strenuous (adj.)

• active
• arduous
• breathless
• burdensome
• demanding
• determined
• difficult
• dogged
• dynamic
• eager
• earnest
• effortful
• energetic
• enthusiastic
• exhausting
• hard
• indefatigable
• intense
• laborious
• lusty
• mean
• operose
• persistent
• pertinacious
• red-blooded
• resolute
• sincere
• taxing
• tenacious
• tireless
• tiring
• toilsome
• tough
• uphill
• vigorous
• vital
• wicked
• zealous