Most of an individual's body mass is located in the upper two-thirds of the body, thereby creating an unstable design that is in constant need of correction to maintain upright erect balance (36). This design is governed by a complex and multifaceted coordination involving the neuromuscular afferent and efferent systems producing the necessary responses to keep the center of mass within the base of support (36). This upright maintenance of balance is not only important for humans to participate in daily activities of living but also critical in competitive sports. Sporting activities challenge the body's postural control system and require the individual to maintain balance in both for effective completion of the sporting task.
Static and dynamic balance is defined as maintaining balance under static-nonmoving and dynamic-moving conditions, respectively (28), both of which are vital to sports and can be trained for, individually and collectively. The nature of the sport played determines the role and importance of these 2 types of balance maintenance (3,9). Multiple factors affect the maintenance of balance and postural control, both intrinsically and extrinsically. Intrinsic factors are within the human body and comprise the physiological changes that affect sensorimotor function, central integration, and specific motor responses. Such changes could be a result of aging, total body mass, speed of walking, fatigue, and more. Extrinsic factors are outside the human body and comprise the physical characteristics of the ground surface and/or the surface with which the foot is in contact.
Human balance performance has been assessed in multiple populations, including healthy adults (2), clinical (30,34), aging (7), occupational (8,9), and athletic populations (15,21,24). Research among athletes have ranged from different sports such as football, soccer, volleyball, basketball, gymnastics, dance-ballet, track and field, etc. Additionally, research with balance has focused on different levels of sport including recreational, amateur, high-school, NCAA (National Collegiate Athletic Association) and professional sports (14,21,24). Balance research among these individuals have been both gender inclusive (32), where studies have analyzed both male and female athletes, and gender specific (6,9,21), where studies have analyzed male or female athletes individually. Specifically, female athletes have been shown to have a greater incidence of lower-extremity musculoskeletal injuries and greater ligament injury risk than the male athletes participating in similar sporting events (19,31). Moreover, the importance of balance training and rehabilitation after such injuries is critical in returning back to the sport. Hence, it is clear that balance performance and training is a crucial component of athletic and sporting performance. However, even with extensive research, there is a wide variety of balance assessments performed and balance training prescribed to female athletes. Furthermore, with increasing participation of women in sports and with the advent of modern technology, this article will provide coaches and practitioners a review of these different types of balance assessments and training recommendations among female athletes that are both unique to the sport played and to the general female athletic population (see supplemental video abstract, Supplemental Digital Content 1, https://links.lww.com/SCJ/A179).
BALANCE PERFORMANCE VARIABLES
There have been various procedures used to quantify balance using relatively simple subjective assessment tests and tools and using more technological objective measures. The subjective balance assessments include assessment tests that are commonly and readily used off-field and on the sidelines, inpatient and outpatient clinics and athletic training and rehabilitation centers, such as the Berg Balance Scale, Tinetti Performance-Oriented Mobility Assessment, Romberg's test, Star Excursion Balance Test (SEBT) (Figure, A), and the Y-balance test (YBT) (Figure, C). These tests have already been discussed in detail elsewhere (4,5,10,16), and this review article will focus on the more recent types of balance assessment in athletes. The more objective tests that use modern technology include assessment of balance under static and dynamic conditions but rely on kinematic and kinetic data to quantify balance, especially in a laboratory setting. These modern balance tests using force plates and computerized dynamic posturography (Figure) can simulate challenging conditions such as a moving surface and visual surround to test balance and postural stability under changing or altering environmental conditions.
Recently, balance machines (Figure, A and D), force plates, and computerized dynamic posturography machines have been heavily relied on because of their low subjective error in assessing balance. A centroid of force application called the center of pressure (CoP) patterns derived from a force plate (17) and kinematic trajectories form accelerometers (1) have been used to quantify human balance performance. The changeable positions of the CoP are registered and calculation of CoP velocity and amplitude is made possible, from which an increased measure of the CoP velocity and amplitude can be interpreted as decreased stability or poor balance. The most common measures used for the assessment of postural stability are the CoP excursions. Center of pressure excursion velocity and area determine the amount of decline in the postural control. A novel approach for this assessment was the time-to-boundary measures of the CoP excursions (17), which is an estimate of the time it takes for the CoP to reach the boundary of the base of support. A lower time-to-boundary measure can be interpreted as postural control instability because of the reduced time available to execute a postural correction (17).
Dynamic posturography involves creating various conflicting sensory conditions by rotating the surface platform and/or the visual surrounding in proportion to the subject's postural sway. This has been used extensively in assessing human balance and posture, especially in clinical practice to differentiate disturbances of vestibular, visual, and proprioceptive functions, including central coordination (22). The sensory organization test (SOT) on the Neurocom Equitest Balance System (NCBS) evaluates the integrity of the 3 sensory modalities by selectively disrupting somatosensory and/or visual information. The contribution of each sensory input toward equilibrium can be ascertained by measuring the equilibrium adjustments of a standing subject, where input from the visual, support surface, and environment is recorded by presenting conflicting visual, proprioceptive, and vestibular stimuli (14). This type of balance assessment gives practitioners a very precise evaluation of the athlete's postural control. Such assessments can be conducted during initial pretraining, during training, after an injury in rehabilitation, and during and after rehabilitation to have estimates of the injury prognosis.
