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A total of 41 high school female softball pitchers from the southern United States agreed to participate in the study (1.69 ± 0.07 m, 76.14 ± 17.08 kg, 15.1 ± 1.1 years, n = 41, n = 35 right-hand dominant). Pitchers were grouped into one of two categories based on their total bf%. The cutoff value determining pitchers with high fat% from pitchers with healthy fat% was set at 32% body fat according to the American College of Sports Medicine criterion-based reports, which define 20–32% body fat as being satisfactory for health in women (ACSM, 2013). Therefore, pitchers were grouped in the healthy bf% group if their bf% was <32% and grouped into the high bf% group if their bf% was ≥ 32%. There were 18 pitchers grouped into the healthy bf% group (1.70 ± 0.07 m, 64.54 ± 9.11 kg, 16 ± 2 years) and 27 pitchers grouped into the high bf% group (1.70 ± 0.07 m, 84.04 ± 15.78 kg, 15 ± 2 years).

Prior to participation, all participants were explained the study protocol and informed consent was signed by the participants' parent/guardian while the participant signed assent documentation. To be eligible for participation, pitchers needed to be injury- and surgery-free for the past 6 months and on a current softball roster. They also needed to have pitched in a game within the past 6 months and have reported to the laboratory fully rested for the previous 24 h.

All protocols were approved by the Institutional Review Board. Pitchers first completed a dual-energy X-ray absorptiometry (DEXA) whole-body scan, which collected whole-body and segmental composition measurements, including fat tissue, lean tissue, and bone mineral content (GE Healthcare, Madison, WI, USA). The standard error of estimate for the DEXA is ±1.8%. Following the dual-energy X-ray measurement, pitchers' bilateral hip and shoulder ISO and ROM were assessed in both the IR and ER directions. Regarding the hips, the push hip was part of the leg that pushes the pitcher off the ground and the stride hip referred to the leg that made foot contact during the pitch (Figure 1).

A handheld dynamometer (Lafayette Instruments, Lafayette, IN, USA) and inclinometer (Fabrication Enterprises, Inc., White Plains, NY, USA) were used to measure ISO and ROM, respectively (Dwelly et al., 2009; Sauers et al., 2014; Oliver et al., 2016, 2019b; Friesen et al., 2019, 2020a). For hip data measurement, pitchers sat on an athletic training table with their hips and knees flexed at 90°. A rolled towel was placed under the distal femur to allow for smooth rotation of the hip joint (see Figure 2). ROM was assessed by having the examiner rotate the shank either toward the contralateral leg (ER measurement) or away from the contralateral leg (IR measurement). The inclinometer was placed on the shaft of the fibula just proximal to the lateral malleolus for IR and on the shaft of the tibia just proximal to the medial malleolus for ER (Friesen et al., 2019). End ROM was determined just prior to when the participants' hip would lift off of the table while seated and with the examiner feeling for firm capsular end feel (Friesen et al., 2019). Hip ISO was also measured in this position (Figure 3). The dynamometer was placed at the same location as the inclinometer and resistance was applied while the participant pushed against the dynamometer and the hip remained in a neutral position during testing.

In gathering shoulder measurements, participants lay supine on an athletic training table with their upper arm abducted 90° and their elbow flexed to 90°. Again, a rolled towel was placed under the distal humerus to ensure smooth shoulder movement. The tester used one hand to limit scapular movement while the other hand slowly rotated the forearm either in the direction of the feet (IR measurement) or in the direction of the head (ER measurement). End ROM was determined just prior to when the participants' scapula would lift off the table for IR and at firm capsular end feel for ER (Figure 4). The inclinometer was positioned on the forearm above the styloid process of the ulna for both measurements. ISO was measured in the same way, with the tester maintaining a neutral arm position for the participant, while they applied pressure to the dynamometer (Figure 5). The tester ensured intrarater reliability, with an intraclass correlation coefficient [ICC(3,k)] of 0.92–0.95 for all measurements. Minimal detectable change (MDC) was calculated with a 95% CI to determine clinical significance. Glenohumeral joint IR and ER ROM MDCs were 6.2 and 7.5, respectively. Hip IR and ER ROM MDCs were 6.6 and 4.9, respectively. IR and ER MDCs for ISO measurements were 3.5 and 3.6, respectively, for the glenohumeral joint, and 9.0 and 2.3 for hip IR and ER.

All statistical analyses were completed in SPSS software package (SPSS Statistics 26 Software, IBM Corp., Armonk, NY, USA). Prior to statistical analyses, all data were checked for normality, linearity, and outliers (defined as those >2 SDs away from the mean). Data were considered normal and linear, with a few outliers present. Analyses were conducted with and without outliers to which no significant differences were observed; therefore, original data with outliers included were used for analyses. Levene's test for equality of variance was conducted and equal variance was consistently observed. Mixed analyses of variance (ANOVA) were conducted to assess the difference between bf% groups and side dominance on several variables, including shoulder and hip external and IR ROM and ISO. Two subsequent mixed ANOVAs were also used to assess arm bone mass and lean mass. Alpha level was set a priori to 0.05.

