Handgrip Strength and Healthspan: Impact of Sports During the Developmental Period on Handgrip Strength (Juntendo Fitness Plus Study)

Handgrip strength as a biomarker is being studied as a factor in predicting disease onset. However, the effect of improving handgrip strength through physical exercises, such as sports during the developmental period, on disease prevention has yet to be fully elucidated. The Juntendo Fitness Plus (J-Fit Plus) Study is a unique database of anthropometric and physical fitness measurements with over 50 years of accumulated data. It has the potential to explore the effects of sports on the association between handgrip strength and morbidity/mortality. We first outline previous studies on the impact of physical exercise interventions on handgrip strength, separated into adulthood and developmental period. We then introduced a unique effort to investigate the effects of sports using the J-Fit Plus Study database and describe the challenges of finally elucidating the impact of exercise on the association between handgrip strength and health status.


Introduction
Handgrip strength is a well-known biomarker of the aging process and health.Many studies have reported the health benefits of higher handgrip strength in middle-aged and older adults over the quarter of a century [1][2][3][4] . Fr example, recent largescale follow-up studies have repeatedly revealed the inverse association between handgrip strength and various diseases, such as heart diseases 1) , diabetes 2) , cancer 3) , and dementia 4) .Although these analyses were adjusted for several covariates (age, education level, body mass index, alcohol, tobacco, medical history, etc.), the association still exists [1][2][3][4] .In adolescents, low handgrip strength is also asso-ciated with poor cardiometabolic health outcomes 5) and premature death 6) .Therefore, achieving and maintaining a high level of handgrip strength may be useful 7,8) .However, low handgrip strength is observed across the lifespan 9) . Ifimproving handgrip strength through physical activity and sports is beneficial for future health, it may be helpful as a strategy for children and adults with low handgrip strength. Threfore, this perspective aims to discuss the following three points: 1) the impact of an exercise training intervention on handgrip strength in adults, 2) the impact of physical activity and sports on handgrip strength in children and adolescents, and 3) future research directions to elucidate the association between handgrip strength and health status.

Impact of exercise training intervention on handgrip strength in adults
Previous studies have examined the effects of nutrition alone, exercise alone, and combining the two on handgrip strength in middle-aged and older adults.For example, Bauer and colleagues 10) investigated the effect of dietary supplementation (i.e., vitamin D and leucine-enriched whey protein) on handgrip strength in 380 independent-living older adults.They reported that changes in handgrip strength did not differ from those in the control group after 13 weeks of the intervention.A metaanalysis has observed that nutrition alone has a small effect on handgrip strength (increased by 1.14 kg) compared to the control group 11) .
On the other hand, Labott and colleagues 12) investigated the effects of exercise interventions on handgrip strength in healthy community-dwelling older adults of 60 years or older.They reported that the impact of exercise interventions, including aerobic and resistance training in different types or multi-dimensional training, was statistically significant, but the change in handgrip strength was small (standardized mean difference = 0.28).However, because this systematic review and meta-analysis include different modes of exercise training, it is necessary to consider the impact of resistance training alone, which might be particularly effective.Several systematic reviews and meta-analyses have recently reported the effects of resistance training on handgrip strength in older adults [13][14][15] . Srprisingly, the impact of resistance training on handgrip strength in older adults was small (the standardized mean difference was less than 0.3).These results suggest that handgrip strength is unlikely to improve with regular resistance training during late adulthood 16) .If this is true, it is thought that increasing handgrip strength as much as possible during the process of reaching young adulthood is effective in maintaining high handgrip strength during adulthood.
Incidentally, in the previous studies investigating the association between handgrip strength and morbidity, the morbidity was observed by dividing handgrip strength into three or four groups (e.g., low, mid, and high).In these studies, the differences in cutoff values or mean values between the low and high handgrip strength groups were approximately 10-15 kg 1,3,17,18) .These values may help inform on what change might be required to see an improvement in handgrip strength (at least one that might be associated with health benefits).However, that is a value that needs to be updated through longitudinal studies.More importantly, it will be important to identify the minimum level of strength required for health (if any).

