A wide variety of training methods may be implemented when training and developing maximal and explosive strength characteristics (e.g., peak force (PF), impulse, rate of force development, and power output) [1
]. One such method for training the lower body includes prescribing partial range of motion exercises, such as the half squat or quarter squat. Implementing these exercises allows individuals to use supramaximal loads (i.e., loads in excess of their one repetition maximum (1RM) that cannot be lifted through a full range motion), which may allow for enhancements in maximal force production via reduced neuromuscular inhibition [3
]. Moreover, when supramaximal loads are lifted from a stationary position (e.g., off of safety pins or boxes), the lifter is forced to overcome its static inertia, which may benefit both impulse and rate of force development characteristics [4
]. In fact, previous research displayed greater increases in maximal strength and early force-time characteristics when implementing partial range of motion squat variations with full range motion squats compared to only using full range of motion squats [5
]. Further research displayed improvements in squat strength, vertical jump height, and 40-yard sprint speed in highly-trained men following 16 weeks of training with partial squats [6
]. Given the potential benefits of incorporating partial squats within resistance training programs, it is no surprise that these exercises are frequently prescribed [7
A training variable that may be overlooked is the intent in which the prescribed exercises are performed. While multi-joint exercises, such as the back squat and bench press, may be implemented often within resistance training programs, whether they are performed in a ballistic (BAL; acceleration throughout the entire movement) or non-ballistic (NBAL; intentionally fast with a negative acceleration at the end of the movement) manner may produce different training adaptations. Previous research has examined the differences between BAL and NBAL upper and lower body exercises [11
]. Newton et al. [11
] indicated that performing bench press throws resulted in greater muscle activation, force production, velocity, and power output compared to traditional bench press repetitions performed at the same load. In contrast, Lake et al. [12
] showed no difference in mean force or power between BAL and NBAL back squats; however, the authors did note that the method used to determine both variables affected the comparison. Further results from this study did show statistically significant differences in mean velocity and acceleration duration that favored back squats performed in a BAL manner. Similarly, Pestaña-Melero et al. [13
] showed that BAL bench press repetitions produced greater velocities, while NBAL repetitions displayed greater force outputs. Despite the mixed evidence above, no research has compared the force-time characteristics of BAL and NBAL partial range of motion movements. To understand how to implement these exercises in training, further research is necessary.
The load prescribed during training may impact an exercise’s force production characteristics. For example, while most literature agrees that heavier loads may produce greater PF and impulse magnitudes [14
], rapid force production characteristics may be emphasized using either lighter [15
] and heavier [16
] loads based on the exercise. Although an abundance of literature has examined the loading effects on exercises performed in a NBAL manner [14
], an even greater amount of literature investigated how the external load affects the force-time characteristics of BAL exercises, such as jump squats [14
], weightlifting movements and their derivatives [14
], and bench press throws [19
]. However, despite the abundance of BAL and NBAL loading literature, there is currently no research that examines how load affects the force-time characteristics of a partial range of motion squatting variation performed from a static starting position, such as the concentric-only half squat (COHS). Moreover, no literature has studied the differences in how load affects the force production characteristics of COHS performed with BAL and NBAL intent. To effectively prescribe partial squat variations, with BAL or NBAL intent, information about how load affects performance is essential. Therefore, the purpose of this study was to compare the PF and impulse characteristics of COHS performed in a BAL or NBAL manner. A secondary purpose was to examine how the external load affects the PF and impulse characteristics of BAL and NBAL COHS. It was hypothesized that BAL COHS would produce greater PF and impulse characteristics across all loads examined and that the external load would substantially impact the PF and impulse produced during COHS. It was also hypothesized that the greatest PF and impulse magnitudes would be produced at the heaviest loads.
All data were normally distributed. The reliability and descriptive statistics for PF, Imp50, Imp90, Imp200, and Imp250 at each load examined for BAL and NBAL COHS are displayed in Table 1
and Table 2
3.1. Intent Effects
Statistically significant main effect differences existed PF (p < 0.001, c = 1.00). Post hoc analysis indicated that load-averaged BAL COHS produced greater PF magnitudes compared to NBAL COHS (p < 0.001, d = 1.30, CI = 6.05–8.80). In addition, statistically significant main effect differences existed for Imp90 (p = 0.006, c = 0.84), Imp200 (p = 0.001, c = 0.96), and Imp250 (p < 0.001, c = 0.98), but not for Imp50 (p = 0.018, c = 0.69) Post hoc analysis indicated that load-averaged BAL COHS produced statistically greater Imp90 (d = 0.25, CI = 0.02–0.11), Imp200 (d = 0.36, CI = 0.11–0.38), and Imp250 (d = 0.41, CI = 0.19–0.55) compared to NBAL COHS.
3.2. Load Effects
Statistically significant main effect differences were found between intent-averaged loads for COHS PF, Imp50, Imp90, Imp200, and Imp250 (all p < 0.001, c = 1.00). Moderate-large effect sizes existed across loads for PF (d = 0.77–2.38), while small-moderate effect sizes existed across loads for Imp50 (d = 0.20–0.92), Imp90 (d = 0.21–0.96), Imp200 (d = 0.21–1.00), and Imp250 (d = 0.20–1.08).
3.3. Intent x Load Interaction Effects
A statistically significant intent x load interaction effect existed for COHS PF (p
< 0.001, c = 1.00). Post hoc analysis indicated that BAL COHS produced statistically greater PF than NBAL COHS at 30% (p
< 0.001, d = 3.37), 50% (p
< 0.001, d = 2.88), 70% (p
< 0.001, d = 2.29), and 90% 1RM (p
< 0.001, d = 1.19). In contrast, there were no statistically significant intent x load interaction effects for Imp50 (p
= 0.263, c = 0.34), Imp90 (p
= 0.341, c = 0.29), Imp200 (p
= 0.509, c = 0.15), or Imp250 (p
= 0.493, c = 0.14). Figure 3
and Figure 4
display the intent and load interactions for PF and Imp50, Imp90, Imp200, and Imp250 during both the BAL and NBAL conditions, respectively.
