Tongue Position matters. Its not what you think.

This very interesting study found that subjects were able to produce much more muscle force when they pushed their tongue into the roof of their mouth. We’re still not 100% sure why this is the case – it may have to do with a unique relationship between the tongue and CNS. In addition, it’s likely that this effect is only beneficial on isolation movements due to this neural pathway. Regardless, give this a shot for yourself and see how you like it! I’ve personally found it to be effective for cranking out a few extra reps on leg curl variations in particular.
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▪️ Source: di Vico, R., Ardigò, L. P., Salernitano, G., Chamari, K., & Padulo, J. (2013). The acute effect of the tongue position in the mouth on knee isokinetic test performance: a highly surprising pilot study. Muscles, Ligaments and Tendons Journal, 3(4), 318.