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The measurement and analysis of heart rate variability (HRV), which is based on the variation between consecutive NN intervals, has become an established procedure over the past two decades. A decrease in HRV has been shown to correlate with an increase in mortality. HRV is influenced by a number of physiological factors such as various diseases. Awareness of these mediators or confounders is of great importance in the analysis and assessment of HRV both in scientific studies and in clinical practice. This document, which is based on a selective survey of references and supplemented by information from national and international guidelines, presents the main endogenous, exogenous and constitutional factors. A decrease in HRV has been observed not only in connection with non-influenceable physiological factors such as age, gender and ethnic origin, but also in conjunction with a large number of acute and chronic diseases. Numerous lifestyle factors have both a positive and a negative influence on HRV. There are also physical influences that affect HRV. They must on no account be disregarded. Although the list of the factors is long and not all of them have yet been fully studied, awareness of them is of crucial importance in the measurement of HRV (both under laboratory conditions and during medical practice), its analysis and its assessment. More research also needs to be carried out to close knowledge gaps.

Background Musculoskeletal injuries (MSkIs) are a leading cause of health care utilization, as well as limited duty and disability in the US military and other armed forces. MSkIs affect members of the military during initial training, operational training, and deployment and have a direct negative impact on overall troop readiness. Currently, a systematic overview of all risk factors for MSkIs in the military is not available. Methods A systematic literature search was carried out using the PubMed, Ovid/Medline, and Web of Science databases from January 1, 2000 to September 10, 2019. Additionally, a reference list scan was performed (using the “snowball method”). Thereafter, an international, multidisciplinary expert panel scored the level of evidence per risk factor, and a classification of modifiable/non-modifiable was made. Results In total, 176 original papers and 3 meta-analyses were included in the review. A list of 57 reported potential risk factors was formed. For 21 risk factors, the level of evidence was considered moderate or strong. Based on this literature review and an in-depth analysis, the expert panel developed a model to display the most relevant risk factors identified, introducing the idea of the “order of importance” and including concepts that are modifiable/non-modifiable, as well as extrinsic/intrinsic risk factors. Conclusions This is the qualitative systematic review of studies on risk factors for MSkIs in the military that has attempted to be all-inclusive. A total of 57 different potential risk factors were identified, and a new, prioritizing injury model was developed. This model may help us to understand risk factors that can be addressed, and in which order they should be prioritized when planning intervention strategies within military groups.