Given the prevalence of those affected by mental health disorders (970 million worldwide) and reported increased rates of mental health-related issues since the COVID-19 pandemic, it should come as no surprise that our awareness of and attention to individual mental health has led to an upsurge in lifestyle management solutions in recent years (1). Of the lifestyle approaches available, physical activity and exercise are backed by a substantial amount of evidence that supports their benefits for depression, anxiety, psychological distress, and overall mood (1–3). Paradoxically, the pressure athletes experience in training and competitions can evoke a myriad of mental health issues including increased stress, anxiety, and depression (4). Hammond et al. (5) found the prevalence of depression among elite athletes (34%–68%) to be significantly higher than that reported in the general population (17% for adults aged 19–34) or intercollegiate athletes [21% (5);]. These mental health issues can deplete athletic performance, reduce the likelihood of competition success, and have a negative effect on an athlete's quality of life and general well-being.

Successful athletes must focus on psychological training that incorporates motivational techniques, goal setting, cognitive and emotional management, and mental focus training to obtain optimal performance (6, 7). It has been suggested that mental resilience training can help athletes mitigate their stress, anxiety, and depressive symptoms as well as optimize performance in sport and life (8). Building mental resilience (or mental toughness) helps improve self-confidence (i.e., one's sense of control) and mental stability, such that they remain relatively impervious to competition or hardship (8, 9). Athletes with a high level of mental toughness, more effectively manage training and competition demands while maintaining confidence and control under pressure (8, 9). In general, healthy coping is required to appropriately deal with stress, anxiety, and adversity (10). Consistent with the idea that athletes should incorporate psychological training alongside their physical training, these results underline the importance of including mental resilience methods and positive coping strategies in athletic training.

Some psychological and social aspects of physically intense, community-based programs like CrossFit have been shown to produce improvements in mental functioning amongst athletes (11–13). The introduction of psychological stress and other physiological variables to training paradigms like those experienced during breath-hold diving and training (14) may produce additional improvements in the mental health of athletes. By incorporating unique stress experienced underwater during breath-holds, Deep End Fitness (DEF) was designed to improve athletes' ability to employ top-down cognitive control techniques over emotional and physiological responses to stressful situations (15). This is achieved through a multi-modal training approach applied in inclusive, community workout sessions at public and private aquatic centers. Training includes breathwork exercises stressing nasal breathing and diaphragmatic breathing leading into breath-holds during land-based exercises before entering the water. The benefits of deep, diaphragmatic (or meditative) breathing are well-established and have been shown to reduce stress while improving heart rate variability, alpha brainwave activity, and top-down emotional control (16). Upon entering the water, training includes treading, static breath-holds, bobbing exercises, and breath-holds during underwater exercises that elicit psychological and physiological stress (fear) by producing “hunger for air” (17, 18). We hypothesize that teaching athletes to focus and accomplish tasks under this reflexive stress can translate to improved mental health and cognitive functioning. To begin testing this hypothesis, in the present study we investigated the influence of CrossFit and DEF training on mental health and coping outcomes in a group of multi-sport athletes.

In this study, we examined the psychological influence of a novel underwater fitness training paradigm by comparing the effects of CrossFit (CF) and Deep End Fitness (DEF) training on mental health outcomes in athletes. In general, we observed that both CF and DEF training produced positive mental health benefits. Our observations indicate however, that exposure to the unique psychophysiological stressors experienced when underwater, breath-holding during DEF training may be responsible for specific improvements in positive affect and increased problem-based coping compared to CF. The community-based nature of CF and DEF training likely contributed to some of the improvements we observed. In fact, community-based or group fitness programs have been shown to foster a sense of belonging and foster other health benefits (24–27). The sense of community has been shown to be one of the most appreciated benefits of individuals engaged in CF training (11). In other work, we have found that more than 70% of athletes participating in DEF training rank the sense of community as one of their favorite aspects of the workouts (data not shown).

We found that DEF training significantly reduced anxiety and stress in athletes, as well as improved positive affect and problem-focused coping. The “hunger for air” and physical and psychological stress experienced during DEF training presents athletes with unique mental and physical challenges (17, 18). DEF training involves the teaching of cognitive control, stress reduction, and focusing techniques that help athletes overcome some of the mental and emotional difficulties of breath-hold diving. Encouraging athletes to set goals for achieving underwater physical challenges and equipping them with a skillset to achieve these goals, builds confidence and reinforces problem-focused coping as indicated by our data (Figure 4B).

