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- The effect of short-term endurance and strength training on motor unit conduction velocityPublication . Vila-Chã, Carolina; Falla, Deborah; Velhote, Miguel C.; Farina, DarioAIM: The aim of this study was to investigate the effect of strength and endurance training on the conduction velocity of vastus medialis obliquus and lateralis single motor units during voluntary sustained knee extensions. METHODS: Seventeen sedentary healthy men (age, mean ± SD, 26.3 ± 3.9 yr) were randomly assigned to one of 2 groups: strength training (ST, n= 8) or endurance training (ET, n= 9). Conventional endurance and strength training was performed three days per week, over a period of 6 weeks. Motor unit conduction velocity (MUCV), maximum voluntary force (MVC) and time-to-task failure at 30% MVC of the knee extensors were measured before and immediately following training. To assess MUCV, multi-channel surface and intramuscular EMG signals were concurrently recorded from the vastus medialis obliquus (VMO) and vastus lateralis (VL) muscles during 60-s isometric knee extensions at 10% and 30% of MVC. RESULTS: After 6 weeks of training, MVC increased in the ST group (16.7 ± 7.4 %; P < 0.05) whereas time to task failure was prolonged in the ET group (33.3 ± 14.2 %; P < 0.05). Both training programs induced an increase in motor unit conduction velocity at both 10% and 30% MVC (P < 0.01). Furthermore after both training programs, the reduction in MUCV over time during the sustained contractions occurred at slower rates compared to baseline (P < 0.01). CONCLUSION: These results indicate that short-term endurance and strength training induce similar alterations of the electrophysiological membrane properties of the muscle fiber and in their changes during sustained contractions
- Electrophysiological assessment of neuromuscular adaptations to trainingPublication . Vila-Chã, CarolinaOver the last decades, it has been shown that the human neuromuscular system is highly adaptive and can be modified in response to different motor training programs. Depending on the demands of the motor training, the adaptations seem to involve distinct structural and functional changes across the motor cortex, spinal cord and skeletal muscle. The technological development observed in the last years, increased the use of electrophysiological techniques to assess the neuromuscular adaptations to motor training. Nonetheless, the current evidences on the neuromuscular adaptations to different motor training are inconsistent and incomplete, in particular regarding endurance and strength training. This is mainly due to lack of studies based on a rigorous consideration of the limitations of the available techniques. Therefore, the main goal of this dissertation is to give new insights on the adaptations of the neuromuscular system by systematically investigating the changes in its central and peripheral properties in response to endurance and strength training. For this purpose, recent developed techniques for recording and processing electromiographycal (EMG) signals were applied. The first study (STUDY I) investigated if 6 weeks of either endurance or strength training alters the motor unit behavior and if such changes were accompanied by alterations in muscle fiber properties. Intramuscular and multichannel surface EMG recordings were used to investigate the motor unit discharge rates and motor unit conduction velocity (MUCV) of the vastus medialis obliquus and vastus lateralis during submaximal isometric contractions. The results demonstrated that endurance training increased endurance capacity and was accompanied by a decrease of the motor unit discharge rates. In contrast, strength training enhanced maximum force output and was accompanied by an increase of the motor unit discharge rates. By the end of 6 weeks of training, both training programs elicited increases in the motor unit conduction velocity, revealing electrophysiological adaptations of the muscle fiber membrane properties in similar directions. However, in the first 3 weeks of training, when changes in motor unit discharge rates were most marked, changes in MUCV were not observed. These findings reveal different time courses of some of the neural and peripheral adaptations in response to different motor training programs. The observed changes may contribute for distinct neuromuscular fatigue profiles among endurance and strength-trained athletes. Therefore, the aim