The stability of individuals is often in a state of flux, moving from relatively stable to unstable conditions and vice versa (37). This transition, known as metastability (37), is imperative for successful movement and balance, whether the movement is initiated by an elderly adult that fears falling or an elite athlete. Metastable training can help to maintain musculoskeletal health and rehabilitate musculoskeletal injuries (4,8,17,23,28) and enhance work and athletic performance (8,9,17).
According to the principle of training specificity (14,15), training must attempt to mimic the demands of the task. Many metastability exercises involve standing on an unstable surface (4,9,11,12,17). Metastable resistance exercises can increase both core (trunk) and limb muscle activation (2,3,30,34,41,42). Proponents of metastability training suggest the relative instability may stress the neuromuscular system to a greater extent than training with more stable devices (19,20,25,31,49,51). However, actions performed on unstable surfaces or using unstable devices can impair force (7,8,10,11,13,43), power (29,38), velocity, and range of motion (29). These force decrements may be attributed to a transition from muscle mobilizing to stabilizing functions (1). More research is necessary to investigate whether a progressive resistance training program, using an instability suspension device with multiple degrees of freedom, can provide training-induced improvements in force, neuromuscular efficiency (the extent of muscle activation needed to perform a task), and endurance. It is possible that more extreme levels of instability might not provide a conducive environment (because of instability-induced decreased force, power, velocity, and range of motion) to promote training adaptations (2,7).
There are many unstable surfaces available for the lower body such as inflatable balls, discs, wobble or balance boards, foam tubes, high-density and low-density foam platforms, as well as many other related devices (4,9–11,17). For the upper body, these aforementioned similar devices have been used as well as suspended chains and ropes (18,22,40). Typically, the ropes and chains for the upper body are suspended from a stable support with the instability derived from their lateral and anteroposterior degrees of freedom of movement. However, suspension devices that can provide unilateral movement in a vertical direction have only been investigated in one study (22). Athletes, such as rowers, boxers, and martial artists, would need movement training expertise in all 3 planes (frontal, sagittal, and transverse). The efficacy of such a device for training needs to be validated and more firmly established.
Most push-up training studies that compare stable vs. unstable training environments report increased lower back (i.e., lumbar erector spinae) (18,22,44), abdominals (40), triceps brachii (TB) (22,40), biceps brachii (BB) (40), upper trapezius (22,47), serratus anterior (SA) (18,45,50), and rectus femoris (18,22) electromyographic (EMG) activity. By contrast, Kalantari et al. (36) found greater trapezius, SA, BB, teres major, and posterior deltoid EMG activity with their stable push-ups. Another study used participants with scapular dyskinesis and reported decreased SA EMG activity with push-ups under unstable conditions (47). Because higher levels of instability can result in higher muscle activity but lower force output (2,7,10), it is unknown whether a highly unstable system with greater degrees of freedom would actually provide greater training benefits.
Training programs must be structured, so that athletes are prepared for the wide variety of postures and external forces. This is best accomplished through performance of exercises that encompass all planes of movement. There is also a need for progressive increases in resistance or complexity (instability). The unstable devices in most of the previous training studies cannot incorporate progressive increases in instability. A new device (Yoak) can be manipulated to suspend with single or double suspension in concert with slings, which allows for a progressive increase of instability. It is unknown whether this progression can provide substantial training improvements.
The objective of the study was to investigate possible training-related changes in strength, endurance, and neuromuscular efficiency after an 8-week push-up training program comparing push-ups preformed under stable and unstable suspension systems. It was hypothesized that the progressive instability (variety of degrees of freedom and planes of movement) and suspension (Yoak) push-up training program would provide greater training improvements than a program performed on a stable floor involving limited movement planes and axes.
Experimental Approach to the Problem
The study involved an 8-week push-up training program performed either with an unstable suspension system (Yoak) or on a stable floor. Pre- and post-training measures included maximal voluntary isometric contraction (MVIC) force with a chest press exercise, push-up endurance (number of repetitions), neuromuscular efficiency (mean EMG activity/muscle activation during the first 3 push-ups), and the fatigue index (EMG activity of the last 3 repetitions vs. first 3 push-up repetitions).
