Related Clues:

• Taking a breather
• Like lowered heart rates
• Reclining
• Proven in trials
• Put through a trial
• Like objects in a Newtonian law
• Tried-and-true
• Not going anywhere?
• Free from anxiety
• Vegging out
• Took for a trial run
• Challenged, or what the theme entries in this puzzle should be
• Took out for a spin
• Stuck a toe in
• Worked on in the lab