Many tasks of daily living and also sporting performances require the involvement of the core trunk muscles. The trunk muscles are of primary importance in terms of performance related activities and also for their involvement in maintaining correct posture, helping reduce the risk of acute and chronic back problems. For example, here the posterior erector muscles associated with the spine and the anterior flexor muscles are responsible for flexing the lumbar spine and creating intraabdominal pressure; together, these muscles aid stability of the trunk and spine. Therefore, it is important to maintain the correct levels and balance of strength and function of these muscles to enable optimal function and reduce the risk of both acute and chronic injury whether in a sporting context or carrying out an everyday task.
Previous work examining trunk muscle activity has identified strengthening exercises, which result in the highest level of electromyographical (EMG) activity in a range of trunk muscles, including the rectus abdominis (RA), erector spinae (ES), and external obliques (EOs) (3,10,12,16). In a study comparing 5 commonly used abdominal conditioning exercises (reverse curl, trunk curl, v-sit, trunk curl with a twist, vacuum), Willett et al. (15) found that the reverse curl resulted in the greatest RA activity; the v-sit and reverse curl resulted in the greatest EO activity. Previous work comparing isometric to dynamic exercises indicates that similar or greater levels of muscle activity are seen in dynamic exercise (1,5,11), which appears to be load dependent (2,5,11). It is also worth noting that even though the aforementioned studies reported muscle activity from surface trunk muscles, McGill et al. (9) previously reported that surface electrodes can adequately represent activity of the deep abdominal muscles.
Some authors have also compared exercise performances in stable vs. unstable conditions, with results that demonstrated that instability does not always result in increased trunk muscle activity (7,8,11,14). Wahl and Behm (14) found that performing either dynamic or static exercises while standing on a variety of unstable surfaces did not significantly (p > 0.05) alter activity of the ES or RA muscles. In contrast, Vera-Garcia et al. (13) reported a significantly greater activity of the RA and EO in an unstable vs. stable condition. Stevens et al. (12) compared RA and EO activity during a bridge, bridge on a Swiss ball, and a unilateral bridge (all isometric exercises) and found minimal differences in RA muscle activity between the bridge and Swiss ball bridge but greater EO muscle activity during the Swiss ball bridge because of the unstable nature of the Swiss ball. Therefore, it would appear that creating instability via alteration of the surface that the individual performs the exercise on has minimal effect on the ES and RA muscles.
There is a need to further understand the relationship between exercise and function to enable optimal strategies to be used in both rehabilitation and conditioning programs. During rehabilitation, the emphasis must be on return to function; hence, the most suitable exercises would appear to be those which are dynamic in nature and mimic the demands of specific functional tasks. It may also be that by appropriate use of specific strengthening exercises, one may be able to help reduce the risk of future injury by causing appropriate muscular adaptation during dynamic movements. Dynamic exercises such as the back squat (BS) and deadlift when performed at 80% of 1-repetition maximum have been shown to generate greater trunk muscle activity (RA and ES) compared to isometric exercises (5), which had previously been found to elicit the greatest activity of the spinal erectors and lower abdominal muscles (superman [SM], prone bridge [PB], respectively) compared to other isometric trunk strengthening exercises (1). Hence, it appears that a high level of trunk muscle activation can be achieved through the use of dynamic exercises (squats, deadlifts, bent over row) used during the main components of a strength training program (4,5,11), and that unilateral and overhead lifts which create instability and increase trunk muscle activity compared to performance in a stable or bilateral condition (1). Clarification, therefore, of level of activity of these functional muscles during different strengthening exercises may help reduce injury risk and plan optimal rehabilitation strategies.
Therefore, the aim of this investigation was to examine trunk muscle activity across isometric trunk strengthening exercises (PB and SM), dynamic strengthening exercises (BS, Front Squat [FS], and Military Press [MP]) performed using a standardized, submaximal, load, which may be more representative of the loads lifted by recreational strength training enthusiasts. It was hypothesized that RA muscle activity would be greatest during the MP, because of a changing center of mass throughout the exercises, resulting in decreased stability and that ES muscle activity would be greatest during the squatting exercises because of the forward lean of the trunk.
Experimental Approach to the Problem
A repeated-measures within-subjects design was used. Subjects repeated each of 5 exercises in a randomized order. Isometric exercises included the PBPB (Figure 1) and SM (Figure 2), because these were identified by Behm et al. (1) as resulting in the highest RA and ES activation, respectively, when compared to other isometric exercises. Dynamic exercises included the BS (Figure 3), FS (Figure 4), and MP (Figure 5).
Ten recreationally trained men (age 21.8 ± 2.6 years; body mass 82.65 ± 10.80 kg, 174.5 ± 7.2 cm), with ≥2 years regular (≥3× week) resistance training, participated in this study. All subjects provided written informed consent before participation, and the investigation was approved by the University of Salford institutional review board. The study conformed to the principles of the World Medical Association's Declaration of Helsinki.
