The force-production characteristics of 3 weight-lifting derivatives were examined by comparing the force–time curves of each exercise. Sixteen resistance-trained men performed repetitions of the hang power clean (HPC), jump shrug (JS), and hang high pull (HHP) on a force platform at several relative loads. Relative peak force (PFRel), relative impulse (IMPRel), peak rate of force development (PRFD), and time-normalized force–time curves of each exercise were compared. The JS produced greater PFRel than the HPC (P < .001, d = 1.38) and HHP (P < .001, d = 1.14), while there was no difference between the HPC and HHP (P = .338, d = 0.26). Similarly, the JS produced greater IMPRel than the HPC (P < .001, d = 0.52) and HHP (P = .019, d = 0.36). The HHP also produced greater IMPRel than the HPC (P = .040, d = 0.18). Finally, the JS produced greater PRFD than the HPC (P < .001, d = 0.73) and HHP (P = .001, d = 0.47), while there was no difference between the HPC and HHP (P = .192, d = 0.22). The HPC, JS, and HHP force–time profiles were similar during the first 75–80% of the movement; however, the JS produced markedly different force–time characteristics in the final 20–25% of the movement. The JS produced superior force-production characteristics, namely PFRel, IMPRel, and PRFD, as well as a unique force–time profile, compared with the HPC and HHP across several loads.
Timothy J. Suchomel and Christopher J. Sole
Timothy J. Suchomel, Christopher B. Taber and Glenn A. Wright
The purpose of this study was to examine the effect that load has on the mechanics of the jump shrug. Fifteen track and field and club/intramural athletes (age 21.7 ± 1.3 y, height 180.9 ± 6.6 cm, body mass 84.7 ± 13.2 kg, 1-repetition-maximum (1RM) hang power clean 109.1 ± 17.2 kg) performed repetitions of the jump shrug at 30%, 45%, 65%, and 80% of their 1RM hang power clean. Jump height, peak landing force, and potential energy of the system at jump-shrug apex were compared between loads using a series of 1-way repeated-measures ANOVAs. Statistical differences in jump height (P < .001), peak landing force (P = .012), and potential energy of the system (P < .001) existed; however, there were no statistically significant pairwise comparisons in peak landing force between loads (P > .05). The greatest magnitudes of jump height, peak landing force, and potential energy of the system at the apex of the jump shrug occurred at 30% 1RM hang power clean and decreased as the external load increased from 45% to 80% 1RM hang power clean. Relationships between peak landing force and potential energy of the system at jump-shrug apex indicate that the landing forces produced during the jump shrug may be due to the landing strategy used by the athletes, especially at lighter loads. Practitioners may prescribe heavier loads during the jump-shrug exercise without viewing landing force as a potential limitation.
Jason Lake, Peter Mundy, Paul Comfort, John J. McMahon, Timothy J. Suchomel and Patrick Carden
This study examined concurrent validity of countermovement vertical jump reactive strength index modified and force–time characteristics recorded using a 1-dimensional portable and laboratory force plate system. Twenty-eight men performed bilateral countermovement vertical jumps on 2 portable force plates placed on top of 2 in-ground force plates, both recording vertical ground reaction force at 1000 Hz. Time to takeoff; jump height; reactive strength index modified; and braking and propulsion impulse, mean net force, and duration were calculated from the vertical force from both force plate systems. Results from both systems were highly correlated (r ≥ .99). There were small (d < 0.12) but significant differences between their respective braking impulse, braking mean net force, propulsion impulse, and propulsion mean net force (P < .001). However, limits of agreement yielded a mean value of 1.7% relative to the laboratory force plate system (95% confidence limits, 0.9%–2.5%), indicating very good agreement across all of the dependent variables. The largest limits of agreement were for jump height (2.1%), time to takeoff (3.4%), and reactive strength index modified (3.8%). The portable force plate system provides a valid method of obtaining reactive strength measures, and several underpinning force–time variables, from unloaded countermovement vertical jump. Thus, practitioners can use both force plates interchangeably.
John J. McMahon, Paul A. Jones, Timothy J. Suchomel, Jason Lake and Paul Comfort
Purpose: The Reactive Strength Index Modified (RSImod) has been recently identified and validated as a method of monitoring countermovement-jump (CMJ) performance. The kinetic and kinematic mechanisms that optimize a higher RSImod score are, however, currently unknown. The purpose of this study, therefore, was to compare entire CMJ force–, power–, velocity–, and displacement–time curves (termed temporal-phase analysis) of athletes who achieve high versus low RSImod scores. Methods: Fifty-three professional male rugby league players performed 3 maximal-effort CMJs on a force platform, and variables of interest were calculated via forward dynamics. The top (high RSImod group) and bottom (low RSImod group) of 20 athletes’ CMJ kinetic- and kinematic-time curves were compared. Results: The high-RSImod group (0.53 ± 0.05 vs 0.36 ± 0.03) jumped higher (37.7 ± 3.9 vs 31.8 ± 3.2 cm) with a shorter time to takeoff (TTT) (0.707 ± 0.043 vs 0.881 ± 0.122 s). This was achieved by a more rapid unweighting phase followed by greater eccentric and concentric force, velocity, and power for large portions (including peak values) of the jump, but a similar countermovement displacement. The attainment of a high RSImod score therefore required a taller, but thinner, active impulse. Conclusions: Athletes who perform the CMJ with a high RSImod, as achieved by high jumps with a short TTT, demonstrate superior force, power, velocity, and impulse during both the eccentric and concentric phases of the jump. Practitioners who include the RSImod calculation in their testing batteries may assume that greater RSImod values are attributed to an increase in these underpinning kinetic and kinematic parameters.
