Neuromuscular characteristics of eccentric contractions of the knee extensors and their muscle damage profiles

Abstract

Muscle can generate greater force with lower muscle activation during eccentric (ECC) than isometric (ISO) and concentric (CON) contractions (actions). Unaccustomed ECC exercise induces muscle damage represented by prolonged impairment of muscle function and delayed onset muscle soreness (DOMS), but also confers protection from muscle damage, known as the repeated bout effect (RBE). However, the neuromuscular characteristics of ECC contractions, and the mechanisms involved in the muscle damage and RBE induced by ECC are not fully understood. Therefore, the purpose of this PhD thesis was to examine the neuromuscular characteristics of ECC contractions of the knee extensors in relation to muscle damage and the RBE profiles. This thesis consisted of five studies as summarised below. Study 1: Short-interval intracortical inhibition (SICI) is often used to examine inhibitory responses in the primary motor cortex representation of the quadriceps, but appropriate pairedpulse transcranial magnetic stimulation (TMS) parameters to optimise SICI measurement were not clear. Using 9 men and 6 women (26.6 ± 4.4 y), responses to single and paired-pulse (3-ms interval) TMS were recorded from vastus lateralis (VL), rectus femoris (RF) and vastus medialis (VM). Test stimulus intensity was 140% of active motor threshold (AMT), and conditioning stimulus intensities (CSIs) ranged from 55-90% (eight intensities) of AMT (5 test and 5 paired responses for each intensity). With CSI of 55% AMT, SICI was minimal (conditioned:test motor evoked potential [MEP]; 1.00, 0.96 and 0.95 for VL, RF and VM, respectively, -1.00 indicates inhibition). Inhibition was greater at 70-90% AMT for VL (0.67- 0.85), at 75-90% AMT for RF (0.70-0.78) and at 80-90% AMT for VM (0.59-0.68) when compared to 55% AMT. The CSIs that elicited maximal and 50% maximal inhibition were ~84% and ~75% AMT, respectively. This method was shown to provide a practical approach to investigate quadriceps inhibitory networks. Study 2: Responses to TMS, twitch forces (TF) and voluntary drive were compared between ECC, ISO and CON contractions of the knee extensors. Sixteen participants (20-33 y) performed submaximal and maximal voluntary contractions (MVCs) for ISO and isokinetic (30˚/s) CON and ECC of knee extensors. EMG was recorded from VL. Supramaximal femoral nerve stimulation during and after MVCs evoked superimposed (ST) and resting TF to calculate voluntary activation (VA). Maximal M-waves (MMAX) were recorded. During 30% MVCs, single- and paired-pulse TMS elicited MEPs and assessed SICI. MVC torque was greater (P Study 3: ECC-only and coupled concentric-eccentric contractions (CON-ECC) of the knee extensors were compared for changes in neurophysiological indices before, immediately after and 1-3 days post-exercise. Twenty participants (19-36 y) were randomly assigned to ECC (n=10) or CON-ECC (n=10) group. ECC group performed 6 sets of 8 ECC-only contractions at 80% of ECC one-repetition maximum (1-RMecc), while CON-ECC group performed 6 sets of 8 alternating CON and ECC (i.e., 8 CON and 8 ECC) contractions at 80% of CON 1-RM and 1-RMecc, respectively. Maximal voluntary isometric contraction (MVIC) force, rate of force development (RFD), TF elicited by femoral nerve stimulation, VA, MEPs, corticospinal silent period (CSP) and SICI assessed by single- and paired-pulse TMS, and muscle soreness were measured before, immediately after, and 1-3 days post-exercise. No significant (p\u3e0.05) differences between ECC and CON-ECC were evident for the changes in any variables post3 exercise. MVIC force decreased immediately post-exercise compared to baseline (ECC: -20.7 ± 12.8%, CON-ECC: -23.6 ± 23.3%) and remained low at 3 days post-exercise (-13.6 ± 13.4%, -3.3 ± 21.2%), and changes in RFD were greater than those of MVIC force (immediately postexercise: ECC: -38.3 ± 33.9%, CON-ECC: -30.7 ± 38.3%). VA, TF and MEP/MMAX decreased and CSP increased post-exercise (p Study 4: Since the participants from Study 3 responded similarly for indirect indicators of muscle damage, the 20 participants were used to examine whether the magnitude of muscle damage indicated by changes in MVIC force 1-3 days after ECC could be predicted by changes in central and peripheral neuromuscular parameters immediately post-ECC. The criterion measures were the same as those in Study 2, and additional analyses of the rate of force development (RFDRT) and rate of relaxation (RRRT) of the TF were performed. Relationships between changes in the variables immediately post-ECC and changes in MVIC force at 1-3 days post-ECC were examined by Pearson product-moment (r) or Spearman correlations. MVIC force decreased (-22.2 ± 18.4%) immediately post-exercise, and remained below baseline at 1 (-16.3 ± 15.2%), 2 (-14.7 ± 13.2%) and 3 days post-ECC (-8.6 ± 15.7%). Immediately post-ECC, RFD (0-30-ms: -38.3 ± 31.4%), TF (-45.9 ± 22.4%), RFDRT (-32.5±40.7%), RRRT (-38.0±39.7%), VA (-21.4 ± 16.5%) and MEP/MMAX at rest (-42.5 ± 23.3%) also decreased, while CSP at 10%-MVIC increased by 26.0 ± 12.2% (p Study 5: Among the 20 participants described in Study 4, 10 participants (6 from the eccentriconly exercise group and 4 from the concentric-eccentric exercise group) returned two weeks after the first exercise bout to perform the second bout of the same exercise. Changes in MVIC force, RFD, muscle soreness, TF, VA, MMAX, MEPs, and SICI before, immediately after and 1–3 days post-exercise were compared between the first (ECC1) and the second bouts (ECC2). ECC2 induced less (P=0.01) muscle soreness (peak: 27.5 ± 26.6 mm) than ECC1 (50.7 ± 31.8 mm), and MVIC force decreased more immediately post-ECC1 (-21.6 ± 23.3%) than ECC2 (-11.0 ± 11.3%) and recovered faster to the baseline after ECC2 than ECC1. RFD decreased immediately after ECC1 (e.g. 0-50-ms: -50.5 ± 52.5%, P0.05). The non-significant differences in VA and MEPs between ECC1 and ECC2, and lack of changes in MMAX and SICI suggest that changes in neural factors after eccentric exercise do not explain the repeated bout effect. Collectively, these studies advance understanding of the characteristics of ECC muscle contractions and their relationship to muscle damage and the RBE, and can be used to inform future research directions