The present study investigated the relative activation levels of major trunk and hip muscles during exercises in four-point kneeling. A classification into three general activity levels was created, and a possible relationship with the anatomical classification of local and global muscles was studied.
The ipsilateral GM and MF show a high relative muscle activity (> 20% MVIC) during all exercises in four-point kneeling. Although this is in accordance with earlier studies [1
], the latter results are only available for certain muscles, and only for exercises 1 and 2. In addition, our results seem to indicate that the chosen sequence of the presented exercises challenging the balance and whole body stability correlates with increased and more varied muscle activity.
The results also show a high activity of the contralateral IO and the ipsilateral EO during exercises 1 and 2. As previous studies showed lower activity levels of these muscles during exercises 1 and 2 [8
], these results raise doubts about the statement that this type of exercise mainly activates the paraspinal muscles and not the abdominal muscles [8
]. It was suggested that the contralateral IO muscles were activated to maintain a neutral pelvis and spine posture, in effect balancing the internal moments and lateral shear forces [8
], but this seems to occur in association with ipsilateral EO activity. In this way, the results of the current study indicate that the abdominal obliques to create a stable unit accomplish an ideal co-operation.
Also dorsally, there seems to be a co-operation between the ipsilateral and contralateral back muscles. Recruitment patterns of the ipsilateral ICLL together with the contralateral ICLT are recognized. This is consistent with the findings of Callaghan et al. [8
], although that group reported lower activity levels concerning the contralateral MF.
The contralateral EO and the bilateral LD show moderate activity levels (10–20% MVIC) during all exercises. In contrast to earlier findings [8
], the results of the present study show that extension of the upper extremity does not seem to influence the LD muscle activity. In general, in contrast to its classification as a “global” muscle, symmetrical activity is confirmed during all exercises. A possible reason for such equalized action of the LD could be the tensioning of the TLF in a cranial direction, needed to control the trunk irrespective of the movement or position of the upper limb.
In exercise 3 there are moderate relative activity levels in both the contralateral IO and the ipsilateral EO, and the difference between them is not significant. It seems as if the contralateral IO and the ipsilateral EO can play a role in supporting the stable position to control the neutral spine position. This can not be confirmed by earlier research, as some researchers did not discriminate between ipsilateral and contralateral muscle activity [1
] and others did not generalize to those terms because of varied results for the contralateral IO [31
]. The hip flexion that is added in exercise 3 in comparison to exercise 2 seems to create a lower abdominal muscle activity (P
≤ 0.005, except for the ipsilateral IO).
The ICLL and ICLT seem to act together and also reach similar moderate activity levels [8
]. By the stretch the hip flexion in exercise 3 causes on the contralateral GM, this muscle exhibits a significant higher muscle activity in exercise 3 in comparison to exercise 2 (P
< 0.001). It seems that to counteract the more challenging body position by adding the hip flexion, there is a compensation of the ipsilateral GM and ICLT and the contralateral MF.
The low-level symmetric activity of the RA throughout all exercises is confirmed by previous studies [8
] and by studies of related exercises [6
]. According to Callaghan et al. [8
] it indicates that this muscle was not functionally active and did not contribute to stability. However, as the muscle is bilaterally active at a constant level during all exercises, stability analysis (including external loads) is needed to assume that the limited activity is irrelevant. In the current study the contralateral GM also seems to show relative low activity levels during exercises 1 and 2. During all exercises (even on the side opposite to the leg extension) the GM muscle is still active, preventing flexion of the hips and thus preventing destabilization of the spine. The ipsilateral IO also creates a small relative muscle activity.
Although a distinction is made between high, moderate and low relative muscle activity, the electromyographic activity never exceeds 32% of MVIC. It is mentioned that useful stabilization exercises for the clinic with the aim to hold and control the lumbar spine in a neutral position, work the trunk muscles at approximately 30% of their maximum [25
The results of this study show that in uncomplicated exercises in four-point kneeling performed by healthy subjects, the investigated muscles seem to work together in a harmonious way. These results tend to confirm the recent findings describing that, based on relative muscle activity, no single muscle appears to be superior in enhancing spine stability, but as loads are applied to the spine there is an integration of the different muscles in order to balance the stability and moment demands [26
]. However, the results of this four-point kneeling position cannot be extrapolated to the erect posture, which is the usual posture for the population being investigated.
It seems relevant that in the present study muscles are active in stabilization exercises that are also strongly related to the main thoracolumbar fascia (TLF), such as the IO, EO, GM and LD. Muscles like the ICLL, ICLT and MF have a hydraulic amplifier effect on the different layers of the TLF. The posterior layer of the TLF is ideally positioned to regulate tension via its extensive muscular attachments to both “local” and “global” muscles [5
]. When loading the TLF for instance by the IO or EO, deformation and structural integrity of the fascia should be protected by muscles like the LD and GM. The overall effect of these muscles acting together could have a positive cascading effect on the stiffening of both the lumbar spine and sacroiliac joint [44
Based on the results of differences in cross-sectional area [12
] and timing [19
], there may be some inhibition of certain muscles and dominance of other muscles [38
] to maintain a stable body position in LBP patients. The RA activity during flexion-extension movements [39
] and the EO activity during both flexion-extension [39
] and left rotation movements [34
], as well as the muscle activity of the left thoracic erector spinae during lateral flexion movements [28
] were higher in LBP patients than in healthy controls. However, during coordination and left rotation exercises the MF showed lower activity levels in LBP patients than in healthy controls [11
]. So-called local muscles might demonstrate lower and so-called global muscles higher activity levels in LBP patients compared to healthy subjects. The present study, describing both local and global muscle activity, provides a normative database, which allows comparison with specific pain populations in future research. Apart from the muscle activity levels, further integrated research on muscle strength, muscle timing and movement patterns in specific LBP populations is necessary to effectively distinguish between normal and abnormal spinal function.