Inter-individual variability in the adaptation of human muscle specific tension to progressive resistance training

Considerable variation exists between people in the muscle response to resistance training, but there are numerous ways muscle might adapt to overload that might explain this variable response. Therefore, the aim of this study was to quantify the range of responses concerning the training-induced change in maximum voluntary contraction (MVC) knee joint torque, quadriceps femoris (QF) maximum muscle force (F), physiological cross-sectional area (PCSA) and specific tension (F/PCSA). It was hypothesized that the variable change in QF specific tension between individuals would be less than that of MVC. Fifty-three untrained young men performed progressive leg-extension training three times a week for 9 weeks. F was determined from MVC torque, voluntary muscle activation level, antagonist muscle co-activation and patellar tendon moment arm. QF specific tension was established by dividing F by QF PCSA, which was calculated from the ratio of QF muscle volume to muscle fascicle length. MVC torque increased by 26 ± 11% (P < 0.0001; range -1 to 52%), while F increased by 22 ± 11% (P < 0.0001; range -1 to 44%). PCSA increased by 6 ± 4% (P < 0.001; range -3 to 18%) and specific tension increased by 17 ± 11% (P < 0.0001; range -5 to 39%). In conclusion, training-induced changes in F and PCSA varied substantially between individuals, giving rise to greater inter-individual variability in the specific tension response compared to that of MVC. Furthermore, it appears that the change in specific tension is responsible for the variable change in MVC.