developing explosive power: a comparison of technique and training

Developing Explosive Power: A Comparison of Technique and Training

John Cronin 1, Peter J McNair 2 & Robert N Marshall 3

1Sport Performance Centre - Auckland University of Technology, Auckland, New Zealand. 2Neuromuscular Research Unit - School of Physiotherapy, Auckland University of Technology, Auckland, New Zealand. 3Department of Sport and

Exercise Science, The University of Auckland, New Zealand.

Cronin, J., McNair, P.j., & Marshall, R.N. (2001). Developing explosive power: A comparison of technique and training. Journal of Science and Medicine in Sport 4 (I): 59-70.

The influence of contraction type and movement type on power output of the upper body musculature was investigated across loads of 30-80% 1RM. Twenty seven males (21.9_+3.1 years, 89.0+12.5 kg, 86.32+13.66 kg 1RM) of an athletic background but with no weight training experience in the previous six months volunteered for the study. The results were compared using multivariate analysis of variance with repeated measures (p<0.05). It was found that the combinations of load, movement and contraction type affected mean and peak power in different capacities. Mean power output for rebound motion was 11.7% greater than concentric only motion. The effect of the rebound was to produce greater peak accelerations (38.5% - mean across loads), greater initial force and peak forces (14.1% - mean across loads) and early termination of the concentric phase. Peak power output was most influenced by the ability to release the bar, the greater mean velocities across all loads (4.4% average velocity and 6.7% peak velocity) attained using such a technique appeared the dominating influence. Loads of 50-70% 1RM were found to maximize mean and peak power. Loading the neuromuscular system to maximize mean or peak power output necessitates an understanding of the force- velocity characteristics of the training movement and the requirements of the individual related to the athletic performance and their training status.

Introduction Muscular power is an impor tant componen t of m a n y athletic pursui ts ; the power an athlete can exert often determines success or at least aids m u s c u l a r f u n c t i o n and motor performance. Improving m u s c u l a r funct ion th rough resis tance s t rength training necessi tates an unders tand ing of how specific techniques affect power output . However, there is m u c h debate as to the mos t effective resistance s t rength training method to develop power. The key issues would seem to be which load expressed as a percentage o f o n e repetition m a x i m u m (% 1RM) and which training technique bes t facilitates power development.

Heavy load training involves lifting loads between 80-100% of 1RM. The physiological basis for the superiority of this type of training has been at tr ibuted to: (1) the recru i tment of the fastest high threshold motor uni ts for the development of large forces to overcome heavy loads (Gollinick, Armstrong, Sembrowich, Shepherd & Saltin, 1973; Thors tensson, Hulten, von Dobeln & Karlsson, 1976); and, (2) neura l pa thway training associated with the m o t o r n e u r o n s Kring high frequency impulses for comparatively long dura t ions (Tidow, 1990; Schmidtbleicher, 1992; Young, 1989). A second training method develops power th rough high velocity movements us ing lighter loads. Many au thors

59

DevelopingExplosive Power: A Comparison of Technique and Training

suggest that the optimal compromise between force and velocity for the development of maximum power is achieved at loads between 30-50% of maximum force (Faulkner, Clafiin & McCully, 1986; Kaneko, Fuchimoto, Toji & Suei, 1983; Moritanni, 1992). It is thought that the fast twitch fibres may be selectively activated during such high velocity movements (Ewing, Wolf. Rogers, Amundson & Stutt, 1990; Nardone, Romano, & Schiepati, 1989; Sale, 1992).

Another issue in power development relates to the velocity and acceleration of the weight that the athlete lifts. During light load power training, large accelerations are achieved at the beginning of the concentric phase of the contraction and consequently large amounts of time are spent in deceleration over the final stages of the contraction (Elliott, Wilson & Kerr, 1989; Newton, Kraemer, Hakkinen, Humphries & Murphy, 1996). Hence when using light loads, high force levels are achieved only through a very small range of movement. Some training techniques aim at decreasing the deceleration phase by allowing the load to be projected as in a throw or a jump. Greater average velocity, peak velocity, peak acceleration, average force, mean power output and peak power output have been recorded for this type of training (Berger, 1963; Newton & Wilson, 1993; Newton, Humphries, Murphy, Wilson & Kraemer, 1994).

