Vector-based data in sport performance include a magnitude and direction. Statistically speaking, they are interval in nature as they may be positive or negative. The coefficient of variation (CV) is a commonly reported measure of variability, but its use with vector data is questionable and may be contraindicated. Limits of agreement (LOA) and standard error of measurement (SEM) may be better alternatives for vector data such as acceleration. The purpose of this study was to demonstrate the issues with quantifying variability of vector data, while also evaluating the utility of commonly used measures. Acceleration data at three intervals from 0 to 27.4 m (0 to 90 ft) were calculated from publicly available sprint performance data from 310 athletes participating in the 2018 and 2019 Major League Baseball seasons. CV, LOA, and SEM were calculated to evaluate inter-season variability. Variability of the first two intervals was acceptable for all measures, but the final interval was unclear as the CV was quite large (50.78%), while the LOA and SEM were only slightly larger than the other interval values. The final interval includes both positive and negative acceleration, contraindicating the usage of the CV. LOA and SEM are more useful for vector data, showing that the final interval was more variable between trials than the others, but not to the extent portrayed by the CV. The CV likely should not be used with vector data unless it is known that the data does not cross zero. LOA and SEM are appealing alternatives for the CV and should be considered since they work with positive and negative data.

Abstract Chae, S, Hill, DW, Bailey, CA, Moses, SA, McMullen, SM, and Vingren, JL. Acute physiological and perceptual responses to rest redistribution with heavier loads in resistance-trained men. J Strength Cond Res 37(5): 994–1000, 2023—The purpose of this study was to explore the effect of rest redistribution with heavier loads (RR + L) on physiological and perceptual responses in resistance-trained men. Eight men who had back squat (BS) 1 repetition maximum (1RM) to body mass ratio; 1.8 ± 0.2 completed 2 BS exercise sessions in a counterbalanced and a randomized order; RR + L: 4 sets of (2 × 5) repetitions with 90-second interset rest and 30-second intraset rest using 75% BS 1RM and traditional sets (TS): 4 sets of 10 repetitions with 120-second interset rest using 70% BS 1RM. Blood samples were collected before exercise, immediately post exercise, and 5, 15, and 30 minutes post exercise for the analysis of growth hormone (GH), total testosterone (TT), cortisol (C), and blood lactate (BL), whereas rating of perceived exertion (RPE) and heart rate (HR) were measured immediately after each set of the BS exercise. While neither main effect of condition nor interaction existed, there was a significant (p < 0.05) main effect of time point (and set) for GH, TT, C, BL, RPE, and HR. Volume load was greater for RR + L compared with TS (4,074.9 ± 786.7 kg vs. 3,796.3 ± 714.8 kg). In conclusion, RR + L increases volume load by approximately 7% but does not seem to influence GH (g = −0.15), TT (g = −0.09), BL (g = −0.22), RPE (g = 0.14), and HR (g = −0.08) responses. Practitioners may consider using RR + L to increase volume load without increasing acute fatigue responses.

Chae, S, Bailey, CA, Hill, DW, McMullen, SM, Moses, SA, and Vingren, JL. Acute kinetic and kinematic responses to rest redistribution with heavier loads in resistance-trained men. J Strength Cond Res 37(5): 987-993, 2023-The purpose of this study was to examine mechanical responses to rest redistribution with heavier loads (RR + L) in resistance-trainedmen. Eight men (23.064.8 years, 1.76 +/- 0.06 m, 78.5 +/- 8.6 kg, back squat [BS] one-repetition maximum [1RM] of 138.7 +/- 27.9 kg) completed 2 BS exercise sessions in a counterbalanced and a randomized order; RR + L: 4 sets of (2*5) repetitionswith 90-second interset rest and 30-second intraset rest using 75% BS 1RM and traditional sets (TS): 4 sets of 10 repetitions with 120-second interset rest using 70% BS 1RM. During the concentric phase, mean force ((-)xF), velocity ((-)xV), and power ((-)xP) were collected for each repetition using a linear position transducer and analyzed the first 3 sets. Comparedwith TS, RR+ L resulted in significantly greater (-)xF (1820 +/- 260Nvs. 1753 +/- 248N; p < 0.001; g=0.25) and lower (-)xV (0.47 +/- 0.07m center dot s(-1) vs. 0.5060.09m center dot s21; p50.005; g520.35). However, no significant difference in xP (836 6165Wvs. 8716197W; p50.082; g520.18) was found between RR + L and TS. In conclusion, the lack of difference in xP might be the result of the combination of greater xF and lower xV for RR + L compared with TS. Therefore, practitioners may consider using RR + L to augment xF without compromising xP and increasing total rest time. However, given the approximate 4% difference in xF, the effect of RR + L training on chronic strength adaptation is expected to be small.

