Neuromuscular and Isometric Force-velocity Adaptations to Concentric and Eccentric Strength Training

Abstract

PURPOSE: To examine the mechanisms underpinning a velocity-specific strength increase after slow-speed concentric or eccentric knee extensor (KE) training, we assessed neuromuscular adaptations to isometric KE contractions performed with and without a maximum rate of force development. METHODS: 10 men and 11 women with no history of resistance training performed 10 wk (3.wk−1) of either eccentric or concentric, isokinetic KE training through 90° at 30o.s−1. Before and after training, concentric and eccentric dynamic KE moment was measured at 30 and 180°.s−1. Also, isometric KE moment was measured in trials with a maximum rate of force development (fast) and in a ramped contraction reaching peak moment after 5 s (slow). EMG was recorded from the superficial quadriceps femoris (QF) and biceps femoris (BF) muscles. Antagonist torque during isometric KE was estimated from the EMG-torque relationship measured during a ramped isometric knee flexion (KF) contraction; KE torque was calculated as the sum of KE moment and KF torque−1. QF volume and patella tendon moment arm were measured from magnetic resonance imaging scans. QF force was calculated as the ratio of KE torque-to-moment arm. RESULTS: Isometric KE moment was greater in fast trials (pre-training: fast = 176.1 ± 44.4 Nm, slow = 147.5 ± 52.9 Nm), which was due to a greater QF muscle force (16.4%) with no difference in BF torque (i.e. decreased KF:KE ratio). Mean QF EMG was greater in fast trials, although differences in individual muscles did not reach significance. After training, isometric force increased in both fast (16.8 Nm, 9.5%) and slow (22.2 Nm, 15.1%) trials in both groups, with the greater increase in slow force mirroring the dynamic force changes (all subjects Δ30°s−1 = 44.1 Nm; Δ180°s−1 = 24.7 Nm). Decreases in KF torque accounted for some of the increase in isometric moment in fast (10.9 Nm, 64.9% of change) and slow (8.9 Nm, 40.1% of change) trials. Despite an increase in QF volume (9.6%; no difference between groups) and a trend toward an increase in average QF EMG in the slow isometric trials (p=0.06; increase in rectus femoris EMG, p <0.05; trend toward increased vastus medialis EMG, p=0.08), training-related increases in isometric QF force or KE torque did not reach statistical significance. CONCLUSIONS: The increased isometric KE moment after slow-speed KE training was greater in trials performed with a slow rate of force development, and was associated with a reduced KF torque with a non-significant increase in KE torque. Proportional increases in dynamic moment were greater than isometric, so larger increases in KE contribution in dynamic movements cannot be ruled out.