Physical activity (PA) levels during childhood have declined as over 70% of American children do not participate in the Center for Disease Control and Prevention (CDC) recommended 60 min of PA, and roughly 20% are considered obese (1). Childhood physical inactivity hinders proper musculoskeletal strength as bones and muscles weaken through decreased limb use (2–5). Inactivity is highly problematic as muscular strength (MusS) is a dominant factor in producing neuromuscular control (NC), which is the brain's ability to recruit muscles efficiently for increased motor and movement skills and decreased injury (6–9). MusS is additionally essential for children due to its ability to help prevent diabetes, obesity, cardiovascular disease, bone mineral density, blood lipid profiles, insulin sensitivity, cancer, and mental health (10–14).

Despite CDC guidelines, childhood inactivity is still prevalent, and intensified as PA inequalities exist across cultural backgrounds, sex, and body fat composition, presenting significant developmental gaps as children mature (15–23). For many children, varying cultural backgrounds and social health determinants play a key role in PA engagement (24). These PA disparities exist due to socio-economic and cultural influences, where those who are more advantaged tend to be more physically active, less sedentary, and therefore, less likely to suffer adverse health conditions (15, 23). Unhealthy housing, unsafe neighborhoods, and fewer convenient community PA locations reduce PA opportunities for varied child populations (25–27). Children in lower-income households appear to have less opportunity for PA participation, creating more significant developmental gaps in fine and gross motor, cognitive, language, and socio-emotional skills as they mature (18). The American Heart Association's “2022 Life Essential 8” assessed the cardiovascular health (CVH) of 74,435 American children ages 2–19 revealed white children (77.2%) participated in more PA and muscle-strengthening activities and were less overweight/obese than Black (75%) and Hispanic (71.1%) children. The study also revealed 78% of the boys, on average, participated in healthy PA while 71.8% of the girls, on average, participated in healthy PA, and girls had a 1.6% higher average BMI than boys (26). Sex plays an important role in a child's PA involvement as boys, on average, participate in roughly 40% more moderate to vigorous PA than females (28). This is a serious disadvantage to females, as MVPA is highly influential in maintaining healthy physical development markers during childhood (29, 30). These statistics leave Hispanic females in a category for the least movement opportunities, and therefore the least physically active demographic group.

PA disparities such as these lead to an increased childhood obesity risk (16, 20, 31). This weight gain accumulation creates an overall PA decline compared to normal-weight non-obese counterparts (7, 32–35). Obesity not only stresses children's joints, it also alters gait mechanics and increased hip pain, further limiting PA levels (19). Decreased PA levels have left American children with less bone mass and consequently less MusS than children four decades ago, preventing a means for lifetime activity (1, 36). Without the MusS foundation, NC is greatly hindered as well, and the ability to advance in movement skills is stifled (6, 8). This is not a hopeful diagnosis for millions of children entering what should be considered the most active time of their lives.

The conditions in which children are born, live, and play impact their PA opportunities, but school settings can provide safe, equitable access, and developmentally appropriate PA programs and movement opportunities daily for many children (7, 15). There are different ways to incorporate movement in the school day, but school recess, or unstructured, outdoor play, or recess, has unique child development benefits that may transcend PA inequalities (37). Recess within the school day provides movement opportunities for children regardless of demographics, income, or local extracurricular opportunities offered. Recess provides the freedom of choice in movements participated in, which is essential for PA opportunities among children of varied demographics, sex or body fat percentages. Although recess provides opportunity for increased PA, not all stakeholders in education understand the value of this entity. Therefore, not all children have access to safe recess facilities, nor have policies in place to assure children are receiving adequate recess time.

This is very unfortunate, as recess provides children with a perfect space to be physically active, coordinate limb movements, and move kinesthetically, while engaging in single or combined motor tasks like walking, running, jumping, dodging, and climbing movements (38–40). This freedom to move leads to increased MusS development naturally through play, which in turn, provides the foundation for motor coordination of gross and fine motor skills (5, 39, 40). These movements during play provide the confidence to engage in more advanced movements over time (38, 40, 41). Research also shows increasing movement opportunities, such as recess during the school day, can improve musculoskeletal traits for up to four years after physical activities in school have ended (36, 42).Therefore, time spent in recess should be considered foundational for proper physical development and MusS during the critical developmental elementary years.

Although previous research has shown recess to be a viable means for limb movement utilization that could enhance muscular and neurological development (40), the long-term effects of recess on MusS development still needs to be established. One successful longitudinal recess intervention showing whole child development is the LiiNK Project® (Let's inspire innovation ‘N Kids). The LiiNK Project implements four 15 min recesses and one 15 min character lesson (Positive Action®⁾ daily in elementary and middle schools (43). Though PA is not the primary goal of this recess project, the LiiNK research team has found moderate to vigorous PA (MVPA) occurs significantly more, postural balance increases and motor competencies increase as children participate in more unstructured, outdoor recess (38, 44, 45). Although many whole child benefits have been found through LiiNK's longitudinal intervention, the MusS and NC recess benefits have yet to be studied.

