Neuromuscular Fatigue in Pitchers Across a Collegiate Baseball Season

Abstract

Stone, BL and Schilling, BK. Neuromuscular fatigue in pitchers across a collegiate baseball season. J Strength Cond Res 34(7): 1933–1937, 2020—Neuromuscular fatigue in baseball pitchers has become an important aspect of injury risk. It is imperative to understand how fatigue is manifested to enhance resiliency and mitigate injury risk. Secondarily, collecting data on neuromuscular characteristics of baseball pitchers provides a framework to address these concerns. Using the countermovement jump, this study observed neuromuscular performance during the preseason, midseason, and postseason of a collegiate baseball season with college baseball pitchers. No statistically significant changes were noted in any of the main variables at any testing timepoint (p > 0.05). However, several variables, including concentric mean force (ConMF, d = 0.59) and concentric peak force (ConPF, d = 0.59) in addition to eccentric mean force (EccMF, d = 0.54) and eccentric mean power (EccMP, d = −0.66), displayed moderate effects from preseason testing to midseason testing. Furthermore, jump height displayed a large negative effect from preseason to midseason (d = −0.89). Secondarily, descriptive data for both concentric and eccentric variables were also determined from the present findings. There were moderate changes in neuromuscular fatigue in Division I collegiate pitchers across a competitive season and has provided descriptive data for neuromuscular characteristics in collegiate baseball pitchers.

Introduction

The ability to quantify, monitor, and manage performance variables has become essential for sport science practitioners. Possessing one of the longest seasons in collegiate or professional sports, baseball presents unique challenges in which changes of neuromuscular performance have recently gained attention (²⁰). This is in part because baseball athletes, specifically pitchers, are exposed to the accumulation of both physical and mental stressors across a season (³). Specifically, these include biomechanical stressors that have elicited a 34% increase of injuries over their fielding counterparts (³). Furthermore, the mechanical strain of repeated high velocity throwing in addition to a variety of on- and off-field stressors that have been noted as contributing to chronic fatigue, which may decrease performance over time (²⁰). Chronic fatigue can be observed as a decrement (or impairment) in neuromuscular function (^13,19). Congruently, the rise in technology has offered an ability to noninvasively measure neuromuscular function to quantify these unique challenges (^6,16).

Collegiate baseball on average plays 56 competitive regular season games (²⁰). When adding in the postseason, upward of 70 games may be played. Owing to the extended nature of the season (early February to mid-June), the ability to understand the longitudinal effects of the season and the ability to maximize performance are pertinent. Furthermore, previous literature has sought to quantify what measures elicit potential degradation in performance across this extensive season. Joint kinematics, range of motion, pitch volume, and other laboratory- and field-based methods have been used in an effort to gauge player's state of “readiness,” maximize performance, and mitigate potential injury across a season (^3,4,14,20,21).

The countermovement jump (CMJ) has often been used as an in-field assessment to understand neuromuscular performance and its relationship to fatigue and on-field performance (¹). Specifically, investigations have aimed at understanding measures derived from the CMJ to understand the manifestation of fatigue in athletes (⁵). Recent investigations have uncovered potential measures by which fatigue may be monitored in basketball (¹⁵). A previous investigation compared CMJ force time variables between football, basketball, baseball, and volleyball (¹⁰). That study (¹⁰) demonstrated high force output (e.g., rate of eccentric force development and concentric force) in addition to high overall jump height (JH) in baseball athletes. Specifically, baseball athletes produced a higher rate of eccentric rate of force development (5.4 ± 0.2 N·s⁻¹·kg⁻¹) than football (4.5 ± 0.4 N·s⁻¹·kg⁻¹), basketball (3.4 ± 0.32 N·s⁻¹·kg⁻¹), or volleyball (3.7 ± 0.3 N·s⁻¹·kg⁻¹) athletes. Also, baseball athletes had greater overall JH (59.1 ± 8.6 cm) than football (50.1 ± 15.9 cm), basketball (46.8 ± 12.7 cm), and volleyball (45.7 ± 11.8 cm) athletes. However, to the authors' knowledge, little literature has uncovered CMJ characteristics of college pitchers. Secondarily, little has been done to examine the effect of a Division I collegiate baseball season on neuromuscular function. Thus, the aim of this study was to quantify CMJ characteristics in Division I collegiate pitchers and to secondarily monitor potential changes in neuromuscular function in this same group of pitchers across a competitive baseball season.

