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The purpose of this review is to provide a detailed and updated description of the FinnTwin16 (FT16) study and its future directions. The Finnish Twin Cohort comprises three different cohorts: the Older Twin Cohort established in the 1970s and the FinnTwin12 and FT16 initiated in the 1990s. FT16 was initiated in 1991 to identify the genetic and environmental precursors of alcoholism, but later the scope of the project expanded to studying the determinants of various health-related behaviors and diseases in different stages of life. The main areas addressed are alcohol use and its consequences, smoking, physical activity, overall physical health, eating behaviors and eating disorders, weight development, obesity, life satisfaction and personality. To date, five waves of data collection have been completed and the sixth is now planned. Data from the FT16 cohort have contributed to several hundred studies and many substudies, with more detailed phenotyping and collection of omics data completed or underway. FT16 has also contributed to many national and international collaborations.

INTRODUCTION Participation in diverse physical activities has beneficial health effects. However, little is known on how genetic and environmental factors affect this trait. Thus, we examined to what extent these factors explain participation in diverse leisure-time physical activities from late adolescence to adulthood using a twin study design. METHODS The participants were Finnish twins who reported their participation in diverse leisure-time physical activities at ages 17 (n = 5429) and 34 yr (n = 4246). The number of physical activities engaged in was analyzed using applications of structural linear modeling for twin data. RESULTS On average, the total number of physical activities engaged in during leisure time was slightly over three at both ages and in both sexes, with moderate heritability estimates (40%-58%) from adolescence to adulthood. Environmental factors shared by co-twins (e.g., childhood family environment) influenced only in adolescence, being higher for women. Environmental influences unique to each co-twin explained the remaining variances (34%-57%), being higher at age 34 yr. Participation in diverse leisure-time physical activities correlated moderately between ages 17 and 34 yr (men: rtrait = 0.30, 95% confidence interval [CI] = 0.25-0.35; women: rtrait = 0.26, 95% CI = 0.22-0.31). In addition, genetic influences on participation in physical activities correlated moderately between adolescence and adulthood (rA = 0.51, 95% CI = 0.39-0.64, and 0.44, 95% CI = 0.34-0.55, respectively). These common genetic influences explained 93% of the trait correlations found in men and 85% in women. CONCLUSIONS Genetic and unique environmental influences explain a large proportion of variation in the number of leisure-time physical activities. However, the estimates vary by age and sex. Common genetic background mainly explains the continuity of the participation in diverse leisure-time physical activities between adolescence and adulthood.

Leisure-time physical activity has a complex relationship with low back pain (LBP). Thus, we aimed to investigate whether the diversity of sport activities is associated with the type of LBP. In the FinnTwin16 study, 4246 (55% females) Finnish twins at mean age 34.1 years replied to a health behaviour survey in 2010-2012. Based on the participation in different sport activities, we created two measures of diversity: quantity (i.e. the number of sport activities: 1, 2, 3, 4 and >= 5) and quality (i.e. the type of sport activity: endurance, strength, body care, etc.). Based on the frequency, duration and type of LBP, we created three groups: no history of LBP lasting more than one day, radiating LBP and non-radiating LBP. The associations between the quantity and quality of sport activities and the type of LBP were investigated with logistic regression analyses. Participation in >= 5 sport activities associated with less radiating and non-radiating LBP in analyses pooled across sex (odds ratio 0.46, 95% CI 0.30-0.69 and 0.66, 0.44-0.99, respectively). However, the associations attenuated after adjusting for several confounders. Participation in endurance sports was associated with less radiating (0.58, 0.43-0.76) and non-radiating (0.60, 0.44-0.81) LBP, whereas strength sports and body care only with less radiating LBP (0.76, 0.58-1.00 and 0.26, 0.09-0.74, respectively) adjusted for all sport types. On a sport-specific level, running and cycling were associated with less radiating and non-radiating LBP. In adulthood, the diversity of sport activities, particularly participation in endurance sports, may be associated with less radiating and non-radiating LBP.

