the assessment is not dependent on the ability of the examiner to provide sufficient resistant force for testing. Also, unlike handheld dynamometry, the burst superimposition technique allows for measurement of quadriceps muscle activation, providing information on the patient’s ability to volitionally activate the quadriceps and whether progression of activity can be recommended. Arthrogenic muscle inhibition, measured by decreased quadriceps muscle activation (Lynch et al., 2012), can limit effective rehabilitation following knee injuries and thus delay return to previous activity levels (Rice & McNair, 2010). Quadriceps activation failure is common following ACL injuries and reconstruction and is often observed bilaterally (Chmielewski, Stackhouse, Axe, & Snyder-Mackler, 2004; Hart, Pietrosimone, Hertel, & Ingersoll, 2010; Snyder-Mackler, Delitto, Bailey, & Stralka, 1995; Williams et al., 2005). For patients who choose to undergo ACL reconstruction, it is essential to regain preoperative quadriceps muscle function (including strength and activation as well as neuromuscular control [see below]) following injury. Preoperative quadriceps strength deficits predict poor quadriceps strength and low self-reported function after surgery (Eitzen, Holm, & Risberg, 2009; Logerstedt, Lynch, Axe, & Snyder-Mackler, 2013a). Eitzen and colleagues (2009) suggest that ACL reconstruction should not be performed until the QI is at least 80% (Eitzen, Holm, & Risberg, 2009). Quadriceps strength is of equal importance to patients who choose nonoperative treatment because it may help prevent early onset osteoarthritis (OA; Ageberg et al., 2008). Whichever technique is used to measure strength, it is important to consider the validity of the muscle strength measurement, because knee pain during testing may decrease the force production capability of the muscle being tested and provide inaccurate information regarding quadriceps muscle strength. Neuromuscular control Neuromuscular adaptations are present following both ACL injury and reconstruction. These adaptations can result from affected mechanoreceptors in the ACL and joint capsule that influence somatosensation, muscle activation, muscle strength and atrophy, balance, and gait biomechanics (Ingersoll, Grindstaff, Pietrosimone, & Hart, 2008). Afferent information sent to the central nervous system can be affected by some of these neuromuscular changes, sometimes leading to impairments in bilateral lower extremities, as seen in some patients with bilateral quadriceps activation failure (Ingersoll et al., 2008). Changes in neuromuscular control patterns may lead to chronic biomechanical changes at the lower extremities, increasing the risk of future osteoarthritis at the involved knee joint (Hurd & Snyder-Mackler, 2007; Ingersoll et al., 2008; Rudolph, Axe, Buchanan, Scholz, & Snyder-Mackler, 2001). Although many neuromuscular adaptations affecting the knee joint can be detected only in a laboratory setting, examination of balance and muscle activation patterns can be used to clinically assess neuromuscular control following ACL injury and reconstruction. Differences in single- leg balance tasks, with eyes open and with eyes closed, have been detected in the involved limb following ACL injury compared to controls; however, differences may not be present between limbs (Lysholm, Ledin, Odkvist, & Good, 1998). Abnormal muscle activation patterns, such as increased activation of the hamstrings, may be seen, indicating cocontraction strategies to achieve knee stabilization (O’Connell, George, & Stock, 1998). When assessing balance, it is important to include perturbations rather than static challenges only, because they may better represent demands required during different activity levels (O’Connell et al., 1998). Reaction times to perturbations may be greater in the involved limb than in the uninvolved limb (Lysholm et al., 1998).
The Star Excursion Balance Test is performed by standing on one leg and reaching maximally with the other leg in eight different directions. During single-leg activities on the involved limb, this test has demonstrated deficiencies in dynamic postural control following ACL injuries in four of the directions tested (anterior, lateral, posteromedial, and medial). However, caution is needed; differences are also present in the medial and lateral directions between the uninvolved limb and controls (Herrington, Hatcher, Hatcher, & McNicholas, 2009). Neuromuscular training based on findings during examination should be integrated into the patient’s rehabilitation program, because it leads to improvements in limb symmetry prior to and after ACL reconstruction (Hartigan, Axe, & Snyder-Mackler, 2009). Performance-based testing Biomechanical limb-to-limb asymmetries during gait are present following ACL injury and reconstruction (Di Stasi, Logerstedt, Gardinier, & Snyder-Mackler, 2013; Hurd & Snyder-Mackler, 2007; Ingersoll et al., 2008; Rudolph et al., 2001; Rudolph, Eastlack, Axe, & Snyder-Mackler, 1998; Capin, Khandha, Zarzycki, Arundale, et al., 2018; Capin, Zarzycki, et al., 2017, Capin, Zarzycki, et al., 2019; Hart et al., 2016; Wellsandt et al., 2020). These abnormalities become exaggerated with the increased demands of jogging and running (Ingersoll et al., 2008). Abnormal movement patterns, which may limit performance during stair ascent and descent, lateral step-up tasks, and vertical jump tasks, are also present and important to examine (Ingersoll et al., 2008). Single-legged hop tests are often used as a measure of activity limitations following ACL injury and reconstruction (Grindem, Eitzen, Moksnes, Snyder-Mackler, & Risberg, 2012; Logerstedt et al., 2012; Logerstedt, Lynch, Axe, & Snyder-Mackler, 2013b; Noyes, Barber, & Mangine, 1991; Reid, Birmingham, Stratford, Alcock, & Giffin, 2007). They can be used to predict dynamic knee stability (Grindem et al., 2011; Fitzgerald, Lephart, Hwang, & Wainner, 2001; Logerstedt et al., 2012). Although preoperative single-legged hop tests cannot predict postoperative outcomes, testing at 6 months following ACL reconstruction is effective at predicting self-reported knee function at 1 year following ACL reconstruction (Logerstedt et al., 2012). Single-legged hop tests can also differentiate between patients who are able to return to previous activity levels following ACL injury and reconstruction and those unable to do so (Ardern et al., 2011a; Fitzgerald, Axe, & Snyder-Mackler, 2000a). As shown in Figure 2, the most common single-legged hop tests are a series of four hops, including a single hop for distance (single hop), crossover hop for distance (crossover hop), triple hop for distance (triple hop), and 6-meter timed hop (6-m timed hop; Barber, Noyes, Mangine, McCloskey, & Hartman, 1990; Noyes et al., 1991). These tests can be used to assess a combination of muscle strength, neuromuscular control, confidence in the injured limb, and ability to complete sport- specific activities (Reid et al., 2007). The single-legged hop tests are completed along a 6-m long and 15-cm-wide strip on the floor, with each test completed two times for each leg. For the single hop, the patient stands on the leg to be tested and hops as far as possible, landing on the same leg. The crossover hop is completed by the patient hopping three consecutive times on the same leg, alternately crossing over the 15-cm-wide strip on each hop with total distance forward measured. For the triple hop, the patient completes three consecutive hops on the same leg as far as possible in a linear direction, with total distance measured. The single hop, crossover hop, and triple hop must be completed with a controlled landing on the leg being tested without additional hops or assistance of the contralateral leg to achieve balance, or the trial is re-done. The 6-m timed hop is completed by the patient hopping on one leg as fast as possible along the 6-m distance. Using a stopwatch, the examiner measures the time from when the patient’s heel leaves the ground to the time the 6-m mark is reached. Each hop test is completed on the uninvolved limb
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