Specifications

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Fm
The force in the contractile element is decreasing throughout the
movement because the moment arm of the external force is
reducing and also because the passive force in the muscle is
contributing more and more to balancing the load.
Ft
The tendon force shows the reduction of the mucle action by
virtue of the reduced external force's moment arm alone. A
simplified explanation is that Ft = Fm + Fp, but this is not
entirely true because we also have to account for the pennation
angle.
Fp
The passive force in the muscle increases as the muscle is
stretched.
Strength
This is the strength of the muscle. It is not corrected for the
tendon elongation.
Ft0
The hypothetical force that the tendon would have if the activity
of the muscle were zero. The reason why this is slightly different
from Fp is that Ft0 acts directly along the action line of the
muscle while Fp is diverted by the pennation angle. This property
is mostly interesting to scientists involved in detailed modeling of
single muscles.
Ft0Grad
The gradient of Ft0 with respect to the muscle activity. For
mathematical reasons this is equal to the Strength, and the two
graphs are identical. The reason why this property is included
under to different names is that the simple muscle model, from
which this model is derived, does not have Ft0Grad and hence
needs a Strength property.