A final issue that may be important in developing power is whether the athlete uses eccentric muscle action prior to concentric muscle action. Findings indicate that this type of training leads to greater initial impulse, higher peak bar velocities, higher accelerations achieved throughout the entire range of movement and increased work and power (Asmussen & Bonde-Petersen, 1974; Bosco & Komi, 1979; Wilson & Newton, 1992). Most of the literature attributes the enhancement to one or a combination of four factors: the recovery of elastic strain energy; a higher active muscle state before the beginning of the concentric muscle action; and, myoelectrical and chemomechanical potentiation (Bosco, Viitasalo, Komi & Luhtanen, 1982; Cavagna, Dusman & Margaria, 1968; Ettema, van Soest & Huijing, 1990).

While studies have examined the effect of load (Newton et al., 1996; Newton & Wilson, 1993), speed of movement (Newton et al., 1996) and contraction type (Newton, Murphy, Humphries, Wilson, Kraemer & Hakkinen, 1997; Wilson & Newton, 1992), no one study has examined the interaction of all three variables in a single research paradigm. Hence the specific purpose of the current study was to: (a) compare the power output of four types of explosive training; (b) investigate the influence of various kinematic and kinetic variables on power output; arid, (c) define the load that maximizes power output. It was hypothesized that rebound throw training would be superior in generating larger velocity, acceleration, force, and power outputs and loads of 30-50% 1RM would maximize power output.

Methods Subjects Twenty-seven males volunteered to participate in this research. The subject's mean (+SD) age, weight and maximal bench press strength (1RM) were 21.9+3.1 years, 89.0+2.5 kg, 86.3_+13.7 kg. All of the subjects had an athletic background, however, none had participated in strength training in the previous six months. The Human Subject Ethics Committee of the University of Auckland approved all the procedures undertaken.

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Testing procedures Procedures used in this investigation were similar to those outlined in previous studies (Newton et al., 1996, 1997). Testing was performed over two sessions, the first of which determined each subject's 1RM. At the completion of the 1RM testing, subjects performed a number of bench presses to familiarize themselves with the power movements. These were a concentric only bench press in which a mechanical brake was positioned so that the bar rested approximately 5cm above each subject's chest, parallel to the nipples. The bar was then projected from rest as fast as possible and was either held at the end of the movement or thrown. For the rebound bench press, subjects were instructed to begin with the weighted barbell at arm's length and then lower the bar as quickly as possible, to just above the nipples, and then immediately push the bar upward. The bar was again either held at the end of the movement or thrown.

The second session began with a generalized warm-up of arm/shoulder mobilization exercises, two sets of 10 bench presses at 40% 1RM, followed by five minutes of pectoral and triceps brachii static stretches. The subject was strapped to the bench and instructed to move the bar as "fast" as possible for all loads and movement types. Four movements were performed for six training loads (30, 40, 50, 60, 70, and 80% 1RM). The four movements were: a concentric bench press (CBP); a concentric bench press throw (CBPT); the concentric bench press was preceded with an eccentric contraction (RBP); and, eccentric-concentric bench press throw (RBPT). One trial was completed per movement type and load, and rest periods of a minute between each explosive movement and two minutes between change of loads were provided. Reliability was established in pilot testing. Data for one and three trials was assessed within and across days. Intraclass correlation coefficients of .85 to .99 were found. For the rebound movements, no pause was allowed between the eccentric and concentric phases and the trial was rejected if the bar touched the chest in any manner, or the eccentric phase was terminated greater than 5cm from the chest. Prior to testing, s ~ g loads and movement types were randomized between subjects to reduce the possible confounding effects of order and fatigue.

Equipment Modified S m i t h Press Subjects performed bench presses on a modified Smith press machine. A linear transducer (Unimeasure, Oregon} was attached to the bar and measured bar displacement with an accuracy of 0.0 lcm. These data were sampled at 200ttz and relayed to a computer based data acquisition and analysis program.

