Discussion
The present study was designed to investigate whether the amount of activity (%MVC) of the different parts of the posterior spine muscle chain is influenced by different extension exercise modalities. Therefore the mean muscle activity was analyzed during four different extension modalities.
The results of this study show that all muscles of the posterior chain were, given the intensity of 60% of 1 RM, active within the expected range during the different trunk and leg extension exercises in healthy individuals. The LD and GM however played a smaller role compared to the paraspinal muscles. The recruitment of the GM and LD during an extension movement of the spine can be clarified by the coupling between these muscles and the paraspinal muscles, which is formed through the fascia thoracolumbalis. The lower activity levels of both GM and LD are in agreement with previous findings and can be explained by the main function of these muscles, which is not back extension but arm and leg extension respectively. In contradiction with our results, other authors suggest a major role of the GM during trunk extension which is dependent upon the intensity of the exercise. These authors suggest that with increasing load and repetitions, the lumbar muscles become less responsible for maintaining the force output, while the GM becomes more powerful and responsible for the force output. In the current study only 5 repetitions were investigated which was probably not sufficient enough to induce similar alterations in the muscle recruitment pattern.
These results indicate that for specific strengthening of the LD or the GM other exercises are more appropriate. Nevertheless, we showed that these muscles are contributing to the extension movement.
In literature, a wide variety of muscle activity levels during trunk and leg extension exercises are reported. Different exercise set –ups (starting angle, contraction modality, hand position) and used methods for measuring muscle activity (electrode placement) have been used, making comparisons between results difficult. In the current study mean thoracic and lumbar muscle activity ranged from 45 to 78% of the MVC. These findings are comparable with the findings for the studies of Arokoski et al. and Ng et al.. However, the observed activation of the lumbar spinal muscles is slightly higher than reported by Plamondon et al.. The higher muscle activity in the present study could be explained by the difference in arm position between the studies. In the current study the arms were positioned further away from the center of gravity compared to the arm placement used in the study of Plamondon et al., which resulted in a bigger lever arm and higher muscle recruitment.
Although the lumbar and thoracic paraspinal muscles can act synergistically to produce an extension force, several studies suggest that the back muscles are not one homogeneous muscle mass. The back muscles are composed of different groups of fascicles with different functions. Therefore a distinction, based on anatomical and functional differences, between the thoracic and lumbar muscle groups is necessary. Both muscle groups cross the lumbar spine, whereas the lumbar muscle parts directly attach on to the lumbar vertebrae, the thoracic parts originate from the thorax and insert in long tendons that form the erector spinae aponeurosis. The thoracic muscles, which are located more superficial, are be more force producing muscles, whereas the deeper lumbar muscles (especially the LM) tend to have a more specific stabilizing function of the spine. Therefore, we decided to investigate the thoracic (LTT and ILT) and lumbar extensor (LTL, ILL, LM) groups separately.
To our knowledge only few researchers have previously investigated the contribution of the LTT and ILT during extension exercises. The amount of thoracic muscle activity (45–64% MVC) in the current study is comparable with findings from previous reports during trunk extension in healthy people, although they did not make a distinction between the LTT or ILT as was done in the present study. The necessity to make a distinction between these thoracic muscles has been demonstrated by Coorevits et al., who showed that the LTT has a higher fatigue rate then the ILT during trunk extension in healthy people. Although the current study did differentiate between the thoracic muscles we did not find any differences between the thoracic muscles during performance of the extension exercise modalities which were previously described. The current study did reveal a higher contribution of the lumbar and thoracic muscles during trunk extension exercises than during leg extension exercise. To our opinion the difference can be attributed to the different kinematics and coupled muscle function between the two exercises. A trunk extension from departing from 45° trunk flexion can be seen as a dynamic pelvic and trunk movement. The leg muscles will extend the pelvis, the lumbar muscles will stabilize and extend the lumbar region on the pelvis, and the thoracic muscles will actually lift the trunk. On the contrary, with a fixed trunk in a horizontal position and the hips in a starting position of 45° flexion, most of the dynamic work is performed by the leg muscles while both back muscles groups deliver more static work. The back muscles need to stabilize the pelvis and spine to make leg lifting possible. Literature provides evidence that during concentric muscle work higher levels of activity are produced than during static work. No earlier study has made the comparison in thoracic and lumbar muscle recruitment during both trunk and leg extension which emphasizes the relevance of the current study.
A homogeneous recruitment pattern of the lumbar muscles was observed during extension exercises. In agreement with Callaghan, we found the LM activity did no differ from ILL and LTL activity. However previous studies showed significant higher recruitment of the LM and the LTL, compared to the more lateral ILL, during trunk extension in healthy subjects. In addition, using MRI, a previous study showed higher activity of the LM compared to ILL and LTL during trunk extension in chronic LBP patients. Moreover Ng et al. found higher activity of the LM compared to Iliocostalis and Longissimus thoracis during respectively a trunk holding and leg holding test.
