In particular situations, a coach may advise performing the valsalva maneuver during exercises which place a load on the spine. The vasalva maneuver consists of closing the windpipe and clenching the abdominal muscles as if exhaling, and is performed naturally and unconsciously by most people when applying great force. It serves to stiffen the abdomen and torso and assist the back muscles and spine in supporting the heavy weight. Although it briefly increases blood pressure, its is still recommended by weightlifting experts such as Rippetoe since the risk of a stroke by aneurysm is far lower than the risk of an orthopedic injury caused by inadequate rigidity of the torso. Some medical experts warn that the mechanism of building "high levels of intra-abdominal pressure (IAP)...produced by breath holding using the Valsava maneuver", to "ensure spine stiffness and stability during these extraordinary demands", "should be considered only for extreme weight-lifting challenges — not for rehabilitation exercise".
Co-ingesting creatine with caffeine partially negated the benefits of creatine supplementation (at 5mg/kg bodyweight) during the loading phase in one study. The exact mechanism responsible for this effect is not known, but might be related to opposing actions on muscle contraction time. However, another study in trained men found that co-ingestion of 300mg caffeine per day during creatine loading at 20g per day (split into 4 doses) had no effect on bench press 1RM, time to fatigue, or sprinting ability. However, this study also found that creatine alone or when combined with caffeine had no effect on any of these parameters over placebo, either. Thus, the study may have been underpowered or done in too short a time frame (the test was done after only 5 days of loading) to observe any possible effects.
McArdle’s disease is a myopathic disorder associated with fatigue and contractile dysfunction as a result of alterations in the release of glucose from glycogen (via defects in myophosphorylase enzyme function) resulting in an inability to conduct high intensity work as easily. Creatine is thought to be therapeutic because beyond the general strength enhancing properties of creatine, people with McArdle’s disease have an upregulation of phosphofructokinase (PFK) enzyme activity  and increasing phosphocreatine storages suppresses the activity of this enzyme.
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