The influence of muscle activation on impact dynamics during lateral falls on the hip

Abstract

Muscle activation has been demonstrated to influence impact dynamics during scenarios including running, automotive impacts, and head impacts. This study investigated the effects of targeted muscle activation magnitude on impact dynamics during low energy falls on the hip with human volunteers. Fifteen university-aged participants (eight females, seven males) underwent 12 lateral pelvis release trials. Half of the trials were muscle-‘relaxed’; in the remaining ‘contracted’ trials participants isometrically contracted their gluteus medius to 20–30% of maximal voluntary contraction before the drop was initiated onto a force plate. Peak force applied to the femur-pelvis complex averaged 9.3% higher in contracted compared to relaxed trials (F = 6.798, p = .022). Muscle activation effects were greater for females, resulting in (on average) an 18.5% increase in effective pelvic stiffness (F = 5.838, p = .046) and a 23.4% decrease in time-to-peak-force (F = 5.109, p = .042). In the relaxed trials, muscle activation naturally increased during the impact event, reaching levels of 12.8, 7.5, 11.1, and 19.1% MVC at the time of peak force for the gluteus medias, vastus lateralis, erector spinae, and external oblique, respectively. These findings demonstrated that contraction of trunk and hip musculature increased peak impact force across sexes. In females, increases in the magnitude and rate of loading were accompanied (and likely driven) by increases in system stiffness. Accordingly, incorporating muscle activation contributions into biomechanical models that investigate loading dynamics in the femur and/or pelvis during lateral impacts may improve estimate accuracy.

Introduction

Hip fractures are a widespread and severe health concern for older adults, with frequency estimates of up to 21 million globally per year by 2050 (Cummings and Melton, 2002). As the risk of hip fracture is directly related to the loads applied to the proximal femur (Tamura et al., 1982), increased knowledge of the factors that influence impact dynamics of the pelvis/femur system could assist with development of interventions aimed at hip fracture prevention.

While recent studies are advancing our understanding of dynamics of the pelvis and proximal femur system during lateral impacts (Bhan et al., 2014, Choi et al., 2014, Laing and Robinovitch, 2010, Sarvi and Luo, 2015) there remain questions about the role of muscle activation on these dynamics. Muscle activation has been demonstrated to play a role in impact dynamics across a breath of scenarios including running, car impacts and head impacts (Laing and Robinovitch, 2010, Newell et al., 2013, Nigg and Liu, 1999, Sarvi and Luo, 2015, van der Horst et al., 1997). Specific to fall-related injuries, Burkhart and Andrews (2013) studied falls onto outstretched hands, and observed significant increases in activation levels of six upper limb muscles between release, start of impact, and moment of peak impact force. However, little research has examined the potential influence of muscle activation on dynamics of the femur-pelvis system during the impact phase of sideways falls. Charactization of activation patterns during lateral falls on the hip could assist in determining the potential importance of including muscle activation into computational models of hip fracture risk.

There are several potential mechanisms by which muscle activation might affect impact dynamics. First, since activation increases local muscle longitudinal stiffness (Tamura et al., 1982), it is possible that this effect translates into increased overall stiffness within the femur/pelvis system. Similarly, co-contraction of antagonistic muscles may serve to increase joint rotational stiffness. Second, muscle activation may influence the effective mass of the pelvis, and ultimately, the inertia involved in the event of a fall-related hip impact. Finally, it is possible that muscular contractions could change the geometry of the body-impact surface interface, potentially influencing load distribution and resulting impact dynamics. There exists some limited literature that provides insights into these issues. While muscle activation of the gluteal muscles has been found to change thickness of the tissues overlying the ischial tuberosity (Hodges et al., 2003, Makhsous et al., 2011), similar effects have not been observed for the region overlying the greater trochanter (GT) (Tamura et al., 1982), attributed in part to the GT’s use as an attachment point for multiple tendinous tissues as opposed to muscle bodies. While Choi et al. report that increased activation of hip abductor muscles decreased peak femoral neck compressive stress by 24% and tensile stress by 56% during simulated sideways falls, these results may not be generalizable as they were generated with a mechanical test system (Choi et al., 2014). In the most direct studies of the influence of muscle activation on impact dynamics, Robinovitch et al. (Robinovitch et al., 1991, Sarvi and Luo, 2015) used pelvis release experiments with live volunteers. They initially reported that impact peak force applied to the proximal femur increased with increased muscle activation (Robinovitch et al., 1991), but found little effect of muscle activation in a subsequent study (Sarvi and Luo, 2015). This lack of consistency could potentially be related to their approach of simply asking participants to “contract [their] trunk and back muscles prior to the start of each ‘contracted state’ trial” (i.e. muscle activation level was not explicitly controlled). Overall, despite plausible mechanisms by which muscle activation could influence impact dynamics during falls on the hip, there is currently no available literature that characterizes activation patterns during the impact phase of sideways falls, or the influence of explicit changes in activation magnitude on impact dynamics.

The effects of muscle activation on impact dynamics could differ across sexes due to differences in body composition. Previous studies have shown that the thickness of soft tissue overlying the greater trochanter is negatively correlated with estimated mechanical risk of hip fracture during a fall on the hip (Majumder et al., 2008, Robinovitch et al., 1995). Compared to males, females have significantly less lean tissue mass, increased fat mass, and a greater proportion of their fat mass located in the lower body (Ley et al., 1992). More local to the proximal femur, females have significantly more soft tissue overlying the greater trochanter when compared to males (Ley et al., 1992, Majumder et al., 2013). However, whether these differences in body composition interact with muscle activation to influence impact dynamics during sideways falls on the hip is unknown. Given the increased incidence of hip fractures in females (Jean et al., 2013), enhanced knowledge of these factors could provide support for the use of gender-specific computational models of pelvic impacts that incorporate muscle activation contributions (Jean et al., 2013).

The objectives of this study were two-fold. The primary goal was to investigate the potential effects of targeted muscle activation magnitude on impact dynamics during low energy falls on the hip with human volunteers. Our first hypothesis was that trials with contracted muscles would produce (a) higher peak force, (b) shorter time to peak force, and (c) increased stiffness of the femur-pelvis system (henceforth referred to as pelvic stiffness). Our second hypothesis was that more pronounced effects of muscle activation would be observed for males compared to females. The study’s secondary goal was to characterize time-varying changes in muscle activation magnitude during the baseline ‘muscle relaxed’ conditions to provide insights into naturally occurring activation profiles during lateral impacts of the femur-pelvis complex.