A. Introduction

Anterior cruciate ligament (ACL) reconstructive surgeries are some of the most common sports medicine procedures (Csintalan et al., 2008), with approximately 127,000 of these surgeries having been performed in the United States in 2006 (Kim et al., 2011). Given how frequently ACL reconstructions are performed, it is important to assess whether they successfully restore the ACL to its pre-injury functional level. The ACL plays a critical role in knee stability, with its primary function being the prevention of anterior tibial translation during flexion (Liu-Ambrose, 2003). This function is especially relevant during midstance of the gait cycle, at which point the ACL reaches its maximal elongation and experiences peak loading (Wu et al., 2010). The gait cycle - defined as heel strike to heel strike of one leg - is split into two phases: stance and swing. Stance is the time from heel strike to toe off and occurs during the first 60% of the cycle, with midstance occurring between 10-30% of the gait cycle when the body’s center of mass is directly over the foot and is supported by a single leg (Hurd & Snyder-Mackler, 2007). Swing occurs during the latter 40% of the gait cycle from the time of toe off to heel strike, with midswing occurring at 73-87% of the gait cycle when the leg in swing phase passes the contralateral leg in midstance (Uustal and Baerga, 2004). While reconstructive surgery aims to restore ACL function, there is evidence suggesting that the function of the affected leg is inferior to that of the unaffected leg following surgery, as evidenced by altered spatial-temporal aspects of gait and joint angles in the affected leg (Webster et al., 2012).

Scanlan et al. (2013) used a within-subjects design to explore how knee flexion during gait is affected by ACL reconstruction, observing that the degree of knee flexion during stance was greater in subjects' affected legs. Given that the ACL is maximally elongated during midstance (Wu et al., 2010) and is more strained at smaller angles of knee flexion (Li et al., 2005), it is likely that subjects recovering from surgery will exhibit a greater degree of knee flexion during stance. This adaptation is developed in response to the heightened pain-related fear of reinjury to the affected leg post-surgery (Chmielewski et al., 2008). In an examination of healthy individuals by Lelas et al. (2003), researchers found that peak knee flexion angle during loading increases with gait speed. This is likely due to the greater forces acting at increased speeds (Damavandi et al., 2012), leading to exaggerated strain on the ACL that subjects counteract with increased knee flexion. We therefore expect a greater degree of knee flexion in the affected leg at the increased speed due to individuals attempting to minimize ACL strain. Differences at the hip and ankle joints have also been observed following ACL reconstruction. Ferber et al. (2002) found that the knees and hips of subjects' affected legs were more flexed during mid to late stance compared to those of a control group. Given the interrelated nature of lower extremity joint angles, it is expected that an increased degree of hip flexion and dorsiflexion will occur during stance in the affected leg. Although previous literature has not examined the effect of increased gait speed on hip and ankle joint angles post-ACL reconstruction, based on research in healthy individuals, we expect the affected leg to maintain greater degrees of flexion during stance at the increased speed, with the hip angles remaining unchanged and the ankle angles decreasing in both legs during stance (Biewener et al., 2004).

The result of these changes in joint angles, specifically the increased degree of knee flexion during midstance in the affected knee, limits the distance the leg travels between midswing and heel strike, thus reducing stride length in the affected leg (Knoll et al., 2003). Stride length typically increases with increasing gait speed, and we expect to observe the same pattern in both legs in this study (Oberg & Karsznia, 1993). ACL reconstruction also affects the amount of time spent in each phase of the gait cycle. Since ACL strain is maximized during midstance (Wu et al., 2010), individuals will spend less time in this phase when bearing weight on the affected leg in order to minimize discomfort. Due to the greater forces acting on the knee at higher speeds (Damavandi et al., 2012), in addition to pain and fear of reinjury of the affected leg (Chmielewski et al., 2008), we expect the deviation in the angles of the knee and ankle joints, stride length, and amount of time spent in midstance of the two legs to be amplified at the increased gait speed, while we expect no change in the deviation in the angles of the hip joint (Biewener et al., 2004).

Figure 1. Gait cycle (Uustal & Baerga, 2004). Stance phase occurs during the first 60% of the gait cycle and swing phase occurs during the latter 40%. Heel strike occurs at both 0% and 100%. Midstance occurs during 10-30% of gait cycle while midswing occurs during 73-87% of the gait cycle.