Abstract Bailey, CA, Sato, K, and McInnis, TC. A Technical report on reliability measurement in asymmetry studies. J Strength Cond Res 35(7): 1779–1783, 2021—Much of the current literature on asymmetry in sport performance may be biased by evaluating reliability before quantifying the asymmetry metric. This technical report aimed to evaluate the reliability of asymmetry measurements in countermovement jumps (CMJs) by providing measures before production of asymmetry metrics, after production of scalar- and vector-based asymmetry metrics, and to analyze the resulting differences that can lead to misinformed decision making. Thirteen collegiate baseball players (19.9 ± 1.3 years, 82.2 ± 10.9 kg) participated in CMJ testing on 2 force plates to evaluate symmetry index (SI) scores as scalar (asymmetry magnitude only) and vector (asymmetry magnitude and direction) quantities. Relative and absolute reliability were evaluated for peak force (PF), scalar PF SI, and vector PF SI. Results showed that reliability measures produced on the constituent force-time variable provide much more favorable results compared with evaluating reliability of the asymmetry measure itself (coefficients of variation of 10.4–15.7% vs. 63.2–1,497.1%). Findings also showed that reliability is altered depending on whether asymmetry is quantified as a scalar or vector. Asymmetry should likely be quantified as a vector for reliability purposes because that allows variability in both magnitude and direction. These findings demonstrate that inadequate evaluation of reliability in asymmetry leads to biased results. Practitioners should use caution when considering the results of asymmetry assessments as they may not be as reliable as they are often portrayed.

Athlete monitoring utilizing strength and conditioning as well as other sport performance data is increasing in practice and in research. While the usage of this data for purposes of creating more informed training programs and producing potential performance prediction models may be promising, there are some statistical considerations that should be addressed by those who hope to use this data. The purpose of this review is to discuss many of the statistical issues faced by practitioners as well as provide best practices recommendations. Single-subject designs (SSD) appear to be more appropriate for monitoring and statistically evaluating athletic performance than traditional group statistical methods. This paper discusses several SSD options available that produce measures of both statistical and practical significance. Additionally, this paper discusses issues related to heteroscedasticity, reliability, validity and provides recommendations for each. Finally, if data are incorporated into the decision-making process, it should be returned and utilized quickly. Data visualizations are often incorporated into this process and this review discusses issues and recommendations related to their clarity, simplicity, and distortion. Awareness of these issues and utilization of some best practice methods will likely result in an enhanced and more efficient decision-making process with more informed athlete development programs.

Previous research has shown that fatigue is associated with decreases in performance and may result in joint instability that is associated with injury. Objectives: The purpose of this study was to determine if lightly loaded jumps could simulate fatigue from a lower body kine matic perspective. Design and Methods: Seventeen NCAA DI baseball players (height 1.8 m ± 0.7, body mass 87.5 kg ± 7.9) performed unloaded and lightly loaded (20 kg) squat jumps, while 3D motion capture data were collected via six infrared cameras and reflective markers. Kinematic data included range of motion (ROM), peak angular velocity (PV), position at PV (PPV), peak angular acceleration (PA), and position at PA (PPA) for both the hip and knee as well as jump height (JH). Comparisons between conditions were completed with paired samples t tests, along with Cohen’s d effect sizes and 95% confidence intervals. Results: Statistical differences were noted between condition’s PV at both joints (hip (p = 0.00, d = 0.63); knee (p = 0.000, d = 0.65) and for PA of the hip (p = 0.002, d = 0.55). A decrement in JH was also noted (p = 0.000, d = 1.13). Conclusions: The results of the current investigation indicate that a 20 kg load is enough to cause jump performance changes similar to those seen with previous research associated with fatigue. This may be particularly useful for coaches and sport scientists seeking to understand how athletes will perform while fatigued.