Based on a statistical power analysis of related training articles (14,21,48), approximately 10 participants would be needed in each training group to achieve an alpha of 0.05 with a power of 0.8. Hence, 22 participants were recruited and 19 participants completed the program (10 suspension subjects and 9 stable push-up subjects). Mean anthropometric characteristics for the suspension and stable push-up groups were as follows (all measurements mean ± SD): Yoak suspension group (5 men; 23.6 ± 3.04 years, 180.25 ± 3.41 cm, 76.86 ± 4.07 kg and 5 women; 22.2 ± 5.06 years, 161.04 ± 7.72 cm, and 64.3 ± 6.69 kg) and stable push-up group (3 men; 21.66 ± 1.52 years, 177.5 ± 4.33 cm, 78.63 ± 1.06 kg and 6 women; 21.16 ± 1.47 years, 164.86 ± 7.55 cm, and 63.3 ± 17.14 kg). All participants reported being recreationally active, engaging in resistance training or aerobic exercise at least twice per week. Exclusion criteria included any history of neurological or musculoskeletal injuries in the past year. Participants were instructed to refrain from vigorous physical activity and from consuming alcoholic beverages 24 hours before testing. Caffeinated beverages and dietary supplements were not to be consumed within 6 hours before testing. All participants were verbally informed of the experimental protocol and gave written informed consent. Ethical approval was obtained through the Memorial University of Newfoundland Institutional Center for Health Research ethics board (20180010-HK) and adhered to the Declaration of Helsinki.
Two pre-testing sessions (separated by at least 24 hours) were conducted the week before the 8-week training program, whereas 2 post-testing sessions using the 2 training devices (testing on Yoak suspension and stable floor for training-specific testing) and the chest press MVIC were conducted the week after the training program.
Before and after training, push-ups were performed to task failure (inability to raise the body to full-elbow extension) on separate days using the Yoak suspension system and on a stable floor. The pacing of the push-ups was consistent and monitored with a metronome at 1 Hz. Proper form (i.e., straight trunk) and consistent push-up depth (elbows flex to 90°) was ensured by visual inspection of the researchers. The number of repetitions performed was recorded.
In accordance with previously published reports from this laboratory (7,13,16) and SENIAM recommendations (33), EMG electrodes were placed collar to collar (2 cm) on the midbellies of the anterior deltoid (AD), BB, TB, and SA on the right side of the body. A ground electrode was placed on the head of the radius. Skin preparation for the electrodes included shaving, light abrading, followed by alcohol swabbing. Electromyography was collected using a Biopac (Biopac Systems, Inc., Holliston, MA, USA) data acquisition system at a sample rate of 2,000 Hz (amplification: 1,000, impedance = 2 MΩ, common mode-rejection ratio >110 dB·min (50/60 Hz), noise >5 μV). A band-pass filter (10–500 Hz) was applied before digital conversion. The mean amplitude of the root mean square (RMS) EMG was calculated and used for analysis. For the MVIC chest press, EMG was analyzed over a 1-second duration that included 500 ms before and after the peak force output. For the neuromuscular efficiency measures, the average RMS EMG of both the concentric and eccentric phases of the push-up was normalized to the chest press MVIC EMG. For the fatigue index, the first and last 3 repetitions of the number of repetitions used in the pre-training test were compared before and after training (mean average of last 3 repetitions/first 3 repetitions).
Chest Press Maximal Voluntary Isometric Contraction Peak Force
Participants performed an MVIC chest press while lying supine on a bench. With each hand, participants grasped handgrips attached to rope-like slings that were chained to a Wheatstone bridge configuration strain gauge (Omega Engineering, Inc., Don Mills, Ontario, Canada), which was connected to a metal plate on the floor. Differential voltage from the strain gauge was amplified (Biopac Systems, Inc., DA 100), A/D converted (AcqKnowledge BioPac MP100WSW; Holliston, MA, USA) and monitored on a computer at a sampling rate of 2,000 Hz. Verbal encouragement was given during the 4-seconds trial to ensure maximal force production. Two trials were separated by a 1-minute rest. The slings were arranged to ensure that the subjects' brachium (upper arm) was parallel to the floor with the elbows at 90°. Participants were requested to contract as fast and as hard as possible.