Whether genetically or environmentally driven, demographics such as sex, race, and body fat play a role in PA behaviors, increasing or decreasing MusS and NC development during childhood. Access to quality movement opportunities is essential for MusS, NC, and the overall health of every child (32). Physical educators' ability to assess MusS and NC development within childhood, and across various interventions or physical education programs is equally essential. Therefore, the purpose of this study was to use the physical education setting to examine MusS and NC factors in the Fall and Spring (Time 1 to Time 2) of one school year in a predominately Hispanic sample of second-grade children who received 60 min (LiiNK intervention) or 20 min (control) of daily recess. It was hypothesized that children in the intervention group would demonstrate greater improvements in MusS and NC scores compared to children in the control group. A second hypothesis was the intervention's positive effects on MusS and NC would be consistent even when controlling for body fat percentages between groups. The independent variable for this study was the recess group and the dependent variables were the mean scores for each of the six tests administered (single hand grip, single leg 3-hop, push-up, vertical jump, side-step, and body fat analysis).

The purpose of this study was to use the physical education class setting to examine factors related to MusS and NC scores over time (Time, Time 2, average scores, and change scores) in a predominately Hispanic sample of second-grade children who received 60 min (LiiNK intervention) or 20 min (control) of daily recess. Research has shown Hispanic children, specifically Hispanic females, have less opportunity for PA and are more inactive than children of other races and sex (24–28). This study provides a unique contribution to literature with a focus on Hispanic children, movement opportunities accessible during the school day, and possible positive MusS and NC outcomes with the increased PA opportunities for this population of children.

Due to focusing on a predominantly Hispanic population across the two districts, race was not compared for differences. Conversely, sex and body fat were not found to be different at Time 1 or Time 2. As sex, race and body fat are shown in research to influence PA opportunities among children, the balanced sample allowed the intervention influence to be better observed (15, 16, 18–20, 23–25). Notably, we found consistent evidence for a positive association between increased time spent in daily recess with increased MusS and NC scores among this sample of predominantly Hispanic, second grade children.

LiiNK intervention children outperformed control children in Time 1, Time 2 and the average scores in single hand grip, the push-up and in the NC side-step (Figure 1). Although movement competency in early elementary years is essential for future movement skills, many studies report low levels of these skills among children (2–5, 62, 63). Second-grade children are a critical developmental age, as by age 7 they are expected to have engaged in adequate levels of movement competency such as running, jumping, kicking and throwing (64). Without this foundation, the ability to advance into more specialized skills such as sport or physical education is greatly hindered (6–9). As most children in this age group lack movement skills, the LiiNK intervention children were found to be more advanced in MusS and NC than their 20 min daily recess counterparts.

Controlling for Time 1, Time 2 scores were significantly better for the LiiNK intervention children in the single leg 3-hop (Figure 2). This significant improvement may indicate the extra time spent in recess readies both individual lower limbs for sport and PA while decreasing injury risk to the entire lower body chain (thigh, knee, ankle and foot). All MusS tests are important measures for assessing childhood fitness and MusS function, but the single leg 3-hop is unique in its assessment capabilities. The single leg 3-hop test is used to measure MusS and power in the individual lower limbs of pre-adolescent children; indicating sport and physical activity readiness, as low scores in the single hop test are associated with increased injury risk in the thigh and knee, ankle and foot (49, 56–58). As second grade children are in a pivotal motor development stage, the performance in the single leg 3-hop gives the LiiNK intervention children an advantage in lower-body motor development.

Controlling for Time 1, Time 2 scores were significantly better for the LiiNK intervention children in the side-step test measuring lower body NC (Figure 2). This means that LiiNK children, who are allowed 60 min of daily recess from early childhood on, may be more prepared cognitively and neurologically for advanced movements and sport skills by second grade. NC is the brain's ability to recruit muscles efficiently for increased motor and movement skills and decreased injury (6–9). Inactivity among children is problematic to NC, as development advances with limb movements (65). This finding once again highlights the importance of these brain-body connections needed by age 7 and the possible advantage to those who participate in more outdoor, unstructured free play.

Control children showed significantly better change scores in the push-up, yet they started and ended with a substantially lower percentage of children capable of completing at least one or more push-ups. By Time 2, only 53% of control children could complete at least one push-up, whereas 85% of the LiiNK children could complete at least one push-up. The push-up test was unique compared to the other four tests in that many control children scored a zero. This was not found in the other measures. The push-up test, as it recruits the core, arms, chest and back, is used in younger children as a discriminatory predictor of the potential participation in future athletic or advanced movement endeavors (66). Additionally, injury rates of children who participate in sport from ages 7–12 have increased in recent years, and is higher than adolescent and adult players (67, 68). This increase in child injury rates may be due to low skills and lack of MusS (69), once again highlighting the importance of MusS testing within this population. As upper body MusS is linked to decreased injury and increased sport readiness, it appears LiiNK children are more prepared for advanced upper-body movement with decreased chance of injury.

