Current and Future Trends in Strength and Conditioning for Female Athletes

articipation by female athletes in competitive sport has increased dramatically since the inception of Title IX, although female athletes are represented signi cantly less than their male counterparts in strength and conditioning (S&C) literature. This is apparent when examining cur- rent identi ed trends in the eld, such as implementation of blood ow restriction (BFR) training, functional assessments to predict injuries, or the ever-increasing use of technology in sports. The aim of this review is to examine three prevalent trends in contemporary S&C literature as they relate to female athletes in order to expose areas lacking in research. We conducted journal and database searches to progressively deepen our examination of available research, starting rst with broad emerging themes within S&C, followed next by an inquiry into literature concerning S&C practices in females, ending nally with a review of emerging topics concerning female athletes. To this end, 534 articles were reviewed from PubMed, Academic Search Complete, Google Scholar, CINAHL, MEDLINE, and Web of Science. Results demonstrate the utility of implementing BFR, functional movement assessments, and various technologies among this population to expand representation of female athletes in S&C literature, improve athletic capabilities and performance, and decrease potential for injury over time. Keywords: women athletes; training; functional movement; blood ow restriction; injury prevention;

PubMed, Academic Search Complete, Google Scholar, CINAHL, MEDLINE, and Web of Science. Results demonstrate the utility of implementing BFR, functional movement assessments, and various technologies among this population to expand representation of female athletes in S&C literature, improve athletic capabilities and performance, and decrease potential for injury over time. Keywords: women athletes; training; functional movement; blood ow restriction; injury prevention; screening; technology 1. Introduction Incorporating female athletes into strength and conditioning (S&C) programs is rela- tively new due in part to the passage of Title IX in 1972. Title IX, a federal civil law, had a large impact on females in many different areas within educational institutions, although it arguably had the largest impact on sports participation [1]. Prior to the passage of Title IX, females did participate in sport, but their participation in S&C was far less than their male counterparts. Female athletes participating in sports prior to 1972 often engaged in more recreation-oriented activities, and few female athletes participated in a competitive environment. Unfortunately, cultural views during the early to mid-20th century did not favor elite performances among female athletes, and therefore female participation in S&C was viewed as unnecessary [2]. Title IX was certainly a catalyst for a large increase in the number of female athletes in sport at the professional and international levels [3]. The 1900 Olympic Games in Paris saw the rst inclusion of female athletes with roughly 2.2% of females representing the total number of athletes at the Games (i.e., 22 females out of 997 total participants). In contrast, the 2012 Olympic Games in London showcased female athletes competing in every sport offered at the Games. The rise in female participation in the Olympic Games has grown immensely from its inception to a total of 45% of all participants in the 2016 Olympic Games in Rio de Janeiro [4]. Despite the growth in the participation of female athletes in sport, it seems that growth in S&C programs for female athletes moves at a slower pace. For example, 50% Int. J. Environ. Res. Public Health2022,19, 2687.

of 45% of all participants in the 2016 Olympic Games in Rio de Janeiro [4]. Despite the growth in the participation of female athletes in sport, it seems that growth in S&C programs for female athletes moves at a slower pace. For example, 50% Int. J. Environ. Res. Public Health2022,19, 2687.

Int. J. Environ. Res. Public Health2022,19, 2687 2 of 23 of high school coaches for male varsity student-athletes required their student-athletes to engage in strength training, compared to only 9% of coaches of female athletes [5]. To compound the issue further, there is a scarcity of published research focused on females. The average percentage of female participant inclusion per article within the sports and exercise medicine literature is between 35 and 37% [6]. It is possible that the historical lack of female participation in sport may contribute to this paucity of data, even as the rate of female participation has increased over the years. For example, blood ow restriction (BFR) as a training mechanism is a relatively new training method that is gaining popularity in S&C and rehabilitative settings. Inclusion of female participants in BFR studies is reported at 14% and 17% in studies of acute and chronic use, respectively [7], but only when searches are expanded to include recreationally active or untrained females outside of the19–44-year-oldage range (see Table below). Additionally, other trending areas of inquiry in S&C, such as functional assessments and wearable technology, also indicated a lack of studies that included female athletes as compared to their male counterparts. The National Strength and Conditioning Association (NSCA) was founded in 1978 and is one of the rst organizations to provide a certi cation for strength and conditioning coaches. In 1989, the NSCA released a position statement focusing on strength training for female athletes. Within the NSCA's 1989 position statement, the organization stressed the importance of female role models in S&C as coaches [7]. A survey of 103 collegiate Division I head S&C coaches in 1990 found that 99% of head S&C coaches were male. Furthermore, the mean ratio of collegiate S&C staff was 2.6 males to 0.25 females at that time [8]. In 2016, 32% of all collegiate Division I (DI) S&C coaching positions were occupied by female S&C coaches [9]. However, DI male student-athletes reported being less comfortable with a female S&C coach, and would prefer working with a male S&C coach over a female S&C coach

