There’s only a week in between the end of the American hard court season in Miami and the beginning of the European clay court season. There’s a few very good reasons why players form drops when they first hit the red dirt or why niggling injuries rear their ugly heads.
When playing clay court tournaments the rallies are longer with the same rest period between points, meaning that the work to rest ratio changes and players need good endurance to survive a clay court tussle, even in best of three. This is part of the reason Rafa Nadal is so good on clay. He has better endurance that any athlete out there, with his VO2max (a measure of how efficiently he uses oxygen) apparently being 85, the same as Lance Armstrong, with a elite player sitting around 65. This just means that for each breath Rafa’s body and working muscles are getting a hell of a lot more oxygen than most other people.
The longer playing time seen on clay courts also affects heart rates, lactic acid build up and shots played, with players trying to catch each other off balance or hit the ball behind the other player. The lower friction resistance of a clay court also means that a player has more time to run around and hit their preferred shot.
Hard court points are generally only a few shots and are of much shorter duration and significantly less forgiving on the joints. When a player tries to stop and change directions in a hard court match, they’re basically relying on the muscles and joints (and a little bit of slide). On a clay court a player can slide to decelerate.
A player also needs to slide on a clay court and be prepared for dramatic changes in court speed during a match if, for example, a wet court dries. Sliding means a player needs to have good lower limb strength to develop power into their shot while sliding as well as good balance.
Sliding is one of the other reasons Rafa Nadal is head and shoulders about the rest on a clay court. He plays left but is naturally right handed. A person generally has better balance to their dominant side, but because Nadal is dominant to both sides, he has a unique ability to slide well to both sides, making him more effective and efficient.
While hard courts have more jarring, and general joint abuse style injuries, injuries on a clay court tend to come from falling if a players balance is average (or if they just fall) or because of fatigue from longer play duration.
A player who plays more clay court tournaments is less likely to have knee injuries than a player who plays a lot of hard court tournaments.