation for the next 4 sessions. For the following 4 visits, BW jumps were measured after 4 randomly assigned rest intervals (30 seconds, 1 minute, 2 minutes, or 4 minutes). At the beginning of each session, the subjects performed a warm-up protocol identical to day 1. After the warm-up, the subjects were attached to the assisted pulley system and then completed 5 consecutive countermovement jumps with hands on their hips. Body weight was reduced by 30% during these jumps. Previous research has shown that this level of reduction results in near-optimal assisted jump performance (31). By using the force plate, we were able to accurately measure each subject’s BW immediately before jump initiation (in the erect standing position before countermovement). After 5 assisted jumps, subjects stood on the force plate and rested for 1 of the 4 rest intervals. Once rest was complete, they performed 3 maximal countermovement BW vertical jumps, separated by a 15 seconds rest.
Assisted Pulley System
For assisted jumping, subjects stood on the force plate and were attached through the full-body harness to 4 0.91-m elastic cords (31). The elastic modulus of the cords (between 0.3 and 0.6 m of stretch) was 188.337 ± 32.65 Nm2 and was calculated by dividing delta force by delta length. The cords were attached to a canyoneering 9-mm static rope that was looped through a double carbo block and then looped through a 40-mm triple carbo block (Harken, Pewaukee, WI, USA), bolted to the ceiling. The rope led into a 57-mm carbo block with a lock cam (Harken) attached to the wall, which enabled the researcher to decrease each subject’s BW by pulling on the rope and stretching the elastic cords.
Assessment of Vertical Jump Performance
Force plate data were sampled at 1,000 Hz and maximum values from the peak velocity repetition for each condition were analyzed. Both the linear velocity transducer and the force plate were connected to a desktop computer running custom LabVIEW data collection and analysis software (version 7.1; National Instruments Corporation, Austin, TX, USA) to analyze the force-time and velocity-time curves. The force plate was used to estimate vertical jump height (JH in cm) using the time in the air equation: distance = (1/2 gt2)/2, where g is gravity at 9.81 m·s−2 and t the flight time. Takeoff velocity (TOV in m·s−1) was recorded when the subject’s feet left contact with the force plate. Relative ground reaction force (rGRF in N·kg−1) was determined by dividing peak ground reaction force by body mass. Relative peak power (rPP in W·kg−1) was determined as the product of force and velocity. This procedure has previously (31) been shown to have high reliability (intracorrelation coefficients between 0.8 and 0.9).
Four 1 × 5 repeated measures analyses of variance were used to examine differences in TOV, rPP, JH, and rGRF between conditions. The SPSS version 20.0 for Windows (SPSS, Inc., Chicago, IL, USA) was used for all analyses, and an a priori alpha level of 0.05 was used to determine statistical significance.
Takeoff velocity demonstrated a main effect for condition, with 1- and 4-minute rest being greater than baseline, whereas no other conditions were significantly different (Figure 1). Relative peak power also demonstrated a main effect for condition, with 1-minute rest being greater than all other conditions, but no other conditions were significantly different from each other (Figure 2). Jump height (Table 1) and rGRF (Table 2) demonstrated no main effects.
The purpose of this study was to examine the influence of rest intervals after assisted jumping on BW vertical jumps. Our results revealed that rPP and TOV were enhanced at 1-minute of rest, whereas JH and rGRF demonstrated no significant differences between rest times. This may be as a result of an acute neuromuscular enhancement because high-velocity (overspeed) movements can cause increased neural activity (11,12,23). Further explanations could include muscle mechanics (increased regulatory light chain phosphorylation or an increase in Ca++ release, allowing greater fast force production via increased cross-bridge cycling rate), decreased antagonist coactivation, or altered jump mechanics.
According to Moritani and Herbert (23), neural factors are primarily responsible for adaptations during the initial stages of novel exercise, leading to maximal neural activation. Two factors that are involved with neural activation are motor unit activation and motor neuron excitability. Sale et al. (27) investigated slow strength training over several weeks and found that reflex potentiation (motor neuron excitability) increased in all trained muscle groups. They concluded that this was a result of motor neuron activation, a form of neural adaptation increasing synchronization of motor units. Furthermore, these neural adaptations were directly related to the movements performed. Therefore, subjects were able to recruit and activate more motor units after training compared with prior. These changes were probably incremental over time but were only measured after chronic training. Therefore, we chose to measure them acutely. Although we did not do a training study, we found similar results and speculate that the acute increase in rPP and TOV were because of an increase in the excitability of motor neurons.
