There is a lot of debate about side arm throwers in disc golf as well as other throwing sports like baseball. The real problem is when you apply max effort to a side arm motion like a baseball pitch or a disc golf drive.
All About Energy
The first issue is that you are applying more force to a smaller area by throwing sidearm. That is, if you throw backhand you create a flow of energy that passes through your torso and exits out your arm. Your arm is more of a vehicle of kinetic energy versus the source of the energy. You do use some of your core when you throw side arm but you rely a great deal on the unstable joint of your shoulder and the smaller joint of your elbow to act as the fulcrum of the throw.
Stopping Said Energy
There is also the issue of the follow through. During a maximum effort back hand you will notice that you spin out of the throw during your follow through. This makes it much easier to disperse the energy you created. When you sidearm you do not have this same opportunity and your muscles in your shoulder and arm must absorb more of this energy putting pressure on them and creating overuse.
Better for Older Players?
You hear a lot of young ball players and disc golfers boasting about their high powered side arm actions but low and behold they usually do not make it out of their twenties before they experience a major injury. You have to understand that there is a lot going on here as far as bio-mechanics go. You can read one study that says sidearm is a more natural way to throw compared to an over hand throw and then another study says you’re asking for trouble if you throw sidearm.
Sidearm may be a more natural throw versus an overhand throw when it comes to sub-maximal effort, once you put as much power as you can on those joints you are at risk (for both side arm and overhead shots actually). In disc golf you have the option to throw back hand and although you could get injured, it is much less likely.
There is a lot of evidence that shows sidearm throwers are more at risk the younger they are. Statistics show that more younger sidearm throwers get hurt than older sidearm throwers, but statistics can be misleading. In baseball many pitchers who began as overhand throwers and developed a 2 o’ clock pitching motion ended their careers successfully, but I know of more than one sidearm only disc golfer who has had severe elbow trouble before they turned 25.
The fact of the matter is that the sidearm has a huge place in the game of disc golf. It is a very useful shot and many sidearmers are super competitive at this game. There are exceptions where people who may throw max effort sidearms every day never have a problem. Strengthening your joints and muscles as well as improving flexibility will no doubt help you out with sidearm throwing, but it is my opinion that repeated maximal sidearm throws (especially in younger players) will put you at a much higher risk than the backhand throw.
It is also my opinion that sub maximal sidearm shots can be used with a much less chance of injury, such as an up shot or drives below 300 feet. It is a very useful shot that I use myself when I need to especially to get out of a jam or for a right fading hyzer. It is up to you to be your own scholar on the matter, there is no exact answer since evidence for both sides is available.
My expertise on this matter come from research about sidearm baseball pitching, a Bachelors in Kinesiology and personal experience.
What are your experiences with sidearm throwing?