Instability training is a common resistance training method used in exercise programs and fitness facilities today. Instability can be obtained through the use of many devices and techniques including, but not limited to, unstable platforms such as Bosu balls or Swiss balls (2,30,38) and/or completing open kinetic chain exercises with the use of free weights (19,25,29). More recently suspension training systems have been added to the list of instability training devices. In suspension training, as the name suggests, straps and/or ropes are used to suspend specific body segments in the air. Individuals then work against their body weight as they complete exercises in the unstable environment created by the suspension straps. Although considerable research has examined more traditional means of instability training (3), little previous research has evaluated the effects of suspension training. This research will therefore examine suspension training, with a focus on the front plank, an exercise known to specifically target trunk musculature. In particular, it will examine the effects of different levels of suspension on trunk and lower limb muscle activation in an effort to provide fitness professionals with exercise progression guidelines when using this training technique.
Strength and endurance training of trunk musculature is an essential area of resistance training that has gained renewed emphasis in scientific literature, athletic training, and rehabilitation fields (4). Although the efficacy of trunk muscle training as an athletic performance enhancement tool lacks sound research support (36), positive effects of such training have been reported in the areas of low back health (1,27), injury reduction (44), ability to transfer power to extremities (39) and, falls prevention in seniors (21,22). Despite the fact that research aimed at quantifying the effects of various types of trunk strengthening exercises is available (4,10,32,37), it is often a challenge for athletes or strength and conditioning practitioners to know which exercises will best meet their needs (41).
One trunk muscle training method that has received considerable attention is the use of instability (11,18,26,40,45). Although some researchers have reported no effect of instability on trunk muscle activation (20,42), research has generally indicated that exercising on unstable surfaces enhances trunk muscle activity (4,6,18,34,45). As reviewed by Behm et al. (4,5), although instability resistance training is not the best choice for all training circumstances (i.e., it can result in a decrease in power output), it is generally recognized as being an important part of any well-rounded exercise program (5,18). As many activities of daily living and most sporting activities are done in nonstatic and thereby relatively unstable conditions, an additional benefit of instability training is that it may provide a more effective transfer of training adaptations to everyday tasks (3). Although research suggests benefits of instability training for both athletic and general populations, research is lacking on whether suspension training systems provide some of the same benefits.
Suspension training is not new. Evidence of rope training date back to the mid 1800's (see ref. 14 for example) and gymnasts and trapeze artists all perform aspects of their sport using suspension. Since the mid-1990's, commercially available suspension training systems have been on the market. Despite this availability of numerous suspension training systems (i.e., FKPro system, Redcord, aeroSling ELITE, The Hook Isometrics/Suspension Trainer, Ztrainer Suspension Fitness System, TRX suspension trainer), there is relatively little research available to fitness professionals on how the use of suspension training impacts muscle activation. Most of the literature that does exist (12,26,35) is limited to reporting long-term performance based measures. Only 1 study (31), examining pushing exercises, provided practitioners with outcomes that could be used for exercise prescription. As such, evidenced-based decision making regarding suspension training exercise prescription and progression is currently very challenging for fitness professionals.
The primary purpose of this study was therefore to examine differences in abdominal muscle activation across 4 plank variations using a suspension training system. Since previous literature has demonstrated greater electromyography (EMG) activity using instability devices and exercises, it was hypothesized that planks performed using suspension would result in greater abdominal muscle activation than stable planks and that the highest levels of activation would be observed when both arms and feet were suspended. The secondary aim of the study was to determine if activation of rectus femoris (RF) and serratus anterior (SA) would be affected when suspension training was employed. As neither of these muscles is considered key agonists in the plank exercise, it was hypothesized that suspension exercises would not change their activation levels.
Experimental Approach to the Problem
A repeated measures design was used to compare muscle activation during the 4 different plank exercises to determine if muscle activation changed as a result of the level of suspension used. Suspension was created using a TRX suspension training system (Fitness Anywhere, Inc., San Francisco, CA, USA). Electromyography was used to measure activation of 4 muscles (a) rectus abdominis (RA), (b) external oblique (EO), (c) (RF), and (d) SA. To determine the effect of suspension training on muscle activation, the degree of suspension was varied by using 4 exercise conditions: (a) stable plank with toes and forearms on floor (Norm), (b) plank performed with forearms suspended and toes on floor (ArmsIn), (c) plank performed feet suspended and forearms on floor (FeetIn), and (d) plank performed with feet and forearms suspended using 2 suspension systems (Full). All exercises are shown in Figure 1.
