A three-dimensional kinematic and kinetic study of the college-level female softball swing.

Author:Milanovich, Monica
Position::Research article - Report
 
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Introduction

Fastpitch softball is one the most popular competitive and recreational sports in the United States (ASA Youth Program, 2010). Females exclusively participate competitively at the high school, collegiate, and professional levels. For example, the sport is played in over 277 Division I college teams nationwide. Despite its widespread popularity, little research has been performed on the fundamental mechanics of the female softball swing especially when compared to baseball (Adiar, 2002). Messier and Owen (1984) stated, "the absence from the biomechanics literature of studies concerning female fast pitch softball batting has left the athlete and her coach with little scientific information on which to base the implantation of various techniques." Their initial biomechanical studies used direct video measurement to quantity and describe the three-dimensional velocity characteristics of eight female fast pitch softball batters. Their results included quantifying the maximum linear and angular velocity and components (fixed axis representation), presenting typical time histories of the linear and angular velocity components, and offering a qualitative description of swing mechanics based upon these data. These findings were compared to baseball batters and were found to be significantly different thus questioning the value of future biomechanical comparisons between batters of opposite genders participating in unique sports with significantly different batting requirements (Messier and Owen, 1984).

With the exception of this one study, the majority of the biomechanics research regarding the softball swing has focused on understanding the properties and performances of the bat (Bahill, 2004; Noble and Eck, 1986; Russell, 2005), and determining the relationships between bat velocity and mass properties (Fleisig et al., 2002; Koenig et al., 2004; Smith et al., 2003). Much of the motivation for this work was to provide the scientific basis for establishing standards of bat performance to balance player performance and safety (Fleisig et al., 2002).

Russell (2005; 2006) has done extensive work describing and quantifying the various relevant mass properties and associated measures of softball bats. He presents a qualitative description of softball swing mechanics from an overhead two-dimensional perspective, and discussions on the influences of the various mass properties and measure on bat performance and swing mechanics. Bahill (2004) and Noble and Eck (1986) investigated the relationships among softball bat mass properties, bat impact behavior, and resulting batted ball speed. Both studies acknowledged the complexity of batter swing mechanics, the interrelationship between bat properties and swing mechanics, and the role that individual swing characteristics have on impact behavior and batted ball speed.

Fleisig et al. (2002) experimentally investigated the relationship between bat mass properties and bat velocity (linear and angular) for 17 female collegiate softball players using bats engineered to have various mass and inertia properties. This study found that linear velocity had a significant correlation with bat moment-of-inertia (measured about the bat handle), but not bat mass. There were no correlations found relative to angular velocity. Smith et al. (2003) conducted a similar experimental study of bat mass properties and bat speed using 16 amateur slow-pitch softball players. This study isolated bat mass properties into two groups. One group varied bat mass for a constant moment of inertia, and the other varied bat moment of inertia for a constant bat mass. This study also found that linear velocity had a significant correlation with bat moment of inertia, but not bat mass. Finally, Koenig et al. (2004) investigated the relationship between bat moment-of-inertia (about the bat handle) and linear bat speed. Ten collegiate female fast-pitch softball players each swinging six different bat configurations were measured experimentally, and additionally analyzed with a planar one degree-of-freedom analytical model. This study found that for the majority of bat inertia values, bat speed was independent of inertia. This finding does not agree with their theoretical predictions which indicated an inverse relationship.

For these three studies bat moment-of-inertia was measured relative to a fixed location on the bat handle (ASTM F2398-04), thus these results may be misleading. Bat moment-of-inertia about this point is a function of bat mass, mass centre location, and mass centre inertia which effectively summarizes these three mass properties into one quantity, and this quantity is dominated by the location of the mass centre (Milanovich, 2010). In addition, for this measure of bat inertia to be more relevant, the location of the bat mass centre should be measured relative to the actual centre-of-rotation of the bat (Milanovich, 2010), which has been shown to be between the batter and bat handle at impact (Smith et al, 2003; Russell, 2005), dependent upon swing trajectory, and constantly moving during the swing (Milanovich, 2010). Thus the lack of consideration of subject swing characteristics may help explain the conflicting findings of these studies.

