Stralka, & Bailey, 1994). During testing, a burst superimposition of electrical stimulation is delivered to the quadriceps muscle while the patient is completing a quadriceps MVIC. Quadriceps muscle activation is then calculated as the ratio of force produced during MVIC to the force produced with addition of the electrical stimulation, which can be used as a measure of arthrogenic muscle inhibition or the inability to fully activate a muscle due to structural changes in a joint (Lynch, Logerstedt, Axe, & Snyder- Mackler, 2012). Testing is completed on each limb until 95% quadriceps activation is achieved, activation levels plateau, or the patient fatigues. After completing testing on each limb, a quadriceps index (QI) can be calculated as the quotient of the involved limb to the uninvolved limb multiplied by 100. The QI provides information on the strength of the quadriceps of the involved limb compared to the contralateral limb. The burst superimposition technique is superior to handheld dynamometry (that is properly secured) because the patient completes an MVIC into a stationary structure that records quadriceps muscle force generation. Thus, 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
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