This study investigates cross-sectional and longitudinal associations between the diversity of leisure-time sport activities and the frequencies of low back pain (LBP) and neck-shoulder region pain (NSP) in twins, including a cross-sectional within-pair design to adjust for potential familial confounding. Finnish twins born in 1975–79 (FinnTwin16 study) reported participation in leisure-time sport activities at the mean ages of 17 (1992–96) (n = 5096, 54% females) and 34 years (2010−12) (n = 3731, 57% females). Diversity assessed as the number of sport activities was categorized as 1, 2, 3, 4, and ≥ 5, excluding inactive individuals. The frequencies of LBP (n = 3201) and NSP (n = 3207), reported at age 34, were categorized as never/seldom, monthly, or weekly pain. Cross-sectional and longitudinal individual-based associations between the number of sport activities and the frequency of LBP and NSP were investigated with multinomial logistic regression analyses, adjusting for multiple confounders. Cross-sectionally, participation in ≥5 sport activities, compared to 1 sport, was associated with significantly less weekly LBP (OR = 0.63, 95%CI = 0.43–0.90), but not with NSP. Longitudinally, participation in several sport activities in adolescence had no significant association with LBP or NSP in adulthood. Cross-sectional within-pair analyses were conducted among twin pairs discordant for LBP (n = 507) and NSP (n = 579). The associations between monozygotic and dizygotic twin pairs were similar in LBP-discordant pairs but differed within NSP-discordant pairs. Participation in ≥5 sport activities in adulthood may be associated with less weekly LBP, but not with monthly LBP or the frequency of NSP. However, within-pair analyses for NSP suggest confounding due to shared familial factors., Highlights • Participation in more sport activities is associated with less low back pain (LBP). • More sport activities is not associated with neck-shoulder pain (NSP). • Shared familial factors may confound the association between sport activities and NSP. • Number of sport activities in adolescence did not predict LBP or NSP in adulthood.

Diverse physical activity is globally recommended across all age groups but is sparsely studied. Previous literature has focused on the dose of physical activity that is required to achieve numerous health benefits, and links have been found between earlier and current physical activity, as well as physical activity and spinal pains. Simultaneously, evidence indicates that frequent participation in only a single sport increases the risk of overuse injuries, burnout, and even dropout from sports among young athletes. Frequent spinal pains are related to both low and high intensity of physical activity, as well as to specific risk sports among athletes. Similar population-based evidence is scarce. This thesis aimed to study the diversity of sport activities in adolescence and leisure-time physical activity in adulthood, as well as, associations between the diversity of sport activities and spinal pains, including low back pain and neck-shoulder region pain, in adulthood. This thesis includes three studies based on the FinnTwin16 study of Finnish twins born in 1975–79. The first survey wave took place in 1991-1995 when twins were 16 years of age and 4 follow-ups have occurred since. The second and fifth survey waves, conducted when twins were 17 and 34 years of age on average, provided information on participation in different leisure-time sport activities. Additionally, the fifth wave included more items on leisure-time physical activity behavior and several items on spinal pains including non-specific low back pain and neck-shoulder region pain, as well as, radiatiang and non-radiating low back pain lasting more than one day. Categorized variables were created for the quantity and quality of sport activities, leisure-time physical activity level, and spinal pains. Aiming to study the participation in a diversity of sport activities in comparison to single sport, the studied sample included only individuals who engaged in leisure-time physical activity at least once month and reported at least one sport activity at the wave of interest. The chosen sample was large and representative with 3734 individuals (57% females). The studies included cross-sectional, longitudinal, and within-pair analyses. Participation in five or more sport activities in adolescence was related to higher levels of leisure-time physical activity in adulthood, but only among females. Shared familial factors, however, seemed to confound the detected association. Neither participation in several sport activities in adolescence nor adulthood was associated with neck–shoulder region pain in adulthood. In contrast, participation in several sport activities in adulthood was related to less weekly low back pain among