Data analysis The displacement time data were filtered using a low pass filter with a cutoff frequency of 10 Hz. The filtered data were then differentiated using a finite differences algorithm (EASYEST TM LX, Asyst Software Technologies, Inc.) to determine velocity and acceleration data. The velocity data were combined with the force data to determine instantaneous power output over the range of motion for each load condition. Pmalysis of the concentric phase of the RBP and RBPT was determined as the time when bar velocity changed from positive to negative. For the BP and BPT, the start of the concentric phase was determined as the point at which the vertical force increased (Newton et al., 1997). The following variables were de te rE~ed for the bar from its kinematic and mass characteristics during

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the concentric phase: average velocity, peak velocity, peak acceleration, average force, peak force, mean power and peak power.

Statistical analysis The results for average velocity, peak velocity, peak acceleration, average force, peak force, mean power and peak power were compared using a 2 x 2 x 6 repeated measures ANOVA (contraction, movement and load). Post-hoc contrasts using the Bonferroni procedure were used to determine significant differences between the means. The criterion level for significance was set at p<0.05.

ReSultS The power outputs as described in this s tudy represent the power t ransmit ted from the subject to the bar. Figure 1 shows that the rebound conditions, whether thrown or held, elicited higher mean power outputs (11.7% - mean across loads), compared to the non-rebound bench presses. The combined rebound and throw condition produced significantly higher mean power output than the rebound condition only. The bench press throw condition produced significantly higher mean power than the bench press that held onto the bar at the end of the concentric phase. Further analysis revealed that the ability to release the bar for the bench press throw increased mean power output on average 5.8% for all loads. Significantly higher average velocities were found across all loads when throw conditions were compared to non-throw (4.4%).

Figure 2 shows no significant difference for peak power between the rebound and non-rebound conditions. The techniques that allowed the load to be released produced significantly higher peak power (9.1% - mean across loads) than the

350 . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . .

250 . . . . . . . . . .

200 l P..~'~m~ Bemh Press Concen~¢ Berth Press 1brow Retx~l~l Berlch Press Rel)ound Bel~ P~ess Tnrow

[ ] 30% IRM

• 40% IRM

[ ] 50% 1RM

[ ] 60% 1RM

[ ] 70% 1RM

[ ] 80% 1RM

Figure 1:

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Comparison of mean power output for different bench press techniques across loads of 30-80% IRM.

750

700

650

500

. ~ 600

~. 5511

450

[ ] 30% 1RM • 40% 1RM [ ] 50% 1RM [ ] 60% 1RM [ ] 7O% iRM [ ] 80Yo°J 1RM 400

_ , ] L r l l / _ l-,T _1111


Figure 2: Comparison of peak power output for different bench press techniques across loads of 30-80% 1RM.

Concentric Bench Press Concentric Bench Press Throw

%111M PV PA PF TPCV DOCC PV PA PF TPCV DOCC (m/s) (m/s 2) (N) (sec) (sec) (m/s) (m/s 2) (N) (sec) (set)

30 40 50 60 70 80

30 40 50 60 70 80

1.49 5.87 414.1 0.401 0.629 1.35 4.63 505.9 0.459 0.683 1.16 3.78 583.8 0.528 0.761 1.01 2.96 660.9 0.62 0.818 0.86 2.43 766.4 0.774 0.964 0.68 1.93 826.3 0.928 1.125

1.63 5.74 411.1 0.429 0.792 1.45 4.73 505.2 0.495 0.816 1.27 3.76 589.2 0.547 0.826 1.05 2.85 657.3 0.648 0.883 0.89 2.45 772.8 0.771 0.953 0.72 2.06 820.5 1.019 1.213

Rebound Bench Press RebOund Bench Press ThrOw

1.52 8.54 491.8 0.348 0.614 1.37 7.11 596.1 0.425 0.642 1.21 6.33 692.7 0.508 0.739 0.99 4.97 786.4 0.593 0.803 0.83 4.12 876.6 0.726 0.945 0.65 3.19 906.5 0.927 1.238

1.68 8.51 488.8 0.415 0.782 1.48 7.34 593.5 0.475 0.779 1.3 6.67 714.3 0.514 0.793

1.08 5.01 777.3 0.568 0.844 0.88 4.33 866.1 0.732 0.944 0.69 3.25 914.1 1.015 1.18

Table 1: Peak force (PF), peak velocity (PVI, time to peak velocity (TPV), peak acceleration (PAl and duration of concentric contraction (DOCC) for the various types of bench press at loads ranging from 30- 80% IRM.

