For cardiac muscle, Vmax can be used as a measure or:

In cardiac muscle Vmax is used as an independent index of cardiac contractility.The velocity of an unloaded contraction (Vmax) is determined by the physicochemical propeies unique to cardiac muscle and is therefore considered a measure of the inotropic state. However, because load always exists, Vmax must be extrapolated from the force-velocity curve. Ref: Hoit B.D., Walsh R.A. (2011). Chapter 5. Normal Physiology of the Cardiovascular System. In V. Fuster, R.A. Walsh, R.A. Harrington (Eds), Hurst's The Hea, 13e.