of the second study (STUDY II) was to investigate the effects of 6 weeks of endurance and a strength training program on acute responses of the muscle fiber membrane properties and discharges rates of low threshold motor units of the vastus medialis obliquus and vastus lateralis muscles during prolonged submaximal isometric contractions. The conduction velocity of the individual motor units was estimated from the averaged multichannel EMG surface potentials by a spike triggered average technique. It was shown that motor unit discharge rate declines over the duration of the sustained contraction and their trend was not significantly affected by training. Conversely, the rate of decline of motor unit conduction velocity during sustained contractions was reduced following six weeks of both endurance and strength training, however a greater reduction is observed following endurance training. These alterations likely contribute to longer times to task failure following endurance training. The third study (STUDY III) intended to clarify the mechanisms involved in the opposite adjustments of the motor unit discharge rate observed in the study I. The results revealed that following 3 weeks of endurance training the excitability in the H-reflex pathway increased but the V-wave amplitude remained unchanged. In contrast, following strength training the V-wave amplitude increased whereas subtle changes were observed in the H-reflex pathway. These results suggest that the elements of the H-reflex pathway are strongly involved in chronic adjustments in response to endurance training, contributing to enhance resistance to fatigue. Conversely, following strength training, it is more likely that increased descending neural drive during MVC and/or modulation in afferents other than Ia afferents contributed to increased motoneuron excitability and maximal voluntary contraction. The present work revealed for the first time that endurance and strength training induces opposite adjustments in the motor unit behavior. Moreover, the distinct adjustments in the spinal cord output, seems to result from changes in different neural mechanisms located at supraspinal and/or spinal level. The neural adjustments following endurance training seems to result from changes at spinal level whereas the adjustments following strength training are likely due to changes at supraspinal level. These adaptations occurred following a short period of training, while no changes in the contractile and electrophysiological properties of the muscle fibers were detectable. Changes at peripheral level occurred only following a longer period of training.
- Changes in H reflex and V wave following short-term endurance and strength trainingPublication . Vila-Chã, Carolina; Falla, Deborah; Velhote, Miguel C.; Farina, DarioThis study examined the effects of 3 wk of either endurance or strength training on plasticity of the neural mechanisms involved in the soleus H reflex and V wave. Twenty-five sedentary healthy subjects were randomized into an endurance group (n 13) or strength group (n 12). Evoked V-wave, H-reflex, and M-wave recruitment curves, maximal voluntary contraction (MVC), and time-to-task-failure (isometric contraction at 40% MVC) of the plantar flexors were recorded before and after training. Following strength training, MVC of the plantar flexors increased by 14.4 5.2% in the strength group (P 0.001), whereas time-to-task-failure was prolonged in the endurance group (22.7 17.1%; P 0.05). The V wave-to-maximal M wave (V/Mmax) ratio increased significantly (55.1 28.3%; P 0.001) following strength training, but the maximal H wave-to-maximal M wave (Hmax/Mmax) ratio remained unchanged. Conversely, in the endurance group the V/Mmax ratio was not altered, whereas the Hmax/Mmax ratio increased by 30.8 21.7% (P 0.05). The endurance training group also displayed a reduction in the H-reflex excitability threshold while the H-reflex amplitude on the ascending limb of the recruitment curve increased. Strength training only elicited a significant decrease in H-reflex excitability threshold, while H-reflex amplitudes over the ascending limb remained unchanged. These observations indicate that the H-reflex pathway is strongly involved in the enhanced endurance resistance that occurs following endurance training. On the contrary, the improvements in MVC following strength training are likely attributed to increased descending drive and/or modulation in afferents other than Ia afferents.
