Shala E. Davis

Effect of static and dynamic stretching on vertical jump performance in collegiate women volleyball players.

Dalrymple, KJ, Davis, SE, Dwyer GB, and Moir, GL. Effect of static and dynamic stretching on vertical jump performance in collegiate women volleyball players. J Strength Cond Res 24(1): 149-155, 2010-The purpose of this study was to determine the effect of stretching on peak jump height during a series of vertical jumps, specifically focusing on a) static stretching (SS), b) dynamic stretching (DS) and c) no stretching (NS) performed immediately before a series of countermovement vertical jumps (CMJ). Twelve female collegiate volleyball players (mean ± SD; age 19.5 ± 1.1 yr; height 1.71 ± 0.06 m; mass 71.3 ± 8.54 kg) volunteered for this study. Data collection lasted a total of 3 weeks, and each subject performed all 3 stretching protocols, 1 session per week, with 1 week between sessions. The order of the stretching protocols was randomized for each subject. During each testing session, all subjects performed a 5-minute light jog as a warm-up, followed by 8 minutes of 1 of the stretching protocols. One minute after the completion of each protocol, 5 maximal CMJ were performed on a force platform, with each jump separated by 1 minute of passive recovery. Jump heights were calculated by integrating the vertical force trace. There were no significant differences between the SS, DS, and NS conditions for any of the jumps (p > 0.05). Despite the lack of significant effects for the group, there were notable individual responses to each of the warm-up conditions. Practitioners should be aware of the individual responses of their athletes to different types of warm-up protocols before athletic performance and the possible impact of prescribing or eliminating certain exercises.

The effects of load on system and lower-body joint kinetics during jump squats

To investigate the effects of different loads on system and lower-body kinetics during jump squats, 12 resistance-trained men performed jumps under different loading conditions: 0%, 12%, 27%, 42%, 56%, 71%, and 85% of 1-repetition maximum (1-RM). System power output was calculated as the product of the vertical component of the ground reaction force and the vertical velocity of the bar during its ascent. Joint power output was calculated during bar ascent for the hip, knee, and ankle joints, and was also summed across the joints. System power output and joint power at knee and ankle joints were maximized at 0% 1-RM (p < 0.001) and followed the linear trends (p < 0.001) caused by power output decreasing as the load increased. Power output at the hip was maximized at 42% 1-RM (p = 0.016) and followed a quadratic trend (p = 0.030). Summed joint power could be predicted from system power (p < 0.05), while system power could predict power at the knee and ankle joints under some of the loading conditions. Power at the hip could not be predicted from system power. System power during loaded jumps reflects the power at the knee and ankle, while power at the hip does not correspond to system power.

The acute effects of manipulating volume and load of back squats on countermovement vertical jump performance.

Moir, GL, Mergy, D, Witmer, CA, and Davis, SE. The acute effects of manipulating volume and load of back squats on countermovement vertical jump performance. J Strength Cond Res 25(6): 1486-1491, 2011—The acute effects of manipulating the volume and load of back squats on subsequent countermovement vertical jump performance were investigated in the present study. Eleven National Collegiate Athletic Association division II female volleyball players performed 10 countermovement vertical jumps (CMJs) on a force platform 2 minutes after the last squat repetition of a high-load (HL) or high-volume (HV) squat protocol. Two minutes of rest was provided between each CMJ. The HL protocol culminated in the subjects having to perform 3 repetitions with a load equivalent to 90% 1 repetition maximum (1RM) back squat, whereas 12 repetitions with a load equivalent to 37% 1RM were performed in the HV protocol. During an initial familiarization session, knee angles were recorded during a series of CMJs, and these angles were used to control the depth of descent during all subsequent back squats. Jump height (JH) and vertical stiffness (VStiff) were calculated during each of the 10 CMJ, and the change in these variables after the 2 squat protocols was assessed using an analysis of variance model with repeated measures on 2 factors (Protocol [2-levels]; Time [2-levels]). There was no significant difference in JH after the HL and HV protocols (p > 0.05). A significant Protocol × Time interaction for VStiff resulted from the increase after the HL protocol being greater than that after the HV protocol (p = 0.03). The knee angles before the HL and HV protocols were significantly greater than those measured during the initial familiarization session (p = 0.001). Although neither squat protocol provided any benefit in improving JH, the heavy squat protocol produced greater increases in VStiff during the CMJ. Because of the increased VStiff caused by the HL protocol, volleyball coaches may consider using such protocols with their players to improve performance in jumps performed from a run such as the spike and on-court agility.

The Yo-Yo IR2 test: physiological response, reliability, and application to elite soccer.

Abstract Oberacker, LM, Davis, SE, Haff, GG, Witmer, CA, and Moir, GL. The Yo-Yo IR2 test: physiological response, reliability, and application to elite soccer. J Strength Cond Res 26(10): 2734–2740, 2012—The purpose of this study was to compare the effects of resistance training performed on either a stable or unstable surface on performance tests in female soccer players. Nineteen National Collegiate Athletic Association Division II female soccer players were assigned to either an unstable training group (UST: 19.0 ± 0.47 years; 1.69 ± 6.4 m; 67.8 ± 7.7 kg) or a stable training group (ST: 19.6 ± 0.49 years; 1.64 ± 3.2 m; 62.7 ± 6.27 kg). Player positions were distributed evenly between the groups. Both the groups followed a 5-week periodized resistance training program designed to develop maximum muscular strength. The groups performed the same exercises during each workout, with the UST performing 2 of the exercises in each session on an unstable surface. Pretraining and posttraining measures of straight-line sprint speed, planned and reactive agility, aerobic capacity, and countermovement vertical jump (CMJ) were taken. Significant main effects for time were reported for straight-line sprint speed, planned agility, and reactive agility with both groups demonstrating improvements during the posttraining testing session. The ST demonstrated a significant increase in CMJ during the posttraining session (change in mean: 0.04 m) in contrast to the decline demonstrated by the UST (change in mean: −0.01 m). Performing resistance training exercises on an unstable surface confers no advantage over traditional resistance training exercises for improving the speed, agility, and aerobic capacity of female soccer players. Furthermore, the use of an unstable surface may inhibit the effects of resistance training on vertical jump height, an important variable in soccer performance.

