For a long time, we've believed that anthropomorphic proportions are the deciding factor in the sumo vs conventional debate. And by this I'm talking primarily about lever lengths--ie: femur to torso ratio, arm length, shin length, etc. Length has been the most talked about issue on this matter, to the neglect of the subject of hip anatomy. This is probably because segment lengths and proportions are easy enough to measure, but who in the world knows what their hip socket looks like? I know few people who can just bust out an MRI image of their pelvis, easily identifying all its angles and distances. How in the world can you figure out what your hip anatomy is without being able to see it? More on that later. First let's talk about a few structures of the hip.
So what are the major differences in hip anatomy? Well! Let me tell you!
One could write an entire book illustrating the different angles of inclination, socket depth, angles of rotation, and femoral neck lengths possible. And from there one could go into detail about how every combination of these variables would determine range of motion in various hip movements, as well as strength capacity in each of these movements. But that would be a very boring book to anybody but me and some other die-hard advanced anatomy powerlifter nerds, so I'm not going to write it here.
But for now we can look at angle of inclination. This could be the biggest determining factor in hip mobility, and what will be the strongest angle of hip abduction/rotation when performing a deadlift.
Though the "normal" angle of inclination is roughly 125 degrees, this hip anatomy is far more rare than the other two. Most likely, there will be room for a little more versatility in the deadlift stance than with the other two hip angles, and both styles will probably work for a person with this hip anatomy. Here, angle of inclination will not be as much of a deciding factor in deadlift stance as much as various other anatomical factors will be, so I won't discuss this one in length.
With a coxa vara anatomy, as seen in Fig. B., the abduction and rotation demands of a sumo deadlift would likely cause the femur itself to ram into the pelvis. As you can see, the femoral neck, which is represented by the solid black line, is relatively long and perpendicular to the hip socket. By abducting the femur, the femoral neck would pinch up against the top of the hip socket. Bone on bone is not generally a comfortable or useful function, so this type of hip angle would almost invariably force the lifter to perform conventional.
With a coxa valga anatomy, even extreme ranges of abduction are likely fluid and comfortable. Because the femoral neck is nowhere near the hip socket, the femoral range of motion is huge in most directions, but maybe a little less with pure hip flexion. This type of body is likely very powerful in the adductors, and able to create massive amounts of tension from the adductor magnus to aid in hip extension. The flexibility afforded by this hip angle gives the sumo stance very strong potential.
With these anatomical differences in mind, clearly the sumo deadlift is a good and necessary choice for some lifters. It's recently become more popular in the powerlifting world, with some of the top deadlifters in the world utilizing a wider stance. But the sumo deadlift is still hotly debated, and often seen as a bastardized movement within powerlifting. I attribute this mostly to ignorance, particularly in regards to the more advanced sciences of the human body.
There's this persistent myth that the sumo deadlift is easier because the range of motion is shorter. I mean... not really. Well, what I actually mean is just no. This myth is mostly perpetuated by people who don't have an advanced understanding of physics or biomechanics. Don't get me wrong, it is totally fine if you don't have an advanced understanding of physics or biomechanics, but don't just SAY things.
In reality, it's not so simple as all that. While it is true that the wider stance brings the hips lower, reducing the range of motion, a pure decrease in mechanical work doesn't necessarily work out to a decrease in the torque required to lift the weight.
If we are looking at hip extension as a purely saggital (front to back) plane movement, it does appear that the moment arm (the horizontal distance between the hip crease and the vertical bar path) in a sumo deadlift is shorter. Because torque is defined as the load times the moment arm, it appears the shorter moment arm in sumo would result in less hip extension torque required to lock out. However, hip extension is not a uniplanar movement, it is multidimensional. Because of the external rotation of the femur in a sumo deadlift, hip extension actually occurs in both the saggital and the frontal plane. So the moment arm is not actually the distance from the hips to the bar in a straight forward line. By now, this is probably going over most people's heads, so take a look at the diagram below, and it will make much more sense.
As you can see, we can't calculate torque by simply looking at the distance from the hip to the bar in the saggital plane like we can with a conventional deadlift; as with any multiplanar movement, we must look at the moment arm drawn in the plane of the joint axis of rotation. Furthermore, when we think about the saggital plane in a deadlift, we tend to think about "front" and "back" as they pertain to the torso. But in a sumo deadlift, the saggital plane should be seen as relative to the femur, because that's the joint axis we're examining--hip extension occurs in the saggital plane relative to the femur. Woah, ok. I know.
I know it sounds all crazy and complicated, but just take that frontal/saggital plane axis and rotate it about 45 degrees, so that now it's viewed relative to the leg. Now when we think about "front" and "back", they occur at the same angle the leg is abducted. With this in mind, the moment arm in both stances are virtually identical, making torque requirements virtually identical.
So, if hip extension torque demand for lockout isn't really the key difference, what is?
2 words: Quads. Spine. In a sumo deadlift, the hips are slightly lower than in a conventional deadlift, placing much more demand on the knee joint through leg extension. This makes the quads the primary mover as the bar breaks ground. In a conventional deadlift, the higher hips place the torso at a steeper incline, requiring a greater amount of spinal erector activation to keep the spine tall as the bars moves off the floor.
So this is all fascinating, but how do you know which style to choose without a hip MRI? Chances are you don't really need to know what your hip anatomy is to choose the right stance. It might seem rather unscientific, but a good rule of thumb is to train both styles regularly, and pretty heavy, and take note of which one feels more "right" and strong. The thing is, when you get to a plateau, you can normally hone in on aspects of your technique to get strong in that movement again. But if you plateau in one stance much earlier than the other, and can't seem to overcome this with significant technical improvements, you are probably just not as strong as you could be in the other stance.
Another glaring metric would be, do you have any unavoidable or persistent, consistent pain with one style more than with the other?
If you have tried sumo and just cannot, for the life of you, get into a position where your hips don't feel like they are being ripped out of their sockets, it might be because there's some bone on bone action going on there. No amount of hip mobility is going to change your bony structures, unless you break them. Yikes!
If you struggle with unavoidable low back pain when pulling conventional, it may be because you are placing more than ideal stress on the spinal erectors when your strong point is really your adductors. If you are very thin or have a very long torso in relation to your legs, the demand on your spinal erectors might just be more than you can brace for at very heavy weights.
If there's any question about it, I am a proponent of sumo deadlifts. I see them as a valuable option in a world of different anatomies that must be considered. There's a whole host of humans with lower back issues or very small frames who might see more gains (and comfort) with sumo deadlifts. This is of course anecdotal, and should not dictate your stance if your experience doesn't reflect what I'm saying. At the end of the day, every body is different, and your body will most likely tell you what it likes and doesn't like. And if you don't listen, it will scream at you, don't worry.
So take the science into account, and by all means get a feel for which style gets you the largest gains and feels most comfortable in the long run!