Reactive strength index-modified (RSImod) is a measure of lower body explosiveness calculated by dividing jump height by time to takeoff. RSImod is different between stronger and weaker athletes and between males and females. The purpose of this study was to evaluate differences in RSImod between males and females while controlling for maximal strength and lower body explosiveness. Forty-three female and fifty-eight male Division-I athletes performed countermovement jumps on a force plate during unloaded (0kg) and loaded (20kg) conditions. We used an ANCOVA to test whether RSImod is different between sexes conditioning on relative maximum strength (PFa) and average RFD 0-200ms (RFD200) measured during the isometric mid-thigh pull (IMTP). Differences of 0.087 (95% CI: 0.040 - 0.134; p = 0.0005) and 0.075 (95% CI: 0.040 - 0.109, p < 0.0001) were observed for RSImod between sexes in unloaded and loaded conditions, respectively. A male with PFa of 186 (grand mean of the sample) and RFD200 of 6602 N/s (grand mean of the sample) is predicted to have 28% greater RSImod than a female of similar PFa and RFD200. Maximum strength development should be a primary aim of training in female athletes, in addition to other trainable factors, such as stiffness and RFD.

Objectives: The purpose of this investigation was to evaluate the intra- and intersession reliability of a new inertial measurement unit (ZEPP Sensor (ZS)) for bat swing mechanical analysis. Design and Methods: This investigation included 16 male collegiate baseball players (89.53 ± 12.5 kg, 180.61 ± 6.5 cm, 20.12 ± 0.8 years). Following a whole body dynamic warm-up and 5 dry swings, 5 swings were recorded where athletes hit balls off of a tee with bats instrumented with a ZS. Data collection took place on 3 days. Intrasession reliability analyzed trial to trial data, while intersession reliability analyzed data from each of the 3 sessions. Both relative and absolute measures of reliability were calculated with intraclass correlation coefficients (ICC), coefficients of variation (CV), and limits of agreement (LOA). Results: Within and between session acceleration/time related variables produced excellent relative reliability measurements (ICCs 0.882-0.988) as well as acceptable absolute measures of reliability (CVs 1.9%-9.2%). Variables derived from the gyro scope did not display the same consistency (intrasession vertical angle ICC = 0.492, intersession attack angle CV = 108.6%). Conclusions: Considering acceleration/time data, the ZS appears to be a reliable method of monitoring bat swing mechanics, but not for the angular position variables. Changes in body position have been demonstrated with fatigue in jumping performance. Similarly, bat position changes may occur with fatigue and should be monitored, but coaches and sport scientists should use caution when selecting variables to monitor and only select those that are reliable.

Bricker, JC, Bailey, CA, Driggers, AR, McInnis, TC, and Alami, A. A new method for the evaluation and prediction of base stealing performance. J Strength Cond Res 30(11): 3044-3050, 2016-The purposes of this study were to evaluate a new method using electronic timing gates to monitor base stealing performance in terms of reliability, differences between it and traditional stopwatch-collected times, and its ability to predict base stealing performance. Twenty-five healthy collegiate baseball players performed maximal effort base stealing trials with a right and left-handed pitcher. An infrared electronic timing system was used to calculate the reaction time (RT) and total time (TT), whereas coaches' times (CT) were recorded with digital stopwatches. Reliability of the TGM was evaluated with intraclass correlation coefficients (ICCs) and coefficient of variation (CV). Differences between the TGM and traditional CT were calculated with paired samples t tests Cohen's d effect size estimates. Base stealing performance predictability of the TGM was evaluated with Pearson's bivariate correlations. Acceptable relative reliability was observed (ICCs 0.74-0.84). Absolute reliability measures were acceptable for TT (CVs = 4.4-4.8%), but measures were elevated for RT (CVs = 32.3-35.5%). Statistical and practical differences were found between TT and CT (right p = 0.00, d = 1.28 and left p = 0.00, d = 1.49). The TGM TT seems to be a decent predictor of base stealing performance (r = -0.49 to -0.61). The authors recommend using the TGM used in this investigation for athlete monitoring because it was found to be reliable, seems to be more precise than traditional CT measured with a stopwatch, provides an additional variable of value (RT), and may predict future performance.

