It has been documented (1,4) that elastic energy stored in muscle–tendon units during the braking phase of a stretch-shortening cycle (SSC) enhances the mechanical efficiency and power output during the push-off phase.
Former studies were directed on the mechanics and energetics of the stretch-shorting cycle during running and jumping (1,3,6). For instance, Cavagna et al. (5) found that during sprinting from start to peak running velocity (9.4 m·s−1), the power output increased up to an average of 5 m·s−1 through intrinsic properties of muscle contraction. The remaining increases in power output necessary to obtain maximal velocity were attributed to the storage and subsequent release of elastic energy in leg muscles.
The increased prestretch intensity (e.g., jumping from a higher dropping height) has a considerable influence on the process of storage and subsequent recoil of the elastic energy because of the activation of the SSC (13). Likewise, countermovement weight exercise involving SSC leads to a higher power production in the concentric phase as compared with the lift performed from rest (4). The mechanics and energetics of the SSC has been devoted a special issue of Journal of Applied Biomechanics (Vol. 13, No. 4, 1997). In general, the increased production of muscular power has been explained by 2 proposed models: mechanical and neurophysiological (26). However, the degree of their contribution on force production remains a speculation.
Several studies have analyzed the influence of parameters, such as ground contact time, muscle preactivation, or additional weight on the SSC, to define a range where the energy-storage potential of the SSC—and therefore the efficiency of the movement—is maximized. One important factor that determines how much energy can be stored and then released during the SSC is the load on the tendomuscular system. When the weight lifted is too high, the contact and coupling time is too long for the storage of energy (11). Using optimal weight that allows the maximal use of elastic energy may provide conditions for the most effective development of explosive power.
Although the force output increases as a function of load, it has been demonstrated that even lighter loads may result in considerable force because of the high accelerations produced (20). One example is a study wherein increasing the acceleration from 0.7 to 1.3 g during reactive jumps on a sledge jump system resulted in increased peak forces (+15% when comparing the value at 1.3 g with the one at 0.7 g), longer contact times (+10%), increased muscular activity (+7 to 87%, depending on the muscle), increased momentum (+28%), unchanged leg stiffness, and increased joint excursions in the ankle and knee joints (+3%) (17).
Using a lighter load during the rebound bench press throw produces a greater initial enhancement of power output, but the decay of this enhancement is more rapid (8). The heavier 80% 1-repetition maximum (1RM) loading prolonged time to peak power enhancement (80 vs. 20 milliseconds) and slower decay of the stretch-induced augmentation (460 vs. 260 milliseconds). It may be assumed that the degree of SSC enhancement of power (200–780% in the first 100 milliseconds) differs not only in the weight lifted but also in the time-dependent characteristics of the motion. A high correlation (r = 0.70) has been reported between eccentric maximum strength and peak power when using the SSC during elbow flexion; however, this parameter did not correlate with the initial power (100 milliseconds) during elbow flexion performed in the SSC condition (19).
These findings raise the importance of analyzing not only peak power but also the mean power during the entire concentric phase of lifting, and if possible, during the acceleration phase. Understanding the relationship between the maximal force produced (or weight lifted) and the magnitude of power enhancement in the concentric phase of countermovement exercise can serve as a basis for the design of an efficient training program, namely, when implementing novel devices, methods, or exercises in training routines.
Theoretically, power enhancement depends on the amount of energy stored in the eccentric phase and reused in the subsequent concentric phase of exercise. Under the assumption of an unchanged spring constant, the amount of energy stored in an elastic system depends mainly on the force applied during the eccentric phase. Lifting heavier weights eliciting a higher force during the eccentric phase is most likely associated with a more pronounced enhancement of power in the subsequent concentric phase.
Therefore, it is of special importance to determine the optimal load at which maximal potentiation of power occurs (28). Usually, the maximal power is estimated from peak values, whereas the mean values in the concentric phase of lifting are often neglected. However, in some cases, the mean power is considered as a more reliable and more sensitive parameter than peak power (29). Provided that the maximal power estimated from mean values is applied in the functional assessment of strength capabilities, it is necessary to know whether the maximum is achieved at different weights than the one estimated from peak values. It may be assumed that maximal delta power (the difference in power during the concentric phase of a resistance exercise with and without countermovement) is achieved at lower weights when calculated from peak values and at higher weights when calculated from mean values. The hypothesis was verified by the comparison of delta peak and mean power during the concentric phase of resistance exercises with different weights.