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techniques that did not project the load. These results indicated that throwing the bar was most influential to gains in peak power.

As there was no interaction effects found for both mean power and peak power, the measures were collapsed across all loads. Comparisons between groups revealed that loads of 50-70% 1RM were found to be superior for generating greater mean power output. Loads of 50-60% 1RM were most effective in producing greater peak power.

Table 1 describes the effect of load and bench press technique on various kinematic and tdnetic variables. Peak velocities differed significantly across loads, the heavier loading conditions resulting in decreased velocities. A mean increase in peak velocity of 6.7% (p_<0.05) across loads and movement types was recorded for the throw conditions. The enhancement in bar velocity however, decreased with the utilization of heavier loads. Smaller but significant differences were found for the rebound conditions; the effect of rebound however, having no effect on peak velocity for the heavier loading intensities (60-80% 1RM).

As load increased, peak acceleration decreased and peak force increased for all conditions (p<O.05). Mean increases in peak acceleration of 38.5% was found across loads for both traditional and throw bench presses for motion that used rebound. The potentiation afforded by rebound was similar across all loading intensities. A smaller mean increase (14.1%) was found for peak force across loads and contraction types for motion that used rebound. The potentiating effects of rebound decreased with heavier masses (e.g. 30% 1RM =15.85%; 80% 1RM = 9.6%). The throw conditions had no significant effect on peak acceleration or peak force. Interestingly no significant differences were found between average force output across movement and contraction types though heavier loads produced significantly greater average force. As expected the time to peak velocity

16

14

12

10 Percentage

I.crease in Bar

Velocity (%)

8,

6

4

2

0 3O

®

40 50 60 70 80

% 1RM

]Rebound

B Throw

Figure 3: The effect of rebound and throwing the bar in terms of percentage enhancement of average velocity across loads of 30-80% IRM.

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6 0 0

• , e - - e

5OO

4O0

P o w e r 300 (watts)

20O

0 1 O0 200 300 400 500 600 700 800 900 Time (ms)

= CBP --o--CBPT • RBP --~--RBPT

Figure 4: Power-time curve of a representative subject for the concentric phase of the concentric bench press (CBP), concentric bench press throw (CBPT), rebound bench press (RBP) and rebound bench press throw (RBPT).

and duration of concentric contraction increased with heavier loading intensities. The effect of rebound and release on average velocity is shown in Figure 3.

Average velocity was found to differ significantly across all loads, the rebound and release conditions superior to the concentric and held bench presses. Though it seems that rebound improves average velocity to greater effect, the variance is not significantly different to the enhancement afforded by the bench press throw conditions.

Figure 4 depicts the concentric power-time curve of a representative subject for all four conditions at 40% 1RM. The curve shows the higher initial power output recorded during rebound motion (RBP and RBPT). Also if the RBP and CBP are compared, motion that utilizes rebound is terminated earlier (lOOms). It appears that the effect of rebound is to shift the power-time curve to the left.

DiSCuSSiOn Effect of Load In previous studies the load, contraction type and technique were examined. Conflicting results were found in this research with respect to the influence of these three factors on power production. Some research indicated that mammal power output occurred approximately 30% of max imum isometric strength and 30% of ma x i m um shortening velocity (Faulkner, Claflin & McCully, 1986; Moritanni, 1992; Perrine, 1986). Other research suggested that heavier loads (80- 100% 1RM} provided a greater training st imulus for the development of power provided that the contractions were performed as explosively as possible

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(Schmidtbleicher & Buehrle, 1987; Young, 1989). Hence, some authors recom- mend an integrated approach, utilizing explosive heavy and light load training (Newton & Kraemer, 1994). The mean power output and peak power results of this study suggest that intermediate loads (50-70% 1RM) maximize power output for the bench press motion using isoinertial equipment. Light (30-40% 1RM) and heavy (80% 1RM) loads were inferior to the 50-70% 1RM loads in optimizing peak and mean power output for all techniques. Such findings are not uncommon and it has been suggested that during more traditional lifts (e.g. b en ch press or squat) that power is maximized at loads of 60% of 1RM or higher (Newton et al., 1996; Santa Maria, Gryzbinski & Hatfield, 1985). The results of this study certainly support this contention.