The 2 groups (Yoak suspension vs. stable push-ups) performed a series of push-ups 2 times per week over an 8-week period. The training program progressed from 1 set of push-ups to task failure in the first week, to 2 and 3 sets of each exercise in the second and subsequent weeks, respectively. Participants performed each set to failure to ensure that an overload stress was placed on the musculature with the goal to promote an overcompensatory training adaptation (5). The number of repetitions performed by each participant was recorded for every set and training session. Rest intervals consisted of 5 minutes between sets. Push-up training exercises were performed either with the suspension Yoak device or against an Olympic bar or on a stable floor using body mass as resistance. The Yoak device is a bowed, 1.2-m (4 feet) long, flat wooden cross-piece manufactured with an aluminum core, and multiple holes spanning its breadth for ease of grasping and hanging weights. When anchored overhead with slings, it can be manipulated to single or double suspension in concert with slings, which allow for a progressive increase of instability (Figure 1). The Yoak push-up strap height was altered dependent on the push-up capability of the participant to provide a progressive upper-body, metastable push-up training program. The first strap progression had the handles 1 meter above the floor placing the body in an approximate 45° angle to the floor. Once a participant could perform 15 or more repetitions at this height, they were progressed to the next strap height at 70 cm from the floor. Once again, if the individual could perform 15 or more push-ups, then they were progressed to a strap height of 40 cm from the floor (Figure 1). Henceforth, the participants would continue focusing on increasing the number of push-ups with no further strap height progressions. Similarly, with the stable push-up group, if 15 push-ups could not be accomplished, the participant would begin by performing push-ups against an olympic bar placed in a squat rack 1 meter above the floor (Figure 2). Once 15 or more push-ups were achieved, then they were similarly progressed to 40 cm from the floor and then to the floor. The unstable suspension system provided a greater challenge for executing the prescribed push-up movement. Thus, the third or final level for the unstable push-up training was performed at 40 cm from the floor, which provided a similar level of exertion as performing a stable push-up from the floor.
With the Yoak suspension device, the strap(s) suspended from the ceiling securing the Yoak could also be altered to provide a more stable configuration (2 suspension straps) or a relatively more unstable configuration (1 suspension strap) (Figure 3). Yoak suspension push-up training progressed from a double strap to combinations of double and single strap(s) (i.e., 2 sets with double suspension and 1 set with single suspension followed in the next week by 1 set with double suspension and 2 sets with single suspension). Researchers monitored participant form to ensure that a straight back was maintained, hands were placed immediately outside the width of the shoulders, and the pacing was consistent as monitored with a metronome (1 Hz: 1 second up and 1 second down). (Table 1 for training program details). Subjects were monitored to ensure that the push-up action descended until the elbows were at a right angle and ascended to a fully extended arm position.
The normality and homogeneity of variances within the data were confirmed with the Shapiro–Wilk and Levene tests, respectively. The chest press MVIC force was analyzed with a 2-way analysis of variance (ANOVA) (2 training groups × 2 times). The push-up endurance (number of repetitions), neuromuscular efficiency, and fatigue index were analyzed with a 3-way ANOVA (2 training groups × 2 testing conditions × 2 times). Training conditions included the Yoak suspension vs. the stable push-ups, testing conditions involved testing on the Yoak suspension vs. stable floor, with time conditions including pre- and post-training. Significant main effects were detected with a Bonferroni post hoc analysis, whereas interactions were identified with Student's paired t-test. The level of significance was set at p ≤ 0.05. Effect sizes (ESs) were calculated and reported. Effect size calculations involved dividing the change score by the mean SD of the raw data to arrive at a standard ES (26). Cohen's d values equal to or greater than 0.2, 0.5, and 0.8 were used to determine whether the effect sizes (magnitude of change) were small, medium, or large, respectively.
Push-up Endurance (Number of Repetitions)
A training condition × test condition interaction (p = 0.02) demonstrated that the Yoak suspension–trained group performed significantly more Yoak push-up repetitions (67.7%) compared with the stable group. There was a near significant training condition x time (p = 0.08) interaction with the Yoak suspension group demonstrating a greater training response than the stable group. The number of push-ups increased by 81.6 and 30.6% for the Yoak suspension and stable groups, respectively. A 3-way interaction (p = 0.03) illustrated that the stable trained group when performing push-ups on the Yoak suspension system performed 153.3, and 33.8% less repetitions than the Yoak suspension–trained group performing push-ups on the Yoak or on the stable floor, respectively, as well, the stable trained group performed 46.4% more push-ups on the stable floor compared with the Yoak testing condition (Figure 4).