Two separate experiments were conducted. In each experiment 440 one day-old commercial broiler cbicks were fed starter diets formulated to be isonitrogenous and isocaloric, and containing either commercially processed soybean meal or ground full-fat raw soybeans heated at 100 C and 121 C for 30 and 60 min. The cbicks were separated by sexes and reared in battery cages, ten per group, Weekly body weights and feed consumption were obtained on each treatment group. Body weight and feed utilization were significantly reduced when the ground full-fat raw soybeans were used in tbe starter diet. Autoclaved cooking temperatures of 100 C and a cooking time of 60 min appeared to be the best combination used for cooking the ground full-fat raw soybeans. The response of broiler chicks to feeding ground full-fat raw soybeans appears to be age related. The chicks overcame much of tbe body weight depression after 3 weeks of age.

This study sought to determine the level of association between bilateral force production symmetry assessment methods (standing weight distribution [WtD], unloaded and lightly loaded jumps, and isometric strength) and to determine whether the amount of symmetry carry-over between these tasks differs for strong and weak athletes. Subjects for this study included male (n = 31) and female (n = 32) athletes from National Collegiate Athletic Association Division I sports. Athletes performed WtD, unloaded and lightly loaded (20 kg) static and countermovement jumps, and isometric midthigh pull (IMTP) assessments on 2 adjacent force plates. Ground reaction force data were used to calculate symmetry variables and performance-related variables. Using Pearson zero order correlations, evaluations of the amount of symmetry carry-over were made. Weight distribution correlated strongly with jump peak force (PF) (r = 0.628-0.664). Strong relationships were also observed between loading conditions for jump variables (r = 0.568-0.957) as were the relationships between jump types for PF, peak power, and net impulse (r = 0.506-0.834). Based on the pooled sample, there was a lack of association between IMTP and WtD for jump symmetry variables. However, when examining strong and weak groups, rate of force development showed moderate to strong symmetry carry-over in the strongest athletes (r = 0.416-0.589). Stronger athletes appear to display similar explosive strength symmetry characteristics in dynamic and isometric assessments, unlike weaker athletes. Strength seems to influence the amount of force production symmetry carry-over between bilateral assessments. There may be optimal loads and variables for symmetry assessment, but these may differ based on population characteristics.

The purpose of this investigation was to determine the existence of bilateral strength and force-production asymmetry and evaluate possible differences based on sex, as well as strength level. Asymmetry was assessed during weight-distribution (WtD) testing, unloaded and lightly loaded static- (SJ) and countermovement-jump (CMJ) testing, and isometric midthigh-pull (IMTP) strength testing. Subjects included 63 athletes (31 male, 32 female) for WtD, SJ, and CMJ tests, while 129 athletes (64 male, 65 female) participated in IMTP testing. Independent-samples t tests were used to determine possible differences in asymmetry magnitude between males and females, as well as between strong and weak athletes. Cohen d effect-size (ES) estimates were also used to estimate difference magnitudes. Statistically different asymmetry levels with moderate to strong ESs were seen between males and females in WtD, 0-kg SJ (peak force [PF]), 20-kg SJ (peak power [PP]), 0-kg CMJ (PF, PP, net impulse), and 20-kg CMJ (PF), but no statistical differences were observed in IMTP variables. Dividing the sample into strong and weak groups produced statistically significant differences with strong ES estimates in IMTP PF and rate of force development, and many ESs in jump symmetry variables increased. The results of this investigation indicate that females may be more prone to producing forces asymmetrically than males during WtD and jumping tasks. Similarly, weaker athletes displayed more asymmetry than stronger athletes. This may indicate that absolute strength may play a larger role in influencing asymmetry magnitude than sex.

This study examined the reliability of reactive strength index-modified (RSImod), relationships between RSImod and force-time variables, and difference in RSImod between male and female collegiate athletes. 106 Division I collegiate athletes performed unloaded and loaded countermovement jumps. Intraclass correlation coefficients and coefficients of variation were used to establish the reliability of RSImod. Correlations were calculated between RSImod and rate of force development, peak force, and peak power. RSImod appears to be a reliable performance measurement in male and female athletes. Furthermore, RSImod has moderate to very large relationships with rate of force development, peak force, and peak power. Statistically significant differences in RSImod existed between males and females during both unloaded and loaded CMJs.

