surgeries, meniscus repair, or chondral surgeries. In these cases, the ROM might be limited to 90° for 6 to 8 weeks after surgery to prevent failure of these surgeries (Adams et al., 2012). In pa- tients who had reconstruction surgery using hamstrings-gracilis autograft, the hamstrings strengthening exercises should not be initiated until 8 weeks after surgery to ensure tendon regrowth (Adams et al., 2012). Imbalanced quadriceps-hamstrings muscles strength was re- ported as a predictive measure to identify female athletes with high risk for ACL injury (Myer et al., 2010). Clinicians may consider providing balanced strength training to both the hamstrings and quadriceps muscles throughout the rehabilitative process. As a re- sult, the hamstrings may be able to decrease the anterior tension load applied on the graft tissue during quadriceps activation. This might suggest assessing the strength difference between quadri- ceps and hamstrings muscles to determine patients’ readiness to return to preinjury activities and to ensure better outcomes. Neuromuscular electrical stimulation NMES is commonly used for treating quadriceps muscle inhibi- tion and strengthening the quadriceps muscles following ACL in- jury and ACL reconstruction (Adams et al., 2012; Kim et al., 2010; Lynch et al., 2012; Rice & McNair, 2010; Williams et al., 2005). Electrical stimulation to increase quadriceps muscle strength is most beneficial when quadriceps weakness is due to arthrogenic muscle inhibition (Kim et al., 2010; Lynch et al., 2012; Rice & Mc- Nair, 2010). The NMES device provides an alternating electrical current that resembles the electrical current of action potential coming from the motor cortex. The electrical current will initiate an action potential in the nerve branches that innervate the quad- riceps muscle. As a result, the muscle will develop an involuntary muscle contraction (Trimble & Enoka, 1991). Early incorporation of NMES into the treatment program after ACL injury or recon- struction surgery is recommended to avoid quadriceps muscle in- hibition and its impact on the functional recovery (Rice & McNair, 2010). Administering strength training augmented with NMES after ACL reconstruction was shown to be more effective in improving the quadriceps strength compared to the strengthening training alone (Kim et al., 2010). Therefore, NMES training is recommended as an adjunct treatment for patients whose injured limb quadriceps muscles exert a maximum voluntary isometric force of less than 80% of the uninjured limb (Adams et al., 2012; Delitto et al., 1988; Snyder-Mackler et al., 1995; Snyder-Mackler et al., 1994). There is an inconsistency in the reported studies about the treat- ment parameters for NMES (treatment time, frequency, phase duration, and the on-to-off ratio). A systematic review suggests that a stimulus waveform of 1.0- to 2.5-kHz frequency alternating current, with a 2- to 4-millisecond burst, may yield the best torque output with the least patient discomfort (Kim et al., 2010; Ward, Robertson, & Ioannou, 2004). Gorgey and Dudley (2008) sug- gested that a pulse duration of 450 microseconds resulted with 38% higher knee extensor torque compared to a short-phased duration of 250 milliseconds. A commonly used NMES protocol consists of 10 electrically stimulated isometric contractions of the quadriceps muscles, with 10 seconds on and 50 seconds off, with a 2,500-Hz stimulus delivered at 75 bursts per second (Fitzger- ald, Piva, & Irrgang, 2003; Lepley & Palmieri-Smith, 2013; Snyder- Mackler et al., 1994). The self-adhesive stimulating electrodes can be place on the vastus medialis distally and on the vastus lateralis proximally (Lepley & Palmieri-Smith, 2013; Snyder-Mackler et al., 1994). For the NMES to be effective in improving the quadriceps muscle strength, high-intensity electrical stimulation up to the patient’s tolerance is recommended to be administered (Risberg, Lewek, & Snyder-Mackler, 2004). Fitzgerald and colleagues (2003) reported that electrical current amplitude for NMES must be equivalent to the electrical current needed to produce 50% of maximum vol- untary isometric contraction of the reconstructed limb’s quadri- ceps muscle in order to improve the quadriceps muscle strength. NMES training can be performed on a dynamometer with the