Methods

Experimental Approach to the Problem

The present investigation examined baseline and changes in neuromuscular function in Division I collegiate baseball pitchers across a collegiate baseball season. The CMJ was selected based on previous investigations supporting its validity and reliability of both performance and fatigue markers (^1,5,8,13,20). Absolute changes in CMJ variables from preseason baseline testing were determined in 4 assessments divided throughout the season as outlined in Figure 1.

Figure 1.: Testing timepoints. The first 2 arrows represent the 2 preseason timepoints, each separated by one week. The 2 timepoints in March occurred 48 hours apart, exactly at the half-way point of the season, and the last timepoint occurred on the same day of the week as the first and third timepoint, exactly at the end of the regular season. The same testing protocol was performed at each testing timepoint.

Subjects

Division I collegiate baseball pitchers (mean ± SD: n = 17, 20.5 ± 0.5 years, 189 ± 5 cm, 91.8 ± 12.8 kg) participated in this study. Pitchers were only studied within this analysis due to their specific performance and sport training regimens and schedules, and none played any other position. This is a convenience sample for pitchers in that there was no a priori sample size estimation for statistical analysis. This secondary analysis was approved by the University of Oklahoma's institutional review board in compliance with the Declaration of Helsinki, based on the fact that these data were not collected for research purposes. Data were collected from January through May prior to and across the NCAA spring season. Each subject had undergone a physical and had been cleared by team medical staff. Consent not needed as these data were analyzed post-hoc and not collected for the purposes of research.

Procedures

Assessments were conducted at the same location during the same times for each subject across seasons. All subjects performed a standardized dynamic warm-up performed before each testing session, which included dynamic stretching and locomotion patterns (i.e., skipping, jogging, and running), similar to previous literature (⁹). This allowed for appropriate preparation for maximal performance during the jump testing. After the warm-up, detailed instructions were given (described below), and the subjects performed 3 practice CMJs as an additional warm-up. All jumps were performed on the ForceDecks FD4000 (ForceDecks, Newstead, AUS) at a sampling rate of 1,000 Hz, with body mass recorded to the nearest 0.1 kg. Two separate pretest practice sessions were completed 14 days preceding the first in-season assessment. These practice sessions were performed in the same exact manner as the testing protocol. During each testing session, subjects wore the same standard issue shorts, t-shirt, and tennis shoes.

The CMJ testing protocol was performed as previously described (¹¹). Briefly, subjects performed all jumps from a starting position of standing with their feet hip width relative to shoulder width with hands placed on the hips (hands akimbo), in a tall standing position so that equal weight was distributed on both plates of the FD4000. A visual representation of weight distribution was displayed on a monitor in front of the subject to provide visual feedback to obtain a neutral standing position. On the “go” command by the tester, individuals performed a maximal countermovement to a self-selected depth, followed by a maximal jump while maintaining hand placement on the hips. Subjects landed in a quarter squat athletic position followed by a full return to the initial standing position with knees and hips extended. This completed a successful CMJ and was repeated for 2 subsequent jumps completed for a total of 3 jumps. If a subject exhibited excessive knee flexion at any point during flight, that jump was considered invalid and the subject repeated the jump. Furthermore, the same tester completed all jumps throughout each testing timepoint.