The purpose of this study was to evaluate the relationship between force production asymmetry and performance of athletes with differing strength levels in males and females. Collegiate athletes (n=129) from various sports were ranked according to isometric mid-thigh pull peak force and the top (‘strong’) and bottom (‘weak’) 25% were used for analysis. Symmetry index (SI) scores were calculated and correlated with their respective force-time characteristics using bivariate correlations. For the weaker males, several negative moderate correlations were observed; however, no statistically significant correlations were observed for the females in either group. These findings indicate that force production asymmetry is inversely related to performance in weaker male athletes during isometric strength testing; however, similar to previous findings, this relationship is not apparent in stronger males.

Suchomel, TJ, Bailey, CA, Sole, CJ, Grazer, JL, and Beckham, GK. Using reactive strength index-modified as an explosive performance measurement tool in Division I athletes. J Strength Cond Res 29(4): 899-904, 2015-The purposes of this study included examining the reliability of reactive strength index-modified (RSImod), the relationships between RSImod and force-time variables, and the differences in RSImod between male and female collegiate athletes. One hundred six Division I collegiate athletes performed unloaded and loaded countermovement jumps (CMJs). Intraclass correlation coefficients and typical error expressed as a coefficient of variation were used to establish the relative and absolute reliability of RSImod, respectively. Pearson zero-order product-moment correlation coefficients were used to examine the relationships between RSImod and rate of force development, peak force (PF), and peak power (PP) during unloaded and loaded jumping conditions. Finally, independent samples t-tests were used to examine the sex differences in RSImod between male and female athletes. Intraclass correlation coefficient values for RSImod ranged from 0.96 to 0.98, and typical error values ranged from 7.5 to 9.3% during all jumping conditions. Statistically significant correlations existed between RSImod and all force-time variables examined for male and female athletes during both jumping conditions (p <= 0.05). Statistically significant differences in RSImod existed between male and female athletes during both unloaded and loaded CMJs (p < 0.001). Reactive strength index-modified seems to be a reliable performance measurement in male and female athletes. Reactive strength index-modified may be described and used as a measure of explosiveness. Stronger relationships between RSImod, PF, and PP existed in female athletes as compared with that in male athletes; however, further evidence investigating these relationships is needed before conclusive statements can be made. Male athletes produced greater RSImod values as compared with that produced by female athletes.

Chronic fatigue affects athletes' abilities to maintain force and power capabilities over the course of a season. The ability for sport scientists and coaches to monitor and manage fatigue is beneficial for all sports, especially those with lengthy seasons. Although methods of monitoring and managing fatigue have been suggested for various sports, there is currently limited research examining methods of monitoring fatigue in baseball over the course of a season. To effectively maintain peak performance, potential methods for monitoring and managing fatigue in baseball players need to be discussed.

The purpose of this study was to examine the differences in reactive strength index-modified (RSImod), jump height (JH), and time to takeoff (TTT) between 6 U.S. collegiate sport teams. One hundred six male and female Division I collegiate athletes performed unloaded (<1 kg) and loaded (20 kg) countermovement jumps as part of an ongoing athlete monitoring program. Reactive strength index-modified, JH, and TTT values for each team were compared using 1-way analysis of variance. Statistically significant differences in RSImod (p < 0.001), JH (p < 0.001), and TTT (p = 0.003) existed between teams during the unloaded jumping condition. Similarly, statistically significant differences in RSImod (p < 0.001), JH (p < 0.001), and TTT (p = 0.028) existed between teams during the loaded jumping condition. Men's soccer and baseball produced the greatest RSImod values during both the unloaded and loaded jumping conditions followed by women's volleyball, men's tennis, women's soccer, and women's tennis. The greatest JH during unloaded and loaded jumping conditions was produced by men's baseball followed by men's soccer, women's volleyball, men's tennis, women's soccer, and women's tennis. Men's soccer produced shorter TTT compared with men's baseball (12.7%) and women's soccer (13.3%) during the unloaded and loaded jumping conditions, respectively. Collegiate sport teams exhibit varying reactive strength characteristics during unloaded and loaded jumping conditions. Understanding the differences in RSImod between sports may help direct the creation of training and monitoring programs more effectively for various sports.