Support for the utilization of 50-70% 1RM loading intensity may be inferred from the findings of McDonagh and Davies (1984), who stated that a muscle must be activated at an intensity of at least 66% of maximum before there will be an increase in strength. Utilization of such loading requires greater force production for a longer duration, thus a greater proportion of the motor unit pool is activated with the use of both slow- and fast-twitch motor units. If power is the product of strength and speed, the heavier loading proposed by this research would seem a better option for inducing hypertrophic as well as neurotrophic adaptation. Support for this notion is noted in research that has suggested strength may be more trainable than speed (Kaneko et al., 1983). Furthermore, as all explosive movements start from zero or slow velocities and progress to higher shortening velocities, intermediate training loads and training velocities may best optimize adaptation at both ends of the speed-strength cont inuum (Behm, 1991; Ewing et al., 1990).

The apparent difference between the results of this research and those proposing that lighter loads maximize power output may also be explained by differing force-velocity responses according to the training status of subjects (Komi & Hakkinen, 1988). Most of the research cited by Newton and Wilson for example used experienced subjects who could lift up to 1.3 times their body weight. The force-velocity responses of the weaker, inexperienced subjects of this research may very well differ with respect to their more experienced counterparts.

Effect of Projection of Bar In an effort to gain a sport specific training effect, athletes may attempt to move the bar rapidly dining the lift. If the bar is not released, such movements result in shorter acceleration and longer deceleration phases, as the athlete mus t slow the bar earlier in the movement (Newton & Wilson, 1993; Newton et al., 1996). The longer deceleration results from shorter agonist activation and greater antagonist co-activation, particularly when lighter loads are utilized (Newton et al., 1994). As such, this type of training offers a sub-optimal training stimulus. Newton and colleagues (1996) found the bench press throw allowed the bar to be accelerated for 96% of the throw movement as opposed to 60% for the traditional bench press. Throw training allowed the muscles to produce tension over a greater portion of the concentric movement, thus offering a superior training stimulus.

Figure 2 shows greater peak power output for the RBPY and CBPT, with the ability to release the load increasing peak power output on average by 9.1% for all loads. Interestingly this potentiation due to release was not evident in the mean

66


power output data (Figure 1). Greater mean power output values were found in motion that used rebound. To explain these differences, acceleration and velocity data were analyzed, realizing that power is the product of force and velocity and because acceleration and force are proportional, the power-time profile associated with the bench press movements are related to the velocity and acceleration signals. Analysis revealed there were no significant differences between the CBP and CBPT, and RBP and RBPT in te rms of acceleration (see Table 1). It would appear that the only possible explanation for the superior peak power outputs of the CBPT and RBPT were due to the greater velocities achieved across all loads (4.4% mean velocity and 6.7% peak velocity) for the throw. The higher peak velocities were achieved later in the concentric contraction for the throws, indicating that the bar was being accelerated over a greater portion of the concentric phase. The use of a SSC, however, did not enhance peak velocity between non-rebound and rebound conditions. This is similar to findings of Newton et al. (1997) who explained that the recovery of stored elastic energy and reflex potentiation only enhance the initial phase of the concentric movement and that peak velocity occurs later whereby the effect of the SSC has diminished. This suggests that ma_~i_mizing velocity through a longer acceleration phase is important for the generation of higher peak velocities and peak power outputs.