A main effect for time (p < 0.0001) indicated that there were 56% more repetitions performed post-test compared with pre-test. The Yoak suspension–trained group executed a near significant 21.3% greater number of repetitions than the stable group (main effect for training condition: p = 0.1). A greater number of repetitions were performed (27.7%) when tested on the stable floor vs. on the Yoak suspension system (main effect for testing condition: p = 0.04).
Chest Press Maximal Voluntary Isometric Contraction Force
There was a significant training condition x time interaction (p = 0.03), whereby the Yoak suspension–trained group increased their chest press MVIC force by 9.2% compared with a 0.5% increase with the stable trained group (Figure 5). A main effect for time showed a significant (p < 0.05) 4.8% increase in chest press MVIC force.
Significant training condition × time interactions showed that the BB of the Yoak suspension–trained group exhibited a 30.4% (p = 0.013) decrease in EMG activity (improved neuromuscular efficiency with the first 3 repetitions), whereas the stable trained group demonstrated a 97.8% (p = 0.02) increase in EMG activity (diminished neuromuscular efficiency) when performing Yoak suspension push-ups when comparing pre- to post-training results. Both the AD and SA EMG activity indicated significant and near significant 3-way interactions. With the AD, the Yoak suspension–trained group had near significant 32.8% (p = 0.07) and 20.6% (p = 0.1) decreases in EMG activity with the first 3 Yoak suspension and stable floor push-up repetitions, respectively, after training. The Yoak suspension–trained group had significant 51.9% (p = 0.02) and 41.8% (p = 0.1) decreases in SA EMG activity with the first 3 Yoak suspension and stable floor push-up repetitions, respectively, after training. Furthermore, the stable trained group exhibited 62.2% (p = 0.04) less SA EMG activity after training when performing the first 3 push-up repetitions on the Yoak suspension system (Table 2). Main effects for training group were evident with improved neuromuscular efficiency for the BB (64.1%, p = 0.04) with the Yoak vs. stable training groups. A main effect for time for SA (p < 0.05) indicated 51.9% improved neuromuscular efficiency pre- to post-training.
There were significant main effects for training condition, test condition, and time but no significant interactions for the TB. A near-significant (p = 0.09) main effect for training condition indicated that, for the TB, the Yoak suspension–trained group had a 12.5% lower fatigue index than the stable trained group. Post-training results showed a 23.1% decrease in the TB fatigue index (main effect for time; p = 0.03). With the AD, a main effect for test condition (p = 0.026) showed an 8.9% lower fatigue index with the Yoak suspension system. There were no significant results for the BB or the SA.
The most important findings in this study were that the Yoak suspension training group showed training-specific responses with superior improvements for push-up endurance, and neuromuscular efficiency vs. the stable trained group. There was also evidence for some nontraining-specific, Yoak suspension training advantages with chest press MVIC force and near-significant improvements with the neuromuscular efficiency of the AD and SA and the TB fatigue index. Unlike a number of other stable and unstable resistance training systems, the Yoak suspension system can provide additional instability training progressions by advancing from a double suspension (2 bands) to a single suspension system. With the unencumbered exercise straps, resisted movement occurs over multiple planes and axes progressing from a bilateral emphasis (double suspension) to greater unilateral demands (single suspension).
The Yoak suspension group exhibited substantially higher (81.6%) training-induced increases in the number of push-up repetitions performed on the Yoak suspension system than the stable group (30.6%). This result follows the concept of training specificity (5,15), whereby training in an environment similar to the testing environment provides greater gains than a dissimilar testing environment. The Yoak suspension system, especially when suspended with a single sling, forces the individual to use each arm independently to ensure that balance is maintained. Stable push-ups permit an over-reliance on a dominant arm, for example, without the possibility of losing balance. This improved unilateral independent control of upper limbs with the Yoak system would be an important asset for sports that involve the coordinated control of both arms such as combat sports (boxing and martial arts), rowing, and other sports.