- Effect of unaccustomed eccentric exercise on proprioception of the knee in weight and non-weight bearing tasksPublication . Vila-Chã, Carolina; Riis, Simone; Lund, Ditte; Møller, Anders; Farina, Dario; Falla, DeborahThe study investigates the effects of eccentric exercise of the quadriceps on proprioception of the knee in weight and non-weight bearing tasks. Proprioception of the exercised leg was assessed at 120 and 150 of knee extension in 15 healthy adults (age 25.0 ± 3.6 yrs) before, immediately after, and 24 h following eccentric exercise of the quadriceps. Three tests of proprioception were performed: 1. matching the position of the exercised leg (right leg) to the reference leg (left leg) in sitting (non-weight bearing matching task); 2. repositioning the exercised leg after active movement in sitting (non-weight bearing repositioning task); 3. repositioning the exercised leg after active movement in standing (weight bearing task). Maximum knee extension force was reduced by 77.0 ± 12.3 % immediately after the exercise, and by 82.7 ± 16.2% 24 h post exercise, with respect to baseline (P < 0.001). The absolute error in the non-weight bearing matching task at 120 of knee extension was greater immediately following eccentric exercise (12.3 ± 5.6, P < 0.001) and 24 h after exercise (8.1 ± 4.5, P < 0.05) compared to baseline (5.8 ± 2.7). Similarly, the absolute error in the non-weight bearing repositioning task at 120 was greater both immediately (5.9 ± 3.1 , P < 0.01) and 24 h post exercise (5.2 ± 3.0 , P < 0.05) compared to baseline (4.5 ± 2.6 ). Therefore, in both non-weight bearing tasks, the subjects matched the position of their leg after eccentric exercise by adopting a more extended knee position of the exercised limb. Furthermore, the subjects showed higher variability in their performance immediately post exercise (P < 0.05, compared to baseline) but not 24 h after. In contrast, eccentric exercise did not affect the repositioning errors in the weight bearing task. In conclusion, eccentric exercise of the quadriceps impairs proprioception of the knee both immediately after and 24 h post exercise, but only in non-weight bearing tasks.
- Eccentric exercise and delayed onset muscle soreness of the quadriceps induce adjustments in agonist–antagonist activity, which are dependent on the motor taskPublication . Vila-Chã, Carolina; Hassanlouei, H.; Farina, Dario; Falla, DeborahThis study investigates the effects of eccentric exercise and delayed onset muscle soreness (DOMS) of the quadriceps on agonist–antagonist activity during a range of motor tasks. Ten healthy volunteers (age, mean ± SD, 24.9 ± 3.2 years) performed maximum voluntary contractions (MVC) and explosive isometric contractions of the knee extensors followed by isometric contractions at 2.5, 5, 10, 15, 20, and 30% MVC at baseline, immediately after and 24 h after eccentric exercise of the quadriceps. During each task, force of the knee extensors and surface EMG of the vasti and hamstrings muscles were recorded concurrently. Rate of force development (RFD) was computed from the explosive isometric contraction, and the coefficient of variation of the force (CoV) signal was estimated from the submaximal contractions. Twenty-four hours after exercise, the subjects rated their perceived pain intensity as 4.1 ± 1.2 (score out of 10). The maximum RFD and MVC of the knee extensors was reduced immediately post- and 24 h after eccentric exercise compared to baseline (average across both time points: 19.1 ± 17.1% and 11.9 ± 9.8% lower, respectively, P\0.05). The CoV for force during the submaximal contractions was greater immediately after eccentric exercise (up to 66% higher than baseline, P\0.001) and remained higher 24 h postexercise during the presence of DOMS (P\0.01). For the explosive and MVC tasks, the EMG amplitude of the vasti muscles decreased immediately after exercise and was accompanied by increased antagonist EMG for the explosive contraction only. On the contrary, reduced force steadiness was accompanied by a general increase in EMG amplitude of the vasti muscles and was accompanied by increased antagonist activity, but only at higher force levels ([15% MVC). This study shows that eccentric exercise and subsequent DOMS of the quadriceps reduce the maximal force, rate of force development and force steadiness of the knee extensors, and is accompanied by different