LiiNK intervention children shifted into healthier categories by 5% from Time 1 to Time 2 (overweight to healthy and obese to overweight), but had no shifts towards obesity. Control children shifted 2% from the healthy group to overweight or obese, had no change among the overweight group, and shifted 4% into the obese group from Time 1 to Time 2. Recess aids in increasing limb movements and MVPA for elementary-aged children (44, 61). Through recess, these PA increases may be more associated with lower body fat percentages in children (70, 71). Extra time spent in recess may be a factor in children shifting to the healthy body fat category and preventing a shift into the obese category over time.

When controlling for bodyfat, LiiNK intervention children continued to outperform control children in the single leg 3-hop. Increases of daily recess may allow children regardless of body fat percentages, to recruit and utilize lower body MusS to a significantly greater degree. Obese children are typically found to have less MusS, which hinders advanced movement skills, increases injury and decreases risk of all-cause mortality (7, 9–14, 19, 32–34). When controlling for body fat, children who participate in 60 min of daily recess maintained higher lower-limb MusS. This finding points to recess defined as unstructured, outdoor play as a potential PA opportunity to increase limb usage in children with higher body fat compositions especially in Hispanic populations.

PA discrepancies exist across race, sex and body fat percentages in children. A lack of equitable PA opportunities, where children of varied race, sex, and body fat compositions can participate is evident with the inactivity epidemic in America. Unstructured, outdoor play, or school recess, can provide an equitable PA opportunity due to its accessibility across demographics and its self-directed nature that spans comfort levels, cultures and intrinsic values of each individual child (72–74). The findings showed school recess to be a strong avenue for PA that has shown a preliminary positive influence on MusS and NC development among Hispanic, second grade children. This is an exceptional addition to the literature, as second grade students are at a pivotal developmental time in movement capabilities, where increases in MusS and NC set them up for more advanced sport and movement opportunities in the future, with less chance of injury. Whereas without this PA opportunity, typical trends within this age group point to movement deficiencies, leading to less MusS and increased injury. This study shows recess as a viable, equitable, PA opportunity across sex, race and obese children for MusS and NC advancement.

Feedback from the physical educators showed the MusS and NC tests were within the physical education class scope, and were simple to administer to students. For a class size of 50, the physical educator and one aid believed it would take no longer than one school week of regular scheduled physical classes (in Texas that is mandated at 150 min per week) to administer all tests. Students viewed the MusS and NC tests as fun “challenges” and repeatedly asked the PI and physical educators to repeat attempts for “fun” after the tests were completed.

Great MusS and NC gains were noted in this study among elementary children who received 60 min of daily recess, opposed to those who participated in 20 min of daily recess. To reap the benefits of whole child development, including appropriate MusS and NC development among a sedentary generation of children, it is recommended that elementary schools follow the United Nations Convention on the Rights of the Child (UNCRC) as a bare minimum recess policy (75): •

No less than 40 daily recess minutes separated into a minimum of two recess breaks daily

In order for the results of this study to be generalized into larger population groups, this study should be repeated with a much larger sample size within a physical education setting across various demographics and geographical areas. In these larger studies, it would also be beneficial to collect at a minimum three time points (as opposed to two), as well as continuing to assess the intervention longitudionally. Additionally, this study was intended for typically developing children in elementary schools. The studies effects on non-typically developing children (those who need adaptations or inclusion teachers to aid in physical education activities) would be beneficial.

Although supplies for the MusS and NC tests are roughly $120 total, and therefore assessable in terms of cost, the testing protocol may not be easily understood by all physical educators. Therefore, the development of a training manual with video descriptions of how to administer procedures will need to be created if the tests are going to be assessable to all physical educators.

Other limitations were found in testing administration. The PI, three trained assistants, and trained physical educators administered the test, nevertheless, an error occurred in the push-up test that caused the deletion of 17 samples from the intervention group. This error occurred because the physical educator counted the push-ups similar to class procedures instead of the research training procedures. In order to prevent this in future studies, the PI and physical education teacher need to communicate clearly what a FITNESSGRAM push up looks like for research purposes, opposed to classroom protocol.

The last limitation was found in the vertical jump training. The PI noticed discrepancies in the ability to administer this test properly during training. Therefore, the PI solely administered the vertical test to control the potential for error. The PI suggests the two-foot vertical jump be replaced with the two-foot broad jump for future studies.