Recently, resistance exercises performed on unstable surface have become a part of athletic training and rehabilitation. Accordingly, their role in performance and health-oriented strength training started to be a matter of interest among conditioning specialists and researchers. More pronounced activation of stabilizing muscles has been assumed as the main feature of resistance exercises performed under unstable conditions. This assumption has been proved by electromyographic (EMG) studies showing significantly greater EMG activity of trunk-stabilizing muscles under unstable than stable conditions during exercises, such as curl-up (16), bridge (5,12), dumbbell chest press (13), and squat (2,14). More effective improvement of trunk stability after short-term training program using the Swiss ball as compared with floor exercises may be also documented by intervention studies (6,15). These findings indicate that instability resistance training may facilitate the neural adaptation of trunk-stabilizing muscles, resulting in improvement of trunk stability.
On the other hand, both acute and long-term responses of primarilly activated muscles to exercises performed on unstable surface remain a matter of debate. A significantly lower peak isometric force has been observed under unstable than under stable conditions during dumbbell chest press (60%) (1), isometric knee extension (70%), plantar flexion (30%) (4), squat (46%) (14), and rate of force development during squats (40.5%) (14).
Because of lower force production during instability resistance exercises, some authors (13,14) recommended resistance exercises on a stable base to improve muscular strength and athletic performance. The main reason is that 80% of the maximum muscular strength required for its enhancement in trained individuals (10) is not met during instability resistance exercises (14).
Nevertheless, it should be noted that most of these studies used isometric exercises. When dynamic chest press was performed on an unstable surface, the reduction rates were smaller (∼6% in force and 10% in velocity and power outputs) (8). However, this was proved only in the case of upper body instability resistance exercise, and there is no information on strength parameters during those performed with lower limbs. It may be assumed that the mechanism of power production in the upper limbs is different from the lower limbs (antigravity muscles), in which some tension has to be continuously exerted to maintain standing posture on unstable support and where the body weight is used as the load. Besides this, the question also remains as to what role in power production plays the degree of instability of devices used (e.g., Swiss ball and Bosu ball). Moreover, force and power outputs during a single chest press or squat on stable and unstable surface have been usually evaluated; however, little is known about these parameters while performing resistance exercises in interval mode.
Therefore, the aim of the study was to compare the power outputs in concentric phase of chest presses and squats performed in 6 sets of 8 reps on stable and unstable support surface.
Experimental Approach to the Problem
The study evaluates the power outputs in the concentric phase of resistance exercises performed in the interval mode under stable and unstable conditions. The subjects performed randomly on different days 6 sets of 8 reps of (a) chest presses on the bench and Swiss ball, respectively, and (b) squats on stable support base and Bosu ball, respectively. Rest interval of 2 minutes was applied between particular sets. The exercises were performed with previously established 70% of 1 repetition maximum (1RM) under stable conditions. A PC-based system FiTRO Dyne Premium was used to monitor force and velocity and to calculate power. Peak power and mean power in acceleration and entire concentric phase of lifting were analyzed.
A group of 16 physical education students (age 23.4 ± 1.9 years, height 181.5 ± 6.1 cm, weight 75.1 ± 6.1 kg) volunteered to participate in the study. All of them had experience with resistance training involving exercises such as chest presses and squats. However, they had no experience with instability resistance exercises. They were asked to avoid any strenuous exercises during the study. All the participants were informed on 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 approved by an institutional review board.
Before the study, the subjects were exposed to a familiarization session, during which the techniques of both exercises, namely, on unstable surfaces, were explained and trial sets carried out. Emphasis was placed on proper technique of exercises and achieving a knee angle of 90° during squats. Exercises were performed with countermovement using maximal effort in concentric phase. A metronome was employed to guide the frequency of repetitions.
Afterward, they performed randomly in different days 6 sets of 8 repetitions of (a) barbell chest presses on the bench and Swiss ball, respectively, and (b) barbell squats on stable support base and Bosu ball, respectively. A rest interval of 2 minutes was applied between particular sets. The weight lifted was calculated as a percentage of their 1RM. All the exercises were performed with previously established 70% of 1RM under stable conditions. According to Goodman et al. (7), there are no significant differences in 1RM strength and muscle activation during the barbell chest press on a stable and unstable surface.
