If you are going to purchase one piece of equipment, buy a set of light or medium weight dumbbells. You can use a set of dumbbells for both upper and lower body workouts as well as some cardio moves. I usually try to have a set of three dumbbells: light (3-5 lbs), medium (8-10 lbs) and heavy (12-20 lbs). Depending on where you are at on your fitness journey, you might just want 3 lb, 5 lb and 8 lb dumbbells. Or, you might need to go heavier than 20 lbs.
Another piece of equipment that I find to be really useful is the swiss ball. In fact, I love doing entire workouts just using the swiss ball. The only downside to a swiss ball is that if you have tight quarters, these balls are hard to store out of the way! I have a friend that purchased a fabulous cover for hers, and she uses it as an additional seat in her living room!
Swiss balls come in three sizes, so you will want to look at the description on the packaging to make sure you are purchasing a ball that's ideal for your height. It makes it much easier if you buy one that is appropriate for your size. I have a workout using the swiss ball on my site if you want some ideas on how to incorporate it into your workouts.
Theraband resistance bands are another great piece of equipment because they can be thrown into a purse, suitcase or gym bag. They take up little to no room, yet depending on the resistance that you chose, they can be very challenging. The different colors of the bands equate to different resistances. For example, a green band is 4.6-6.7 lbs in resistance; at 100 percent elongation, it provides 4.6 lbs resistance. If you elongate it to 200 percent, your resistance increases to 6.7 lbs. If you need more resistance, then a black band would provide you with 7.3-10.2 lbs of resistance. As you use the band for multiple repetitions and sets, you will feel the resistance! As a bonus, they are also helpful for stretching.
Medicine Balls can be incredibly useful and versatile. You can get them in a variety of sizes and weights. Some common uses are slams (slam the ball on the ground and squat down to get it), tosses or using them for a base. For example, when having clients do a medicine ball push-up, I have them place one hand on the ground and the other hand on the medicine ball. The height differentiation and instability of the ball makes one side work harder than the other, and the movement is much more challenging. We also hold them for additional weight or swing them in a circular motion (liked hands of a clock) for more challenging arm workouts. The options are endless, and I think they add another level of intensity to a workout. As always, make sure the addition of a piece of equipment to your workout doesn't impact your form. If it does, grab a lower weight or take out the piece of equipment.
A foam roller isn't something you would typically think of when listing pieces of equipment to buy; however, it is worth the investment. Foam rollers can be used in a workout, but I think they provide the most benefit during a warm-up or a cool down portion of a workout. Foam rollers help break up scar tissue in tight muscles allowing for more mobility and healing. In some ways, it's like giving yourself a deep tissue massage. Foam rollers are great for relaxing tight muscles in hard-to-reach locations like your Iliotibial (IT) Band or back. There are numerous "how to" videos online as well as articles for ideas on how to use one.
If you have some room in your "home gym" budget, you might want to consider checking out a TRX (Total Body Resistance Exercise) suspension trainer. A TRX suspension trainer is comprised of adjustable straps that can be hung from an anchor point. Originally developed by a Navy Seal, this compact piece of equipment can be used in a variety of places and utilizes your bodyweight for resistance. Many parks and recreation programs have TRX classes as well as traditional gyms. It would be worth it to take a few classes to figure out how to get the most use out of this piece of equipment. (You could also hire a personal trainer for a few sessions. They should be able to instruct you on how to use the equipment and come up with some sample workouts.)
I love using the TRX because it's great for individuals who have a chronically injured area. Because you are holding onto the straps, you can offset some of the weight from a sore/ injured body part and do a movement you can't normally do. I have several clients who have sore knees and hips. By holding on to the straps of a TRX, they can sink into a squat position with no pain. It helps work their quadriceps and allows them to do a movement that they can't normally do. You rely on your core to help stabilize you for all of the movements, so it's a great abdominal strengthener, too. I've seen them priced anywhere from $150-$250. I travel with mine - I just need a secure place to hang it from such as a huge tree branch, goal post, fence, etc.
Bosu Balance Trainer:
My other favorite piece of equipment that is a bit of an investment is a Bosu Balance Trainer. Bosu Balance Trainers are dome shaped balls with a flat base. They are great for working on your balance and adding a bit of difficulty to an exercise movement because of their instability. You can stand on them and do squats, bicep curls, med ball tosses, etc. Or, you can flip them over and attempt the same movements while trying to balance with the ball portion against the floor. They are great for abdominal exercises, too.
Bosu Balance Trainers are a bit of a financial investment. They usually cost about $99-$110. If you have the finances to purchase one, consider adding it to your gym. There are lots of Bosu workouts on Pinterest, but I think the easiest way to get familiar with one is to hire a personal trainer for 1-2 sessions. A trainer can help you get your form correct and provide you with some sample workouts.
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Bosu Balance Trainer