RESEARCH ON BALANCE ASSESSMENTS IN FEMALE ATHLETES
A few studies have shed light on the importance of balance performance in female athletes. Research among female athletes has focused on different sports played and has used different assessment protocols. The goal of this section is to provide a condensed summary of such research (3,6,9,11–13,20,25,26,29,31,33,35) (Table). Acute differences in balance performance were analyzed among basketball players, soccer players, and gymnasts using the balance error scoring system to test static balance and the SEBT to assess dynamic balance (6). This study found that basketball players had worse static balance compared with gymnasts and worse dynamic balance compared with soccer players, and it was suggested this was due to sport specificity of gymnastics and soccer using more single-leg movements. Another study compared soccer, volleyball, and dance athletes' static and dynamic balance performance using the SOT and motor control test by means of the NCBS (9). The authors found that soccer players had worse static balance compared with volleyball and dance groups; however, soccer and volleyball players performed better in dynamic balance compared to dancers. Some researchers compared single-sport athletes and multisport female athletes (13) in dynamic balance using the YBT and found no differences between the 2 groups. Balance differences in these athletes were attributed to the nature of the sport and the type of training.
Research on balance training and their effects on balance performance among female athletes has also been conducted (11,29). One study had soccer players perform an 8-week neuromuscular training program, targeting various lower-body core exercises, and measured balance before and after training using SEBT (11). The researchers found that the 8-week training program improved dynamic balance in those that completed the training program. Another study implemented a 13-week Indo Board training program with volleyball players and soccer players acting as a control group. Balance was recorded before and after using the BioDex Balance System for both groups and showed that the intervention group increased their stability measures (29). These studies have shown that training for balance will most likely improve balance performance and can potentially reduce injury in female athletes (11,29). Overall, it is agreed that balance performance is different among sports with the need to train specifically for static and dynamic balance and that training for balance improves performance and helps in reducing injury risk.
IMPLEMENTATION OF BALANCE TRAINING
As previously established, multiple research studies have shown that a balance training program will improve the individual's balance performance and aid in injury prevention. Although this is true, different types of balance training have been used to gain improvements. A 5-phase balance training program, including 5 exercise sessions per week for 4 weeks (phases 1–4) and 3 exercise sessions during the final maintenance week (phase 5), among high school soccer and basketball players reduced the risk of ankle sprains. The exercise sessions included single-leg stance in eyes open and closed conditions, dribbling, squatting and swinging on a single leg, and performing functional activities (23). Balance and neuromuscular training among female athletes have also been shown to reduce the overall incidence of serious knee injuries (18). The recommendation for young female athletes in sports is to use a training program that entails jumping, pivoting, and cutting, which are a set of similar occurrences in sports such as soccer, volleyball, and basketball (18). Similar neuromuscular training programs have also been shown to be effective in female handball players in improving and maintaining dynamic balance performance even 1 year after training (20).
Balance training is often administered with an unstable surface to challenge the proprioceptive and somatosensory balance systems. Long duration balance training has also been shown to be effective to improve balance performance among female athletes. Myer et al. (26) compared a 7-week plyometric and a 7-week balance program on female athletes and demonstrated an improvement in the athlete's neuromuscular power and control. The successful balance training protocol used both the flat and round side of a BOSU ball and involved multiple exercises such as single and double-leg squats, hops, deep holds, and resisting perturbations (26). Forty-one female basketball, soccer, and volleyball players were trained for 6 weeks with a combination of plyometrics, core strengthening and balance, resistance training, and speed training, with the balance training incorporating standing, jumping, squatting, and hopping exercises that focus on neuromuscular control using the BOSU and Swiss ball. The results suggest a comprehensive training protocol that includes balance training can improve performance (27). Another study showed an improved balance performance in SEBT after a biweekly, 8-week neuromuscular training program (11).
Coaches and trainers can formulate a training protocol using the above suggestions and incorporating balance training among female athletes in an attempt to improve performance and reduce injuries. The authors suggest incorporating balance training during strength and conditioning sessions; however, the type of exercises may need to vary based on sport demands. Balance training exercises should include but not be limited to double- and single-leg squats, hops, deep holds, resistive perturbations, and possible variations using a BOSU ball. Exercises during training should be modified for the athlete's ability to complete the tasks successfully with still being challenged.
The purpose of this article was to provide a condensed summary of the different types of balance performance, assessment, and training among female athletes and to emphasize its importance in improving performance and preventing any undue injuries. Balance assessments among female athletes with different sporting backgrounds can also be used as a screening tool during preseason, in-season, and postseason to identify individuals at a greater risk of injuries because of balance decrements. This article also emphasizes the importance of including balance assessments and training as a routine protocol in the female athlete's strength and conditioning program. The addition of a balance training aspect is crucial in training and a comprehensive neuromuscular training program that includes components of plyometrics, strength and resistance training, and balance and core training can have simultaneous performance gains and injury reductions.
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