Our bodies do not produce linear force. Our muscles produce rotational forces which we call torque, to move our limbs that create movement.
Some people often ask, why do Powerlifters Sumo deadlift? The answer lies in bio-mechanics!
To stand up with the weighted bar, our musculo-skeletal system must produce forces that overcome the force of gravity. If they cannot, then we cannot pick the weight off the floor.
Gravity is the main external force acting on the athlete-bar system. It produces a linear force downward, perpendicular to the surface of the earth (for all intent and purpose). In the image attached, the force vector acts downwards, through the red line. This force applies a torque (rotational force) to the hips, that the glutes need to overcome to be able to stand up. The glutes extend the hip, which is the opposite direction to which gravity acts on the hips.
The torque applied by gravity, can be calculated. Torque is simply mathematically explained as Force x Distance. The Distance (d) is measured perpendicular to the line of action of the Force (gravity in this case). This distance is also known as the moment, or, moment arm. The moment arm in this case is represented by the cyan line, and is measured from the point of rotation (hips), to the line of action (red line). Therefore, the longer the moment arm, the more torque applied to the point of rotation, therefore the greater the effect of gravity and the stronger our hip extension has to be.
With a Sumo deadlift, we effectively reduce the moment arm, by squeezing the hips close to the bar and maintaining a synergistic movement at the shoulder and hips (both move up as you squeeze the hips forward). This is where most people fail when attempting Sumo. In effect, using more of a sumo-conventional style, allowing their hips to move backward, and not squeeze them forwards.
Two other things change with sumo as well!
Firstly, we are able to sustain a comparatively open hip angle compared to conventional. Therefore our hip extensor musculature are much more efficient. This is why we can half squat more then we can full squat.
Secondly, we effectively reduce the range of motion. That is, we move the bar less distance to lockout. Since Work = Force x Distance, we reduce the amount of work required to move the same weight.
So why Sumo?
1. Reduce the hip moment. 2. Open the hip up. 3. Reduce the range of motion.
In theory, we should all Sumo, but practically things change. For example some athletes simply cannot get into a good sumo position to take advantage of the above. Sumo also requires much more skill.
Either way, we should all be using the conventional deadlift to get strong!