Effect o f Rebound As stated earlier, greater mean power values were recorded for the rebound training techniques. Figure 1 illustrates this point, showing that pre-stretch increased mean power output for all loads on average 11.7%. This is similar to the pre-stretch augmentat ion found in other studies (Asmussen et al., 1974; Wilson et al., 1991). Potentiation from the preceding eccentric muscle action was more marked when peak acceleration was compared between conditions. When non- rebound bench presses were compared to the rebound bench presses, it was found that the average velocities (12.4%) and peak accelerations (38.5%) were greater across all loads for the rebound conditions. Furthermore, the peak accelerations were recorded early in the concentric contraction immediately following the eccentric muscle action. It is recognized that rate of concentric contraction is augmented in SSC movements, especially in the initial period of the concentric phase. The findings of this research certainly suppor t early augmentat ion of the concentric contraction when preceded by rebound motion. Temporal analysis of the concentric only and rebound power data revealed that the effect of rebound was to produce a phase shift of the power signal to the left, producing greater initial and mean power outputs (see Table 1 and Figure 4). Temporal analysis also revealed that the concentric phase of the RBP was terminated earlier than the CBP conditions, a similar finding (but not to the same magnitude) to Newton and colleagues (1997).

The augmentat ion of concentric muscle action from a pre-stretch is typically attributed to: the storage and re-utilisation of elastic energy stored in the series elastic component (SEC) of the musculo-tendinous system (Asmussen & Bonde- Petersen, 1974; Komi & Bosco, 1978); spinal reflexes (Dietz, Noth & Schmidtbleicher, 1981) as well as long latency responses (Mel~lle-Jones & Watt, 1971) that increase muscle stimulation, a l lo~ng the muscle to reach max imum activation before the onset of the concentric muscle action (Van Ingen Schenau, 1984). Any one, or combination of the above factors could be proposed to explain the early enhancement afforded by rebound motion. Research has shown that

67


most of the augmentat ion was lost within the first 0.2s of the concentric muscle action (Thys, Cavagna & Margaria, 1975; Chapman & Caldwell, 1985; Wilson, Elli0tt & Wood, 1991). The diminishing contribution of the SSC is evident toward the later portion of the power-time curves as shown in Figure 4. The loss of this enhancement could be attributed to the dissipation of elastic energy or the inability of the muscle to generate force at high shortening velocities according to the force-velocity relationship of muscle. ~

The interaction effect noted between load, movement and contraction was attributed to the heavier loading intensities decreasing the potentiating effects of rebound and projection of load. As loading intensity increases, the ability to release the load decreases due to the development of slower shortening velocities as described by the force-velocity relationship of muscle (see Table 1). The decreased potentiation of rebound in heavier SSC contractions may be attributed to a slower rate of eccentric muscle action, longer duration eccentric muscle action, and slower coupling times. Bosco and colleagues (1982) suggested that stretching phases greater than 500 ms produced longer Coupling times, resulting in decreased recovery of elastic energy. Alternatively, it may be that the relative importance of the prior eccentric movement is greater for tight load movements. As the movement time is shorter, being at a high active muscle state at the s tar t of the concentric phase would appear advantageous for such short duration high- velocity movement.

Conclusion In conclusion, the combinations of load, movement and contraction type affect velocity, acceleration, force and ultimately, power in different capacities. Mean power output was mos t influenced by rebound motion. The effect of the rebound was to produce greater peak accelerations (35% across all loads), greater initial force and peak forces (14.1% across all loads) and early termination of the concentric phase. Peak power output was most influenced by the ability to release the bar, the greater mean velocities across all loads (4.4% average velocity and 6.7% peak velocity) attained using such a technique appeared the dominating influence. Loads of 50-70% 1RM were found to maximize mean and peak power. A significant interaction effect was found between load, movement and contraction, the heavier loading intensities decreasing the potentiating effects of rebound and projection of load. Loading the neuromuscular system to maximize mean or peak power output necessitates an understanding of the force-velocity characteristics of the training movement and the requirements of the individual related to' the athletic performance and their training status. Specific power techniques and loads can then be utilized to stimulate the required adaptation.

Acknowledgments This research was funded by the Sport Science New Zealand Research Program.

The research as described above complies with the current laws of the country in which the experiment was performed.

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