Cardiorespiratory response to traditional and instability resistance exercises

INTRODUCTION
Recently, resistance exercises performed on an unstable surface became a part of athletic training and rehabilitation. Accordingly, their role in performance and health has become a matter of interest among conditioning specialists and researchers [14, 17]. They have measured and analyzed strength and balance parameters during various instability resistance exercises. It has been demonstrated that there is a significantly greater electromyographic (EMG) activity of trunk-stabilizing muscles under unstable than stable conditions during exercises, such as curl-up [20], bridge [5, 14], dumbbell chest press [15], and squat [2, 16].
However, the responses of upper and lower limb muscles to such exercises performed on an unstable surface remain a matter of debate. Significantly lower isometric force output has been demonstrated under unstable than stable conditions during strength exercises, such as dumbbell chest press (60% in peak isometric force) [1], isometric knee extension (70% in force), plantarflexion (30% in force) [4], and squat (46% in force and 40.5% in rate of force development) [16]. However, when the dynamic chest press was performed on an unstable surface the reduction rates were lower (approximately 6% in force and 10% in velocity and power outputs) [13]. These findings indicate that various factors play a role in the amount of force reduction, such as the type of exercise, the form of muscle contraction, the degree of instability during the task, and so forth.

Likewise, the postural sway response to resistance exercise depends on various factors, including the type of exercise, its intensity (e.g., the weight lifted), the rate of movement, the muscle mass activated, the intensity of proprioceptive stimulation, the number of repetitions and sets [24]. More specifically, higher sway velocity has been found [22] after exercises performed with lower- (squats and calf rises) than with upper limbs (biceps curls and behind the neck presses). Such an impairment of balance in the early phase of recovery after resistance exercises is, rather than fatigue, a consequence of more marked ventilation [23]. However, this was proved only under stable conditions and there is no information on changes in balance and ventilatory parameters induced by strength exercises performed on an unstable surface.

Yet, taking into account significant differences in the EMG activity of trunk muscles, strength parameters of upper and lower limbs and probably also in sway variables during stability and instability resistance exercises, one may also expect a different cardiorespiratory and metabolic response to such exercises. However, the only to-date comparison of the kinetics of oxygen uptake (VO2), heart rate (HR) and related parameters during interval running and weight lifting has been investigated [10, 11]. It has been found that during active intervals oxygen uptake increases in both exercise forms, though the increase is less pronounced during lifting. The slighter response in the latter is compensated by a further increase in the early phase of recovery, whereas in running oxygen uptake diminishes immediately after interruption. The HR increases throughout active intervals of both exercise types; however, contrary to VO2, this increase continues only during running, whereas in resistance exercise a decrease sets in immediately after the final rep. Reflecting these changes, VO2/HR in running as compared to lifting is significantly higher during active phases, but lower in the early phase of recovery. It means that while in running the oxygen pulse follows the actual energy demand, in heavy resistance exercise the impaired heart pumping function caused by increased intrathoracic pressure has to be compensated by a further increase in oxygen pulse and very probably also of stroke volume in the early phase of recovery.

Since the current literature offers no information whether instability resistance exercises differ substantially in this respect from typical strength exercises, the aim of the study was to compare the cardiorespiratory response to upper and lower body resistance exercises performed under stable and unstable conditions, respectively.

MATERIALS AND METHODS
SUBJECTS
A group of 16 physical education students (age 23.4 ± 1.9 yrs., 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 among many others. However, they had no experience with instability resistance exercises. They were asked to avoid any strenuous exercises during the study. All participants were informed on the procedures and on the main purpose of the study. The procedures presented were in accordance with the ethical standards on human experimentation.

MEASUREMENTS AND PROCEDURE
STUDY SETTING
Before the beginning of the study, the subjects had a familiarization session, during which the technique of both exercises, particularly on unstable surfaces, was explained. Emphasis was placed on achieving the knee angle of 90° during squats, and on the rate of movement during both exercises. Exercises were performed with countermovement using maximal effort.

Afterwards they performed randomly on different days 6 sets of 8 repetitions of a) barbell chest presses on bench and Swiss ball, respectively, and b) barbell squats on a stable surface and Bosu ball, respectively. The weight lifted was calculated as a percentage of their 1 repetition maximum (1RM). All exercises were performed with previously established 70% of 1RM. According to the instructions, sets were completed in approximately 20 seconds and followed by 2 minutes of rest period.

Under unstable conditions, the barbell chest presses were performed in the supine position placing the Swiss ball in the thoracic area with the feet on the floor (Figure 1), which engaged a wider base of support than squats performed in the standing position. Squats were performed from full extension to the knee angle of 90° while supporting a barbell on the back. A laboratory assistant stood behind the subjects to prevent a possible fall.