Participants performed 1 set of 3 repetitions of each of the dynamic and overhead exercises and performed both of the isometric exercises for duration of 30 seconds (independent variables). The dynamic and overhead exercises were performed using a standardized load of 40 kg, to ensure that all lifts were comparable (with a load suitable for all participants to complete the required number of repetitions of each exercise at the required tempo) and to permit generalization of findings to a wider population. A heavier load was not selected as an increased load has been shown to result in an increase in muscle activity during dynamic exercises (2,5,11), which would therefore increase the likelihood of higher EMG values (dependent variable) during the dynamic exercises. Although dynamic exercises would not normally be compared to isometric exercises, the trunk muscles would be contracted isometrically during both the dynamic and isometric exercises.
To maintain controlled and comparable performances of all exercises, all subjects were familiarized with the protocol that included a 3-second count for the descent and ascent, independently.
Two electrodes with a 20-mm electrode spacing (Ambu blue sensors-N10A) were placed midline of the muscle belly on each of the RA and ES muscles (Table 1), with a reference electrode being placed around the ankle (Ambu blue sensor Q10A); all electrodes were placed on the right side of the participants. Before electrode attachment the skin was prepared by shaving, abrading, and cleaning with an alcohol-based solution to minimize skin impedance.
Muscle activity, during the ascent and descent phase, was recorded via the Neurolog system (Digitimer Ltd NL900D) sampling at 2,000 Hz per channel, preamplified and band pass filtered (10-500 Hz), the root mean square value (RMS) for the duration of each phase of the dynamic lifts (ca. 3 seconds) was determined. For the isometric exercises, a 3-second duration window was used (from 14 to 17 seconds of the duration). Analysis of the initial 3 seconds and final 3 seconds of the exercise showed no significant change (p > 0.05) in muscle activity across the duration of the exercise.
Muscle EMG activity was analyzed using Acqknowledge (BIOPAC system version 184.108.40.206) where the EMG signal was recorded from each repetition during both the ascent and descent phases. Paired samples t-tests revealed that there were no significant differences (p > 0.05) in RA and ES muscle activity between descent and ascent phases (Tables 2 and 3) of each of the dynamic lifts, and therefore, mean (ascent and descent phases) EMG values were used for further analysis.
Intraclass correlation coefficients (ICCs) were conducted to determine reliability across each of the 3 repetitions for each exercise. One-way repeated-measures analysis of variance, with Bonferroni post hoc analysis, was conducted to determine differences in muscle activity between exercises. The alpha level was set to p ≤ 0.05.
The ICCs demonstrated the highest levels of reliability for muscle activity during the isometric exercises. All exercises demonstrated a significant (p ≤ 0.01) high level of reliability (r = 0.764-0.998) (Table 4).
Rectus Abdominis Electromyographical Comparisons
The 1-way repeated-measures analysis of variance demonstrated significant differences (p < 0.001) in muscle activity across exercises for both the RA. Post hoc analysis revealed significant differences (p < 0.01) in RA muscle activity across all exercises and identified that the PB resulted in a significantly greater (p < 0.001) RA muscle activity (0.454 ± 0.211 RMS[V]) compared to the BS (0.047 ± 0.025 RMS[V]), FS (0.060 ± 0.027 RMS[V]), and MP (0.125 ± 0.072 RMS[V]) and compared to the SM exercise (0.035 ± 0.008 RMS[V]) (Figure 6).
Significantly greater activity of the ES was seen during the SM exercise (0.951 ± 0.217 RMS[V], p < 0.01) and the FS (1.010 ± 0.308 RMS[V], p < 0.01) compared to the BS (0.749 ± 0.276 RMS[V]) and MP (0.150 ± 0.089 RMS[V]), and also the PB (0.067 ± 0.058 RMS[V]). There was no significant difference (p > 0.05) in ES muscle activity between the FS and the SM exercise (Figure 7).
The BS resulted in a significantly greater (p < 0.001) activity of the ES (0.749 ± 0.276 RMS[V]) compared to the MP (0.150 ± 0.089 RMS[V]) during the descent phase and compared to the PB (0.067 ± 0.058 RMS[V]). The MP also demonstrated a significantly higher (p = 0.003) level of ES muscle activity (0.150 ± 0.089 RMS[V]) compared to the PB (0.067 ± 0.058 RMS[V]), which resulted in the lowest level of ES activity (Figure 7).
