Short answer. Just their condition at the end, not their original condition. Article 573 of the Civil Code lets the usufructuary use such items according to their intended purpose and requires returning them only in whatever state they are in when the usufruct ends — unless the usufructuary's own fraud or negligence caused extra deterioration.
What the law says
Whenever the usufruct includes things which, without being consumed, gradually deteriorate through wear and tear, the usufructuary shall have the right to make use thereof in accordance with the purpose for which they are intended, and shall not be obliged to return them at the termination of the usufruct except in their condition at that time
Civil Code, Article 573 — Things That Deteriorate With Use. Read the full provision →
Ordinary use is expected, not penalized
Article 573 covers things which, without being consumed, gradually deteriorate through wear and tear — machinery, vehicles, tools, and similar items that wear down through normal operation rather than being used up in a single act. For these, the usufructuary shall have the right to make use thereof in accordance with the purpose for which they are intended. Using the item as it is meant to be used is exactly what the usufruct permits, wear included.
Return is judged by condition at the end, not the start
The article then answers the return-condition question directly: the usufructuary shall not be obliged to return them at the termination of the usufruct except in their condition at that time. Ordinary wear accumulated through the intended use of the item is simply accepted as the state it is in when the usufruct ends. The owner is not entitled to get the item back as good as new; they are entitled to get it back as it stands, worn down by legitimate use.
The exception for fraud or negligence
That protection is not unconditional. Article 573 closes with: the usufructuary shall be obliged to indemnify the owner for any deterioration they may have suffered by reason of his fraud or negligence. So while ordinary wear and tear from proper use is on the house, extra damage caused by the usufructuary's own fraud or carelessness is not. That deterioration has to be paid for separately, on top of returning the item in whatever condition it is otherwise in.
What this means practically
The dividing line under this article is between deterioration that comes from using the item as intended, and deterioration caused by fraud or negligence on the usufructuary's part. Since only the second kind creates a duty to indemnify, it is worth being able to show that any damage to a wearing item reflects ordinary use consistent with its purpose, rather than mishandling, if a dispute arises when the usufruct ends.