The purpose of this study was to determine the changes that occur in tendinous tissue properties during the early phase of tetanic summation in the in vivo human tibialis anterior muscle (TA). The torque response and tendinous tissue elongation following single stimuli, two-pulse trains, and three-pulse trains were recorded in the TA during isometric contractions. The elongation, compliance, and lengthening velocity of tendinous tissue were determined by real-time ultrasonography. The contribution of the response to the second stimulation (C2) was obtained by subtracting the response to the single stimulation (Cl) from the response of doublet. The third contribution (C3) was obtained by subtracting the response to the doublet from that of the triplet. C2 (7.8 +/- 0.5 Nm) and C3 (7.3 +/- 0.6 Nm) had torque responses significantly higher than Cl (3.6 +/- 0.7 Nm). In contrast, the elongations of tendinous tissue for C2 (2.8 +/- 0.4 mm) and C3 (1.7 +/- 0.2 mm) were significantly lower than for Cl (4.9 +/- 0.3 mm), indicating that the summation pattern of tendinous tissue elongation is different from the summation pattern of torque response. In addition, this showed considerable difference both between C1 (0.12 +/- 0.01 mm/N; 83 +/- 4.6 mm/s) and C2 (0.03 +/- 0.005 mm/N; 50 +/- 6.3 mm/s) and between Cl and C3 (0.02 +/- 0.002 mm/N; 39 +/- 6.4 mm/s) in the compliance and lengthening velocity of tendinous tissue. These results suggest that changes in tendinous tissue properties between first and second contraction are related to different summation patterns of force and tendinous tissue elongation during early phase of tetanic summation. (C) 2009 Elsevier Ltd. All rights reserved.