1. Baseline Level of Arousal
The goal is to put your mind-body in that optimal arousal state where things get done and you experience sustained high energy levels throughout the workout. For more information on the relationship between arousal and performance, check out the Yerkes-Dotson law in psychology.
If your arousal level is low, or you are feeling tired, then an increase in arousal should enhance performance. For most people, the extra boost provided by the coffee in the morning puts them right where they need to be before a workout. So, you should feel free to enjoy a good cup of brew.
On the other hand, if you are someone who wakes up in a naturally high state of energy or arousal, then the coffee before workout may not be needed or even advised. This is because too much arousal could have potential detrimental effects on performance. We know that too much arousal can be just as detrimental as too little.
Yes, you want to have drive but not so much that you feel like your head might burst or eyes pop out of your head while lifting. Most likely, you have experienced the feeling of consuming so much caffeine that you have trouble concentrating. This is not ideal from a performance standpoint, as focus is a critical factor in lifting, and if you are doing cardio, you do not want to put too much strain on the heart muscle.
Too much stress being placed on the system, primarily from the caffeine in coffee, while training could lead to a headache or other unwanted symptoms, such as gastric upset or sleep disruptions.
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