Possible explanations for the contradicting results are differences in exercise and measuring protocol. Coorevits et al. objectified muscle fatigue whereas the present study measures the averaged muscle recruitment. Furthermore Coorevits et al. and Ng et al. studied muscle activity during isometric contraction, while in the present study dynamic and dynamic–static contractions were used. A second explanation of the homogeneous lumbar muscle usage found in the present study, could be the relative high intensity of the exercise (60% 1-RM). Since Mayer et al. demonstrated that the contribution of the lumbar parts of the erector spinae compared to the LM was higher with increasing intensity, it is possible that in order to obtain a force output at 60% of the RM all the muscles are recruited at a comparable intensity. Therefore, further investigation regarding lumbar muscle activity in low load conditions is recommended. It is possible that, in agreement with the evidence of functional differences between the lumbar muscles, these low load conditions are more sensitive for differences in recruitment.
In contrast with a previous investigation, this study shows that lumbar muscle activity was higher during trunk than during leg extension. Discrepancies in exercises intensity and starting angle could explain the contradicting results. In the study of Plamondon et al. the weight of the body part was not taken into account, which complicates the comparison with the current results. In the present study, based on the results of the pretest, all exercises were set at an equal intensity (60% of 1-RM) by adding weight or assisting the body part. Moreover, in the study of Plamondon et al. leg extension was performed at 60° and trunk extension at 45° of flexion, while in our study both exercises were performed at 45° flexion. As suggested by Mannion et al. changes in muscle length, induced by differences in starting angle, have a significant effect on force output of these muscles. Based on the Borg score, subjects experienced trunk extension as more intensive than leg extension, although the intensity of both exercises was equal. An explanation could be found in the muscles activity levels. Logically, because thoracic and lumbar muscles were recruited at a higher degree during trunk compared to leg extension, trunk extension was experienced as more fatiguing. The subjective feeling of heaviness, is normally determined by the weakest link. However, we did not inquire the region (upper, lower back or legs) of heaviness, so no judgment can be made about which muscle group is determining the feeling of heaviness. Further research into this aspect is warranted.
Our results also indicates that the modality of contraction (dynamic or dynamic-static) does not affect posterior muscle chain recruitment patterns. To our knowledge a comparison of back muscle activity between dynamic and dynamic-static extension exercises has not been investigated earlier. But in line with these results regarding muscle recruitment, Danneels et al. found no difference in increase of the lumbar spinal muscle cross sectional area between dynamic and dynamic-static extension training.
Inspired by the basic principles of muscle training, when the goal of the exercises is to train muscles in terms of endurance, the intensity must be drawn up to a percentage of 60. The results of the current study show that when extension exercises are performed at 60% 1-RM, the amount of thoracic muscle activity during all exercises was comparable with the predetermined intensity. Therefore all types of extension exercises are suitable to improve the endurance capacity of the thoracic muscles. The level of lumbar muscle activity during leg extension exercises was also in agreement with this level of the exercise intensity (±60% 1-RM). On the contrary, during trunk extension, the amount of lumbar muscle activity clearly exceeded this level. This means that in clinical practice leg extension can be used to train lumbar muscle endurance, whereas trunk extension exercises at 60% of the 1-RM target the lumbar muscles at a higher training level.
The recruitment of the GM and LD remained far below 60%MVC, so to enhance the endurance of these muscles other exercises will be more appropriate.
Regarding the recruitment of the posterior muscle chain during the different phases of contraction, the present study showed higher levels of recruitment of all paraspinal muscles during the concentric compared to the eccentric contraction phase of the extension exercises. Higher muscle activation during concentric versus eccentric contraction was already demonstrated by other authors. Plamondon et al. found the highest ES activity levels at L5/S1 near the horizontal position of the trunk, so during the concentric phase of the prone back extension exercises, and the lowest levels during the eccentric phase. However, they did not report statistical significant differences. Moreover, Babault et al. reported lower activation levels of the knee-extensors during an eccentric compared to a concentric and isometric contraction of these muscles, which is probably due to a decreased voluntary activation during eccentric contractions. Another explanation could be that during dynamic conditions there is a lower recruitment threshold, so full recruitment in dynamic conditions achieved at lower relative force levels compared to an isometric condition. However, this statement cannot explain the higher activity levels of the ILT, LTL and ILL during isometric compared to eccentric contraction.
In the present study we studied a young healthy population. Since altered muscle activation patterns within specific populations are demonstrated, the results of the current study cannot be generalized to LBP patients.