The Yoak suspension–trained group had significantly superior post-training force increases with the chest press MVIC. Although this action was performed while supine on a stable bench, it might also be ascribed to training specificity. The isometric bench press MVC was performed with 2 separate unilateral straps. Thus, the action was similar to the Yoak suspension push-up that was performed with 2 straps suspended from the Yoak. The Yoak-trained individuals would have become more proficient at coordinating 2 unilateral straps, which must have transferred more efficiently to this testing situation. If a single bar had been used rather than 2 independent straps, the bar would have emphasized a more bilateral action and might have provided a more similar action to the stable training push-up group. It could be speculated that the Yoak suspension training advantage might have been nullified with a single bar.
Although isometric force is typically reduced under unstable conditions (4,7,8,11,17), studies using dynamic unstable chest press exercises (similar muscle groups as the dynamic push-ups in this study) report either preserved isokinetic bench press strength (performed on a physioball) (27,32) or small force and power decrements (6–10%) (39) that might not compromise the training effect. Hence, the dynamic Yoak suspension push-ups during training may not have induced as substantial force impairments compared with previous isometric studies (4,7,8,11,17).
Similar training specificity was evident with the significantly improved neuromuscular efficiency of the BB (30.4%) with the Yoak suspension–trained group compared with the stable trained group, which demonstrated a 97.8% impairment in neuromuscular efficiency when performing Yoak suspension push-ups. Previous training studies have reported post-training lower antagonist EMG levels during lifting activities (24,46). If the BB works as an antagonist in the push-up exercise, why would there be an improved neuromuscular efficiency? The role of the antagonist with the push-ups would be to control arm and shoulder position. The short and long heads of the biceps can both contribute as stabilizers of the glenohumeral joint and their roles in stabilization increases as joint stability decreases (35). Unstable suspension training in this study may have improved the efficiency of the BB in its role for stabilization and motor control. Similarly, there were near significant improvements with the neuromuscular efficiency of the AD and SA. During push-ups, both muscles can provide stabilization for the shoulder and scapula, respectively, and thus tended to become more efficient after the Yoak suspension training program. The improved neuromuscular efficiency of these muscles (BB, AD, and SA) would decrease the extent of activation needed to perform the task (i.e., less motor unit recruitment and firing frequency) (6), decreasing the energy output needed for each push-up, and thus contributing to the increased number of repetitions performed.
The significant main effects for decreased fatigue indexes of the TB and AD also indicate that, for a similar number of push-ups after training and before training, the EMG increase due to fatiguing contractions was lower. With submaximal fatiguing contractions, there is an increased recruitment and rate coding of motor units to compensate for the initially fatigued motor units resulting in an increased EMG signal (6). Similar to the neuromuscular efficiency exhibited with the first 3 contractions, the lower degree of muscle activation and associated lower energy output would reduce the amplitude of the RMS EMG signal illustrating an improved efficiency not only with nonfatigued but also under more fatiguing conditions.
A limitation of the study was the unequal sex ratio in each group (Yoak: 5/5 vs. Stable 3/6). However, although the ratio did not precisely match the general population (50%/50%), there was representation from both sexes. Because most studies suffer from inadequate female representation, this study actually had slightly greater female numbers.
In conclusion, the progressive metastable Yoak suspension training program provided greater training-specific improvements for push-up endurance (number of repetitions performed), chest press MVIC force (using 2 independent unilateral straps), as well as increased neuromuscular efficiency and lower fatigue indexes. It is suggested that the challenges of the metastable environment of the Yoak suspension system evoked greater training adaptations than stable push-ups by enabling improved stabilization and motor control. Although the greater chest press MVIC would suggest greater strength increases with the Yoak suspension system, it is also possible that the aforementioned possible improvements in stabilization and coordination could have made substantial contributions to the MVIC force increases.
It is recommended that training devices such as the Yoak suspension system, which enable varying degrees of instability, can provide superior strength and endurance training results especially when tested under similar metastable conditions. Although all individuals could benefit from such training, athletes who use unilateral arm movements such as combat athletes (i.e., boxers, wrestlers, and martial arts), rowers, racquet athletes (i.e., tennis, squash), and others could especially benefit from the training-specific effects. Individuals can use this system to progress from stable bilateral exercises to unstable exercises beginning with a double suspension system to a single suspension system. Hence, the concept of progressive resistance training can be incorporated even after switching to an unstable device such as the Yoak.
The authors declare no conflicts of interest with the contents of the article.
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