Introduction: Stiffer connective tissues (muscle-aponeurosis and tendon) theoretically facilitate the rate of force transmission from the contractile apparatus to the skeleton and thus the capability of the muscle-tendon unit (MTU) to rapidly develop torque (i.e. explosive strength, ES). However, an association between in vivo tissue stiffness (k) and ES remains unsubstantiated. Recent work suggests MTU k may not discriminate inter-individual differences in ES (1). Whether tendon k separately influences ES is unexplored. Purpose: To examine the relationship between absolute and relative tissue k (both MTU and tendon) and ES. Methods: Following familiarisation, 52 healthy untrained males (18-30 yrs) completed duplicate test sessions. Isometric knee extension contractions were performed (~110° knee angle) with a strain gauge perpendicular to the tibia sampling external force. Maximal voluntary torque (MVT) was assessed before a series of explosive voluntary and involuntary (supra-maximal octets [8 pulses at 300Hz] electrically evoked via femoral nerve stimulation) contractions. Measures of ES were time to absolute and relative torque levels (50 Nm & 25% MVT increments), and sequential time periods (e.g. t25-50% MVT). Constant loading-rate ramp contractions were performed with simultaneous force and ultrasound recordings of both vastus lateralis aponeurosis and patellar tendon (PT) elongation to derive MTU and PT k. Absolute (N/mm) and relative (to MVT and resting tissue length) k were measured over identical torque ranges as the ES measures. Pearson correlations tested relationships between tissue k and ES. Results: Absolute MTU and PT k were unrelated to absolute voluntary or involuntary ES measures (-0.243>r< -0.002, 0.092<P<-0.99). Relative PT k was also unrelated to any voluntary or involuntary ES measure (-0.246>r< -0.004, 0.085<P<-0.976). In contrast, relative MTU k was related to some measures of relative voluntary ES: t25-50% (r=-0.411, P=0.003), t50-75% (r=-0.314, P=0.028) and t75% (r=-0.298, P=0.038). Relative MTU k was related to involuntary t25-50% (r=-0.389, P=0.006) with a tendency to relate to involuntary t50% (r=-0.273, P=0.06). Conclusions: Absolute and relative PT k were not associated with ES, and this was also the case for absolute MTU k. Relative MTU k was however associated with relative measures of voluntary and involuntary ES. These results suggest a differential influence of tissue components (muscle-aponeurosis vs tendon) on relative ES. However the overriding influence of maximum strength and/or tissue length seemingly negate any relationship between absolute MTU k and ES.