| strong(adjective, sound, reasonable, strain, rational) |
| firm(adjective, strain, true, firmness, constant) |
| hard(adjective, strain, firmness, labour, hardiness) |
| sound(adjective, reasonable, strain, firm, true) |
| stable(adjective, strain, firm, true, firmness) |
| stiff(adjective, strain, firmness, labour, hardiness) |
| fast(adjective, strain, firm, firmness, constant) |
| tenable(adjective, strain, firmness, labour, safety) |
| unyielding(adjective, strain, firmness, labour, hardiness) |
| proof(adjective, strain, firmness, labour, hardiness) |
| resistant(adjective, strain, firmness, labour, hardiness) |
| steadfast(adjective, strain, firmness, labour, hardiness) |
| rigid(adjective, strain, firmness, labour, hardiness) |
| robust(adjective, strain, firmness, labour, hardiness) |
| substantial(adjective, strain, firmness, labour, hardiness) |
| tight(adjective, strain, firm, true, firmness) |
| constant(adjective, strain, firmness, labour, hardiness) |
| durable(adjective, strain, firmness, labour, hardiness) |
| immune(adjective, strain, firmness, safe, labour) |
| lasting(adjective, strain, firmness, labour, hardiness) |