Lower-body maximal force and the rapid expression of force are important for many athletes, because these factors are related to many types of athletic performance (e.g., jumping). Rapid movements against loads are important to develop lower-body maximal force and the rapid expression of force regardless of whether the load is light or heavy (Behm and Sale, 1993; Fielding et al., 2002). During rapid movements, it is necessary to accelerate the barbell in the first half of the concentric phase, which is known as the "acceleration sub-phase." However, it is thought that rapid movements during free-weight exercises cause a "deceleration sub-phase" (or sometimes the "decelerating phase") at the end of the concentric phase (Elliott et al., 1989; Lake et al., 2012; Sanchez-Medina et al., 2010). The acceleration reaches less than zero as a result of the barbell decelerating at the end of the concentric phase. In other words, as force is equal to mass multiplied by acceleration, muscular forces reach less than that in the standing position during the deceleration sub-phase (Sanchez-Medina et al., 2010). Therefore, a deceleration sub-phase during the concentric phase makes it difficult to stimulate muscles throughout the full range of motion.
The use of variable resistance training, weightlifting movements, and ballistic training exercises that minimize the deceleration sub-phase and maximize the acceleration sub-phase has been recommended by many sports scientists and coaches (Israetel et al., 2010; Joy et al., 2016; Kawamori and Newton, 2006; Riviere et al., 2017; Stevenson et al., 2010). Previous studies have been based on the assumption that a deceleration subphase occurs whether the loads are light or heavy during back squat (BSQ); however, to the best of our knowledge, a deceleration sub-phase has been reported only for one load during BSQ (45% one repetition maximum [1RM]) (Lake et al., 2012). Therefore, it is necessary to investigate the characteristics of the deceleration sub-phase with different loads, including the optimal loads for lower-body maximal force and the rapid expression of force during BSQ, which have a large range (30-70% 1RM) (Soriano et al., 2015). If the loads have different kinetic and kinematic characteristics, the use of this exercise for the development of specific physical characteristics is recommended in order to enhance power.
Assuming that a deceleration sub-phase occurs during BSQ with different loads, the duration of the deceleration sub-phase may be expected to increase as the load decreases. As the load decreases, velocity increases (Gonzalez-Badillo and Sanchez-Medina, 2010); thus, a large deceleration may be needed to stop the barbell at the end of the concentric phase during BSQ. On the basis of this assumption, we hypothesized that 1) as the load increases, the absolute duration of the deceleration sub-phase will decrease, and 2) as the load increases, the relative duration of the deceleration sub-phase will decrease. Previous studies have examined the duration of the deceleration sub-phase, but not the magnitude of impulse during the deceleration sub-phase (negative impulse). The negative impulse represents the area under force-time curve (Brady et al., 2017) during deceleration sub-phase. Therefore, we also investigated the negative impulse during the deceleration sub-phase in this study.
To investigate a wide range of loads, subjects performed trials in a random order, with 30%, 40%, 50%, 60%, 70%, 80%, and 90% of the maximal dynamic strength (MDS = 1RM + body mass--[shank mass]) (Cormie et al., 2007) on a force plate. These loads represented 0%, 12%, 27%, 42%, 56%, 71%, and 85% of each subject's 1RM. The body mass and external load moved together as a unit during BSQ, although the lower leg and feet remain relatively static. Therefore, the loads excluding the mass of the shanks (12% of body mass) were used in this study. Force signals from the force plate were collected, and the deceleration sub-phase duration and sub-phase impulse were calculated from the force signals.
Sixteen healthy men (powerlifters: n = 6; bodybuilders: n = 2; recreationally trained men: n = 8) (mean [+ or -] standard deviation: age: 25 [+ or -] 3 years; height: 1.73 [+ or -] 0.07 m; mass: 83.2 [+ or -] 16.1 kg; BSQ 1RM: 163.8 [+ or -] 36.6 kg; BSQ 1RM/body mass: 2.0 [+ or -] 0.4) who had performed resistance training for at least 1 year were recruited for this study. All subjects were informed of the experimental procedure, potential risks, and purpose of this study and signed an informed consent document before the investigation. The study was approved by a local Ethical Review Committee on Research with Human Subjects.
The recreationally trained men attended two sessions: a 1RM session was conducted on the first day and a testing session and measurements of height and mass were conducted on the second day. The powerlifters and bodybuilders attended one session, which included measurements of height and mass. The 1RMs of the powerlifters and bodybuilders were determined through the season records. The 1RM session for recreationally trained men was conducted at least a week before the testing session by following the National Strength and Conditioning Association guidelines (McGuigan...