- Adjustments in motor unit properties during fatiguing contractions after trainingPublication . Vila-Chã, Carolina; Falla, Deborah; Velhote, Miguel C.; Farina, DarioThe objective of the study was to investigate the effect of strength and endurance training on muscle fiber membrane properties and discharge rates of low-threshold motor units of the vasti muscles during fatiguing contractions. Methods: Twenty-five sedentary healthy men (age (mean T SD) = 26.3 T 3.9 yr) were randomly assigned to one of three groups: strength training, endurance training, or a control group. Conventional endurance and strength training was performed 3 dIwkj1, during a period of 6 wk. Motor unit conduction velocity and EMG amplitude of the vastus medialis obliquus and lateralis muscles and biceps femoris were measured during sustained isometric knee extensions at 10% and 30% of the maximum voluntary contraction before and immediately after training. Results: After 6 wk of training, the reduction in motor unit conduction velocity during the sustained contractions at 30% of the maximum voluntary force occurred at slower rates compared with baseline (P G 0.05). However, the rate of decrease was lower after endurance training compared with strength training (P G 0.01). For all groups, motor unit discharge rates declined during the sustained contraction (P G 0.001), and their trend was not altered by training. In addition, the biceps femoris–vasti coactivation ratio declined after the endurance training. Conclusions: Short-term strength and endurance training induces alterations of the electrophysiological membrane properties of the muscle fiber. In particular, endurance training lowers the rate of decline of motor unit conduction velocity during sustained contractions more than strength training
- Motor unit conduction velocity during sustained contraction after eccentric exercisePublication . Hedayatpour, Nosratollah; Falla, Deborah; Arendt-Nielsen, Lars; Vila-Chã, Carolina; Farina, DarioBACKGROUND:Eccentric contractions induce muscle fiber damage that is associated with a decreased capacity to generate voluntary force and increased fiber membrane permeability. Changes in fiber membrane permeability results in cell depolarization that is expected to have an effect on the action potential propagation velocity of the muscle fibers. PURPOSE:The aim of the study was to investigate the action potential propagation velocity in individual motor units before and 24 and 48 h after eccentric exercise. METHODS:Multichannel surface and fine-wire intramuscular EMG signals were concurrently recorded from two locations of the right vastus medialis muscle of 10 healthy men during 60-s isometric contractions at 10% and 30% of the maximal force. RESULTS:The maximal force decreased by 26.1 ± 16.1% (P < 0.0001) at 24 h and remained reduced by 23.6 ± 14.5% (P < 0.0001) 48 h after exercise with respect to baseline. With respect to baseline, motor unit conduction velocity decreased (P < 0.05) by (average over 24 and 48 h after exercise) 7.7 ± 2.7% (10% maximal voluntary contraction (MVC), proximal), 7.2 ± 2.8% (10% MVC, distal), 8.6 ± 3.8% (30% MVC, proximal), and 6.2 ± 1.5% (30% MVC, distal). Moreover, motor unit conduction velocity decreased over time during the sustained contractions at faster rates when assessed 24 and 48 h after exercise with respect to baseline for both contraction forces and locations (P < 0.05). CONCLUSIONS:These results indicate that the electrophysiological membrane properties of muscle fibers are altered by exercise-induced muscle fiber damage.