The barbell chest presses were performed in the supine position with placement of the Swiss ball in the thoracic area and with the feet placed on the floor, which provided a wider base of support than squats performed in the standing position. Squats were performed from full extension to a knee angle of 90° while holding a barbell on the back. To provide similar unstable conditions as during chest presses on Swiss ball, the subjects during squats stand on bladder side of Bosu ball. According to Laudner and Koschnitzky (11), there are no significant differences in EMG data for any muscle (tibialis anterior, peroneus longus, medial gastrocnemius) during single-leg stance on each side of the Bosu balance trainer, which demonstrate no benefit to the amount of ankle muscle activity resulting from flipping the Bosu balance trainer onto the bladder side. The laboratory assistant stood behind the subjects to impede a possible fall.
A computer-based system FiTRO Dyne Premium was used to monitor basic biomechanical parameters involved in lifting exercise (FiTRONiC s.r.o., SK).
The systems consists of a sensor unit based on precise encoder mechanically coupled with reel. While pulling the tether (connected by means of small hook to barbell axis) out of the reel, it rotates and measures velocity. Rewinding of the reel is guaranteed by string producing force of about 2 N. Signals from the sensor unit are conveyed to the PC by means of a USB cable.
The system operates on Newton's law of universal gravitation (force equals mass multiplied by gravitational constant) and Newton's law of motion (force equals mass multiplied by acceleration). Instant force while moving barbell of the mass in vertical direction is calculated as a sum of gravitational force (mass multiplied by gravitational constant) and acceleration force (mass multiplied by acceleration). Acceleration of vertical movements (positive or negative) is obtained by derivation of vertical velocity, measured by highly precise device, mechanically coupled with the barbell. Power is calculated as a product of force and velocity and the actual position by integration of velocity. Comprehensive software allows one to collect, calculate, and on-line display the basic biomechanical parameters involved in weight exercise.
The device was placed on the floor and anchored to the bar by a nylon tether. The subjects performed exercises while pulling a nylon tether of the device (Figure 1). Peak and mean values of power and velocity were obtained from entire concentric phase of lifting, and from its acceleration segment.
Data analyses were performed using statistical program SPSS for Windows version 18.0. Ordinary statistical methods including average and SD were used. A paired t-test was employed to determine the statistical significance of differences between power outputs during chest presses and squats performed on stable and unstable surfaces, respectively. The criterion level for significance was set at p ≤ 0.05.
The results showed (Table 1) that peak and mean power in the entire concentric phase of lifting and in its acceleration segment were significantly higher under stable than unstable conditions.
In the initial set, there was greater decline of mean power in the entire concentric phase of chest presses on the Swiss ball than on the bench (13.2 and 7.7%, respectively) (Figure 2), and squats on Bosu ball than on the stable support base (10.3 and 7.2%, respectively) (Figure 3).
In the final set, the mean power in the entire concentric phase of lifting decreased more profoundy when chest presses were performed on a Swiss ball (from 394.7 ± 33.2 W to 316.3 ± 38.1 W) than on the bench (from 437.7 ± 42.3 W to 385.7 ± 34.4 W) (Figure 4). Consequently, fatigue index was significantly (p < 0.05) higher under unstable than under stable conditions (19.9 and 11.8%, respectively).
Contrary to this, the mean power in the entire concentric phase of squats decreased similarly on the Bosu ball (from 418.7 ± 37.4 W to 371.1 ± 32.5 W) and stable support base (from 500.1 ± 48.0 W to 452.1 ± 44.1 W) (Figure 5), which may be corroborated by no significant differences in fatigue index (11.4 and 9.6%, respectively).
Lower power in the concentric phase of resistance exercises with CM performed on an unstable surface may be ascribed to delayed amortization phase of stretch-shortening cycle (SSC). It is known, that the activation of SSC during exercise with countermovement enhances the power output in the concentric phase of the lifting exercise. The mechanism of power production using SSC employs the energy storage capabilities of series of elastic component and the stimulation of stretch reflex to facilitate the muscle contraction over a minimal amount of time. If a concentric muscle action does not occur immediately after the eccentric one, the stored energy dissipates and is lost as a heat and also the potentiating stretch reflex fails to be activated. Instability resistance exercise may compromise all 3 phases of SSCs, namely, the amortization phase. Around this turning point, where the eccentric phase changes into the concentric one, maximal force is produced. At the same time, the subjects must stabilize torso on an unstable surface to provide firm support for contracting muscles. This additional task may compromise the contraction of muscles acting on the barbell. Their less intensive contraction not only prolongs the change of movement direction, but because of lower peak force, negatively impairs accumulation of elastic energy. The consequence is lower velocity and power in the subsequent concentric phase. This imparing effect is more evident during chest presses on the Swiss ball than during squats on the Bosu ball.