Other physiological aspects of submerged, breath-hold diving may have contributed to the specific improvements in coping strategies we observed following DEF training compared to CF. As mentioned above, DEF training facilitates the learning of strategies for enduring psychological and physical stress that require participants to remain focused and goal-oriented when completing submerged exercises. Underwater, an athlete's mammalian diving reflex (DR) is initiated through stimulation of the trigeminal and vagus nerves to produce bradycardia, vasoconstriction, and slow oxygen metabolism (28–32). Breath-hold diving and underwater immersion, as methods of natural vagus nerve stimulation, encourage physiological relaxation and perhaps reinforces learning. Recently, multiple lines of evidence demonstrate that pairing brief periods (seconds to minutes) of non-invasive electrical vagus nerve stimulation with cognitive or physical tasks enhances brain plasticity, as well as training and learning outcomes (33–36). This poses the intriguing possibility that natural, vagus nerve stimulation experienced during DEF training reinforces the use of certain cognitive strategies leading to the significant improvements in problem-focused coping we observed (Figure 4B). Other lines of evidence show that when conducted with proper training, supervision, and safety support, breath holding can produce distinct neurophysiological states, which can benefit performance under stressful conditions (14, 37).

Strict safety protocols and supervised training are required to engage in any aquatic activity especially breath-hold diving. Blackouts, narcosis, barotrauma, decompression sickness, drowning, and other pulmonary trauma are known adverse events to occur amongst deep free breathing divers (30, 38–41). Improper training, poor safety preparation, inadequate supervision, and other environmental factors are leading causes of these traumatic injuries, accidents, and adverse events (41–43). DEF training sessions are always conducted with at least two certified instructors and under the supervision of a lifeguard and safety divers. Every training session begins with a safety briefing and all participants must use buddy diving practices. The depth and length of time breath-hold divers go to also causes complications. DEF training occurs in pools with average depths of four to six meters and breath-holds typically do not exceed two minutes during workouts. Hyperventilation or “air-packing” techniques are not allowed during DEF training as these poor practices are known to increase hypercapnia and blackouts. We observed no blackouts during our study. Independently, we are working with Red Cross to improve education, regulations, and procedures to enhance drowning prevention and preparedness. Between 19% and 38% of individuals participating in CF have experienced an injury during training (44–46). These injuries are similar across a variety of other high impact sports. We propose the low-impact nature of DEF training, even during submerged breath-hold physical exertion, provides athletes with a safe and unique alternative to other high-impact, community-based training programs prone to injuries.

There are several limitations to our study and observations. First, we only explored the impact of CF and DEF training across multiple weeks. Future studies are required to explore the chronic influence of DEF training across month long time scales. Second, we examined the influence of CF and DF in a limited, but diverse group of conditioned athletes. It is worth pursuing studies of DEF in larger cohorts of specific athletes to determine if some sport disciplines are affected more than others. Similarly, future investigations are required to study the impact of DEF training on individuals, who are not competitively active in sports. Additional limitations of our study are imposed by the difficulty of matching physical exertion levels across CF and DEF training conditions. While DEF includes moderately intense, interval exercises underwater, CF includes high-intensity interval training on land. It is difficult therefore to draw more specific conclusions about differences between CF and DEF at this point. It will be important to treat physical exertion and workload as variables to truly understand how DEF differs from other training regimens. Investigations into the impacts of DEF on cardiometabolic activity are also important to pursue. Despite the limitations of our study, the data do provide initial evidence that DEF can produce positive benefits for athletes.

Although the athletes in the present study did not suffer from any diagnosed mental health disorders, we are planning studies to evaluate the impact of DEF training on athletes diagnosed with generalized anxiety disorder, post-traumatic stress disorder, and depression. These studies represent a major growing area of interest due to the crucial need for new solutions to the mental health crises amongst competitive athletes and the broader public. As such, practical applications of DEF training may be experienced in employee team building exercises, confidence building, executive coaching, and mental resiliency training. In ongoing efforts, DEF training is being applied in this manner to improve water confidence and mental resiliency in professional athletes, tactical operators, and first responders. Indeed our data indicate that DEF training may positively impact mental health in populations beyond athletes, which warrants further investigations.