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).
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 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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