Biarticular and monoarticular muscle activation and injury in human quadriceps muscle

We hypothesized that activation of the quadriceps femoris muscle group duri ng eccentric exercise is related to the increase in magnitude of several ma rkers of muscle injury that developed during the next week. Fourteen male s ubjects performed six to eight sets of five to ten repetitions of single-le g eccentric-only seated knee extension exercise. Magnetic resonance (MR) im ages were collected before and immediately after exercise and on days 2-4 a nd 6 after eccentric exercise. Changes in maximal voluntary contraction (MV C), perceived soreness, muscle volume and muscle transverse relaxation of w ater protons (T-2) were determined for the quadriceps femoris muscle group each day. Changes in muscle volume and T-2 were determined every day for ea ch muscle [vastus lateralis (VL), vastus medialis (VM), vastus intermedius (VI), rectus femoris (RF)] of the quadriceps femoris group. Post-exercise T -2 was greater than pre-exercise T-2 (P < 0.05) for all muscles. The acute <Delta>T-2 (Post-Pre) was similar (P > 0.05) among VL, VM, VI, and RF [5.5 (0.3) ms], suggesting that the four muscles were equally activated during e ccentric exercise. In the week after eccentric exercise, subjects experienc ed delayed-onset muscle soreness (DOMS) and all muscles demonstrated a dela yed increase in T-2 above pre-exercise values (P < 0.05), suggesting that m uscle injury had occurred. For the quadriceps femoris muscle group, there w as no correlation between acute <Delta>T-2 and delayed (peak T-2 during day s 2, 3, 4, 6 minus pre-exercise T-2) DeltaT(2) (r = -0.04, P > 0.05). Simil ar results were obtained when VL, VM, VI and RF were examined separately. O f the four muscles in quadriceps femoris, the biarticular RF experienced gr eater muscle injury [delayed DeltaT(2) = 15.2 (2.0) ms] compared to the thr ee monoarticular vasti muscles [delayed DeltaT(2) = 7.7 (1.3) ms; P < 0.05] . We propose that the disproportionate muscle injury to RF resulted from an ineffective transfer of torque from the knee to hip joint during seated ec centric knee extension exercise, thus causing RF to dissipate greater energ y than normal. We conclude that in humans, muscle activation is not a uniqu e determinant of muscle injury.