(Palmieri-Smith & Lepley, 2015; Schmitt et al., 2012; Zwolski et al., 2015). However, quadriceps strength alone is not sufficient to restore symmetrical movement patterns (Arhos et al., 2021). Several strength training and electrotherapy modalities have been used to help resolve quadriceps strength deficits and activation failure in patients with ACL injury and ACL reconstruction surgery. Isometric quadriceps exercise is a safe strengthening exercise that can be administered immediately after surgery and it has no neg- ative impact on knee laxity (Isberg et al., 2006; Shaw, Williams, & Chipchase, 2005). Early initiation of high-intensity neuromuscular electrical stimulation (NMES) has been found to be effective in improving quadriceps muscle activation and strength after ACL injury and construction surgery (Kim, Croy, Hertel, & Saliba, 2010; Lynch et al., 2012; Rice & McNair, 2010; Snyder-Mackler et al., 1994). In addition to NMES, quadriceps strength training may include the use of high-intensity, low-repetition weightbearing (WB), non- weightbearing (NWB), and eccentric exercises. These strengthening techniques may be augmented with NMES. Pro- gression through quadriceps strength training is based on criteri- on-based guidelines to maximize the quadriceps strength. Hamstrings strengthening Hamstrings muscles work as an ACL agonist in controlling the anterior translation of the tibia (Imran & O’Connor, 1997, 1998). Inability of the hamstrings muscles to provide adequate force to stiffen the knee joint during dynamic activities has contributed to the ACL injury in female athletes (Beynnon & Fleming, 1998; Hewett et al., 2005). Therefore, training the hamstrings muscles in patients with ACL injury may contribute to knee stability dur- ing participation in functional activities and prevent subsequent injury to the knee structures. In patients who are managed opera- tively, hamstrings weakness may fail to provide enough force to stabilize the knee joint and to control the forward forced anterior tibia translation. This in turn may place more tension load on the graft tissue and lead to graft failure. Increased muscle strength difference between quadriceps and hamstrings muscles may con- tribute to graft failure after ACL reconstruction surgery in patients who return to preinjury activity levels (Kyritsis et al., 2016). The rehabilitation programs for operative and nonoperative programs after ACL injury should address hamstrings weakness to improve patients’ outcomes and to prevent unnecessary knee injuries. Hamstrings strengthening exercises can be integrated early in the ACL injury rehabilitation to improve muscle strength. Strength exercises for the hamstrings include isometric hamstring contrac- tion, hamstrings curl in sitting and supine positions, reverse leg curl, and Nordic/Russian hamstrings exercises. Exercises that em- phasis hip extension may also strengthen the hamstrings muscles, including squatting, single-leg ball pickup, lunge with weight, and pelvic bridging exercises. Isometric hamstrings contraction and exercises that do not include loads are performed first, then progression is made toward exercise with loads. The progression strategies for strengthening the hamstrings muscles include in- creasing the weight load and the numbers of repetitions as pa- tients proceed toward the late phase of the rehabilitation pro- gram. Nordic hamstrings exercise can be performed on a low mat ta- ble and requires a one-on-one treatment from the therapist. To perform the exercise, the patient takes a kneeling position while the therapist holds the patient’s ankles to provide stability. The first phase of the exercise consists of eccentric hamstrings activity as the patient lowers his or her torso down toward the mat. The second phase of the exercises is a concentric contraction as the patient moves up to the initial kneeling position. In the early stage of the rehabilitation program, the exercise is performed to 30° to 45° of knee flexion position. Then it is progressed by moving the patient’s body to a lower degree of knee flexion angle and increasing the number of repetitions (White, Di Stasi, Smith, & Snyder-Mackler, 2013). Postoperative hamstrings strength exercises involve concerns re- lated to the ROM of the exercise for patients with concomitant meniscus injury or patients with additional PCL reconstruction
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