The RA muscle activity was found to be significantly greater (p < 0.001) for the PB (0.454 ± 0.211 RMS[V]) compared to the SM (0.035 ± 0.008 RMS[V]) and also compared to each of the dynamic exercises when performed with a standardized load of 40 kg. These findings are in contrast to previous results of Hamlyn et al. (5) and Nuzzo et al. (11) who found greater RA activity during the BS compared to during isometric trunk strengthening exercises including the PB, although higher loads (≥70% 1-repetition maximum) were used to elicit this level of RA muscle activity. In the present study, the loads used are perhaps more aligned with those prescribed for early rehabilitation. Thus, it appears that the relationship of muscle activity and exercise type may not be similar and is perhaps load dependent (5,11). Interestingly, the MP resulted in a significantly greater (p < 0.01) RA muscle activity (0.125 ± 0.0724 RMS[V]) compared to the BS (0.047 ± 0.025 RMS[V]) and FS (0.060 ± 0.027 RMS[V]). This increase in RA muscle activity may be attributable to the changing bar position in relation to the body resulting in a changing center of mass during the MP compared to during the FS and BS. Of the exercises evaluated in this study, the PB results in the greatest RA activity and would therefore be the most advantageous of these exercises for recruiting the RA in comparison to dynamic exercises with a submaximal load (40 kg). The loads used during the dynamic exercises in this investigation may be representative of loads used by the general population; they do not represent the higher training loads of athletes, which are likely to result in greater muscle activity (5,11).
The FS resulted in significantly greater (p < 0.001) ES activity (1.010 ± 0.308 RMS[V]) compared to all other exercises, excluding the SM exercise where there was no significant difference (p > 0.05) in muscle activity (0.951 ± 0.215 RMS[V]).
The BS resulted in significantly greater (p < 0.05) activity of the ES (0.749 ± 0.276 RMS[V]) compared to the MP (0.150 ± 0.089 RMS[V]) and PB (0.067 ± 0.058 RMS[V]). These findings are in agreement with previous findings of Hamlyn et al. (5) and Nuzzo et al. (11) who found that both the BS and deadlift, performed with loads ≥70% 1-repetition maximum, resulted in greater ES muscle activity compared to isometric trunk strengthening exercises including the PB and SM.
The results of this investigation suggest that dynamic exercise (FS) may be preferred in terms of strengthening the ES muscles, because of the high relative level of muscle activity during this exercise. As previously mentioned, the loads used during the dynamic exercises in this investigation are not representative of training loads in well-trained individuals, which are likely to result in greater activity of the ES (5,11) and therefore further strengthen the ES. This is also supported by Keogh et al. (6) who explain that performance in simple stability tasks may not be related to complex multijoint stability tasks. An additional benefit of incorporating exercises such as squats is the increased functional requirement of stabilizing the trunk while performing a dynamic multijoint movement, which is more representative of activities of daily living than purely isometric exercises. Squat exercises also have the advantage of training ‘other’ muscles across the functional length tension range, possibly allowing greater power generation at all levels of movement velocity and force. In addition, it could be reasonably argued that the dynamic exercises have an accelerative component; here rate of force development could be improved. This has implications for individuals prone to falling and in situations where high rates of force development may be advantageous.
The SM exercise may be a good exercise to start strengthening the ES muscles, however, to ensure progressive overload incorporating exercises such as the FS, with a moderate load, is likely to result in greater muscle activity. The performance of the FS can then be progressed via increased loading.
Although the loads used here may be reasonably representative when designing rehabilitation programs, for those actively engaged in sport to a high level, these load levels may not be ideal. It would be useful to observe the muscle activity parameters at a range of loads to include near maximal. Therefore, it is recommended that future research in this area consider the use of a range of loads, which will be representative of the loads used by athletic populations. This will inform the relationship between the measured muscles to examine if the activation ratios seen here for different exercises holds when a range of loads are used.
It is likely that these higher loads will result in greater levels of muscle activity as reported by Hamlyn et al. (5) and Nuzzo et al. (11).
When trunk muscle strengthening is the focus of rehabilitation, it may be reasonable to begin with static exercises, such as the PB and SM, to increase strength and control of the muscles being exercised in a stable environment and then progress to more dynamic movements. Such isometric exercises give adequate stimulus to the core muscles of the trunk but limited stimulus to the prime movers or functional activities (i.e., lower limb muscles).
According to the principles of specificity, the use of dynamic exercise for strengthening such as the squatting and press type exercises, appear to be a useful adjunct or progression to training the trunk musculature. The additional benefit here is that core muscles are also affected to a similar level as that during the execution of certain isometric exercises, giving simultaneous progressive exercise stimulus to not only the primary movers but also the core stabilizing musculature. It is also worth noting that the level of trunk muscle activity has previously been reported to be load dependent (5,11) and would therefore be expected to increase with heavier loads than those reported here.
Where specific strengthening of the RA is required, then the PB appears to be the most favorable exercise for strengthening because a higher level of RA muscle activity is seen relative to the other exercises in this study.
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