Effect of cluster set configurations on mechanical variables during the deadlift exercise.

The purpose of the present study was to investigate the effects of different configurations of repetitions within a set of deadlifts on the mechanical variables of concentric force, concentric time under tension, impulse, work, power, and fatigue. Eleven resistance trained men (age: 21.9 ± 1.0 years; deadlift 1 repetition maximum: 183.2 ± 38.3 kg) performed four repetitions of the deadlift exercise with a load equivalent to 90% of 1 repetition maximum under three different set configurations: Traditional (continuous repetitions); Doubles cluster (repetitions 1 and 2, and 3 and 4 performed continuously with a 30 s rest inserted between repetitions 2 and 3); Singles cluster (30 s rest provided between repetitions). The order of the sessions was counterbalanced across the subjects and the mechanical variables were calculated during each repetition from the synchronized signals recorded from force platforms and a motion analysis system. Relative to the Traditional set, the insertion of rest periods in the cluster set configurations resulted in greater time under tension (p < 0.001) and therefore, greater impulse (p < 0.001) during the repetitions. Reductions in power were observed during the cluster sets compared to the Traditional set (p = 0.001). The Doubles cluster set resulted in greater fatigue scores for power compared to the Traditional set (p = 0.04). The influence of cluster sets on mechanical variables appears to be mediated by the mechanical characteristics of the exercise (i.e. stretch-shortening cycle) and the competing physiological mechanisms of fatigue and potentiation.

The development of a repetition-load scheme for the eccentric-only bench press exercise.

Abstract The purpose of the present study was to develop a repetition-load scheme for the eccentric-only bench press exercise. Nine resistance trained men (age: 21.6 ± 1.0 years; 1-repetition maximum [RM] bench press: 137.7 ± 30.4 kg) attended four testing sessions during a four week period. During the first session each subject’s 1-RM bench press load utilizing the stretch-shortening cycle was determined. During the remaining sessions they performed eccentric-only repetitions to failure using supra-maximal loads equivalent to 110%, 120% and 130% of their 1-RM value with a constant cadence (30 reps·min-1). Force plates and a three dimensional motion analysis system were used during these final three sessions in order to evaluate kinematic and kinetic variables. More repetitions were completed during the 110% 1-RM condition compared to the 130% 1-RM condition (p=0.01). Mean total work (p=0.046) as well as vertical force (p=0.049), vertical work (p=0.017), and vertical power output (p=0.05) were significantly greater during the 130% 1-RM condition compared to the 110% 1-RM condition. A linear function was fitted to the number of repetitions completed under each load condition that allowed the determination of the maximum number of repetitions that could be completed under other supra-maximal loads. This linear function predicted an eccentric-only 1-RM in the bench press with a load equivalent to 164.8% 1-RM, producing a load of 227.0 ± 50.0 kg. The repetition-load scheme presented here should provide a starting point for researchers to investigate the kinematic, kinetic and metabolic responses to eccentric-only bench press workouts.

The Effects of Ballistic and Non-ballistic Bench Press on Mechanical Variables.

Abstract Moir, GL, Munford, SN, Moroski, LL, Davis, SE. The effects of ballistic and nonballistic bench press on mechanical variables. J Strength Cond Res 32(12): 3333–3339, 2018—The purpose of this study was to investigate the effects of ballistic and nonballistic bench press performed with loads equivalent to 30 and 90% 1 repetition maximum (1RM) on mechanical variables. Eleven resistance-trained men (age: 23.0 ± 1.4 years; mass: 98.4 ± 14.4 kg) attended 4 testing sessions where they performed one of the following sessions: (a) 3 sets of 5 nonballistic repetitions performed with a load equivalent to 30% 1RM (30N-B), (b) 3 sets of 5 ballistic repetitions performed with a load equivalent to 30% 1RM (30B), (c) 3 sets of 4 nonballistic repetitions with a load equivalent to 90% 1RM (90N-B), or (d) 3 sets of 4 ballistic repetitions with a load equivalent to 90% 1RM (90B). Force plates and a 3-dimensional motion analysis system were used to determine the velocity, force, power output (PO), and work during each repetition. The heavier loads resulted in significantly greater forces applied to the barbell (mean differences: 472–783 N, p < 0.001), but lower barbell velocities (mean differences: 0.85–1.20 m·s−1, p < 0.001) and PO (mean differences: 118–492 W, p ⩽ 0.022). The ballistic conditions enhanced the mechanical variables only at the lower load, with 30B producing significantly greater force (mean difference: 263 N, p < 0.001), velocity (mean difference: 0.33 m·s−1, p < 0.001), and PO (mean difference: 335 W, p < 0.001) compared with 30N-B. Furthermore, the increase in PO across the 3 sets in 30B was significantly different from all other conditions (p = 0.013). The total mechanical work performed was significantly greater for the conditions with the heavier loads compared with those with the lighter loads (mean differences: 362–5,600 J, p < 0.001) and that performed during the ballistic conditions was significantly greater than that performed during the nonballistic conditions with the same load (mean differences: 945–1,030 J, p < 0.001). Ballistic bench press may be an effective exercise for developing PO, and multiple sets may elicit postactivation potentiation that enhances force production. However, these benefits may be negated at heavier loads.