Our group has been engineering cell-scaffold constructs to improve tendon repair by contracting mesenchymal stem cells (MSCs) in collagen gels and then evaluating their repair potential in wound sites in rabbits. Because the construct's initial conditions may influence the ultimate repair outcome, this two-part study sought to distinguish which factors most influence contraction kinetics in culture. (1)We optically determined if varying cell-to-collagen ratio significantly affected construct contraction. Temporal changes in construct area were monitored up to 168 h for 4 cell-to-collagen ratios (HK = 0.04, LK = 0.08, HM = 0.4, and LM = 0.8, where H, L = 2.6, 1.3 mg/mL collagen and K,M = 0.1, 1 million cells/mL, respectively).A mathematical model was created with terms that represent the different combinations of cell densities and collagen concentrations in order to predict the contraction kinetics as a function of time. Highly significant differences in construct areas were found among all 4 ratios after 8 h of contraction with the exception of the LK (0.08) vs. HM(0.4) conditions. This similar pattern raised the question of whether cell density or collagen concentration more influenced these events. (2) To isolate these effects, the contraction kinetics of the HM construct were compared to those of a new construct (L5K) with equivalent cell-to-collagen ratio (0.4) but half the cell density (500 K MSCs/mL) and half the collagen concentration (1.3 mg/mL). The L5K construct contracted significantly faster and more completely than the HM construct but no differently than the LM construct. These results indicate that above a threshold value of cell density, percentage reductions in collagen concentration influence contraction kinetics more than equivalent percentage increases in cell seeding density. The fact that our model successfully predicted intermediate time points of contraction suggests its utility for examining other cell and collagen densities. Controlling scaffold as well as cellular initial conditions will be critical in achieving our goal of functional tissue engineering (FTE) a successful tendon repair.