| persistent(adjective, feature, state, characteristic, strain) |
| stiff(adjective, feature, state, characteristic, tough) |
| tough(adjective, person, dull, mean, body) |
| firm(noun, adjective, adverb, feature, state) |
| strong(adjective, person, body, strength, tough) |
| difficult(adjective, dull, great, tough, easy) |
| stable(adjective, strong, strain, firm, solid) |
| sound(noun, adjective, strong, reasonable, strain) |
| rigid(adjective, feature, state, characteristic, mean) |
| severe(adjective, nature, mean, strong, tough) |
| tight(adjective, adverb, strong, severe, strain) |
| unyielding(adjective, mean, strong, tough, easy) |
| cruel(adjective, person, mean, strong, cool) |
| arduous(adjective, dull, great, tough, easy) |
| harsh(adjective, mean, strong, cool, tough) |
| stout(adjective, person, body, strong, strength) |
| laborious(adjective, dull, great, tough, easy) |
| steady(adjective, feature, state, characteristic, strain) |
| substantial(adjective, tough, strain, difficult, solid) |
| robust(adjective, person, body, strong, strength) |