¨No Pain, No Gain¨ is true for most sports and training, but not so much the spirit of Yoga practice, or more specifically, the Asana practice. Yoga in general and the practice of Asana is meant to bring harmony and calmness and therefore improve health and wellness. The Asanas can even be healing and therapeutic to injuries and traumas in our body and mind.
In my opinion, many Yoga Asanas poses and movements such as the Sun Salutations or the Vinyasa Flow are pretty similar to poses or movements in other sports and training like Squat, Lunges, Burpee, and such. However, Yoga movements are very slow and with long holds so that we can have full control of how the body bearing weight in the pose, and how the pose will benefit (strengthen or stretch) the right part of the body. Once we gained control of slow movement, we can perform better in faster movements such as continuous Sun Salutations with very low risk of injury. In this way, Yoga is a good supplement to other sports too.
Unlike other competitive sports such as football, running, skiing, etc. with much weight and fast movements putting on the joints that even a sudden turning can twist and tear some ligaments, Yoga often has very slow and gentle movement. Yoga is ¨the safest sport¨, but there's always a risk of injury because that risk is always there in life and even if someone doesn't do anything at all, they still risk injuring themselves. Actually, do nothing leads to more injuries than doing Yoga. However, injury in Yoga is rarely by sudden accidents, but in most cases, it's wear and tear injury that happens through a lot of times not putting the joints in a healthy position.
Of course, there are still accidents in Yoga, I have heard of people who fall out of inversions and get dislocated neck joints or wrist joints. But even these accidents show a lack of body awareness and therefore putting much weight (as heavy as the body) into unstable joints, often happened many times during one's Yoga practice.
Think of someone who bears weight in unstable shoulder joints in all the Planks and Chaturangas throughout their Yoga journey, that's a lot of times putting weight on unstable shoulders. Same example to the knees and ankle joints: if someone is walking around with unstable knees and ankles, they can still walk around, but problems will arrive very slowly later on. The joints become weaker and weaker in a long and slow process, and it's one last hit to make them scream into pain to get our attention.
One good news is that injury in Stretching often happens to people who are more flexible because their limits are too vague to see and they could pass the limit any time. To stiff people, they have very clear signals of STOP: the pain when they stretch, so they almost can not pass their limit. It would be too painful before they even get anything torn.
However, Strengthening activities are when we put weight on our joints to strengthen them and having unstable joints will start the wear-out process to the joints. To sum up, unless you already were injured before you do Yoga, in most cases, the injury will happen very slowly. Yoga is a practice of body awareness too, and you should be able to listen to your body and constantly check in with your body. But most of the time the unstable joint positions feel comfortable and easy - that's why it's so tempting to go autopilot to those joint positions all the time!
Take an example of myself: I have flat feet, and that's because my ankles are in an unstable position: they are collapsing inside. That is due to my unstable knees - they are collapsing in, too. And that is, one more time, due to my unstable hips: my hip external rotator muscle groups are too weak to turn my thigh bone out. With my flat feet, I can do all the standing Yoga poses easily, even the balancing poses on one leg. But if I put my joints in the correct positions, including ankles, knees, and hips, it's insanely hard work for muscles around my ankles and my buttock (glute - the main hip external rotator) and I keep falling out of those poses.
So what can we do to avoid injury in Yoga?
First of all: understand how your body works, especially the joints and the muscles. I try to explain in themes and topics in my Vinyasa Yoga classes. The two biggest and main weight-bearing joints in our body are hips and shoulders, and they should be the most stable ones.
Second of all: if you doubt something doesn't seem right, go to a physical therapist or doctor to check out your posture. As a Yoga teacher, I can tell you somewhat about your joints based on which muscles are too long or too short and the way you hold your posture. But I am not qualified as someone who can diagnose.
And one thing we can do: use props in doing Yoga! Props give awesome support to keep you in the pose so that you have time and peace to check what's going on in your most important joints in the pose. I wrote a blog post about how to use props in basic balancing poses: Warrior 3, Half Moon, Twisted Half Moon. Sometimes, props can support you fully and even if you are not in a perfect joint position, no major problems will happen. Some examples are in my post about using props in Chaturanga Dandasana, Tripod Headstand and Forearm Stand - poses that bear weight on shoulder joints and the latter two are a bit risky.