of collegiate S&C staff was 2.6 males to 0.25 females at that time [8]. In 2016, 32% of all collegiate Division I (DI) S&C coaching positions were occupied by female S&C coaches [9]. However, DI male student-athletes reported being less comfortable with a female S&C coach, and would prefer working with a male S&C coach over a female S&C coach regardless of their quali cations [10]. Clearly, while there has been growth in the eld of S&C for both female coaches and athletes, there are still attitudes towards females in S&C that may hinder their development. Addressing the social and structural determinants for females in S&C should help increase opportunities for both female athletes and coaches. Given the increasing female participation in sports, there is a need to examine the implementation of current S&C practices in this population. The overarching goal of this narrative review is to identify the effects of emerging S&C training modalities on female athletes' performance. More speci cally, this narrative review synthesizes available literature regarding three identi ed prevalent trends in S&C (i.e., BFR training, functional assessments and screening, and the use of technology in sports) and the utilization of these trends with female athletes in order to guide future research for females participating in sports. 2. Methods To prepare for the narrative review, a preliminary literature search for recent articles on training modalities employed among female athletes was conducted. Initially, we targeted two speci c journals in which research of this nature is generally published in order to identify trends in the eld of S&C in female athletes; namely,Medicine & Science in Sports & Exercise(MSSE) and theJournal of Strength and Conditioning Research(JSCR). Search criteria regarding publication date included articles that were published in the past eight years; this is the most distal time interval available on each journal's Advanced Search Builder [11,12]. This broad initial search revealed approximately 2000 results. Three topics were then selected based on their prevalence in these peer-reviewed publications (see Table and screening, and the use of technology in sports. After these topics were established, search terms for female subjects were

eight years; this is the most distal time interval available on each journal's Advanced Search Builder [11,12]. This broad initial search revealed approximately 2000 results. Three topics were then selected based on their prevalence in these peer-reviewed publications (see Table and screening, and the use of technology in sports. After these topics were established, search terms for female subjects were added to further explore emerging S&C practices in females. Finally, a search term targeting practices with female athletes was added to specify this group as the focus of this review. Values presented in Table December 2021.

Int. J. Environ. Res. Public Health2022,19, 2687 3 of 23 Table 1.Initial literature and database search strategies. Literature Search Strategies a Journal Search Terms Results ( n) MSSE Blood ow restriction training 184 (female OR women) AND blood ow restriction training 70 (female OR women) AND athlete* AND blood ow restriction training 13 Functional assessment AND screening 188 (female OR women) AND functional assessment AND screening 126 (female OR women) AND athlete* AND functional assessment AND screening 63 Technology in Sports 470 (female OR women) AND technology in sports 378 (female OR women) AND athlete* AND technology in sports 153 JSCR Blood ow restriction training 93 (female OR women) AND blood ow restriction training 44 (female OR women) AND athlete* AND blood ow restriction training 33 Functional assessment AND screening 168 (female OR women) AND functional assessment AND screening 113 (female OR women) AND athlete* AND functional assessment AND screening 87 Technology in Sports 580 (female OR women) AND technology in sports 323 (female OR women) AND athlete* AND technology in sports 285 Academic Database Search Strategies Database Search Terms b Filters Applied PubMed (“female” OR women) AND athlete* AND (“strength and conditioning”) AND (“blood ow restriction training” OR functional (assessment OR screening) OR technology OR sports) Free full text, Full text, Humans, English, Adult: 19–44 years, Female, from 2011–2021. Google Scholar Custom date range: 2011–2021. Sort by relevance. Articles: any type. Include citations. EBSCOHost c Full Text; Scholarly (Peer Reviewed) Journals; Published Date: 1 January 2011–31 December 2021; Language: English; English Language; Human; Sex: Female; Age Groups: Adult: 19–44 years; English Language; Human; Sex: Female; Age Related: Adult: 19–44 years. Expanders—Apply related words; Apply equivalent subjects. Search modes—Boolean/Phrase. Web of Science Timespan: 1 January 2011 to 31 December 2021 (Index Date). Languages: English. Note. JSCR:Journal of Strength and Conditioning Research; MSSE:Medicine & Science in Sport & Exercise. a “Articles” were selected for Content Type while “Last 8 Years” was selected for Publication Date for both journals. b Search terms were applied to all elds during database searches. c Academic Search Complete, CINAHL Plus, and MEDLINE were accessed via EBSCOHost

