Throughout the lifespan, maximum hand grip strength reflects general health and vigor and is therefore commonly suggested to be a biomarker of aging. Although aging results in declines in muscle strength, individual strength changes follow a predictable pattern, as shown by strong correlations between measurements carried out decades apart. Family and twin studies have also shown that muscle strength is a moderately (30-65%) heritable phenotype. This suggests the genetic component of maximal strength may be used to characterize the intrinsic capacity for lifetime health.
Previous genetic studies have not been able to estimate genetic variation in muscle strength, which is a multifactorial phenotype. This means that variation in muscle strength is explained by several environmental factors, including exercise, but also by the contributions of hundreds or thousands of genetic variants, each of which has a small effect size. Using novel polygenic scoring methodology, the measured genetic variation can be summarized in a single score, usually called a polygenic risk score or a polygenic score. The score describes an individual’s genetic liability to a trait or disease, in this case genetic liability to muscle strength.
In our study, published in the November 2022 issue of Medicine & Science in Sports & Exercise®, we constructed a polygenic score for grip strength and validated it against measured grip strength and heritability estimates derived from an independent twin study. We further showed that polygenic score for grip strength also predicts variation in other measures of muscle strength and can be used as an estimate of muscle strength genotype.
The polygenic score summarizes variation of over 1 million nucleotides and was able to explain 6% of the variation in grip strength and 5% in knee extension, respectively. The predictive value of the polygenic score for grip strength was significantly higher than what has been observed for physical activity polygenic scores, possibly due to the ease of obtaining standardized grip strength measurements.
In our study we also showed that a muscle strength genotype that supports higher muscle strength was associated with better physical functioning and lower risk of functional limitations among older women. This suggest that genotype may have significant impact on health throughout the lifespan, at least in functional tasks that require good muscle strength.
While the construction of polygenic scores requires very large, genotyped datasets, after validation, scores can be calculated and used in any dataset that has been genotyped. Polygenic scores may, for example, be used in epidemiological designs to partly adjust for underlying genotype, to study how genotype affects training response or to study gene-lifestyle-disease interactions. In health promotion, these scoring methods can possibly be used for screening high-risk individuals, who need more intensive monitoring with respect to functional limitations or specific diseases. However, to date, we have a very limited amount of information on how genotype affects individual responses to treatment. Therefore, careful ethical consideration is needed before scoring methodology is used in health care.
Elina Sillanpää, PhD, is an associate professor of health promotion and Academy of Finland research fellow working in the faculty of sport and health sciences at the University of Jyväskylä, Finland. She has a Ph.D. in sport sciences. Dr. Sillanpää leads a research group that focuses on genetic and molecular studies of physical activity and exercise in relation to biological aging, functional disabilities and aging-associated diseases, using advanced statistical methods and novel bioinformatics to analyze gene-environment interactions. Dr. Sillanpää is an active member of large consortiums, including the Interplay of Genes and Environment Across Multiple Studies (IGEMS) and the Genomics and Biology of Physical Activity Consortium (GenBioPAC). Dr. Sillanpää is a member of ACSM.
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