The benefits of squat exercises have been reported in studies of athletic training (Grooms et al., 2013), rehabilitation (Escamilla et al., 2001), and locomotive syndrome in elderly people (Nakamura, 2011). The squat is a wide entity, comprising the partial squat (Escamilla et al., 2001), half squat (Hartmann et al., 2013), parallel squat (Caterisano et al., 2002; Contreras et al., 2016; Hartmann et al., 2013), full squat (Caterisano et al., 2002; Contreras et al., 2016), and deep squat (Hartmann et al., 2013). Furthermore, there are front (Aspe and Swinton 2014; Clark et al., 2012; Contreras et al., 2016), back (Aspe and Swinton 2014; Clark et al., 2012; Gullett et al., 2009) and overhead squats (Gullett et al., 2009) with various bar positioning, which have been previously analyzed. In the squat exercise, several factors or conditions can influence the muscle activity, including the knee angle (Gryzlo et al., 1994), foot width (Escamilla et al., 2001; McCaw and Melrose, 1999), rotation of the lower limbs Escamilla et al., 2001; Ninos et al., 1997; Signorile et al., 1995), or whether the knees going over the toes or not (Fry et al., 2003; Isear et al., 1997). In addition, the position of the center of gravity (COG) has been reported as a critical factor for electromyographic activity of lower extremity muscles (Kvist and Gillquist, 2001; Nishiwaki et al., 2006), although active control of the posture was not examined in these reports.
In patients with anterior cruciate ligament (ACL) reconstruction, exercise for activities of the hamstrings and gastrocnemius are especially important in the closed kinetic chain (CKC), and squat exercise is widely used in the rehabilitation program. (Palmitier et al., 1991). However, compared to that of other muscles, the activations of these muscles are relatively low in squat exercises, even with the addition of the barbell (Aspe and Swinton, 2014; Contreras et al., 2016; Gullett et al., 2009). Thus, a new strategy that better focuses on the target muscles is clearly required for effective squat training. We hypothesized that the active control of the COP position could be used to increase the training efficacy of the squat in targeting specific muscles. Herein, we measured the electromyographic muscle activity of the lower limbs during isometric squat hold with two alternative positions of the COP (normal squat posture and front shifted posture), and three angles of the knee joint (30, 60, and 90 degrees). The muscle activity patterns were analyzed and the possible target muscles in each squat variation were assessed.
Experimental approach to the problem
This is a prospective observational study. To evaluate the effect of changing the COP in the squat posture on muscle activity patterns, we compared target muscle activities in the normal squat posture (NSP) to those in a squat posture designed to move the COP forward as far as possible (forward-shifted posture: FSP) at three flexion angles of the knee joint (30, 60, and 90 degrees). Subjects held a squat position and flexion angles of the knee joint were controlled by a goniometer. We set acceptable range of plus/minus 5 degrees.
Ten healthy men with no history of knee injury, knee pain, or surgery participated in this study (Table 1). All had squat experience. The content of the experiment was sufficiently explained to the subjects, and only those who voluntarily agreed to participate in the experiment were enrolled. All subjects provided informed consent for the study procedures. Participation was voluntary and in agreement. This study was approved by the ethics committee of Nara Medical University (No. 1969). No subjects had difficulty to hold the squat position for 5 seconds.
The location of the COP was measured using a stabilometer (Twin Gravicorder G-6100; Anima Corp., Tokyo, Japan) at a sampling frequency of 100 Hz. The participants could visually check the COP during the squats. In the NSP condition, the shoulder joint was abducted as an external rotation and the middle finger was placed on the temple, and the squat posture was adopted with the COP located just between feet (Figure 1). In the FSP condition, the squat posture was adopted with palms faced downward and the COP shifted forward as far as possible (Figure 1). The foot width in the squat posture was 45 cm, between the left and right fifth metatarsal bone bottoms. The participants were told that the long axis of the femur was on the long axis of the third metatarsal bone of the foot. In addition, the participants were directed to look frontward.
Measurement of the COP
The measurement of the COP was performed barefoot, using a stabilometer. The ratio of the distance from the heel to the COP and the foot...