Using some principles in biomechanics and my understanding based on my research into the start position of the snatch, I will try to explain how to look at your start position from a simple biomechanical perspective. Also, you need to understand that there will be individual differences due to body type, height, limb lengths and even the type of lifts performed.
The purpose of the start position is to allow us to effectively use the muscles in the legs such as the knee and hip extensors to produce force to overcome inertia to get the barbell moving in the first pull. The predominant movement is knee extension with little hip extension (NOT no hip extension). This means more of the quads are being used to overcome the inertia as compared to the glutes. Hence the common cue of pushing with the legs. More importantly, in the start position, the slack in the body due to not feeling the resistance of the weight or barbell needs to be removed and that position needs to be maintained in order for force to be applied in the right direction to overcome inertia.
From this, the change in angle in the knee is larger than than in the hip, showing the use of the legs. Picture by Rob Macklem.
There are many way to get into a start position. We have dynamic starts, static starts, high hip, low hip. Which one works for you and which one should you use? How do we adopt a start position suitable for ourselves accounting for individual differences? Here are some concepts to consider:
Your flexibility would determine what kind of a start position you CAN adopt. Why highlight the word "can"? Sometimes getting into a start position might be easy. But for you to be able to produce force in that position might not be possible. If it is not, how are you going to be able to overcome inertia for the first pull?
Thus, flexibility is important because like at the bottom of a squat, you need to adopt a position suitable to adequately use your legs to push the weight back up. Flexibility also determines how well you adopt that position. If you have tight hamstrings, it would be impossible for you to squat low in the first place, or even if you are able to squat low, you may not be able to roll your hips enough to maintain a neutral spine.
Similarly, a lack of flexibility in areas like the knees and shins as well as ankles would affect the way you start. A low hip start position requires your knees to be flexed past 90 degrees and your ankles to be dorsiflexed to a certain extent. If this is not achieved, it is quite difficult to get into that start position. If your shins or the anterior portion of your ankle are tight, you would not be able to adopt that position as well.
2. Levers of the Body.
What is this? This is basically what creates movement in the body. Your joints and segments in the body. So in the weightlifting movements, this is basically the torso, the femur (thigh bone), the tibia/fibula (lower leg), the arms. In other words, that's your whole body. So the start position is to use these levers to properly create leverage to allow the bar to be lifted off the ground. How you create leverage is based on the length of the lever arm to the joint. So how long your torso is determines whether you should lean over the barbell more and have the hip higher or the length of your femur determines how upright you sit in that start position. Understanding the anthropometry of your physical structure allows you to find the best lever position to produce sufficient force and apply that force in the right direction.
3. COM and BOS.
Now we dabble into some concepts of motor control and biomechanics. Understanding the centre of mass (COM) and base of support (BOS) will allow you to understand how your start position actually affects your pulling mechanics. In a normal standing anatomical position, your centre of mass would be situated somewhere around your tummy or belly button. This is why your spine column takes up most of the support in holding your body upright. However, in the start position, the weight of the barbell is added resistance to your centre of mass. This means that the position of the barbell needs to be adjusted to ensure that the combined resistance with the centre of mass is situated over the base of support. This then allows better leverage to use the levers of the body to lift the barbell off the floor.
This is one of the last things to consider but it is as important. Firstly, it is important because if you do not have the strength to hold the start position, the moment you exert force to overcome inertia, you will end up not being able to hold the position, and that would subsequently affect the other positions to be adopted. It is one of the last things to consider because if all the other factors were in place (i.e having good flexibility to utilise leverage to ensure the COM is over the BOS), you would be able to have a solid start position which would not change much as the slack is taken on from the resistance of the barbell and the weight is initially lifted off the ground.
So this is just the basic concept of applying some simple biomechanics to understanding the science of weightlifting. There are more details for each of the factors/concepts mentioned which I will be discussing in an upcoming workshop (The Training Geek Weightlifting Biomechanics Workshop: Understand Your Start Position) running out of The Strong Room. But I hope you can see the picture of how weightlifting is basically similar to physics just that we are using the body to create a force to a resistance while acting against gravity. :)
Stay Strong and Keep That Start Position,
The Training Geek