Trunk muscle coactivation in preparation for sudden load

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Abstract

Biomechanical stability of the lumbar spine is an important factor in the etiology and control of low-back disorders. A principle component of biomechanical stability is the musculoskeletal stiffening generated by preparatory muscle coactivation. The goal of this investigation was to quantify preparatory behavior, evaluating trunk muscle activity immediately prior to sudden trunk flexion loading during static extension tasks compared to activity observed when subjects were informed no sudden load would occur. Coactive excitation was also examined as a function of fatigue and gender. Results demonstrated increased extensor muscle and flexor muscle coactivation following static fatiguing exertions, potentially compensating for reduced trunk stiffness. Female subjects produced greater flexor antagonism than in the males. No difference in the preparatory coactive muscle recruitment patterns were observed when subjects were expecting a sudden flexion load compared to recruitment patterns observed in similar static postures when subjects were informed no sudden load would be applied. This indicates the neuromuscular system relies greatly on response characteristics for the maintenance of stability in dynamic loading conditions.

Introduction

Tolerance to spinal injury can be dramatically influenced by musculoskeletal stability. Although NIOSH suggests spinal loads below 3400 N may be considered safe for a majority of the working age population [31] the spinal column will become unstable and fail at compressive loads less than 100 N without muscles to provide stability [8]. Fortunately, the neuromuscular system can control mechanical stability of the spine, thereby allowing the structure to safely withstand extreme compressive loads.

Increased muscle activation and antagonistic co-activation may be recruited to augment spinal stability in an unstable environment [7], [11], [14]. Empirical measurements suggest preparatory coactivation and intra-abdominal pressure reduced kinematic displacement following sudden flexion load, implying increased trunk stiffness associated with co-contraction [6], [36]. Recent measurements demonstrate antagonistic coactivation is actively recruited in response to changes in stability [32]. Thus, in an unstable environment or when a sudden load is anticipated it is expected that increased co-contraction must be recruited to stabilize the spine in a pre-emptive manner to reduce injury risk.

Sudden load paradigms are designed to investigate the neuromuscular preparation and response to biomechanical trunk perturbations. When a sudden flexion load is unexpectedly applied to the trunk a response in the form of antagonistic co-activation has been reported at levels up to 140% of the equivalent static value [24]. The response is influenced by asymmetry, fatigue, and the subject's history of low-back pain [28], [39]. Unfortunately, preparatory myoelectric behavior has been rarely reported. Measurements by Lavender et al. [23] revealed some subjects increased preparatory antagonistic co-activation while others demonstrated no preparatory myoelectric activity. Thomas et al. [36] reported that activity in both extensor and flexor muscles ramped up in time to meet the kinetic impact. Conversely, no preparatory myoactivation was observed when the timing of impact was unknown. It was curious that increased coactivation was not observed in both unblinded and blinded conditions as the subjects were aware that a sudden load would occur in both conditions, but lacked timing information in the latter. Recognizing that coactivation augments trunk stiffness and stability, one might expect increased antagonism in preparation for both blinded and unblinded sudden loading conditions.

Preparatory coactive recruitment may be influenced by fatigue and gender. It has been established that fatigue influences muscle spindle behavior [27] and associated reflex mechanics including stretch sensitivity, electromechanical delay and myoelectric response amplitude [1], [12], [16]. Fatigue has also been shown to affect muscle response time in the low-back [39]. All of these factors modulate stability. Epidemiologic data support these biomechanical concepts, demonstrating correlations between risk of LBD and muscular endurance [2]. Thus, fatigue depression of neuromotor response may require compensation by means of modified preparatory myoelectric behavior to maintain spinal stability. Gender is also a risk factor with females suffering more than twice the rate of musculoskeletal and low-back injuries than equivalently trained males [10], [22]. To improve gender inclusion in the workplace it is necessary to understand potential factors influencing biomechanical stability. Gender differences in passive joint stiffness have been established [4] and recent measurements indicate gender differences in muscle-controlled active joint stiffness [15]. To compensate for reduced active muscle stiffness, it is hypothesized that females may perform lifting tasks with greater coactivation to augment trunk stiffness and stability. The influences of fatigue and gender on preparatory myoelectric activity have not been reported.

