heart
'tireless' is an adjective. Adjectives describe things or 'nouns'example -The runner was tireless. 'Tireless' describes the condition of the runner ie. they do not seem to get tired . 'Runner' is a noun; 'tireless' describes the noun.p.s. If a car was without tyres it would be 'tyreless' :)
It means done without ceasing, without stopping; continuously.
The current Guinness World Record for the longest time jumping rope without stopping is 30 hours and 40 minutes.
stopping
you have more than 600 muscles in your body. They help you do almost everything - from pumping blood throughout your body to lifting your heavy backpack. You control some of your muscles and others, like your heart, do their jobs without you thinking about them at all.
Without stopping to think, he pulled the dog away from the oncoming car.
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Involuntary muscles are those that move without conscious control.
Nonstop is an adjective that describes something that continues without stopping or pausing.
Involuntary muscles.
cardiac muscles
Involuntary muscles