| stiff(adjective, strong, hard, strain, firm) |
| strong(adjective, tough, strain, fit, fast) |
| firm(adjective, strong, tough, strain, fit) |
| hard(adjective, strong, strain, firm, firmness) |
| stable(adjective, strain, firmness, labour, hardiness) |
| sound(adjective, strain, firmness, labour, hardiness) |
| immune(adjective, strain, resist, firmness, safe) |
| unyielding(adjective, strong, strain, firm, firmness) |
| proof(adjective, strain, resist, firmness, labour) |
| refractory(adjective, strain, firm, solid, firmness) |
| rigid(adjective, strong, strain, firm, stiff) |
| durable(adjective, strain, firmness, sturdy, labour) |
| impregnable(adjective, strain, fast, firmness, labour) |
| resilient(adjective, strong, tough, strain, stiff) |
| steadfast(adjective, strain, firmness, labour, hardiness) |
| tight(adjective, strong, strain, firm, solid) |
| tolerant(adjective, strain, firmness, labour, hardiness) |
| lasting(adjective, strain, firmness, labour, hardiness) |
| robust(adjective, strain, firmness, labour, hardiness) |
| withstanding(adjective, strong, strain, stiff, firmness) |