The goal of this investigation was to quantify trunk muscle electromyographic (EMG) activity in preparation for a sudden flexion load. It was hypothesized that increased coactivation would be observed when subjects were preparing for an impending flexion moment impact compared to equivalent conditions wherein subjects were informed no sudden load was to be applied. Furthermore, it was hypothesized that increased preparatory coactivation must be observed in a fatigued state and greater coactivation may be demonstrated by female subjects to maintain biomechanical stability. Improved understanding of neuromuscular preparatory behavior may contribute to enhanced assessment of spinal stability and control of LBD risk.

Section snippets

Subjects

Eleven males and 14 females, 19–40 years of age, with no prior history of low back pain voluntarily participated in this experiment. Mean (±SD) subject height and weight was 172.9±10.9 cm and 71.9±13.9 kg respectively. A secondary study included nine subjects, four male and five females, 21.2–26.0 years of age and mean (±SD) height and weight of 166.8±9.8 cm and 66.8±11.8 kg. All subjects provided informed consent approved by Human Investigations Committee of the university.

Protocol

EMG and motion data

Results

Measured EMG was influenced by pre-load and task asymmetry (Table 1). The kinetic impact was sufficient to disturb the equilibrium of the trunk musculoskeletal system, generating a myoelectric response with similar characteristics and latency as described in the literature (Fig. 1). Post-hoc analyses demonstrated pre-load weight was associated with a significant increase in preparatory EMG activity for all of the measured trunk muscles, including trunk extensor and antagonistic flexor muscle

Discussion

Biomechanical stability describes the potential of the musculoskeletal system to maintain equilibrium in the presence of kinematic or kinetic disturbances. When the equilibrium posture is in a state of minimum potential energy, the system will return to this minimum energy level if perturbed [37]. One method of achieving this minimum energy state and stability condition is to establish increased stiffness. The stiffness component of active muscle is well recognized and contributes to voluntary

Conclusions

Scientific evidence suggest biomechanical stability of the lumbar spine is an important factor when considering the risk of LBD. Two of the primary components in spinal stability include the preparatory muscle recruitment and the response or feedback behavior. Results from the current study demonstrate no difference in the preparatory coactive muscle recruitment patterns when subjects were waiting in anticipation of a sudden flexion load compared to recruitment patterns observed in similar

Acknowledgements

We wish to thank S. Wilson, Ph.D. and D. Padua for technical assistance in this effort. This research was supported by grant K01 OH00158-03 from NIOSH of the Centers of Disease Control and Prevention, and grant R01 AR46111-03 from NAIMS of the National Institutes of Health.

Kevin Granata received a M.S. in physics from Purdue University and a Ph.D. in biomechanics from The Ohio State University. He serves as the Research and Engineering Director of the Motion Analysis and Motor Performance Laboratory at the University of Virginia where he is an assistant professor in the departments of Orthopaedic Surgery and Biomedical Engineering. His research focuses on neuromotor control and modeling of movement with specific interests in musculoskeletal stability and

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    Kevin Granata received a M.S. in physics from Purdue University and a Ph.D. in biomechanics from The Ohio State University. He serves as the Research and Engineering Director of the Motion Analysis and Motor Performance Laboratory at the University of Virginia where he is an assistant professor in the departments of Orthopaedic Surgery and Biomedical Engineering. His research focuses on neuromotor control and modeling of movement with specific interests in musculoskeletal stability and associated injury mechanisms.

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    Karl Orishimo received a B.S. from the University of Pennsylvania and a M.S. in biomedical engineering from the University of Virginia, working in the Motion Analysis and Motor Performance Laboratory. His MS thesis focused on the stability of the lumbar spine during lifting exertions. He is currently a Research Engineer at Anderson Orthopaedic Research Institute.

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    Adam Sanford received a B.S. from Cornell University and is finishing a M.S. in biomedical engineering with the Motion Analysis and Motor Performance Laboratory at the University of Virginia. His MS thesis focuses on the dynamic motion of the lumbar spine during lifting exertions. He is employed as a Research Engineer at Zimmer Orthopaedics

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