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Hi,

I have a question concerning the results of my studies:
I have weakened the rectus femoris (reduced the controls to 10% of their maximum value) and implemented a path spring parallel to the rectus femoris muscle in the model 'Gait2354' to support the weakened muscle. I focused on the phase terminal stance to initial swing (time 1.3-1.5). In this phase the knee and the hip of the right leg flex. I saw that vastus intermedius and psoas compensate for rectus femoris weakening. With spring support the force that the vastus intermedius produces goes down compared to no spring support. The value of the psoas also goes down with spring support but only between 1.3-1.375. Then the force value of psoas with spring support exceeds the value of psoas without spring support. I saw that all hip flexors have higher values from 1.375-1.5 in the support case than without support. Does anybody know what could be the reason for this?

Thank you very much.
Pia

I would suggest plotting the length, and forces of the spring, during stance. You should also plot the hip and knee angles- are you sure that the hip is flexing during stance?? It is important to make sure you know what is actually happening with model kinematics, and the spring. Having a spring force in parallel with the Rec Fem doesn't necessarily 'support' this muscle, muscles aren't just springs.

Ill guess- your spring is going to produce force as the hip extends and the knee flexs, effectively adding resistance to hip extension during stance. To balance the required moments, the model would have to increase hip extensor and knee flexor muscle activations. Since there are bi-articular muscles, this could lead to increases in hip extensor moments that need to be balanced out by an appropriate increase in hip flexor activation.

Thank you for your answer.
I'm sorry I did not mention that the simulation is performed during walking and not stance. In this phase of the gait cycle (terminal stance to initial swing) the knee and the hip flex. I plotted the hip and knee angles and the spring force. From these two pictures I can see ( I hope this is right ) that the spring acts as a hip flexor. (The spring produces force from about 1.2 - 1.65 and the hip flexes at that time) But this makes it even more incomprehensible for me to understand the behaviour of m. psoas because this muscle acts as a hip flexor.

To find out what the spring does, I compared the forces of all knee flexors/extensors and all hip flexors/extensors of the original Gait2354 model with the knee flexors/extensors and hip flexors/extensors of Gait 2354 model with a spring implementation. I saw that with the spring implementation the hip flexors and knee flexors have to work more and the hip and knee extensors are unaffected. Does this mean that the spring works as a knee and hip extensor or does is just mean that the spring works as a knee and hip flexion resistor?