The only functional characteristic to present differences according to bf% group was IR ROM of the shoulder, although it is important to note that these differences were smaller than the MDC calculated previously. Discussion of results should therefore be viewed with this in mind. Data revealed that pitchers with a healthy bf% had less dominant arm IR ROM than the high bf% group (albeit not of clinical significance). The difference between bf% groups suggests that the amount of body fat a pitcher possesses might have a slight influence on functional characteristic adaptations. Loss of IR ROM of the dominant shoulder is common among those athletes who regularly perform throwing tasks (Shanley et al., 2011), but the healthy bf% group displaying less throwing shoulder IR ROM than the high fat% was not expected. Originally, it was hypothesized that the high fat% group might display decreased ROM in general, due to the impingement issues that arise from exhibiting increased fatty tissue. Research points out that those pitchers with a higher body mass index possess less bilateral hip ROM, hypothesized to be the result of greater impinging tissue (Friesen et al., 2020a). It was also expected that the greater mass associated with those in the high bf% group might accrue greater adaptation due to increased loading on the joints. With current data revealing the opposite, it might suggest that the pitchers with a healthy bf% might be accruing greater functional adaptations than the high fat% group. Perhaps healthy fat% pitchers encounter higher repetition and greater pitch counts that might result in greater adaptation (e.g., more throwing shoulder ER ROM and less IR ROM). Conversely, this might suggest that the high fat% group performs less repetitions, which could be either, or both, a cause and effect of carrying extra fat mass. Those who possess extra fat mass might be less active and, as a result, perform fewer and less intense repetitions. Interestingly, a previous report also found that pitchers with higher body mass index did not achieve as much dominant shoulder ER ROM as those with lower body mass index (Friesen et al., 2020a). Therefore, adaptation maybe does not occur the same in a population of pitchers with increased mass and body fat. Future studies are necessary to understand why those softball pitchers with more body fat accrue less throwing shoulder IR ROM as the discussion of causation is merely speculation. It should also be noted that since the acquisition of less IR ROM is a common adaptation of throwers, and our two groups displayed opposite trends of adaptation, future research should account for body mass or bf% as a potential covariate when analyzing functional adaptations of throwers.

Data revealed that there were side-to-side asymmetries in IR ROM between the throwing and glove shoulders. Interestingly, the high bf% group displayed more IR ROM in their throwing shoulder compared to the glove shoulder, opposite of the healthy fat% group. While drastic shoulder asymmetries can lead to injury among throwers (Shanley et al., 2011), the varied asymmetry between bf% groups suggests a potential effect of body fat on shoulder adaptations. However, as asymmetry was present within both populations, we cannot assume that body fat alters symmetry. We can, however, state that body fat might influence functional adaptations, namely, at the shoulder joint. As we know that higher bf% has been demonstrated to be related to higher injury risk (Chalmers et al., 2015; Oliver et al., 2018), this also provides some evidence that perhaps the altered functional characteristics within those with higher body fat could be a potential mechanism for injury; however, more studies are needed to examine mechanisms of injury among those with higher body fat.

Notable side-to-side differences were observed in shoulder ER ISO and hip IR ISO. Based on the pitching motion, asymmetry in IR ISO of the shoulder was expected as another typical adaptation for throwers. During the windmill pitch, the throwing arm shoulder completes rapid IR during the acceleration phase of the pitch (Maffet et al., 1997; Barrentine et al., 1998). To slow the rapid IR, ER strength is necessary to bring the pitcher to a safe stop to finish the pitch. The repeated pitching exposure can strengthen the musculature of the throwing shoulder while in this position, as opposed to the glove arm that performs less overall motion. It was also reported that there was more IR strength in the push hip than in the stride hip. Again, this was expected as the drive off the ground occurs while the pitchers' push hip is internally rotated (Friesen et al., 2020a). In contrast, during the stride, the stride hip needs ER strength more than IR strength to keep the stride leg hip from collapsing and exhibiting greater IR upon aggressive ground contact.

The final findings noted asymmetry in throwing arm lean tissue and bone for both groups of pitchers. As might be expected, bone and lean tissue was heavier within the throwing arm. As this did not vary between pitcher bf% groups, this again highlights the importance of other factors that might influence adaptation to functional characteristics. However, descriptive analysis of the mean and SD data reported in Table 1 shows that the high fat% pitchers did possess greater mean weight of both lean tissue and bone than the healthy fat% group. Perhaps this is an adaptation required to move the generally larger limb of those who are bigger in stature and carry increased body fat.

While research notes that pitchers with increased bf% exhibit higher rates of pitching-related injury (Oliver et al., 2019a; Friesen K. B. et al., 2021) and altered biomechanics during the windmill pitch (Friesen et al., 2020b), there were minimal differences in functional characteristics between bf% groups. There were, however, many asymmetries reported, as could be expected. As bf% does not seem to greatly influence player adaptation, but injury reports suggest an increased injury-susceptibility for larger athletes, additional research is needed to understand the mechanism of injury among softball pitchers. Similarly, more studies are needed to examine the influence of specifically fat mass on softball pitch biomechanics in effort to best inform practitioners working with the development of softball pitchers. Also, this might suggest that other factors such as biomechanics, effort, and practice/game exposure influence functional characteristic adaptations more than the presence of body fat.

This study has some limitations. First, there are potential inconsistencies in maximum effort required during the ISO measurement, and similarly the consistency of the applied rater pressure to the device can vary given the handheld nature of the device. However, all participants were given the same instruction, “to push as hard as possible,” and the rater too gave maximal effort during each measurement. Second, the expertise and development of participants was not controlled; therefore, pitchers with various mechanics and pitching success rates might influence the aggregate data analysis. However, the recruitment sample did come from one general area of the southern United States; therefore, it might be assumed that these players were of a similar skill and competition level. Similarly, the study sample involved a relatively small age range, and it might be expected that older pitchers with more substantial bf% differences could present different and more substantial findings. It is also important to keep in mind that there were no drastic functional characteristic differences between the bf% groups, nor was there a significant number of other variables that differed between bf% groups. Finally, our functional characteristics were measured in static positions and will therefore be limited in their application of the dynamic pitching motion under consideration.