The sit-up: complex kinematics and muscle activity in voluntary axial movement

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Abstract

This paper describes the kinematics and muscle activity associated with the standard sit-up, as a first step in the investigation of complex motor coordination. Eight normal human subjects lay on a force table and performed at least 15 sit-ups, with the arms across the chest and the legs straight and unconstrained. Several subjects also performed sit-ups with an additional weight added to the head. Support surface forces were recorded to calculate the location of the center of pressure and center of gravity; conventional motion analysis was used to measure segmental positions; and surface EMG was recorded from eight muscles. While the sit-up consists of two serial components, ‘trunk curling’ and ‘footward pelvic rotation’, it can be further subdivided into five phases, based on the kinematics. Phases I and II comprise trunk curling. Phase I consists of neck and upper trunk flexion, and phase II consists of lumbar trunk lifting. Phase II corresponds to the point of peak muscle contraction and maximum postural instability, the ‘critical point’ of the sit-up. Phases III–V comprise footward pelvic rotation. Phase III begins with pelvic rotation towards the feet, phase IV with leg lowering, and phase V with contact between the legs and the support surface. The overall pattern of muscle activity was complex with times of EMG onset, peak activity, offset, and duration differing for different muscles. This complex pattern changed qualitatively from one phase to the next, suggesting that the roles of different muscles and, as a consequence, the overall form of coordination, change during the sit-up.

Introduction

We investigated the sit-up because it represents a useful behavioral model for studying how the central nervous system (CNS) coordinates natural voluntary movements, which often involve large numbers of joints, degrees of freedom in joint motion, and muscles. Most previous studies of motor coordination focused on simple movements involving just one or a few joints, usually in one extremity. The study of coordinated movements at individual joints presupposes that the coordination of complex, multi-joint movements can be reconstructed from the summation of coordination at individual joints. Such a simplistic perspective was shown by Bernstein [8] to apply predominately to laboratory analogs of coordinated movement, rather than to natural motor behavior. Consequently, at this time there still exists little understanding of how complex movements involving multi-link kinematic chains are coordinated, particularly those involving proximal and axial muscles.

The goal of the study presented here was to identify qualitatively the different components of the sit-up that need to be coordinated for the movement to succeed or, stated another way, to identify the most critical loci of coordination in the sit-up. By the term ‘coordination’, we mean the imposition of a consistent, temporal or spatial relationship among different components of a movement, be those components structural (e.g. central vs. peripheral control), kinematic, or neuromuscular. By first establishing what elements of the sit-up are coordinated, we will then be able to analyze each element in detail in order to clarify the underlying mechanisms of coordination (see Ref. [16]).

Currently, relatively little is known about the coordination of natural, complex movements. Previous studies of complex axial movements focused more on their biomechanics and exercise value than on their mechanisms of coordination. For example, a number of studies examined the sit-up, to determine its exercise value [[5], [20], [21], [22], [25], [30], [34]] and its physiological risk [19], [22], [28]. Similarly, weight lifting was evaluated (e.g. Ref. [37]) to determine how to minimize the risk of back injury during its execution. However, a number of studies have examined patterns of muscle activity in complex movements [[5], [19], [20], [21], [22], [25], [34]], which has provided insight into the underlying mechanisms of coordination. The principal insight from these studies is that muscles in complex movements are not activated and deactivated simultaneously, as typically found in simpler movements involving a single extremity.

The experimental approach used in our study was to first decompose the sit-up into phases based on kinematics. Two major components of the sit-up were identified, and these were further subdivided into five ‘phases’, based primarily on trunk and leg motion. Within each phase, we also identified subcomponents that appeared to have a specific mechanical function and that might be discretely controlled by the actions of specific muscles. We also studied the pattern of muscle activity within each phase, across a number of muscles from the neck to the legs, to determine whether the activation pattern changed in each phase. We hypothesize that changes in the activation pattern across muscle populations indicate changes in CNS control of the movement.

The experimental analysis addressed four questions, based on our preliminary observation [14], [15] that this movement has two distinct parts—one flexing the trunk and the other rolling the pelvis and trunk into a sitting position. The first question addressed was: does the CNS control these two parts of the sit-up differently? Because the center of gravity (CoG) is located rostral to the pelvis in the supine individual, a major subtask of the sit-up is to move the CoG caudally through the pelvic axis of rotation in order to provide the leverage necessary for the sit-up to be successfully executed. The second question investigated was: when does the CoG pass through the rotational axis in the sit-up? A substantial number of individuals, with and without neurological disorders [6], are unable to perform a complete sit-up. The third question addressed was: is there a particular (i.e. critical) point in the sit-up at which the sit-up will most likely fail without the appropriate coordination? Finally, previous qualitative analyses of muscle activity during the sit-up have shown this activity lacks simultaneity. The fourth question addressed was: does the intramuscular pattern of muscle activity change significantly during the sit-up? Such changes would indicate the presence of serial control, in addition to the more typical parallel control observed in simpler movements.

Preliminary reports of this study have been presented in abstract form [14], [15], and a review of complex movements has been published as a book chapter [16].

Section snippets

Methods

Eight human subjects (ages 18–49; five, male; three, female) with no known neuromuscular disorders participated in this study after signing an informed consent form approved by the OHSU Institutional Review Board. Subjects accepted into the study were relatively fit and capable of performing sit-ups without difficulty. Each subject—wearing brief shorts and, in the case of female volunteers, a sports bra (Fig. 1)—lay supine upon a force plate, on a force table. Recordings were made of horizontal

Results

The sit-up appears to the eye as a single, fluid motion of the body axis (e.g. Fig. 2A), despite the large number of muscles recruited and joints moved. However, results indicate otherwise. Each stick figure in Fig. 2A illustrates the position of the body axis at a 100-ms interval for a representative sit-up. Shortly after sit-up onset, the total trunk angular velocity peaked (greater distance between the stick figures), then slowed to a minimum just before the mid-point (small distance between

Discussion

The study presented in this paper represents an initial investigation into how the human CNS coordinates voluntary movements that are biomechanically complex, a characteristic of many natural voluntary movements. We have argued, primarily on theoretical grounds, that there may exist many aspects of motor coordination that might only be revealed in complex movements [16]. The purpose of this study was to analyze the kinematics and EMG activity associated with sit-ups in order to identify what

Acknowledgements

The authors gratefully acknowledge Graham Kerr, Ph.D. and Ron Jacobs, Ph.D. for their contributions to the development of the force table and to the National Institutes of Health for funding this project (AR31017, AR46007). S.V. was supported by the Fund for Scientific Research, Flanders, Belgium. S.B. was supported by the Research Council of the Katholieke Universiteit Leuven.

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