How does weight position effect muscle activation?
Biomechanics and physics play a huge role when it comes to training, however they’re seldom considered.
We all need to take into account when training is the levers of our body and load position.
The muscles we activate will depend on how we position our joints, if a joint is parallel to the line of action of the weight it won’t be recruited to the extent if it was at a greater angle than 0.
The muscles at the joint will still be active in an isometric state but won’t be actively working to lift the weight.
5 Kinetics example
For example if we look at the row on the left the forearm is almost parallel to the line of pull of the barbell (directly down), this will lead to minimal activation of the biceps and other elbow flexors.
On the other hand if we look at the image on the right, the forearm is at an angle (into elbow flexion) this will cause the elbow flexors to work to hold the weigh into this position or potentially lead to more elbow flexion taking away from the load on the lats.
This can also be applied in different ways when it comes to exercising specifically with cable machines as there line of pull can be adjusted to directions other than towards the ground.
We can choose at what angle the biggest torque happens at a joint, this is when the load is perpendicular to the lever. So for example when doing a bicep curl with a barbell, the load is greatest at 90 degrees of flexion.
If we use a cable machine and have the cable directly behind the arm the greatest resistance is now at 0 degrees.