Chenmiao,

My thoughts:
I tend to think of muscle strength as the maximum isometric force that can be produced. As opposed to maximum power. I would also see it as more functional measure such as the maximum force generated during an isometric mid-thigh pull (IMTP).

I would see muscle force as specific to the muscle and the force generating at any point in time, this may be anywhere from at rest to maximal force. This is distinct from the torque generated at a joint or summation of torques across multiple joints as seen in the IMTP. The calculation of muscle force would therefore include joint torques and muscle moment arms. However, depending on how detailed or specific you wanted to get in describing force you could include force sharing among agonists, co-contraction of ant-agonists, muscle cross section areas, elastic components and contractile elements, pennation, force-length, force-velocity, and activation characteristics. This is delving into muscle modelling.

Cheers
Allan