1. Holding on burns 20 percent fewer calories than letting go at the same speed.
2. It can throw off your walking gait and posture.
3. It can cause repetitive stress injuries in the hips and shoulders. I once had a new client who complained of mysterious shoulder pain. I found out she always held onto the treadmill while walking. I told her to let go. After she began walking hands off, after a few days, the shoulder pain disappeared.
4. When you hold on, you are not really, truly walking, because in everyday walking, you're not holding onto anything for support. So if you hold onto the treadmill, your body is not being trained to do anything. In fact, it's being UN-trained.
5. It Un-teaches your body how to balance. Your balance will become worse if you hold on. When you hold on, the machine becomes an external support system to your body. This teaches your body to rely on an external agent for balance. So when you're outside somewhere, and you have to balance or walk on uneven surfaces, or step around things or go down stairs, etc., your body won't be efficient at handling the demands of self-support without that external agent to hold onto.
6. Holding on at fast speeds can raise blood pressure, because you are gripping at something. A tight grip, especially, will raise blood pressure.
7. You will be tricked into thinking you're far more fit than you actually are, because no matter how high you set the incline, even at a fast speed, if you hold on tightly enough, you can keep up with the tread without any challenge if you hold on. This will fake you out into thinking you can handle actual hills outdoors.
8. Holding on can aggravate a pre-existing back problem, or knee problem. When you hold on, the entire kinetic chain is disrupted.
9. Holding on creates a false sense of accomplishment. You're not really doing anything. Even the most frail person can use a treadmill if he or she grasps the machine.
10. It looks ridiculous. It's like, "Gee, what is he SCARED of? Let go!" Or, "You call that walking?! That ain't walking!"