Physical conditioning to enhance +Gz tolerance: issues and current understanding.
INTRODUCTION: Although Canadian Forces (CF) efforts directed at developing new G-protection strategies have often raised the question of potential benefits of physical conditioning (PC) on G tolerance (GT), a fatality in a CF fighter aircraft accident, in which it was suggested the pilot may have had 'sub-optimal GT,' sparked renewed interest in this topic. METHODS: A two-part review was conducted: 1) a survey of the literature on the effects of PC on GT; and 2) a determination of further research required to resolve uncertainties on the subject. RESULTS: Five key themes surfaced: 1) GT as a concept is complex, and has different connotations for different users; 2) the term 'PC' likewise has a variety of meanings, and precise definitions are necessary to compare research results; 3) in examining the relationship between PC and GT, the roles of strength training, muscle fatigue, and aerobic fitness are not as clear as some studies seem to suggest; 4) in designing PC programs to enhance GT, issues such as palatability, efficacy, and intended target population must be addressed for the program to be operationally useful; and 5) there is a requirement for investigations that have controlled important influences such as intercurrent +Gz-stress exposure, proficiency in performing the anti-G straining maneuver, and the wide inter- and intra-individual variation for PC and GT measurements. DISCUSSION: The effects of PC on GT are not well established. Further research with more robust experimental designs and/or analyses than those used to date must be conducted (on new or existing data) to clarify this relationship. Conducting such work with sound experimental design and controls is more complex and time-consuming than some may appreciate.