Muscle tension is the primary reason for a lack of dexterity and a reduction in endurance during playing; quite simply, the muscles get tired during the warm up phase and then start aching and become stiff during the performance phase, limiting execution.
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Everyone has a so-called 'resting muscle tension (tone)' level. No matter your physical activity (boxing, climbing, etc.), performance is better when it is higher. How to get it higher? Focused physical exercise.

Here are three articles with tips and thoughts on how to enhance your arm muscles AWAY from the piano which will in turn, when combined with the exercises AT the piano (as given in this video), increase your fluidity, endurance, precision and touch. Read them by replacing the activity discussed with the word 'piano' and see if you can modify a proposed exercise to your own life at/away from the piano.

1. Boxing: http://www.expertboxing.com/boxing-training/boxing-workouts/10-arm-endurance-exercises-for-boxing
2. Mountain Climbing: http://nicros.com/training/training-articles/training-muscular-endurance-part-1/
3. Muscle 'Twitch': https://www.acefitness.org/blog/5714/slow-twitch-vs-fast-twitch-muscle-fibers

Too much attention is given to 'playing the piano'... so this video is a big wake-up call for you all to make you realise that a lot of your difficulties stem from poor muscle management and issues which you may never have associated with playing the piano such as stress levels, sleep, water intake and diet.

This is a very enlightening article: http://www.musicandhealth.co.uk/stress.html

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