In a related study, Hakkinen and Komi (11) investigated explosive versus strength training with previously weight-trained men. They divided their subjects into 3 groups: control, explosive, or heavy resistance. They found that acute neuromuscular enhancement was dependent on the type of activity performed. To increase fast force production, high-velocity muscle actions were essential. Additionally, they recorded electromyographic (EMG; root mean square) activity and showed significant increases in motor unit activity after training. They concluded that high-velocity movements were directly related to recruitment patterns of the motor units. Explosive activity has been shown to increase motor neuron excitability, which may result in motor unit synchronization during high-velocity movements (11,12,23) and thereby enhance subsequent performance. Even though we did not record EMG activity, our study was similar in that we used high-velocity explosive movements, albeit not in a training environment, which may have enhanced motor unit recruitment and acute neural adaptations.
In another study by Hakkinen et al. (12), experienced lifters completed explosive and heavy resistance types of training. Their results demonstrated increased neural activation (through increased EMG root mean square activation), resulting in force production enhancement. They concluded that improvements in force production might have been caused by neural adaptations that increased firing frequency and motor unit activation. Unlike our study, Hakkinen et al. compared strength and explosive training, not overspeed. However, the physiological mechanism of neuromuscular adaptations may be similar. This may also be related to decreased antagonist coactivation leading to increased force production of the agonist, thereby resulting in increased rPP.
An alternate explanation may be altered muscle mechanics via a PAP mechanism. Postactivation potentiation can be explained by increased regulatory light chain phosphorylation. With this, increased Ca++ is released, allowing greater fast force production to occur (6,10,11,15,20,26). Both would acutely enhance performance. This increased potentiation is highly individualized by rest intervals (9), which may explain the significant enhancement found in TOV at 4 minutes of rest.
Our study utilized an overspeed method of activity. This is in contrast to overload, where heavy resistance and slow movements lead to performance enhancement after longer rest intervals (e.g., 8–15 minutes) (6,17,20). In contrast, performance enhancement in our overspeed study occurred after 1 minute of rest. Overspeed jumping is a reduction of BW, thereby increasing velocity (31). Bartolini et al. (2) examined optimal elastic cord assistance for sprinting and demonstrated improved sprint times at 30% BW reduction. In a similar study, Tran et al. (31) investigated the effects of elastic cord assistance on vertical jumping and found that TOV and JH were greatest at 30% BW reduction. Our results were similar as we found significant increases in TOV during BW jumps after 30% BW reduction overspeed jumps. However, JH and rGRF were not increased. Jump height may not have increased because we used the force plate to estimate JH, which assumes that vertical takeoff and landing joint angles are the same, with no difference in mechanics. Moreover, the force plate calculates JH using center of gravity. Thus, because jump angles may have been different between takeoff and landing, calculations may be altered. Relative ground reaction force may have not demonstrated any increase because we measured only the peak value and not the entire force-time curve.
Montoya et al. (22) demonstrated the use of an overspeed concept by investigating the effect of different weighted bats on normal bat velocity. They had subjects perform baseline testing with a normal bat and then warmed up with different weighted bats (light, normal, or heavy), followed by a normal bat swing test. Their results demonstrated that warming up with a light bat (overspeed) increased subsequent swing velocity of a normal bat. Our study was similar in that we used BW reduction (overspeed) and found increases in BW vertical jump TOV and rPP. An additional similarity was that Montoya et al. also used 5 overspeed movements (bat swings) and a short rest period (30 seconds compared with our 1 minute) before testing the normal condition.
In summary, previous studies have concluded that explosive movements increase high-velocity fast force production both acutely and chronically. This may be because of increased motor neuron excitability, firing rate, motor unit activation, or muscle mechanics.
Coaches should consider incorporating 5 overspeed jumps, allowing 1 minute of rest before BW jumping. This technique can be implemented using a bathroom weight scale and a resisted band attached overhead. Weigh the athlete to calculate 30% of his or her BW and then stretch the band till he or she reaches 30% BW reduction. Our study demonstrates that using 5 overspeed assisted vertical jumps with a 1-minute rest period improves TOV and rPP in subsequent BW vertical jumps. These findings suggest that high-velocity movements can enhance acute vertical jump parameters. Future investigations should examine the overspeed concept in a chronic training protocol.
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Keywords:© 2013 National Strength and Conditioning Association
overspeed; velocity; power