Twenty-one (10 women, 11 men) university aged individuals (age, 21.9 ± 2.4 years; height, 175.5 ± 10.13 cm; mass, 74.2 ± 12.61 kg) participated in this study. Participants were excluded if: (a) correct posture could not be maintained during the plank exercises or (b) participants reported a previous history of low back, abdominal, or shoulder injury or any pain that prevented correct completion of the plank conditions. This screening process reduced the potential for the occurrence of injury, along with any shortcomings which may have occurred due to participant fatigue. All participants signed an informed consent form prior to participating in the study. The research was approved by the local research ethics board and as such conformed to Code of Ethics of the World Medical Association.
Participants attended 2 sessions. During session 1, participants were introduced to the 4 plank exercises (see descriptions below and Figure 1) to ensure that they could perform all exercises using a proper form. On the second day of testing, participants were first prepared for collection of EMG data. All electrodes were placed on the right side of the body. Surface electrodes were positioned in accordance with SENIAM (surface EMG for the noninvasive assessment of muscles) recommendations and were confirmed by muscle palpation (23). The skin at the electrode sites was shaved, abraded, and cleaned with alcohol swabs to enhance signal quality. Two silver-silver chloride electrodes (Kendall Medi trace, Medi-Trace, Covidien, Mansfield, MA, USA) were placed per muscle. All EMG data were collected at a rate of 2,000 Hz using the BIOPAC MP-100 data acquisition system with Acknowledge 4 software.
Participants were then required to perform maximal voluntary contractions (MVCs) for each of the muscles studied. Data collected during these MVCs were used to amplitude normalize the EMG. During each MVC, manual resistance was applied by investigators as the participants contracted with maximum effort. All MVCs were collected as individuals performed the following isometric contractions, each of which lasted for approximately 3 seconds: (a) RA: participants performed a partial curl-up with the feet secured and resistance applied to the shoulders (15); (b) EO: the participants performed an oblique curl-up, attempting to move the resisted shoulder toward the opposite knee (15); (c) SA: the participant lay in a supine position with their right shoulder flexed at 90° and elbow fully extended, as they attempted to protract their scapula with resistance applied at the hand and elbow (16), and (d) RF: the individual was asked to sit on the edge of a raised surface with both legs hanging freely and attempted to extend their right knee against the resistance provided by the researcher at the ankle joint.
Before performing the plank trials, the suspension system was anchored to a single ring located in the ceiling, and straps were adjusted so that the handles were elevated approximately 10 cm above the floor. The participants then performed 2 trials of each of the 4-plank exercises in a randomized block design, with 90 seconds rest between each plank. Participants were given an additional 120-second rest period between sets. In both the ArmsIn and Full conditions, the straps were positioned at the radial tuberosity of each of the participant's forearms. For the FeetIn and Full conditions, the suspension straps were placed approximately 2–3 cm above the ankle joint. Investigators monitored and adjusted the placement of the straps during completion of each plank condition. Any shifting of strap position during a trial meant that plank was repeated after a rest period was given.
For all planks, participants were instructed to assume a position with the shoulders and elbows flexed at 90°. They had to maintain a straight, strong line from head to toes with no lowering of the hips and keep the neck in a neutral position. They were required to keep their elbows directly beneath their shoulders, while ensuring there was no rounding of the shoulders or elevation of scapula off the thorax. Participants were also told to contract their core musculature and gluteal muscles slightly to remain stable, while maintaining normal breathing patterns (8). Investigators observed all planks to ensure proper body positioning, any planks that did not meet the above criterion were repeated. Electromyography data collection was not begun until participants were in the required position for each plank. Once begun, data collection lasted for 3 seconds.
The EMG was bandpass filtered (10–500 Hz) during the data collection process. Additional high-pass filtering (20 Hz) was done post collection to remove cable movement artifact, using a dual-pass Butterworth filter. Raw EMG data from all trials were amplitude normalized by dividing the raw EMG data from plank trials by the maximum EMG amplitude that occurred during each MVC trial. Maximum EMG was determined as per Burden and Bartlett (7). Briefly, a 50-millisecond moving window was used to calculate the root mean square (RMS) EMG. The resulting smoothed signal was then examined to determine maximum EMG for each muscle, and this was the value that was used for amplitude normalization of plank trial data. The normalized data were further processed to determine the percentage of muscle activation that occurred during each of the exercises. Again RMS EMG was determined, this time using by calculating the RMS for the full 3 seconds of plank data collected.