(Index Date). Languages: English. Note. JSCR:Journal of Strength and Conditioning Research; MSSE:Medicine & Science in Sport & Exercise. a “Articles” were selected for Content Type while “Last 8 Years” was selected for Publication Date for both journals. b Search terms were applied to all elds during database searches. c Academic Search Complete, CINAHL Plus, and MEDLINE were accessed via EBSCOHost at Northern Arizona University. A database search of the three trending S&C topics in female athletes was then con- ducted using PubMed, Google Scholar, EBSCOHost (Academic Search Complete, CINAHL Plus, and MEDLINE via Northern Arizona University's Cline Library), and Web of Science. Search terms and lters utilized are described in Table. An emphasis was placed on gathering publications regarding adult female athletes, between the ages of 19 and 44 years, who were a part of a professional, national, major regional, or collegiate organization. Other inclusion criteria included full-text availability, peer-reviewed journal article (excluding

Int. J. Environ. Res. Public Health2022,19, 2687 4 of 23 review articles or dissertations, for instance), and English language (see Figure criteria). Articles were excluded based on lacking focus on female athletes, inclusion of untrained or recreationally athletic participants, publication date earlier than 2011, and lack of relevance toward any of the trending topics chosen for review. Records were screened for duplicates and 534 publications were processed overall.Int. J. Environ. Res. Public Health 2022, 19, x 4 of 25 female athletes, inclusion of untrained or recreationally athletic participants, publication date earlier than 2011, and lack of relevance toward any of the trending topics chosen for review. Records were screened for duplicates and 534 publications were processed overall. Figure 1. Flowchart of study selection. 3. Blood Flow Restriction Training BFR training is used in both S&C and rehabilitative settings. BFR aims to occlude venous return to the heart while allowing some arterial blood flow to the occluded area [13]. The reduction of arterial blood flow causes an ischemic state within the muscles. By further eliminating venous return to the heart, BFR forces swelling in the muscle distal to the cuff placement. The reduction of arterial blood flow, ischemic state, and occlusion of venous return increase the metabolic stress within the working muscles and lead to swelling of the muscle cells [14]. Therefore, occlusion pressure, or cuff pressure, is an important factor to consider when using BFR. Many research studies use a protocol of measuring arterial occlusion pressure (in mmHg) using an ultrasound Doppler probe. Cuff pressure is then inflated on the individual at 50–80% of the pressure required for complete arterial occlusion. It is important to recognize that arterial occlusion pressure values vary from person to person and thus arterial occlusion pressure should be tailored to the individual female athlete. Cuff placement is another important factor to consider when implementing BFR. Re- Figure 1.Flowchart of study selection. 3. Blood Flow Restriction Training BFR training is used in both S&C and rehabilitative settings. BFR aims to occlude venous return to the heart while allowing some arterial blood ow to the occluded area