It may be therefore assumed that the Swiss ball imposed a greater degree of instability resulting in a higher difficulty of the task. This may be documented by significantly greater EMG activity of trunk-stabilizing muscles under unstable than under stable conditions during the dumbbell chest press (13). The high muscle activation during exercises performed on an unstable surface can be attributed to their increased stabilization function. This is because of additional stresses imposed on the synergistic and stabilizing muscles of the trunk during chest presses with the back supported by unstable Swiss ball (3).
On the other hand, the Bosu ball likely did not provide a sufficient level of difficulty to induce significant changes in the neuromuscular system. This assumption may be corroborated by study of Wahl and Behm (17) showing no significant differences in the EMG activity of the lower body and trunk musculature between standing and squatting on stable base and on Dyna discs and Bosu balls. According to the authors, these moderately unstable devices are not as effective as Swiss balls and wobble boards in increasing muscle activation with highly resistance-trained individuals.
The degree of instability very probably played a role also in a more profound decrease of power output during chest presses than during squats with increasing number of repetitions (13.2 and 10.3%, respectively) and sets (19.9 and 11.4%, respectively). This greater decline in power output with repeated chest presses was likely because of higher difficulty of the task on the Swiss ball eliciting greater fatigue of the neuromuscular system. It is known that fatigue-induced decrease in power production during exercise involving countermovement has a more pronounced influence on the concentric phase of SSCs. This effect may be ascribed mainly to the altered sensory feedback of the periphery to the central nervous system that could contribute to the less precise stiffness regulation of the relevant muscles. This mechanism may be more compromised by chest presses performed on the Swiss ball than squats on the Bosu ball.
However, besides the type of exercise and the degree of instability of devices used, also other factors such as intensity of exercise (e.g., weight lifted), cadence of movement, muscle mass activated, number of repetitions and sets, or duration of rest periods have to be taken into account. Their effect on power production during instability resistance exercises can substantially differ from those performed on stable support surface. Taking this fact into account, conditioning programs in high-velocity sports should prefer the means that better simulate movement rates achieved with actual competition.
Although our findings indicate that instability resistance exercises should not be used for strength and power training, research has established their benefits in prevention of injuries and improvement of core stability and balance.
Power outputs in the concentric phase of resistance exercises are significantly lower on unstable than stable support surface. When instability resistance exercises are performed in interval mode, in the initial sets, there is a similar decline of mean power in the entire concentric phase of chest presses and squats. However, its values in the final sets decrease more profoundy when chest presses are performed on the Swiss ball than on the bench. Consequently, the fatigue index is significantly higher under unstable than under stable conditions. Contrary to this, the mean power in entire concentric phase of squats decreases similarly on the Bosu ball and on stable support base; thus, there are no significant differences in the fatigue index.
Having this information can serve as a basis for the elaboration of exercise programs applicable in sports and rehabilitation. Although there are several studies supporting the implementation of instability resistance exercises into rehabilitation program, only scarce reports exist concerning their use in sports training. However, research conducted in the rehabilitation sector cannot be simply applied to the sport environment. This is mainly because of different demands on strength and power during everyday activities (low load, slow movements) and sport activities (high load, resisted, dynamic movements). As shown, the power output is compromised during instability resistance exercise. The reduction rates seem to be similar (∼10% for chest press and 16% for squat) as has been reported in previous studies (∼6% in force and 10% in velocity and power outputs for chest press) (8). These differences in strength parameters may depend on the type of exercise and instability devices used (18,20), weight lifted (19), population tested (9), and their training background (21). These findings have to be taken into account when instability resistance exercises are implemented into the training program, namely, for sports that require production of maximal force in short time. It is known that for improvement of athletic performance in sports involving high-velocity movements, workouts should be tailored with respect to movement rates of the particular sport. Therefore, prospective studies are necessary to propose which instability resistance training is effective for improvement of athletic performance.
This project was supported by a Slovak Research and Development Agency (No. SK-SRB-0023-09).
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Keywords:© 2012 National Strength and Conditioning Association
Bosu ball; muscular power; chest presses; squats; stable surface; Swiss ball