A repeated measures 1-way analysis of variance (ANOVA) was performed to determine if any significant difference in EMG activity was present between the normal plank condition and each of the suspended plank conditions. A significance level of p ≤ 0.05 was used. Post hoc t-tests, using a Bonferroni correction, were used to further assess any significant effects found using the ANOVA. Additionally, Cohen's effect size (ES) values were determined to evaluate the magnitude of the changes in muscle activation for the 2 exercise conditions. The ES were evaluated in accordance to the criterion of >0.70 large; 0.40–0.70 medium, and <0.40 small (9).
A total of 25 participants were recruited for the study. Of these, 22 were able to perform all plank exercises correctly and thus continued to day 2 of testing. During day 2 of testing, 1 participant reported back pain when trying to complete the MVCs and as a result could not continue with the study. All individuals indicated that they participated in core-strengthening exercises at least twice a week. None had previous experience with the TRX suspension training system used in the study.
When muscle activation data were examined, a significant main effect of condition was demonstrated for RA (p < 0.001), EO (p < 0.001), RF (p = 0.006), and SA (p = 0.011). Post hoc analysis of this main effect for RA and EO (Figure 2A) revealed both muscles had lower RMS EMG during the Norm condition than during FeetIn (p < 0.001, ES RA = 1.1; ES EO 0.5), ArmsIn (p < 0.001, ES RA = 2.9; ES EO = 1.9), and Full (p < 0.001, ES RA = 1.9; ES EO = 1.3) plank conditions. Activation of the abdominals was also less during FeetIn planks than it was for both ArmsIn (p < 0.001, ES RA = 1.1; ES EO = 0.9) and Full (p < 0.001, ES RA = 1.02; ES EO = 0.83) conditions. There was no difference in activation for either of the abdominals between ArmsIn and Full (p > 0.05, ES RA = 0.02; ES EO = 0.05) conditions. Post hoc and ES examination of the main effect of condition for RF (Figure 2B) determined that RF activation during the ArmsIn plank was significantly greater than activation during both the FeetIn (p < 0.001, ES = 0.68) and Norm (p < 0.001, ES = 1.3) plank variations. For SA, (Figure 2B) activation during FeetIn planks was significantly greater than the activation observed during ArmsIn (p < 0.001, ES = 1.0) and Full (p < 0.001, ES = 0.91) planks. All other comparisons lacked statistical significance and exhibited small ESs with the exception of ArmsIn vs. Norm plank which resulted in a large ES (ES = 1.1) for SA activation.
Suspension training has become increasingly popular as a training tool. Despite this popularity, relatively little research exists on the effects of such training on muscle activation magnitudes. This study examined the effects of a commercially available system (a TRX suspension trainer) on muscle activation in an effort to provide fitness professionals with evidence on which to base suspension training exercise prescription and progression. The results of this study confirmed the hypothesis that the instability provided by a suspension training system would produce significant increases in muscle activation levels in RA and EO. Contrary to our hypothesis regarding RF and SA, suspension training did result in significant increases in activation of both muscles, although not to the same extent as the changes observed in RA and EO.
In general, the results suggest that suspension training and the instability it creates do increase demands placed RA and EO. As such, utilization of suspension training in the manner depicted in this study should be considered as a viable means of increasing exercise intensity for these muscles. The results of this research are supported by similar findings in the current body of literature surrounding instability training. In a comparable study conducted by Marshall and Murphy (30), increased muscle activation was found in RA when core-strengthening exercises were performed on an unstable surface using the Swiss ball. Similarly, Wahl and Behm (43) found greater EMG activity in the lower abdominals when participants were exposed to an unstable environment using a wobble board.
Before beginning data collection, it was hypothesized that both ArmsIn and FeetIn conditions would produce similar demands on the musculature. This was based on the fact that both planks would create the same relative degree of instability (i.e., 1 unstable point of contact each). Findings did not agree with this as ArmsIn planks resulted in 21% greater RA activation and 12% greater EO activation than the FeetIn planks did. Similarly, ESs were large for both RA (ES = 1.1) and EO (ES = 0.95) when activation was compared between ArmsIn and FeetIn conditions. Why did this occur? One obvious difference between the 2 conditions was the trunk angle. Because of the elevation of the upper body required during arms in suspension, the angle between torso and floor was greater for the arms suspended conditions (Figure 1). Consideration of the biomechanics of the 2 positions however confirmed that the increased angle of the body in the arms suspended position would result in a shorter moment arm for the force of gravity. This would in turn reduce the moments created by this force therefore reducing the load that needed to be maintained. As a result, we would have expected this difference in angle to result in less, not more, muscle activation. Future studies replicating these exercises should endeavor to ensure that trunk angles are kept constant between conditions to definitively rule this out as a contributing factor.