While exercising and during recovery periods, the cardiorespiratory parameters (VO2, VCO2, VE, O2-Pulse, HR) were monitored by means of breath-by-breath system Spiroergometry CS 200. After the lifts were completed, the post-exercise oxygen consumption was recorded until its values fell below the resting O2 uptake (approximately 6.0 ml•kg-1•min-1). Blood samples from the fingertip were taken in the 6th minute of recovery after the last set for the estimation of lactate concentration (in 6 subjects only). Enzymatic method (Boehringer sets) was used for the analysis.

STATISTICAL ANALYSIS
Ordinary statistical methods including average and standard deviation were used. A paired Students t-test was employed to determine the statistical significance of differences between cardiorespiratory parameters during resistance exercises performed on stable and unstable surfaces, respectively, p < 0.05 was considered significant.

RESULTS
The kinetics of most of the cardiorespiratory parameters revealed (Figure 2) only slight changes during active lifting period compensated by a rather pronounced increase in an early phase of recovery. More specifically, oxygen uptake after both stability and instability resistance exercises increased and it was only after about 30 to 40 seconds that a gradual decrease back to the resting level set in. On the other hand, the heart rate reached the maximum at the end of exercise and started to decline immediately in the recovery phase, as can be seen in detail in Figure 3.

Even more delayed was the response of oxygen pulse which remained relatively unchanged during exercise and started to increase in recovery reaching the maximum after some 40 to 50 second (Figure 4).
However, the values of VO2/kg obtained during active lifting intervals were significantly (p ≤ 0.01) higher when chest presses were performed on an unstable than on a stable surface (Figure 5). Contrary to this, no significant differences were observed during squats performed under unstable and stable conditions (Figure 6). The same trend was observed for VCO2/kg.

In spite of these differences observed during initial repetitions, there was a gradual increase in cardiorespiratory parameters (VO2/kg, VCO2/kg, VE/kg, O2-Pulse, HR) from the 1st to the 6th set, during both instability and stability resistance exercises. For instance, peak values of VO2 (Figures 7 and 8) increased similarly under stable as well as unstable conditions during chest presses (from 11.5 ± 1.5 ml•kg-1•min-1 to 12.6 ± 2.2 ml•kg-1•min-1 and from 13.6 ± 1.9 ml•kg-1•min-1 to 14.9 ± 2.4 ml•kg-1•min-1, respectively) and squats (from 17.8 ± 2.9 ml•kg-1•min-1 to 19.2 ± 2.9 ml•kg-1•min-1 and from 19.4 ± 2.7 ml•kg-1•min-1 to 21.2 ± 2.9 ml•kg-1•min-1, respectively).

In comparison with the increase in VO2 (about 6-7%), even greater increase in ventilation from the 1st to the 6th set of chest presses and squats performed on stable and unstable surfaces was observed (about 19-20%) (Figures 9 and 10).

DISCUSSION
As reported in the previous study by Hamar et al [11] cardiorespiratory response to typical resistance exercise is characterized by a low increase in pulmonary ventilation, oxygen uptake and cardiac output during active lifting period. However, this dampened response is compensated by delayed activation of these functional parameters in an early phase of the subsequent recovery interval. The delayed response of cardiorespiratory parameters during strength exercise occurs due to a substantial increase in intrathoracic pressure necessary for stabilizing chest walls that provide support for muscles of upper and lower extremities. High intrathoracic pressure has a detrimental effect not only on pulmonary ventilation, but namely on the heart function (decreased filling and emptying of the ventricles). The result is a decrease in stroke volume prohibiting an adequate increase in cardiac output despite a compensatory increase in the heart rate.
The present study showed similar kinetics of cardiorespiratory parameters during instability resistance exercises. It has been found that there is a marked difference between a rather fast response of the heart rate reaching the maximum at the end of exercise and the delayed increase in oxygen uptake with maximum levels after some 30 to 40 seconds of recovery. Even more delayed was the response of the oxygen pulse, which remained relatively unchanged during exercise and started to increase in recovery reaching the maximum after about 40 to 50 seconds.

However, cardiorespiratory parameters obtained during active lifting interval and in the following recovery period were significantly higher when barbell chest presses were performed under unstable versus stable conditions. These differences, namely in the parameters of ventilation, are very probably due to higher blood lactate after chest presses performed on an unstable than on a stable surface (on average 6.2 mmol/l and 4.9 mmol/l, respectively). Despite the fact that increased concentration of hydrogen ions is partly compensated by the buffer systems, it also increases ventilation due to the stimulation of the respiratory center. Besides higher blood lactate after instability chest presses, higher oxygen deficit may be also assumed. Consequently, the higher oxygen deficit had to be paid off during recovery, which also contributed to a more pronounced elevation in ventilation. These phenomena are responsible for more marked ventilation induced by more intensive instability resistance exercise with higher contribution of anaerobic glycolysis. Since the intensity of exercises in term of the weight lifted was the same under stable and unstable conditions (70% of 1RM), the higher difficulty of chest presses performed on a Swiss ball may be assumed. This may be documented by a significantly greater EMG activity of trunk-stabilizing muscles under unstable versus stable conditions during dumbbell chest presses [15]. The high muscle activation during exercises performed on an unstable surface can be attributed to their increased stabilization function. This is due to additional stresses imposed on the synergistic and stabilizing muscles of the trunk during chest presses on a Swiss ball placed in the upper thoracic area with the feet placed on the floor [6].