Experimental Approach to the Problem
The study compares the enhancement of peak power and mean power during the concentric phase of resistance exercises with different weights. Subjects randomly performed bench presses and squats on different days. The initial weight of 20 kg was increased by 10 or 5 kg (at higher loads) up to at least 85% of the previously established 1RM. Rest intervals of 2 minutes were applied between repetitions with particular weights. The best result of 3 trials (2 at higher loads) without and with countermovement was taken for evaluation. The FiTRO Dyne Premium system (FiTRONiC, Bratislava, Slovakia) was used to monitor force and velocity and to calculate power. Peak power and mean power during the acceleration and the entire concentric phase of lifting were analyzed. Delta power (the difference in the power during the concentric phase of resistance exercise with and without countermovement) for each weight was calculated.
A group of 27 fit men (age 23.4 ± 3.7 years, height 183.7 ± 8.5 cm, weight 83.0 ± 13.1 kg) volunteered to participate in the study. Subjects' inclusion and exclusion criteria were set up before the intervention. All subjects had experience in resistance training involving exercises such as bench presses and squats (an average of 6.5 years). They were asked to refrain from performing any strenuous exercises during the study. All the subjects were informed about the procedures and the possible risks and gave their written informed consent. The procedures presented were in accordance with the ethical standards on human experimentation and were approved by an institutional review board.
The subjects underwent a familiarization session during which the test protocol was explained and trial resistance exercises were performed. Emphasis was placed on using the proper technique of exercises, as described below. The exercises were performed without and with countermovements using maximal effort during the concentric phase.
Afterward, the testing on 2 different days, with 2–3 days in between, was undertaken. After a standardized warm-up, the subjects performed either barbell bench presses or barbell squats. The initial weight of 20 kg was increased by 10 or 5 kg (at higher loads) up to at least 85% of the previously established 1RM. Rest intervals of 2 minutes were applied between repetitions with particular weights. The best result of 3 trials (2 at higher loads) without and with countermovement was taken for evaluation.
The countermovement (CM) bench presses required that the subjects lower the barbell to their chests without making any contact when transitioning from the eccentric to concentric phase. Any repetitions that made contact with the chest or failed to come within 0.05 m of the chest were disregarded and repeated after 1 minute of rest. The bench press performed without a CM started from an initial position on the chest (barbell positioned ∼0.05 m from the chest), and once this was achieved, the subjects held the position for approximately 2 seconds before performing an upward movement, on the command of the tester. Each participant was visually observed during the exercises to ensure that no countermovement was implemented. Real-time analysis allowed for the monitoring of all possible movements with the barbell. The subjects were required to maintain the same grip width throughout the entire testing protocol. Emphasis was placed on maintaining contact between the hips and the back with the bench.
The CM squats were performed from full extension to a knee angle of 90° while holding a barbell on the back, followed immediately by an upward movement. Squats without CM started from an initial semisquat position (90° knee flexion), as determined from visual inspection, and once achieved, the subjects held the position for approximately 2 seconds before performing an upward movement, on the command of the tester. A laboratory assistant stood behind the subjects to impede a possible fall.
Basic biomechanical parameters during the tests were monitored using the FiTRO Dyne Premium system (FiTRONiC, Bratislava, Slovakia). For this system, Gažovič (10) reported a test–retest correlation coefficient and measurement error of 0.89 and 13.5%, respectively, for peak power and 0.87 and 7.28%, respectively, for the mean power in the concentric phase of bench presses with a weight of 60 kg. The study of Jennings et al. (14) showed intraclass correlation coefficients of 0.97 (95% confidence interval [CI], 0.95–0.98) for maximal power during squat jump and 0.97 (95% CI, 0.95–0.98) for biceps curl with limits of agreement of −17 ± 96 W and 0.11 ± 13.90 W, respectively.
The system consists of a precise analog rotary sensor connected to a reel. When pulling the tether (connected by a small hook to the barbell), the reel is wound and the sensor that is connected measures the velocity. Rewinding of the reel is guaranteed by a string producing force of about 2 N. Signals from the sensor unit are converted from analog to digital (12 bit) and conveyed to a personal computer by means of a USB cable. Comprehensive software allows one to collect, calculate, and read an on-line display of the basic biomechanical parameters involved in the exercise.