This study compared the differences between COHS performed in a BAL or NBAL manner and the effect of load on PF and impulse characteristics. The findings of the current study support our hypotheses. BAL COHS produced statistically greater PF compared to NBAL COHS. Load-averaged BAL COHS produced statistically greater Imp90, Imp200, and Imp250 compared to NBAL COHS, while no difference existed for Imp50. Finally, statistically significant differences existed between loads for intent-averaged COHS for all the examined variables, with the heaviest loads resulting in the greater magnitudes of PF and impulse.
Very large effect sizes existed when comparing the PF between BAL and NBAL COHS [56
]. Similar to previous findings [11
], but in contrast to others [12
], the BAL COHS in the current study produced greater PF magnitudes compared to the NBAL COHS at each of the relative loads examined. These findings may be based on the physiological nature of BAL exercise. Previous research has indicated that BAL exercise may lower the recruitment threshold of motor units [57
], and allow the motor neuron pool to be fully activated within milliseconds [59
], ultimately resulting in greater force production. Thus, it should come as no surprise that the heaviest loads used during the COHS (90% 1RM or 108.6 ± 9.6% of the participants’ 1RM back squat) combined with BAL intent produced the greatest PF magnitude. An interesting finding of the current study is that the largest effect size magnitudes for PF between BAL and NBAL COHS existed at the lightest load (30% 1RM), while the smallest, albeit still large effect, existed at the heaviest load (90% 1RM). This finding may be explained by the ability of an individual to develop high rates of force development with lighter loads during BAL exercises [15
]. In addition, it is possible that the lighter loading conditions required participants during the NBAL condition to stop applying force earlier to maintain contact with force plates. This notion may be supported by the non-statistically significant Imp50 findings and statistically significant Imp90, Imp200, and Imp250 findings. Although rate of force development was not compared in the current study, the impulse characteristics of the current study support this notion. From practical standpoint, these findings show that light and heavy training loads should still be moved with maximal intent to receive the greatest training stimulus. Thus, it may be beneficial to cue athletes to move every load with maximal intent during the concentric portion of the movement (e.g., warm-up sets, working sets, and warm-down/drop sets).
The impulse generated by an individual may ultimately determine the performance of specific tasks, such as vertical jump and weightlifting movements [60
]. Except for Imp50, the findings of the current study indicated that when averaged across loads, BAL COHS produced greater impulse magnitudes compared to NBAL COHS. Although only small effects were predominantly present, it should be noted that the practical differences increased as the time interval increased. Given that the participants in the current study had to perform each COHS repetition from a static starting position and without the benefit of a stretch-shortening cycle, the length of time for each repetition was considerably longer than the time intervals examined. While the time intervals examined in the current study are related to striking [52
], sprinting [53
], and jumping [54
] tasks, the early force production characteristics of BAL and NBAL COHS cannot be overlooked. Specifically, the differences in early force characteristics during the BAL condition may ultimately culminate in larger peak force production, and greater forces throughout the movement, which is supported by our results.
Similar to the PF results, the heaviest load examined produced the largest impulse magnitudes at each of the examined time intervals. However, in contrast to the PF results, only small-moderate effect size magnitudes existed between loads. These findings may be due to the large variation in relative impulse that was present among the participants at each time interval. A recent review discussed the influence that muscular strength can have on general and specific sport performance tasks; however, the authors also discussed how greater muscular strength may positively influence rapid force production characteristics [61
]. The relative 1RM back squat and COHS of the participants in the current study ranged from 1.4–2.4 and 1.8–2.7 times their body weight, respectively. Although the effects of strength on early impulse qualities was not examined in the current study, the wide range of relative strength characteristics may have resulted in a larger variation between the participants. Future research may consider investigating the force-time characteristics of BAL and NBAL COHS between stronger and weaker individuals.
A potential limitation to the current study may have been the non-randomization of load order. Although the likelihood is small based on the single set performed at each load with five or fewer repetitions, it is possible that some of the weaker participants fatigued during the progressive sets, which may have negatively impacted their performance at heavier loads. However, as noted above, the progressive sets used within this study provide a good representation of how COHS may be used within an actual training setting (e.g., warm-up sets followed by working sets). Future research may consider examining the performance differences between stronger and weaker individuals during BAL and NBAL COHS as well as other full and partial range of motion exercises.
COHS performed with BAL intent produced greater relative PF and impulse magnitudes compared to COHS performed with NBAL intent. Greater force and impulse magnitudes (except for Imp50) may be achieved at early time intervals and maintained throughout a COHS by performing the exercise with BAL intent. The external load used during BAL and NBAL COHS may have a moderate to large practical effect on PF and a small-moderate effect on impulse magnitudes produced at various time intervals.
BAL COHS produced larger PF and impulse magnitudes at several relative loads. Thus, performing COHS with BAL intent may provide a favorable training stimulus compared to COHS performed with NBAL intent, especially when performed at lighter loads. Thus, athletes should be cued to move the external load as fast as possible during the concentric phase of the movement to receive the greatest training stimulus during all exercise sets (e.g., warm-up, working, and warm-down/drop sets). Given the reduced range of motion, potential use of heavier loads (>1RM back squat), and BAL intent, BAL COHS may be best implemented during resistance training phases in which the goal is to develop maximal force production, impulse, and rate of force development characteristics.