Control of the lateral abdominal muscles during walking

Abstract

Transversus abdominis (TA), obliquus internus (OI), and obliquus externus (OE) are involved in multiple functions: breathing, control of trunk orientation, and stabilization of the pelvis and spine. How these functions are coordinated has received limited attention. We studied electromyographic (EMG) activity of right-sided muscles and 3-dimensional moments during treadmill walking at six different speeds (1.4–5.4 km/h) in sixteen healthy young women. PCA revealed time series of trunk moments to be consistent across speeds and subjects though somewhat less in the sagittal plane. All three muscles were active during ⩾75% of the stride cycle, indicative of a stabilizing function. Clear phasic modulations were observed, with TA more active during ipsilateral, and OE during contralateral swing, while OI activity was largely symmetrical. Fourier analysis revealed four main frequencies in muscle activity: respiration, stride frequency, step frequency, and a triphasic pattern. With increasing speed, the absolute power of all frequencies remained constant or increased; the relative power of respiration and stride-related activities decreased, while that of step-related activity and the triphasic pattern increased. Effects of speed were gradual, and EMG linear envelopes had considerable common variance (>70%) across speeds within subjects, suggesting that the same functions were performed at all speeds. Maximum cross-correlations between moments and muscle activity were 0.2–0.6, and further analyses in the time domain revealed both simultaneous and consecutive task execution. To deal with conflicting constraints, the activity of the three muscles was clearly coordinated, with co-contraction of antagonists to offset unwanted mechanical side-effects of each individual muscle.

Introduction

The human body has three lateral abdominal muscles: transversus abdominis (TA), obliquus internus (OI), and obliquus externus (OE). Their anatomy suggests a range of functions. In the frontal plane, unilateral OI and OE activity may be involved in ipsilateral bending of the trunk (Standring, 2008). In the transverse plane, OE contributes to contralateral rotation, whereas OI and TA contribute to ipsilateral rotation (Benninghoff and Goerttler, 1964, Hodges, 2008, Urquhart and Hodges, 2005). In the sagittal plane, bilateral activity of the lateral abdominal muscles can contribute substantially to trunk flexion (McGill, 1996).

Bilateral activity of the lateral abdominal muscles may also be involved in stability. Bilateral TA activity was suggested to stabilize the lumbar spine (Hodges, 1999, Hodges and Richardson, 1997a), and concerted action of the lateral abdominal muscles can press the iliac bones against the sacrum, thus providing pelvic stability (“force closure”, cf. Hu et al., 2010, Snijders et al., 1993, Vleeming et al., 1990).

Together with the rectus abdominis, the lateral abdominal muscles form the abdominal wall. Continuous activity of the abdominal wall can be seen as holding the viscera; it regulates intra-abdominal pressure, which implies a role in breathing (Standring, 2008). Finally, activity of the abdominal wall plus the diaphragm and the pelvic floor, contributes to spinal stability (Cholewicki et al., 1999, Hodges et al., 1997, Hodges et al., 2005, Pel et al., 2008).

Several studies have attempted to verify the roles of the lateral abdominal muscles through electromyographic recordings during the performance of specific tasks. In subjects holding loads, OE was involved in lateral bending (Seroussi & Pope, 1987). OI and OE played a role in trunk rotation during twisting efforts against resistance (McGill, 1991), and TA was active during ipsilateral rotation (Urquhart & Hodges, 2005). Considerable activity of all three muscles was reported in flexion exercises (Juker, McGill, Kropf, & Steffen, 1998). OE was involved in expiration (Campbell, 1952), and the activity of all three muscles was modulated at the frequency of breathing (Saunders, Schache, Rath, & Hodges, 2004). Bilateral TA activity preceded rapid movements of upper and lower extremities, which supports the idea that TA stabilizes the spine (Hodges and Richardson, 1996, Hodges and Richardson, 1997a, Hodges and Richardson, 1997b). Furthermore, the use of a pelvic belt led to decreased EMG amplitudes of these muscles during the Active Straight Leg Raise, in accordance with their assumed function in sacroiliac stabilization (Hu et al., 2010).

As a rule, the mechanical effects of any single muscle will not coincide with the exact 3-dimensional demands of a task, and when constraints are conflicting, there will be no perfect solution. More important, the lateral abdominal muscles may subserve multiple functions simultaneously (Benninghoff and Goerttler, 1964, Hodges, 2008). Some evidence for such multitasking has been provided for trunk postural control plus breathing (Hodges et al., 2002, Saunders et al., 2004). Still, how multiple functions are coordinated has received limited attention only. In multitasking, or dealing with conflicting constraints, the control system may prioritize one function and ignore the other (cf. Hodges, Heijnen, & Gandevia, 2001). Also, the control system may exploit muscle redundancy by assigning specific functions to specific muscles or parts thereof (Puckree, Cerny, & Bishop, 1998). Finally, problems of multitasking may be alleviated when the system can deal with different task demands consecutively (task switching) rather than simultaneously (multitasking).

The present study focused on control of the lateral abdominal muscles during gait, which is an activity that involves all functions mentioned above (cf., e.g., Callaghan et al., 1999, Saunders et al., 1953, Saunders et al., 2004). We used electromyography (EMG), calculated trunk moments using a dynamic 3-dimensional linked segment model, and cross-correlated EMG linear envelopes with moment data. The stride cycle was split into different phases (cf. Ivanenko et al., 2004, Perry, 1992), with peak detection in the time series of moments and of EMG. Fourier analysis of the EMG linear envelopes was performed to determine absolute and relative power at the main frequencies (Saunders et al., 2004). Since the relative importance of different functions changes with speed (Anders et al., 2007, Saunders et al., 2004), walking speed was manipulated.

The goal of this study was to understand the coordination of the lateral abdominal muscles during gait. We expected that these muscles would be engaged in multitasking most of the time. Because normal walking is relatively undemanding, we expected that no function would be ignored, and, given the literature (Hodges, 2008), that no (part of a) specific muscle would be specifically dedicated to a single task. On the contrary, we hypothesized that the control system co-activates muscles to produce the desired effect, dealing with conflicting constraints by co-contraction to offset unwanted mechanical side-effects of any individual muscle.