The system operates on Newton's law of universal gravitation (force equals mass multiplied by the gravitational constant) and Newton's law of motion (force equals mass multiplied by acceleration). Instant force when moving the barbell in the vertical direction is calculated as the sum of the gravitational force (mass multiplied by the gravitational constant) and acceleration force (mass multiplied by acceleration). Acceleration of the vertical movements (positive or negative) is obtained as a derivation of vertical velocity. Power is calculated as a product of force and velocity and the actual position by integration of velocity.
The device was placed on the floor and attached to the barbell by a nylon tether. The subjects performed exercises while pulling a nylon tether of the device. Peak values (Figure 1) and mean values of power and velocity were obtained from the entire concentric phase of lifting (Figure 2) and from its acceleration segment (Figure 3).
Statistical analysis of the collected data was performed using the SPSS program for Windows version 18.0 (SPSS, Inc., Chicago, IL, USA). The Kolmogorov–Smirnov test for normality and the Levenne test for equality of error variances were performed on all variables, to find that the data were normally distributed and that no significant differences in sample variance were detected. Statistical power was determined to be >0.80 at the 0.05 alpha level. The data were analyzed using a 2-way analysis of variance with repeated measures. Factors included contraction type (concentric-only, eccentric–concentric) × resistance (from 20 kg to at least 85% of 1RM). The differences in the power during the concentric phase of resistance exercises performed with and without countermovement (ΔP) for each weight were analyzed. Bench presses were analyzed separately from squats because the mechanism of power production in the upper limbs is different from that of the lower limbs (antigravity muscles), in which some tension has to be continuously exerted to maintain standing posture and where the body weight is used as the load. Where significant differences were detected at p ≤ 0.05, post hoc analysis was performed using the Tukey honest significance test. All values are presented as group mean values ± SD.
Figures 4 and 5 depict an individual example of force, velocity, power, and position collected at a sampling rate of 100 Hz by the FiTRO Dyne Premium system while performing the bench press without and with countermovement, respectively.
In the initial phase of eccentric contraction, the force decreases under the gravitational value, but after reaching a maximal downward velocity, the force increases to brake and decelerate the movement. Maximal force is reached around the turning point, where the eccentric phase changes into a concentric one. The high force at the beginning of this phase accelerates the upward movement till maximal velocity is reached. As the force decreases under the gravity value, deceleration sets in resulting in zero velocity in the upper position (12).
In the eccentric phase, there are also negative values of power. They become positive during the phase of concentric contraction. There is a rather steep increase in its values followed by a sort of plateau despite the presence of a rapidly changing force and velocity. This is because the product of high force and low velocity (beginning of the concentric phase), moderate force and moderate velocity (middle of the concentric phase), and low force but high velocity (around two-thirds of the distance between the lower and upper positions) yields about the same values of power.
As expected, there was a higher power output during the CM than during concentric-only exercises. This potentiating effect of power during the concentric phase of bench presses was rather modest at lower weights and becomes more pronounced with increasing weights, reaching a maximum of 118.4 ± 19.0 W at 47% of 1RM for peak values (Figure 6A), 116.2 ± 15.3 W at 48% of 1RM for the mean values during the acceleration phase (Figure 7A), and 114.8 ± 14.8 W at 57% 1RM for the mean values during the entire concentric phase (Figure 8A). Lifting heavier weights not only failed to increase the enhancing effect but it also led to its decline.
A similar trend for the enhancement of power during the concentric phase of squats was observed. The delta power increased from lower weights, reached a maximum of 127.7 ± 20.4 W at 67% of 1RM for peak values (Figure 6B), 124.3 ± 22.1 W at 69% of 1RM for the mean values during the acceleration phase (Figure 7B), and 125.0 ± 19.2 W at 77% of 1RM for the mean values during the entire concentric phase (Figure 8B), and then toward higher weights, decreased again.
However, there were no significant (p > 0.05) differences in the maximal values of delta peak and mean power during both bench presses and squats.
It is known that concentric contraction using the SSC produces greater power output than does a simple concentric contraction itself (15,21,27). An effective SSC requires 3 critical elements, including a well-timed preactivation of the muscle(s) before the eccentric phase, a short and fast eccentric phase, and an immediate transition (short delay) between the stretch (eccentric) and shortening (concentric) phase.