An alternative hypothesis that may explain the increased activation in arms vs. feet suspension conditions relates to the fact that the musculature of the upper limb is significantly smaller relative to that of the lower limb. As a result, upper limb muscles have a decreased capability to provide stability and contend with forces (13). Consequently, when instability is applied to the upper limb, the muscles may be less able to maintain stability through contractions at the shoulder alone. Synergistic activation of the abdominal muscles may thus be required to keep the whole body stable and in the required position. However, when instability is applied to the lower limb, its larger muscle mass is able to generate adequate forces to keep the body in a more stable position. This enhanced stability means less synergistic activation may be required from the abdominal muscles. In this study, it is hypothesized that the increased higher activation levels observed in RA and EO during arms suspended conditions may have arisen due to this need for greater contribution from the core to ensure stability was maintained. This response may be lower in the feet suspended condition, as the stronger musculature of the lower limb may not require the same synergistic assistance from the core musculature.
From Figure 2B, it is clear that the demands placed on RF and SA during all versions of the planks examined in this study were substantially less than those on the abdominal muscles, with activation levels of less than 25% MVC for all exercise variations. As a result of these relatively low activation levels, none of the plank variations would produce enough loading to have a substantial strengthening effect (28); however, possible endurance benefits may be observed. Despite this fact, the demands placed RF and SA did vary with exercise condition and as such are worthy of further discussion.
Activation of RF followed a pattern similar to the 2 abdominal muscles examined, activation least in Norm, followed by FeetIn, ArmsIn, and Full. This was not surprising considering the synergistic nature of activation that has been reported between RF and the abdominals (24,33). As reviewed by Neumann (33), acting through a force couple RF helps to keep the pelvic in a neutral position when the abdominals are contracted strongly. Such synergistic activation is needed as the abdominals would have a tendency to rotate the pelvis posteriorly. If this were allowed to happen, the spine would flatten and a neutral spine would not be maintained as is required during a plank.
Unlike the other muscles examined in this study, SA does not play a direct role in lumbar spine position. It was included in the study primarily because of its role in keeping the scapula fixed to the thorax (17) during activities similar to the plank. In this study, SA activation was greatest in the conditions when the forearms were rested on the floor (i.e., FeetIn > ArmsIn and > Full). One possible explanation for this finding could be related to slight differences in the force distribution on the forearm during the plank variations. In ArmsIn and Norm conditions forearm weight bearing occurred through the straps that were placed just distal to the elbow joint. This is in contrast to the FeetIn and Full planks, where forearm weight was likely distributed over a surface area spreading from the elbow to the mid-forearm. This difference in weight distribution may have changed the direction of the force vector being transmitted from the ground to the shoulder girdle. Such a change in direction of force application may have resulted in more or less destabilization forces acting on the scapula thus affecting the activation of SA.
Results of this study have potential implications for exercise prescription and progression with respect to the training of the abdominal muscles examined. Exercise prescription should vary based on the phase of training of the individual. The goal of exercise progression is to advance individuals systematically and safely through particular exercises to improve components of fitness and health (28). The results of this study suggest that the method of suspension training used may be a useful means to increase exercise intensity (i.e., muscle activation levels) during performance of the front plank exercise. Results of the study indicated increased muscle activation of RA and EO as the participants progressed from the normal condition into the feet suspended and arms suspended condition. These findings have important implications for exercise prescription. For inexperienced exercisers, a stable plank performed on the floor seems to be the best choice as it places the least demand on the abdominal musculature. As endurance improves with training, suspended variations of the plank could be prescribed, beginning with the less challenging feet suspended position, followed by the arms suspended position as the final progression of the plank exercise. Despite the large degree of instability present during the Full suspension condition, this suspension mode did not result in maximal muscle activation levels. As such, the use of 2 suspension training systems, to produce both upper and lower limb instability during front planks, does not seem warranted if the goal of the exercise is to increase demand on the RA and EO musculature. Maximal demand on these muscles can be achieved using the ArmsIn condition, making the use of the second training system seem unwarranted.
Considerable research has quantified how instability training can result in increased muscle activation when compared with performing the same exercises under stable conditions. This study is the first, to the best of our knowledge, to confirm that similar increases in activation occur when using suspension training. Performing plank exercises using a suspension training system evoked higher RA and EO activation, with the arms suspended condition creating the highest activation levels. Surprisingly, planks performed with upper and lower limbs simultaneously suspended did not seem to have any additional benefits.
The authors acknowledge TRX for providing one of the TRX Suspension Trainers used in this study. The results of the current study do not constitute endorsement of TRX Suspension Trainers or other suspension training devices by the authors or the National Strength and Conditioning Association.
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