The biomechanical modeling done in support of these studies has been limited to either treating the swing as a planar rotation about a fixed axis relative to the bat (Noble and Eck, 1986), or a pure planar rotation of the body and the bat about a vertical axis through the batter (Koenig et al., 2004). Modeling complex three-dimensional sports motions as planar fixed point of rotation motions is often done to simplify the resulting computer models and equations of motion (Nesbit, 2005). Koenig et al. (2004) indicated a lack of confidence in their model, with additional development warranted based upon the conflicting conclusions between their experimental results and model predictions. It was further stated that the inclusion of additional degrees-of-freedom to their model (one DOF model) would potentially improve its accuracy in determining swing speed as a function of bat inertia. The general difficulty in modeling the swing of the bat is noted by Smith et al. (2003), Fleisig et al. (2002), and Bahill (2004). A model of the softball swing which does not restrict the motion to a plane about a fixed point of rotation may result in a more accurate and comprehensive description of the swing mechanics as has been performed in other sports motion analyses.

Thus there is an obvious and compelling need for an in-depth and comprehensive description of female softball swing mechanics. A more representative computer model of the swing would aid such a study. A detailed understanding of the mechanics of the female softball swing would be beneficial for scientifically informing various batting techniques, providing a basis for understanding the complex interrelationships among bat properties, swing characteristics, and bat performance, and providing a basis for further study of the motion. Such information would benefit the scientist, player, coach, and equipment manufacturer.

This paper presents a description of the fundamental kinematics and kinetics of a female softball swing for 14 college level participants using an unrestricted three-dimensional rigid model of the bat that was developed for this study. Specifically, the purposes of this study are the following:

* Present a more representative softball swing computer model

* Provide a detailed quantitative description of the kinetics and kinematics of the swing

* Analyze a group of subjects for basic statistical information

* Identify typical similarities and differences in swing mechanics among subjects

* Gain insight to the role of bat properties on swing mechanics

* Attempt to describe the female softball swing from a mechanics perspective

Methods

The subjects of this experiment were fourteen female college level players of various experience (12.3 [+ or -] 4.4 years), height (1.65 [+ or -] 0.06 m), and weight (62.4 [+ or -] 7.8 kg). The subjects were a combination of left-handed and right-handed, with one switch hitter. No effort was made to quantity skill level (Fleisig et al., 2002; Koenig et al., 2004; Messier and Owen, 1984; Smith et al., 2003). This number of subjects is consistent with all previous studies of female softball batting which ranged from 8 to 17 subjects (Fleisig et al., 2002; Koenig et al., 2004; Messier and Owen, 1984; Smith et al., 2003). All subjects were informed of the purposes of the study, and gave written consent for the use of their data for research purposes, in accordance with local IRB requirements.

The subjects stretched and warmed up for a minimum of 10 minutes in accordance with their normal practice habits which followed normal warm-up protocols (Fleisig et al., 2002: Smith et al., 2003). The Motion Analysis system was calibrated until the combined 3D residual for all cameras was less than 1.00 mm. (Test/retest of static marker locations varied by less than 0.20 mm for a given calibration.) The subjects were asked to execute a series of "competitive effort" swings that consisted of hitting a ball placed on a batting tee into a net (Koenig et al., 2004). Tee height and location relative to the tee were chosen by the subjects. Grip offset from the triad was measured between the hands. A marker was placed on the ball to determine the time of impact.

Two distinct bats (aluminum and composite) with significantly different inertial properties were used for the subject trials (see Table 1). These bats were measured for mass, mass centre location, and mass centre inertias using the apparatus, methods, and calibrations described in Nesbit and Serrano (2006). From these quantities, the centre-of-percussion and grip point inertia (Igrip) values were determined using the protocols specified in ASTM F2398. The mass properties of these two bats are consistent with bats used in other studies (Bahill,...

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