Even more profound respiration as a result of the anaerobic acidosis compensation was induced by squats. Such an exercise also causes an intensive stimulus for the respiratory system due to accumulation of lactate. However its values did not differ significantly after squats performed on stable and unstable surfaces (on average 6.7 mmol/L and 7.0 mmol/L, respectively). This might partly explain no significant differences in the parameters of ventilation when squats were performed under stable and unstable conditions. It may be assumed that using an unstable device (Bosu ball) did not provide sufficient challenges to the neuromuscular system. This assumption may be corroborated by findings of Wahl and Behm [21] who showed no significant differences in EMG activity of the lower body and trunk musculature between standing and squatting on a 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.

Our experience also speaks in favor of this. It has been found [25] that there is a significantly lower power output, namely in the acceleration phase, during chest presses performed on a Swiss ball rather than on the bench. Production of the lower work rate along with higher oxygen uptake during instability versus stability resistance exercises indicates lower mechanical efficiency of muscular work. This lower efficiency may be explained by the compromised reuse of elastic energy when chest presses with countermovement were performed on an unstable surface. In such a case the concentric muscle action very probably did not occur immediately following the eccentric one as the subjects tried to stabilize themselves on the ball. As a result, most of the stored energy dissipated and was lost as heat and the potentiating ability of the stretch reflex was negated. This consequently yields lower power in the acceleration phase of chest presses performed on a Swiss ball. In addition, provided that utilization of elastic energy stored in eccentric phase was less efficient, more amount of energy was needed for subsequent concentric contraction. The result is lower mechanical efficiency of muscular work during chest presses performed on unstable versus stable surfaces.
Although it would appear that cardiorespiratory response to instability resistance exercises depends mainly on its type and the degree of instability, the role of other factors such as the intensity of exercise (e.g., weight lifted), the rate of movement, the muscle mass activated, and the number repetitions and sets should be also taken into account.

CONCLUSION
1. Kinetics of cardiorespiratory parameters during and after upper and lower body resistance exercises is similar under stable and unstable conditions. There are only slight changes in cardiorespiratory parameters during the active lifting period compensated by a rather pronounced increase in an early phase of recovery. While oxygen uptake after both stability and instability resistance exercises increases and it is only after about 30 to 40 seconds that a gradual decrease back to the resting level sets in, the heart rate reaches the maximum at the end of exercise and starts to decline immediately in the recovery phase. Even more delayed is the response of oxygen pulse which remains relatively unchanged during exercise and starts to increase in recovery reaching the maximum after some 40 to 50 seconds.

2. Oxygen uptake during exercises performed with upper extremities is significantly higher under unstable than under stable conditions, indicating that such exercise represents a more intensive stimulus for cardiorespiratory functions providing oxygen for working muscles as well as for anaerobic glycolysis as compared to the same exercise performed on a stable surface. In contrast, there are no differences in the cardiorespiratory response to stability and instability resistance exercises performed with lower extremities.

3. Cardiorespiratory parameters gradually increase with increasing the number of sets, similarly under stable and unstable conditions in both exercises.

PRACTICAL APPLICATION
These findings indicate that the cardiorespiratory response to instability resistance exercises depends not only on its type but also on the degree of instability providing a different level of difficulty. However, further studies are needed to investigate the role in cardiorespiratory and metabolic responses to resistance exercises performed under unstable conditions played by other factors, such as the intensity of exercise (e.g., weight lifted), the rate of movement, the muscle mass activated, the number of repetitions and sets, etc. Using 6 sets of 8 reps in the present study showed that cardiorespiratory parameters increased with increasing the number of sets similarly under stable and unstable conditions in both exercises.

Taking into account different cardiorespiratory response to instability resistance exercises performed with upper and lower extremities, the functional adaptation to such exercises should be also investigated. Cressay et al [8] suggest that unstable surface training would be best utilized in the upper body, which typically operates in an open-chain fashion in the majority of sporting movements. Conversely, similar interventions could prove to negatively affect performance in the lower extremities, which typically operate in a closed-chain fashion.

Since physiological response to instability resistance exercises seems to be task-specific, it is also important to evaluate the adaptive changes in cardiorespiratory parameters induced by exercises performed on unstable surfaces. It has been found [19] that such 6-week training does not significantly improve treadmill VO2max, running economy or running posture. Conversely, the importance has been previously established of structural free weight exercises, such as squats [3, 7, 9, 12, 18] and deadlifts [18] in training programs focused on improvement of muscular strength, hypertrophy and running economy. These findings indicate that even more studies are needed to quantify the positive as well as negative effects of instability resistance exercises on sport performance.

ACKNOWLEDGEMENTS
This project was supported by the Slovak Research and Development Agency (No. SK-SRB-0023-09).