Females exhibit shorter paraspinal reflex latencies than males in response to sudden trunk flexion perturbations
Introduction
Females have a higher risk of experiencing low back pain (LBP) or injury than males (Feuerstein et al., 1997, Hillman et al., 1996, Jones et al., 1993, Macfarlane et al., 1997) and are more likely to have chronic (Butler et al., 1995, Volinn et al., 1991, Zwerling et al., 1993) or more severe cases (Von Korff et al., 1993). In fact, females experience LBP or injury 240% more frequently than males in occupational settings that involve lifting (Daltroy et al., 1991, Macfarlane et al., 1997). Females are also more likely to experience limited activity capabilities due to LBP one year following an initial incident (Von Korff et al., 1993). The mechanisms behind these gender differences in rate and severity are unclear, yet they are important to discover in order to address this disparity in injury risk.
Paraspinal reflexes are one of many mechanisms that contribute to the control of spinal stability (Panjabi, 1992a), and altered reflexes may contribute to LBP. Longer paraspinal reflex latencies have been reported in LBP patients, compared to healthy controls, after either sudden loading (Wilder et al., 1996) or unloading (Radebold et al., 2000, Radebold et al., 2001) of the trunk that resulted in trunk flexion. Similarly, patients with unilateral low back pain exhibit longer erector spinae reflex latencies on the painful side than on the non-painful side (Wilder et al., 1996). Longer paraspinal muscle reflex latencies in LBP patients have also been correlated with impaired postural control of the trunk during unstable sitting (Radebold et al., 2001). While it remains unclear whether LBP lead to these increased paraspinal reflex latencies or if altered reflexes contribute directly to the development of LBP, these studies demonstrate an association between LBP and increased paraspinal reflex latency.
Gender differences in paraspinal reflexes may be related to gender differences in LBP. Unfortunately, the few studies investigating gender differences in paraspinal reflexes have not yielded consistent results, perhaps due to confounding experimental factors. Wilder et al. (1996) reported longer paraspinal reflex latencies in females compared to males in response to sudden trunk flexion loading within both LBP patients and healthy controls. However, their subjects participated in a rehabilitation program involving physical conditioning and a cognitive–behavioral approach. Post hoc analyses revealed that males shortened their reflex latencies more than females after the rehabilitation program. Therefore, the gender difference reported could have resulted from the ability of males to shorten their reflex latency more than females following the rehabilitation program, rather than the existence of a true gender difference. Granata et al. (2005) found no gender differences in paraspinal reflex latency following trunk flexion perturbations in individuals without low back symptoms. However, the isotonic trunk extension preloads generated by participants prior to the perturbations could have masked any gender differences in reflex latency by stiffening the spine (Lee et al., 2007) and possibly reduced the need for quicker reflexes. Both Wilder et al., 1996, Granata et al., 2005 also exposed males and females to the same trunk perturbation magnitudes, which can result in greater trunk flexion velocities in females due to a smaller trunk mass (Granata and Rogers, 2007). Since stretch reflexes are heavily dependent upon the rate of change of muscle length (i.e. trunk flexion velocity) (Kearney & Hunter, 1983, Kearney et al., 1997, Moorhouse & Granata, 2007), comparisons of reflex latency could be confounded when males and females experience different trunk kinematics. Gender differences in reflex latencies have also been reported in other muscles. Women exhibited shorter reflex latencies in the posterior neck musculature following sudden whiplash simulations (Siegmund et al., 2003) and in the quadriceps in response to body weight-scaled perturbations that induced internal and external rotations of a single knee (Shultz et al., 2001).
It is important to recognize any gender differences in paraspinal reflexes to further our understanding of gender differences in LBP. However, existing studies do not provide consistent results and may be confounded by extraneous experimental factors. As a result, the goal of this study was to investigate gender differences in paraspinal reflex latency while avoiding potentially confounding factors, and while controlling for potential gender differences in trunk kinematics resulting from gender differences in trunk mass. Based upon gender differences in reflex latency in the neck musculature (Siegmund et al., 2003) and the quadriceps (Shultz et al., 2001), it was hypothesized that females would exhibit shorter paraspinal reflex latencies than males in response to sudden trunk flexion perturbations.
Section snippets
Methods
Participants included ten males (mass: mean 73.4 (SD 1.5) kg; age: mean 22.6 (SD 2.8) years) and ten females (mass: mean 65.0 (SD 1.4) kg; age: mean 27.2 (SD 7.8) years) recruited from the university community. Participants had no history of low back pain or injury. This study was approved by the Virginia Tech Institutional Review Board, and written consent was obtained from all participants prior to participation.
Paraspinal muscle activity and trunk kinematics were recorded following repeated
Results
Reflexes were detected after 94% of the perturbations. Prior to each perturbation, baseline muscle activity did not differ between females (mean 1.07 (SD 0.15) %MVC) and males (mean 1.31 (SD 0.15) %MVC; P = 0.26) or by impulse (P = 0.28). Analyzing reflex latency with respect to impulse (Fig. 3), reflex latency was 18.7% shorter in females (mean 48.8 (SD 3.0) ms) than males (mean 60.0 (SD 3.2) ms; P = 0.02) and did not vary by impulse (P = 0.38). However, maximum trunk flexion velocity (Fig. 4) was
Discussion
Our goal was to investigate gender differences in paraspinal reflex latency in an effort to improve our understanding of the gender disparity in LBP. Following trunk flexion perturbations, females responded with shorter latencies than males. They also exhibited a faster maximum trunk flexion velocity, which was not unexpected due to a smaller trunk mass in females than males. When controlling for this gender difference in maximum trunk flexion velocity, latency remained shorter in females than
Conclusion
Females exhibited shorter paraspinal reflex latencies than males after a trunk flexion perturbation, even when controlling for gender differences in kinematics. Based upon the longer latencies reported in the injured lower back (Radebold et al., 2000, Radebold et al., 2001, Wilder et al., 1996), these results do not appear to support the notion that gender differences in paraspinal reflex latency alone directly contribute to gender differences in LBP. Additional research is needed to elucidate
Acknowledgements
This research was funded by grant number 2 RO1 AR046111 from the NIAMS/NIH. The sponsors had no role in the study design, data collection/analysis/interpretation, manuscript writing, or the decision to submit the manuscript for publication.
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