Assuming Recruitment Of An Entire Muscle Underestimates Specific Tension

Specific tension (ST) reflects the contributions of neural and musculoskeletal adaptations to loss or gain in strength. It is often assumed when expressing ST that the entire muscle can be recruited. This essentially negates neural factors contributing to performance. Although there are methods to quantify muscle size, none of them actually assess the amount of the muscle that was activated and involved during exercise. PURPOSE: To express ST of m. quadriceps femoris (QF) group assuming the entire muscle is involved in a maximal voluntary effort vs. using magnetic resonance imaging (MRI) to measure the activated muscle during surface neuromuscular electrical stimulation (SNMES)-induced isometric actions. METHODS: Seven able-bodied subjects (28 ± 4 yrs old, height 173 ± 9 cm and weight 68 ± 9 kg, mean ± SD) performed three trials of maximum voluntary isometric torque (MVIT) of knee extensor muscle group. They also demonstrated their tolerance to SNMES, 100 Hz, 450 JJ.S pulses and the amplitude of current set to evoke 75% of QF MVIT. MRI scans were taken prior to and after SNMES to determine activated muscle cross-sectional area (CSA). Total CSA of the QF was also obtained from these scans. ST was expressed using the MVIT to the total CSA vs. torque developed during SNMES to the activated CSA. Both anatomical and physiological methods were used to express ST. RESULTS: Specific tension, using activated vs. the total muscle CSA, was 25 ± 6 vs. 20 ± 3 N/cm2 (p <0.05). Physiological CSA was about 4 times (p <0.0001) anatomical CSA. Torque generated during SNMES was correlated to the activated physiological CSA (r2 = 0.49, p < 0.008). CONCLUSION: The results suggest assuming the entire muscle mass is recruited during a maximal voluntary effort is incorrect and results in underestimation of ST. When SNMES is used to control recruitment at 100% of the stimulated muscle, ST is 20 % higher than when a maximal voluntary effort is made. This suggests recruitment of about 80 % of the knee extensors during maximal voluntary isometric effort. The positive relation between activated CSA and the resultant torque strongly suggests ST has meaning, but the correct factors must be used in its expression.