Statistical Analyses

All CMJ data were calculated and analyzed through commercially available ForceDecks conventional methods described elsewhere (¹³). Jump performance variables (Table 1) were then analyzed through repeated-measures analysis of variance to examine differences from baseline throughout in-season timepoints. In addition, pairwise effect size (ES) calculations were provided (Cohen's d) to measure the magnitude of differences (difference in means/pooled SD) within each measure from preseason to postseason and were interpreted as trivial (0.0), small (0.2), moderate (0.6), large (1.2), very large (2.0), nearly perfect (4.0), and perfect (infinite) (⁹). All statistical analyses were performed using SigmaPlot software (Version 12.5, Systat Inc., Chicago, IL) with an a priori alpha level set at p ≤ 0.05.

Table 1: Countermovement jump (CMJ) variable descriptions.*

Results

Descriptive Characteristics

Pitchers displayed an average BW of 92.1 ± 12.7 kg, concentric mean force (ConMF) of 1752.8 ± 231.0 N, and eccentric mean force (EccMF) of 922.0 ± 186.1 N (Table 2) when data were averaged across the collegiate season. All other measurement averages are displayed within the table.

Table 2: Countermovement Jump Descriptive Statistics.*†

Performance and Fatigue Analysis

No statistically significant changes were noted in any variable at any testing timepoint across the season (Table 3, p > 0.05). However, concentric mean force (ConMF, d = 0.59), concentric peak force (ConPF, d = 0.59), concentric peak velocity (ConPV, d = −0.57), eccentric mean force (EccMF, d = 0.54), eccentric mean power (EccMP, d = −0.66), and force at peak power (FatPP, d = 0.61) all displayed moderate effects from preseason testing to midseason testing, while JH displayed a moderate-to-large negative effect from preseason to midseason (d = −0.89). There were no other moderate or large effects noted for any other measurement at any timepoint.

Table 3: Countermovement jump variables at each testing timepoint.*

Discussion

The aim of this study was to quantify CMJ characteristics in Division I collegiate pitchers and to monitor neuromuscular function in this same group of pitchers across a competitive baseball season. The findings allow descriptive characteristics to be reported for Division I collegiate pitchers (Table 3) and to examine potential changes over the season.

To the best of our knowledge, little has been performed in terms of investigation into neuromuscular characteristics (e.g., force, power, and velocity) in collegiate baseball players. However, a recent study on collegiate and professional athletes (e.g., basketball, football, volleyball, and baseball) performed CMJ assessments after warm-up and before a training session (¹⁰). Their findings demonstrated average JH, total jump time, in addition to high rates of eccentric rate of force development and concentric force in college and professional baseball athletes. Specifically, baseball athletes possessed an average JH of 59.1 cm, rate of eccentric force development of 5.41 N·s⁻¹·kg⁻¹, and concentric force of 21 N·kg⁻¹. In the study by Laffaye et al. (¹¹), sampling frequency was conducted at 500 Hz and both eccentric and concentric force-time variables were divided by body mass, whereas the present investigation used 1,000 Hz of sampling rate and absolute values of both eccentric and concentric variables were obtained. In addition, we used different force plate hardware and software, consistent with recent investigations (^7,8). Owing to the differences in sampling frequencies and potential differences in data reduction of the jump metrics, we are unsure if the results are comparable and thus, may limit the ability to determine similarities in measures. Our study has further elucidated both concentric and eccentric measures (e.g., both mean and peak concentric and eccentric) providing descriptive data for collegiate baseball pitchers.

The present findings display a nonsignificant, moderate-to-large decline in neuromuscular performance from the preseason to midseason (d = −0.89) with no change from the midseason to postseason. Since our sample was small, there was not sufficient statistical power to note these as statistically significant. These findings show negative change in neuromuscular performance, indicative of fatigue, that occurs during the season and does not improve by the end of the season. Of the changes in neuromuscular performance exhibited (e.g., force, power, velocity, and JH), it is JH that has been shown to be linked with changes in neuromuscular performance and overall neuromuscular fatigue (¹).

Jump height has shown to be sensitive to neuromuscular fatigue in a variety of settings, including across a competitive season in addition to acute training sessions (²). A study conducted by Sams et al. (¹⁷) examined changes in JH throughout a collegiate men's soccer season. Although they found no statistically significant changes, JH displayed a moderate decay from the preseason to in-season (ES = 0.70), exhibiting a decrease in overall neuromuscular ability.