Impact of physical activity and sports on handgrip strength in children and adolescents
Resistance-type exercises for children and adolescents include lifting weights, bodyweight movements, and upper-body exercises performed during play from an early age.These physical activities are expected to contribute to developing upper-body and lower-body muscular strength, including handgrip strength for children and adolescents.A recent review 8) investigating the impact of physical activity and sports on handgrip strength in children and adolescents compared changes in handgrip strength in children engaged in extra physical activity and sports (i.e., intervention) to normal development (i.e., control).Types of interventions included family-and school-based physical activity and upper-body resistance training.The children and adolescents of the intervention groups increased the intensity and amount of physical activity at home or school, but no additional effects on handgrip strength were observed 8) .Similarly, boys and girls ages 7-12 who underwent eight weeks of upper and lower body resistance training had no additional improvement in handgrip strength 8) .This might suggest that using resistance training equipment for children is not enough of a stimulus for the agonist muscles of the finger flexors in the forearm and hand.
However, a recent study reported that handgrip strength might increase to a greater extent in those who participated in upper-body exercise (including gripping) during active play as children 19) .There are sports where players hold equipment in their hands and those in which they do not.Therefore, we were interested to know what types of sports are more effective for improvement in handgrip strength during the developmental period.
Fortunately, Juntendo University stores the results of handgrip strength measurements taken by students in school, and all first-year students undergo a handgrip strength test.This data only allows cross-sectional comparisons among sports events (i.e., an inability to know how much strength changed in response to the actual sport).However, first-year students usually have specialized in practicing one sport during their junior high and high school days.

Analysis using Juntendo Fitness Plus (J-Fit Plus) Study data
Faculty of Health and Sports Science, Juntendo University has conducted anthropometric, physical fitness, and motor ability measurements, including handgrip strength, as part of the university curriculum, and the test results have been available since 1973.We used the J-Fit Plus Study data from 1973 to 2018 to investigate the effects of the type of sports practiced on handgrip strength in first-year sport university students.The faculty of Health and Sports Science used to be exclusively male students until 1991, and the initial analysis used data only for males 20) .Privacy measures were maintained through Juntendo University, and all data were anonymized before analysis.We selected two types of sporting events with matching physiques (i.e., height and body mass), soccer (n=1127) targets the lower body, and kendo (n=297) and baseball (n=698) use the lower body simultaneously with upper body movement (including gripping).As a result, those in the lower body-only (soccer) sports had -3.78 (95% CI: -4.27, -3.29) kg lower handgrip strength than those in the lower + upper (kendo and baseball).Comparing each individual sport found that each sport was different from each other with kendo > baseball > soccer (between each sport, p<0.001) 20) .The difference in handgrip strength between kendo and soccer was about 5 kg in the overall sample, but the difference between the two groups has widened since the beginning of data collection.
Next, we tried a similar comparison using data from female athletes.The J-Fit Plus Study database from 1996 to 2018 includes 2301 first-year female students, and we selected two sporting events, i.e., soccer (n=161) and kendo (n=53).The mean and standard deviation of age, height, and body mass in the soccer and kendo groups was 18.3 (0.9) years old, 159.7 (5.5) cm and 54.8 (5.8) kg, and 18.1 (0.3) years old, 159.2 (5.2) cm and 58.0 (7.7) kg, respectively.The handgrip strength of female kendo athletes was 34.0 (4.3) kg, which was significantly higher than that of female soccer athletes [27.9 (4.4) kg; difference of 6 (95% CI: 4.7, 7.4) kg].The results did not appreciably change after height and body mass adjustment.Although these findings are cross-sectional, our female and male results suggest that performing sports activities with upper-body gripping movements may be able to augment strength during the developmental years.It is noted that the sports we selected in this study are natural activities that do not directly train handgrip strength.

Future tasks
There are associations between handgrip strength and markers of health [1][2][3][4][5][6] . Hwever, the overarching question is whether increasing handgrip strength would lead to an improvement in health.Unfortunately, there is not enough evidence at the moment.
In adults, gripping exercise may improve handgrip strength, but this might not influence an individual's health status 8) .At the same time, conventional (traditional) resistance exercise does not have a measurable influence [12][13][14][15] . Terefore, handgrip strength acquired in early adulthood is unlikely to change, except for the influence of aging and illness.In addition, it is reported that low to moderate intensity (30%-50% of maximal voluntary contraction) isometric grip exercise training is associated with decreased blood pressure in men and women 21) .However, this improvement in blood pressure is unlikely to be related to how strong (i.e., improving handgrip strength) someone can get.To the best of our knowledge, no studies have observed the impact of high-intensity grip training on risk factors for lifestyle-related diseases.
On the other hand, during the developmental period, sports such as kendo and baseball, which involve holding a tool with the hand, may positively affect the development of handgrip strength in children and adolescents 20) .However, one of the critical future issues is whether the improvement in handgrip strength obtained by sports activity during the developmental period will change in later adulthood (especially the impact of stopping sports) 20) .In addition, the J-Fit Plus Study is cross-sectional data and does not compare the changes in handgrip strength in each sport during development, even though athletes have been engaged in a single sport.Thus, it is necessary to confirm whether participating in these sports is able to augment handgrip strength over that of normal development using a longitudinal design.Although our cross-sectional work suggests this, it is still possible that those with better handgrip strength self-select sports that require a gripping component (i.e., kendo).Finally, it is possible to observe the morbidity of various diseases and mortality using the difference in handgrip strength according to sports events recognized in the J-Fit Plus Study data.As these studies develop, it may be possible to clarify what utility handgrip strength has as a biomarker.