Paul Comfort, Christopher Thomas, Thomas Dos’Santos, Paul A. Jones, Timothy J. Suchomel and John J. McMahon
Purpose: To determine the reliability and variability of the Dynamic Strength Index (DSI) calculated from squat-jump (SJ) vs countermovement-jump (CMJ) peak force (PF) and to compare DSI values between methods. Methods: Male youth soccer and rugby league players (N = 27; age 17.2 ± 0.7 y, height 173.9 ± 5.7 cm, body mass 71.1 ± 7.2 kg) performed 3 trials of the SJ, CMJ, and isometric midthigh pull (IMTP) on 2 separate days. DSI was calculated by dividing the PF during each jump by the IMTP PF. Results: DSI-SJ exhibited moderate (intraclass correlation coefficient [ICC] = .419) within-session reliability and high variability (percentage coefficient of variation [%CV] = 15.91) during session 1; however, this improved noticeably during session 2 (ICC = .948, %CV = 4.03). In contrast, DSI-CMJ showed nearly perfect within-session reliability (ICC = .920–.952) and low variability (%CV = 3.80–4.57) for both sessions. Moreover, DSI-SJ values demonstrated a small yet significant increase between sessions (P = .01, d = 0.37), whereas only a trivial and nonsignificant increase was observed for DSI-CMJ between sessions (P = .796, d = 0.07). Between-sessions reliability was very high for the DSI-SJ (ICC = .741) and nearly perfect for the DSI-CMJ (ICC = .924). There was no significant or meaningful difference (P = .261, d = 0.12) between DSI-SJ (0.82 ± 0.18) and DSI-CMJ (0.84 ± 0.15). Conclusions: Practitioners should use DSI-CMJ, as it is a more reliable measure than DSI-SJ, although it produces similar ratios.
Timothy J. Suchomel, Kimitake Sato, Brad H. DeWeese, William P. Ebben and Michael H. Stone
The purposes of this study were to examine the effect of ballistic concentric-only half-squats (COHS) on subsequent squat-jump (SJ) performances at various rest intervals and to examine the relationships between changes in SJ performance and bilateral symmetry at peak performance. Thirteen resistance-trained men performed an SJ immediately and every minute up to 10 min on dual force plates after 2 ballistic COHS repetitions at 90% of their 1-repetition-maximum COHS. SJ peak force, peak power, net impulse, and rate of force development (RFD) were compared using a series of 1-way repeated-measures ANOVAs. The percent change in performance at which peak performance occurred for each variable was correlated with the symmetry index scores at the corresponding time point using Pearson correlation coefficients. Statistical differences in peak power (P = .031) existed between rest intervals; however, no statistically significant pairwise comparisons were present (P > .05). No statistical differences in peak force (P = .201), net impulse (P = .064), and RFD (P = .477) were present between rest intervals. The relationships between changes in SJ performance and bilateral symmetry after the rest interval that produced the greatest performance for peak force (r = .300, P = .319), peak power (r = –.041, P = .894), net impulse (r = –.028, P = .927), and RFD (r = –.434, P = .138) were not statistically significant. Ballistic COHS may enhance SJ performance; however, the changes in performance were not related to bilateral symmetry.
Paul Comfort, Thomas Dos’Santos, Paul A. Jones, John J. McMahon, Timothy J. Suchomel, Caleb Bazyler and Michael H. Stone
Purpose: To determine the reliability of early force production (50, 100, 150, 200, and 250 ms) relative to peak force (PF) during an isometric mid-thigh pull and to assess the relationships between these variables. Methods: Male collegiate athletes (N = 29; age 21.1 [2.9] y, height 1.71 [0.07] m, body mass 71.3 [13.6] kg) performed isometric mid-thigh pulls during 2 separate testing sessions. Net PF and net force produced at each epoch were calculated. Within- and between-session reliabilities were determined using intraclass correlation coefficients and coefficient of variation percentages. In addition, Pearson correlation coefficients and coefficient of determination were calculated to examine the relationships between PF and time-specific force production. Results: Net PF and time-specific force demonstrated very high to almost perfect reliability both within and between sessions (intraclass correlation coefficients .82–.97; coefficient of variation percentages 0.35%–1.23%). Similarly, time-specific force expressed as a percentage of PF demonstrated very high to almost perfect reliability both within and between sessions (intraclass correlation coefficients .76–.86; coefficient of variation percentages 0.32%–2.51%). Strong to nearly perfect relationships (r = .615–.881) exist between net PF and time-specific net force, with relationships improving over longer epochs. Conclusion: Based on the smallest detectable difference, a change in force at 50 milliseconds expressed relative to PF > 10% and early force production (100, 150, 200, and 250 ms) expressed relative to PF of >2% should be considered meaningful. Expressing early force production as a percentage of PF is reliable and may provide greater insight into the adaptations to the previous training phase than PF alone.