Acute changes in neuromuscular status have been previously investigated, with findings exhibiting significant decrements in JH at 24-, 48-, and 72-hour postfatiguing exercise (⁵). Specifically, their findings suggest that both eccentric and concentric measures of force and power are reliable indicators of fatigue (a coefficient of variation of <10% for intraday and interday reliability) (⁵). The researchers examined 3 consecutive days of CMJ to assess reliability of CMJ as it pertains to neuromuscular status changes and found that the performance initially recovered at 24-hours with a subsequent decrease in performance at 72 hours (⁵). Furthermore, they indicated 9 measures that possessed intraday and interday coefficients of variation of (<5 ± 1%) (⁵). However, using the CMJ as a means of monitoring neuromuscular status has also brought about technique variations that may or may not alter jump strategy and performance. Specifically, a recent investigation by Heishman et al. (⁸) uncovered that CMJ performed with arm swing technique may provide longitudinal changes in CMJ performance while acute changes, specifically measures of fatigue, may be better served using a nonarm swing technique, as was conducted in our investigation.

One of the most striking points may be that to monitor neuromuscular status across a season, more assessments should be conducted than we were able to perform. A study by Malone et al. (¹²) examined changes in CMJ JH of elite youth soccer during the middle of a competition season finding nonsignificant but small changes in CMJ performance (p > 0.05, ES range −0.04 to −0.22) (¹²), whereas another recent investigation into CMJ and neuromuscular fatigue exhibited that fatigue may manifest itself differently after a fatiguing bout of exercise and thus, the potential of the CMJ to be used consistently as a monitoring tool for neuromuscular status (⁵). These studies both differed from that of this study, which consisted to 2 testing timepoints during the preseason, 2 testing timepoints at the midseason point, and 2 testing timepoints at the end of the regular season (Figure 1). This may limit the ability to discern the transient nature of neuromuscular fatigue, such that time between testing points allowed for the dissipation of fatigue and a return to “normalized” neuromuscular status. The combination of frequent assessments of neuromuscular performance coincide with potential relationships between playing intensity, training load, and body composition and their relationship with neuromuscular status (¹⁸) which may give further insight into potential changes throughout a season.

This study is not without its limitation. This study did not delineate between playing time, pitch volume, and pitching role (e.g., starter versus reliever). Although this limitation may limit the specificity of results, it is intriguing that the changes seen in neuromuscular performance were across the entire group which does warrant further investigation by future studies to examine increased testing timepoints in conjunction with playing time, pitching volume, and playing role.

Although there were no significant changes across timepoints, this study did find moderate decrements in several CMJ variables from the preseason to midseason timepoints with no further changes occurring from midseason to postseason testing. Thus, these negative differences from preseason to midseason allude to a level of chronic neuromuscular fatigue which does not recover by the end of the season. This fatigue may manifest itself into limitations in playing performance and the possibility of increased risk of injury. However, to further understand this complex relationship of fatigue, future investigations into baseball may aim at more consistent assessment timepoints while also examining positional differences (including starter versus relief pitchers), playing time, intensity, and changes in body composition in addition to physiological and biomechanical changes.

Practical Applications

This study has elucidated several descriptive measures of CMJ, which will allow comparison to future investigations that may aim to set normative levels of eccentric and concentric force and power metrics across high school, collegiate, and professional baseball players. Secondarily, although no significant changes were noted, JH did show large negative changes (d = −0.89) in this small sample from preseason to midseason assessments, which is similar to data from soccer and basketball. These findings suggest that there is a level of moderate neuromuscular fatigue induced by the season that does not recover by the end of the season. Future investigations are warranted examining different positions within baseball in addition to further understanding differences between starter and relief pitchers. Further examination into other assessments may provide more insight into the physiological and neuromuscular changes across a competitive season as to allow for optimal training to prepare for the rigors of the season in addition to recovery and load management strategies in baseball.