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Factors contributing to an increase in quadriceps specific tension following resistance training in young men

Erskine, Robert M. and Degens, Hans and Jones, David A. (2008) Factors contributing to an increase in quadriceps specific tension following resistance training in young men. [Conference or Workshop Item]

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BACKGROUND: The maximal force a muscle can generate depends on the number of sarcomeres in parallel and thus its physiological cross sectional area (PCSA). However, the increase in muscle strength with training is widely reported to be greater than expected from the increase in size (1). The aim of the present investigation was to systematically address potential problems that may be caused by changes in voluntary activation and coactivation and changes in muscle architecture during maximum voluntary contraction (MVC). To our knowledge, this is the first study to investigate the effect of resistance training on specific tension taking into account the structural differences between the four quadriceps muscles. METHODS: Fourteen healthy male volunteers aged 21 ± 3 yrs performed unilateral leg-extension (4 sets of 10 repetitions at 80% 1RM), 3 times/wk for 9 weeks. Quadriceps tendon force (Ft) was calculated by correcting maximum isometric torque obtained at the optimum knee angle for antagonist coactivation (estimated from electromyographic activity), voluntary activation (using the interpolated twitch technique), patella tendon moment arm length and the ratio of quadriceps tendon force to patella tendon force (2). The PCSA of each quadriceps muscle was calculated by dividing the volume measured using magnetic resonance imaging by ultrasound measurements of fascicle length at MVC. The effective PCSA of the whole quadriceps was determined as the sum of the PCSAs of the four constituent muscles, each multiplied by the cosine of the appropriate pennation angle. The specific force of the quadriceps femoris muscle was obtained by dividing Ft by the sum of the effective PCSAs. RESULTS: Isometric torque increased by 33 ± 12%, (p<0.05) and total quadriceps ACSA increased by 5 ± 4% (p<0.05); quadriceps volume increased by 6 ± 3% (p<0.05); quadriceps PCSA did not change significantly (+5 ± 11%); Ft increased by 23 ± 18% (p<0.05). Specific tension of the complete quadriceps femoris increased by 22 ± 18%, from 53 ± 9 N/cm2 to 64 ± 10 N/cm2 (p<0.05). DISCUSSION: The increase in specific tension confirms previous reports (see reference 1) and was not explicable by changes in muscle architecture, voluntary activation and/or coactivation. It is still not clear what causes the large specific force values or increase in specific force with training but differences or changes in myofibrillar packing and lateral transmission of force remain possibilities to be examined.

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