First and most important is to start slowly. Begin with some conditioning rides in the arena. Try to keep your first few rides to about 15-20 minutes of walking before you begin any trotting or cantering, so that your horse is thoroughly warmed up. Remember that they will be getting their heart rate up, so you should keep the trotting and cantering work to a minimum. I suggest about 5 minutes of trotting max and then a short walk break to let their heart rate come back down and allow them to catch their breath before about 3-5 minutes of cantering or more trot work. This is all gauged of course, on the amount of lay off period your animal has had. If they seem overly winded, then you can shave a minute or so off all of these times. The other thing to remember is a proper cool down. You should walk your horse to cool him out until he is no longer hot to the touch and his breathing and heart rate has come back down within normal levels. (Resting respiration rate should be between 10-24 breaths/minute and a normal resting heart rate should be between 28-44 beats/minute – take either count for 15 seconds and multiply by 4 to get the total/minute) With a walking cool out period, the rates may be just slightly higher than normal resting rates.
When a horse has worked hard, you can actually stop your horse and feel their sides heaving up and down when on their back and can even feel their heart rate if you take a minute to stop and feel. This is an excellent opportunity to determine if your horse is being overworked. Soft horses lack muscle tone and the ability to handle very hard work and may sweat profusely. This is a signal to you to cut back on the work load.
When laying out your plan for your ride, the first order of business should be your ground work. This is an important first step to have your horse respecting you as a leader which can be crucially important after a lay-up period when the horses may have excessive energy. This is also important because it can help you to know if your horse seems attentive and will also help you gauge his reactiveness level. Another benefit is that it will help you to know if your horse is stiff or rigid on one or both sides. I will even do ground exercises before I get on a strange horse that I don't know so that I know where they may "freeze" up when I am mounted and to lay the foundation for the role I have as the horses leader. If you think your horse will be acting fresh and want to jump around, which most will after some time off, then ground work is crucial.
Once mounted, I will begin my warm up with specific suppling exercises. This helps to stretch my horse out but also helps me to again know if they are going to be resistant on one side or the other. If I have a horse jump around and want to act silly, it is pretty important that I be able to gain control back with a soft and supple horse rather than one who wants to sull up and not give to my rein or leg pressure. I will do several minutes of suppling and leg yielding exercises to start out, before I even begin walking. Then I continue to do these rein and leg yielding exercises while I am walking. This also serves the purpose and is perhaps most important, to keep my horse’s brain on me and what I am asking. It is giving them a job, so that they can stay focused on the work at hand. Once I begin trot work, I will continue with the exercises throughout the trot work and then when I am allowing the rest period in between the trot and canter work, I will usually just allow the horse “free” rest and time to bring the heart rate back down. If I choose to go on with some light canter work, I may even continue with these exercises. However, if I don’t feel the horse is ready, then I may choose to skip the canter until a later date and just build on the trotting.
Once I am in the final cool out phase, I will allow the horse free time to just walk, unless they are still feeling a bit amped up and wanting to be silly, then I may intermittently add back in some yielding and suppling exercises to keep their brain engaged.
The most important thing is to be consistent and slowly bring the horses work load back up to a normal level. Of course, the job you are asking your horse to do would be important to factor in. For instance if you have a jumping horse, you would not just begin jumping right away until you had done the appropriate amount of time working on flat work to properly condition and leg your horse up. Likewise, you would not take a cutting horse and just put them immediately on cows. You would be sure your dry work is completely up to par and the horse had been asked over a long enough period of time to execute the same type of moves you would be asking him to perform while working a cow. You also have to ease into the harder work as well, for example you would not go out and work that cow for 30 minutes you might begin with just a few minutes on a cow until you were satisfied that you had a good enough “quit”, or you might only jump a few smaller jumps to begin with, or do pole work and then gradually build up the height of the jumps as well as how many you are asking your horse to jump.
The final thing I always tell my riders is to lower your expectations. If your horse has had time off, they are usually not going to start out mentally like they did when you stopped riding. I like to just assume they will not be so great and sometimes I am pleasantly surprised when they act better than I expect. This will give you the mindset you will need to be patient as you are bringing them back up to speed. Be consistent with your work, start slowly and do your ground work and suppling exercises. You will run less risk of injury to you both and you will be right back where you left off. Before you know it, both horse and rider will be in shape!