pressure should be tailored to the individual female athlete. Cuff placement is another important factor to consider when implementing BFR. Re- Figure 1.Flowchart of study selection. 3. Blood Flow Restriction Training BFR training is used in both S&C and rehabilitative settings. BFR aims to occlude venous return to the heart while allowing some arterial blood ow to the occluded area [13]. The reduction of arterial blood ow causes an ischemic state within the muscles. By further eliminating venous return to the heart, BFR forces swelling in the muscle distal to the cuff placement. The reduction of arterial blood ow, ischemic state, and occlusion of venous return increase the metabolic stress within the working muscles and lead to swelling of the muscle cells [14]. Therefore, occlusion pressure, or cuff pressure, is an important factor to consider when using BFR. Many research studies use a protocol of measuring arterial occlusion pressure (in mmHg) using an ultrasound Doppler probe. Cuff pressure is then in ated on the individual at 50–80% of the pressure required for complete arterial occlusion. It is important to recognize that arterial occlusion pressure values vary from person to person and thus arterial occlusion pressure should be tailored to the individual female athlete.

Int. J. Environ. Res. Public Health2022,19, 2687 5 of 23 Cuff placement is another important factor to consider when implementing BFR. Research protocols in the available literature typically place the cuff on the proximal portion of the limb, most commonly on the proximal thigh or arm. Proximal placement of the BFR cuff can affect distal muscle groups similarly to proximal muscle groups. BFR has been studied with both unilateral and bilateral cuff placement. Unilateral use of BFR has shown improvements on the contralateral limb that are similar to the improvements on the ipsilateral limb. This crossover effect suggests that results from the use of BFR may be systemic [15]. Various types of cuffs may be used in BFR, ranging from elastic bands to in atable cuffs. Researchers have used a variety of different products in the implementation and study of BFR, though most available literature on females uses a system with an in atable cuff that monitors and adjusts occlusion pressure during movements throughout the duration of the intervention session. Although BFR was used as far back as 1983 by the general public, it has more recently been adopted in clinical settings. Yoshiaki Sato was a pioneer in the development of BFR training through the use of the KAATSU training method in Japan, rst beginning in 1966 with self-experimentation [16]. Initial publications surrounding BFR have examined a predominantly male study population. The most widely cited reason for the exclusion of females in BFR studies is due to the menstrual cycle (MC) [17]. Few recent studies have sought to bridge the knowledge gap on the effects of BFR throughout the MC. One of the rst studies to evaluate the hemodynamic effects of BFR training in untrained females during phases of the MC highlighted the need for future research examining the relationship between BFR and the MC [18]. The effects of BFR training on females are becoming increasingly better known, though use of female study populations remains relatively low. Studies focusing on females and BFR training have utilized objective measurements including the hemodynamic response to BFR training, aerobic effects (e.g., effects on

MC highlighted the need for future research examining the relationship between BFR and the MC [18]. The effects of BFR training on females are becoming increasingly better known, though use of female study populations remains relatively low. Studies focusing on females and BFR training have utilized objective measurements including the hemodynamic response to BFR training, aerobic effects (e.g., effects on VO2max), and anaerobic effects (e.g., effects on muscular power), muscular strength, and muscular hypertrophy. Furthermore, studies have also analyzed subjective measurements such as soreness and ratings of perceived exertion (RPE). Effects of BFR training have been analyzed using various levels of intensities, length of interventions, disease status, age, and more. Hemodynamic responses to BFR have been measured using heart rate, cardiac output, and stroke volume. Low-load resistance training (RT) with BFR among older females has been shown to result in similar heart rate response and myocardial workload as high-load RT without BFR [19]. Among college-aged females, the use of BFR combined with low-load RT resulted in similar growth hormone and cortisol responses when compared to high-load RT without BFR [20]. Oxygen saturation rates (SpO2) are not signi cantly different when using BFR in low-load RT when compared to low-load and high-load RT without BFR [18]. The occlusion of blood that occurs is a key component of BFR, and may be responsible for the decreased amount of oxygen that is transported throughout the bloodstream, and is therefore available to muscles [21]. Table Published research regarding the use of BFR in female athlete populations between 19 and 44 years of age is rare. It should be noted that, while the search strategies in this review followed strict guidelines for inclusion and exclusion of studies, criteria were relaxed here as the database searches only yielded three relevant articles. The table and discussion below are thus mostly populated by recreationally active and/or college-aged females, rather than dedicated female athletes.

The review identifies trends in strength and conditioning for female athletes and suggests areas for future research.