Focusing on warm-up, stretching, mobility, and proper form will likely go a long way.
Warm-Up
Consider doing a light warm-up prior to any exercise (but especially prior to doing heavy compound lifts.) This can be as simple as a light jog for 5 minutes on the treadmill, a few minute jump rope session, or lightly working a punching bag for a round or 2. You would probably want to get your heart rate up to around the 130s-140s; while not trying to do anything too intense. Remember, its a warm-up.
Stretching
Next you want to do some light static stretching. Again, nothing too intense. But you want to stretch out the muscles, tendons, ligaments, and fascia to allow optimization of blood flow prior to the upcoming vigorous activity. If you are working out a specific body part that day, try to focus on that part with a little extra stretching.
Mobility
This fits in with stretching as well. Foam rolling, stick roller work, and elastic band work can be your mainstays here. There are some great Mobility books out there (i.e. Becoming a Supple Leopard, Foam Roller Workbook, Self Myofascial Release, etc.) that you can comb thru to build yourself a personalized short mobility routine to be performed prior to serious exercise. A few sets of air squats, hip lunges, and ankle rolls can go a long way.
I would give a strong recommendation to Yoga in this category as well. Many people note that their mobility actually diminishes as their strength and muscle mass increase. In addition, yoga can help you become more flexible and mobile which will help you hit your lifts (especially squats and deadlifts) with better form. Finally, yoga can be restorative as well by aiding you in recovery and preventing future injury through the development of stronger core musculature.
Form
Finally, I think it's important that you make sure that whatever exercise you are doing is being done with proper form. If you are lifting heavy weights; maybe think about hiring a personal trainer for a few sessions to teach you proper form. If that is not an option, ask a friend to video tape your lifts so that you can self-analyze and consider posting to some online lifting forums to gain feedback. If your exercise is something else you are unfamiliar with (BJJ, Muay Thai, kickboxing, bikram yoga, rock climbing, Crossfit WOD, etc.) ... again, I'd suggest hiring an expert for a short period of time to make sure that you are moving with proper form. A lot of overuse/repetitive use injuries can be avoided by making sure you are performing the exercise/movement/sport/activity with the proper intended form.
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I didn't address your specific injury concerns (forearm/knee), but rather wanted to comment on a general routine that I have personally used. Using the above methods I have continued to exercise and lift weights despite suffering through some mild-moderate shoulder pains, tight muscles/soreness, low back pain, and occasional bouts of sciatica.
Warm-Up
Consider doing a light warm-up prior to any exercise (but especially prior to doing heavy compound lifts.) This can be as simple as a light jog for 5 minutes on the treadmill, a few minute jump rope session, or lightly working a punching bag for a round or 2. You would probably want to get your heart rate up to around the 130s-140s; while not trying to do anything too intense. Remember, its a warm-up.
Stretching
Next you want to do some light static stretching. Again, nothing too intense. But you want to stretch out the muscles, tendons, ligaments, and fascia to allow optimization of blood flow prior to the upcoming vigorous activity. If you are working out a specific body part that day, try to focus on that part with a little extra stretching.
Mobility
This fits in with stretching as well. Foam rolling, stick roller work, and elastic band work can be your mainstays here. There are some great Mobility books out there (i.e. Becoming a Supple Leopard, Foam Roller Workbook, Self Myofascial Release, etc.) that you can comb thru to build yourself a personalized short mobility routine to be performed prior to serious exercise. A few sets of air squats, hip lunges, and ankle rolls can go a long way.
I would give a strong recommendation to Yoga in this category as well. Many people note that their mobility actually diminishes as their strength and muscle mass increase. In addition, yoga can help you become more flexible and mobile which will help you hit your lifts (especially squats and deadlifts) with better form. Finally, yoga can be restorative as well by aiding you in recovery and preventing future injury through the development of stronger core musculature.
Form
Finally, I think it's important that you make sure that whatever exercise you are doing is being done with proper form. If you are lifting heavy weights; maybe think about hiring a personal trainer for a few sessions to teach you proper form. If that is not an option, ask a friend to video tape your lifts so that you can self-analyze and consider posting to some online lifting forums to gain feedback. If your exercise is something else you are unfamiliar with (BJJ, Muay Thai, kickboxing, bikram yoga, rock climbing, Crossfit WOD, etc.) ... again, I'd suggest hiring an expert for a short period of time to make sure that you are moving with proper form. A lot of overuse/repetitive use injuries can be avoided by making sure you are performing the exercise/movement/sport/activity with the proper intended form.
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I didn't address your specific injury concerns (forearm/knee), but rather wanted to comment on a general routine that I have personally used. Using the above methods I have continued to exercise and lift weights despite suffering through some mild-moderate shoulder pains, tight muscles/soreness, low back pain, and occasional bouts of sciatica.