Abstract
BACKGROUND: Head acceleration is often used as a proxy measurement for concussion risk. It is unclear what factors contribute to head acceleration during a purposeful header. OBJECTIVE: The objective of this systematic is to identify what factors influence head acceleration during a header and highlight areas that have not been explored. METHODS: Studies were included if they assessed the effect of an independent variable on head acceleration during a purposeful header. There were no exclusion criteria relating to age, sex, playing level, study design, or publication date. Databases included Scopus, Web of Science, MEDLINE, EMBASE, CINAHL, SPORTDiscus and ClinicalKey, and grey literature was also searched. Searches were re-run in January 2024. Each included study underwent a risk of bias or quality assessment, using several tools owing to varied study designs (ROBINS-I, RoB2, modified Downs and Black and AXIS). Results are presented in tabular form, categorised by independent variable(s) and accompanied by a narrative results section. RESULTS: A total of 60 studies were included. Study designs included one-off measurements with variable(s), cohort, cross-sectional and randomised controlled trials (RCTs). Influencing factors included ball velocity and characteristics, header type, impact location, neck training interventions, fatigue protocols, neck strength, demographics, playing scenario and personal protective equipment. A wide range of heading trials and methods of measuring head acceleration were used. Results were conflicting for several variables. There was some consistent evidence that increased ball speed/mass/inflation increased head acceleration. Female players sustained greater head accelerations than males, and some evidence suggested that increased neck strength reduced head acceleration, but not in all cases. This review confirmed traditional neck strengthening is not effective in reducing head acceleration; however, adding neuromuscular exercises appeared to be effective. There was a lack of evidence investigating other systems that may affect head acceleration such as vestibular function, oculomotor performance, rate of force development and cervical spine proprioception. DISCUSSION: This review highlights the range of studies investigating head acceleration during a purposeful header. Included studies were of varying quality, and one limitation was the lack of transformation of the acceleration measures to a consistent frame of reference. Implications from this study suggest potential measures that may reduce head acceleration and therefore improve the safety of performing a purposeful header. These include reducing ball inflation pressure, limiting headers from long range/high speed balls (especially in children/adolescents and females) and ensuring neuromuscular exercises are incorporated into neck training programmes. Dynamic muscle strength outcomes, including measures such as rate of force development, are challenging to measure but should be considered an essential component of neck training when the aim is to reduce head acceleration in a ballistic task such as a header. The review is registered under the PROSPERO registry (CRD42022359294).