The mechanisms underlying this enhancement of power is usually ascribed to the use of elastic energy stored in the elastic components in combination with reflexively induced neural input (4,15,16,23,24). Alternative explanations propose that the prestretch of an active muscle alters the properties of the contractile machinery and that a prior stretch allows the muscles to build up a maximum active state before the concentric contraction begins (2). However, it has been found that there are no differences in the electromyographic activity between the SSC and isometric condition in the concentric portion of a vertical jump, indicating that reflex activity was not involved in the increased torque seen (9). These findings have led to a number of scientists suggesting that reflex activity is not involved in an increased force output during the SSC (25). It may therefore be assumed that mainly the use of elastic energy can explain the enhancement of power during countermovement resistance exercises.
To assess the ability to use elastic energy, the estimation of the difference in power output during the concentric phase of resistance exercises performed with and without a countermovement can be used. As mentioned, the activation of SSC during CM resistance exercise leads to a higher power production in the concentric phase when compared with the lift performed from the rest (4). The higher the difference between power during resistance exercise performed with and without countermovement (ΔP), the better the ability to use elastic energy.
Such a CM enhancement of power depends on the weight lifted with some optimal load at which maximal potentiation of power occurs (28). Our study showed that the maximal ΔP was achieved at lower weights when calculated from the peak and mean values in acceleration phase of lifting than the one calculated from the mean values in the entire concentric phase of chest presses and squats. Lifting higher weights not only failed to increase the enhancing effect but it actually led to its decline. Such a decrease in potentiation of power during the concentric phase of CM exercises may be because of (a) the full exploitation of the spring capacity of elastic tissues already at weights lower than 1RM, and (b) protective inhibition of muscle force production because of extreme peak forces occurring in CM exercises with the weights approaching a subject's 1RM.
Although the differences in the maximal values of delta peak and mean power during both bench presses and squats were rather small, statistical analysis revealed a significant between-subject variability. However, statistical significance in some cases did not imply that the differences observed were practically meaningful. Therefore, Cohen's effect size statistics (Cohen's d) (7) were applied to assess the magnitude of the differences in the peak and mean power of concentric-only and CM resistance exercises for all weights lifted. Calculated moderate to high effects for both bench presses and squats indicate that observed differences may be important for practitioners.
These findings have to be taken into account when resistance exercises are implemented into the training program. Among other variables (number of repetitions and sets, rest intervals, etc.), the weight lifted is one of the most important factors that determines the training stimuli and consequently load- and velocity-specific adaptations. McBride at al. (18) demonstrated that peak power for the bar, body, and system is differentially affected by the load and movement pattern. When using the power clean, squat, or jump squat for training, the optimal load in each exercise may vary. Throwing athletes or weightlifters may be most concerned with bar power, but jumpers or sprinters may be more concerned with body or system power. Thomas et al. (22) found that 30% of 1RM will elicit peak power outputs for the squat jump, bench press, and hang pull exercises, allowing this standard percentage to be used as a starting point to train maximal mechanical power output capabilities in these lifts in strength trained athletes. As shown in this study, strength and conditioning specialists should be also aware that a maximal enhancing effect of peak and mean power in the acceleration phase of lifting occurs at lower weights than the mean power in the entire concentric phase of chest presses and squats. Specification of optimal weight for the improvement of the capability to use elastic energy during resistance exercises is of special importance for designing the training program and evaluating its efficiency. It is mainly related to sports requiring production of maximal force in a short time where obtaining maximal power is considered to be a more specific alternative than is the traditional 1RM approach (12).
Power output is higher during a CM than during concentric-only resistance exercises. This potentiation of power in the concentric phase of CM exercises may be ascribed to (a) the use of elastic energy accumulated in elastic tissues during the eccentric phase with a marked increase in the braking force, and (b) more pronounced muscle contraction as a consequence of the stretch reflex activated by the stimulation of proprioceptors by a rapid increase of force in the eccentric phase. However, these mechanisms mediate the enhancement of power only until the weight lifted does not exceed some level expressed as a percentage of 1RM. The maximal enhancement of peak and mean power in the acceleration phase of lifting occurs at lower weights than mean power in the entire concentric phase of both bench presses (∼50 and 60% of 1RM, respectively) and squats (∼70 and 80% of 1RM, respectively).
This finding that maximal values of peak and mean power in the acceleration phase of lifting are achieved at lower weights than mean power in the entire concentric phase of resistance exercises (i.e., bench presses, squats) has to be taken into account when training efficiency is evaluated, namely, in sports requiring the production of maximal force in a short time.
This project was supported by a Slovak Research and Development Agency (No. SK-SRB-0023-09).
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Keywords:Copyright © 2014 by the National Strength & Conditioning Association.
bench press; power outputs; squat; weight lifted