both sexes in cross-sectional but not in longitudinal design. Familial factors did not seem to confound the detected cross-sectional association. In further cross-sectional investigation, participation in endurance sports was related to less both radiating and non-radiating low back pain in adulthood. Overall the findings provide moderate support for additional health benefits related to participation in a diversity of sport activities compared to single sport activity. Promoting participation in several sport activities during adolescence may help to better maintain leisure-time physical activity levels through the transition from adolescence to adulthood, especially among females. In adulthood, participation in several sport activities, especially in endurance sports, may be related to a lower prevalence of weekly low back pain. Future studies should confirm and further investigate the direction of the detected associations in longitudinal study designs including objective measurement of leisure-time physical activity and standardized measures of low back and neck–shoulder region pain. Furthermore, the contribution of the shared familial factors on the observed associations remains unexplored. Monipuolista liikuntaa suositellaan kaiken ikäisille, mutta liikunnan monipuolisuutta on tutkittu vain vähän. Aiempi tutkimus on keskittynyt selvittämään terveyshyötyjen saavuttamiseksi vaadittavaa liikunnan määrää. Yhteys on löytynyt aiemman ja nykyisen liikunta-aktiivisuuden välille sekä liikunnan ja selkäkipujen välille. Nuorilla urheilijoilla toistuva, yksipuolinen harjoittelu on liitetty vammautumis- ja loppuunpalamisriskiin, jopa urheilun lopettamiseen. Toistuvat selkäkivut on puolestaan yhdistetty sekä vähäiseen että hyvin runsaaseen liikunta-aktiivisuuteen ja muutamiin riskilajeihin urheilijoiden keskuudessa. Vastaavaa tietoa väestötasolta on vähäisesti. Tämän väitöskirjatyön tavoitteena oli tutkia nuoruuden monipuolisen liikunnan yhteyttä aikuisiän vapaa-ajan liikunta-aktiivisuuteen sekä liikunnan monipuolisuuden yhteyttä alaselän ja niska-hartiaseudun kipuihin poikittais- ja pitkittäisasetelmissa. Tämä väitöskirja koostuu kolmesta osatyöstä. Aineistona on Nuorten Kaksosten Terveystutkimus, johon kuuluvat suomalaiset vuosina 1975-79 syntyneet kaksoset. Ensimmäinen kysely toteutettiin vuosina 1991-95, kun kaksoset olivat 16-vuotiaita. Tämän jälkeen on toteutettu neljä seurantakyselyä. Toinen (1992-96) ja viides (2010-12) kysely keräsivät tietoa eri vapaa-ajan liikuntalajeihin osallistumisesta. Lisäksi viides kysely sisälsi kysymyksiä epäspesifistä alaselän ja niska-hartiaseudun kivusta sekä yli yhden päivän kestäneestä säteilevästä ja säteilemättömästä alaselän kivusta. Luokitellut muuttujat luotiin liikuntalajien määrälle ja tyypille, vapaa-ajan liikunta-aktiivisudelle ja selän alueen kivuille. Tavoitteena oli tutkia liikunnan monipuolisuutta verrattuna yhteen liikuntalajiin, joten tutkimusaineistossa mukana olivat vain ne, jotka osallistuivat vapaa-ajan liikuntaan vähintään kerran kuussa ja ilmoittivat vähintään yhden liikuntalajin. Valittuun otokseen kuului 3734 yksilöä (57% naisia), joten sitä voitiin pitää suurena ja edustavana. Osatöissä tehtiin poikittaisia, pitkittäisiä ja pareittaisia analyysejä. Osallistuminen viiteen tai useampaan liikuntalajiin nuoruudessa oli yhteydessä korkeaan liikunta-aktiivisuuteen aikuisiällä, mutta vain naisilla. Jaetut perhe- ja perintötekijät saattavat vaikuttaa tähän yhteyteen. Osallistuminen moneen liikuntalajiin nuoruudessa tai aikuisiällä ei ollut yhteydessä niska-hartiaseudun kipuihin aikuisiällä. Sen sijaan osallistuminen viiteen tai useampaan liikuntalajiin oli yhteydessä vähäiseen viikoittaisten alaselkäkipujen esiintymiseen miehillä ja naisilla poikkileikkaus-, mutta ei pitkittäisasetelmassa. Perhe- ja perintötekijöiden ei todettu vaikuttavan poikkileikkausasetelmassa havaittuu yhteyteen. Tarkemmin alaselän kipuja analysoineessa poikkileikkaustutkimuksessa kestävyysliikuntalajit olivat yhteydessä vähäiseen säteilevään ja säteilemättömään alaselän kipuun. Kaikkiaan löydökset antavat kohtalaista tukea sille, että lisää terveyshyötyjä voi saavuttaa liikunnan monipuolisuudella yhteen liikuntalajiin verrattuna. Erityisesti tyttöjen kannustaminen useiden liikuntalajien pariin nuoruudessa voi auttaa heitä paremmin säilyttämään liikunta-aktiivisuutensa siirryttäessä nuoruudesta aikuisuuteen. Aikuisiällä osallistuminen useaan liikuntalajiin ja etenkin kestävyysliikuntalajeihin voi liittyä vähäiseen alaselän kipujen esiintymiseen. Tulevaisuudessa tässä väitöskirjassa löydettyjen yhteyksien suunta tulisi vahvistaa pitkittäistutkimuksissa, jotka hyödyntävät objektiivisesti mitattua liikunta-aktiivisuutta sekä standardoituja selän kipumittareita. Lisäksi tarvitaan lisätutkimuksia selvittämään yhteisten perhe- ja perintötekijöiden osuus löydettyjen yhteyksien taustalla.