Ca 2+ -binding to troponin C ultimately controls force in muscle leading to the expectation that the two curves, pCa/force and pCa/Ca 2+ binding, will coincide. Using an improved fluorescence apparatus to measure Ca 2+ -binding, we confirm a displacement between the position and shape of the pCa/Ca 2+ -binding and pCa/force curves. This displacement may be part of a mechanism that reduces the noise inherent in the control process. There must always be some Ca 2+ -binding events even at 10 or 100nM, well below threshold for muscle contraction. To minimize the response to such random binding events we suggest that clusters of adjacent Ca 2+ -binding sites must be filled before contraction is initiated. Clusters promote the reconfiguration of the thin filament to the “On” state; this simultaneously increases thin filaments’ affinity for myosin heads and of troponin C for Ca 2+ producing the highly cooperative pCa/force curve. The cluster requirement displaces the Ca 2+ -binding from the force curve as observed. The thin filament conformational changes and the accompanying affinity increases introduce a discontinuity in the pCa/Ca 2+ -binding curve. The curve, therefore, is most appropriately fit by two separate Hill equations, a simple non-cooperative one (midpoint, pK 1 , n 1 ∼1) for the foot and a second cooperative one (pK 2 , n 2 ∼2.5) for the upper part. With this fit pK 2 is larger than pK 1 as our argument requires, in contrast to fitting to the sum of two Hill equations. It also expresses the idea that there may be three states of the thin filament.