- Differences in motor unit behavior following endurance and strength trainingPublication . Vila-Chã, Carolina; Falla, Deborah; Velhote, Miguel C.; Farina, DarioAim: Strength and endurance training are two common forms of motor training which induce distinct adaptive neuromuscular responses. There is a considerable debate on the neural adaptations which accompany strength training and little is known about the effects of endurance training. This study investigated changes in motor output and motor unit behavior following 6 weeks of either strength or endurance training. Methods: Twenty-eight sedentary healthy men (age, mean ± SD, 26.1 ± 3.9 yr) were randomly assigned to one of 3 groups: strength training (ST); endurance training (ET) and; control (C; no exercise intervention). The training was performed three days per week, over a period of 6 weeks. Maximum voluntary force (MVC), time-to-task failure at 30% MVC, and rate of force development (RDF) of the knee extensors were measured before and post training (week 7). Additionally, motor unit behavior and muscle activity were studied with intramuscular and surface EMG recordings from the vastus medialis obliquus (VMO) and vastus lateralis (VL) muscles during sustained isometric knee extensions at 10% and 30% MVC. The surface EMG was recorded also during MVC and explosive isometric contractions. Results: In the ST group, the percent change post-training in MVC and RFD was significantly greater (13.8 ± 3.6% and 19.8 ± 5.9%, respectively) compared to the ET (P < 0.05) and C group (P < 0.05). In contrast, the time-to-task failure increased significantly only in the ET group (percent increase: 19.8 ± 5.9%; P < 0.05). For the isometric submaximal contractions (30% MVC), the average rectified value of the surface EMG of VL and VMO increased post training for both ET and ST groups (P < 0.05). For the same contractions, strength training but not endurance training resulted in a significant increase in the mean motor unit discharge rates for the VMO (1.22 pps; P = 0.002) and VL (1.59 pps; P = 0.001). On the contrary, mean discharge rates were reduced following training for the ET group (VMO - 1.12 pps; P = 0.03 and VL - 1.09 pps; P = 0.04) despite an increase in surface EMG amplitude. Only the ST group showed an increase in the ARV of the VL and VMO (P < 0.05) during the maximal and explosive isometric conditions. No change was observed for the C group in any of the conditions. Conclusion: Endurance and strength training induce distinct adaptations in the neural control of the vasti muscles. Strength training was shown to increase the discharge rate of vasti motor units during isometric knee extension contractions whereas endurance training resulted in a reduction in motor unit discharge rates.
- The influence of kineanthropometrical profile in deep-water tethered runningPublication . Vila-Chã, Carolina; Morais, J.E.; Barbosa, Tiago M.The purpose of this study was to identify the kineanthropometrical parameters that best predict the maximal horizontal propulsive force during deep-water tethered running. 21 young and healthy males with large experience in aquatic exercises (24.3+/-2.7 years old, 191.9+/-82.6 minutes physical activity per week) performed 3 repetitions of maximal deep-water running for 10-s, using a flotation vest (Golfinho, H-906, Coimbra, Portugal). The subjects were connected to a strain gauge (Globus, Ergo Meter, Codigné, Italy) by a cable of steel with reduced elastic properties. The other end of the cable was fasted to a rubber band and this to a swimming starting block. Dynamometrical data was exported and processed with Matlab v. 6.0. It was evaluated the maximal propulsive force (Fx-max) and computed the maximal horizontal propulsive force through a trigonometric correction, as suggested by Taylor et al. (2003). Body mass (SECA, 884, Hamburg, Germany), height (SECA, 242, Hamburg, Germany), body mass index (BMI) and fat mass (BIA 101, RJL Systems, Florence, Italy) were also measured. Surface area (SA) was calculated according to the procedure of Du Bois and Du Bois (Shuter and Aslani, 2000). The forearms (Globus, Ergo Meter, Codigné, Italy) and hands (TSD 121C, Biopac Systems, California, USA) maximal isometric forces were also measured. Intra-cyclic variation of the Fx-max presented a tetra-modal profile. Computing a stepby- step regression equation, for prediction of the Fx-max, the kineanthropometrical variables that entered the model were the forearms maximal isometric force, the BMI, the body mass and the SA (r^2=0.57, p=0.01). The purpose of the study was achieved. Evidences revealed that some kineanthropometrical parameters related to buoyancy force (e.g., fat mass), to drag force (e.g., SA and height), to weight force (e.g., body mass) and to propulsive force (e.g., segmental forces) predicted the Fx-max. This means that, besides physical fitness and technical level, often described in the literature, kineanthropometrical characteristics of the runner also affect significantly his performance during a training session. The main conclusion is that tethered running is significantly associated to kineanthropometrical profile of the runner. So, instructors should pay attention to kineanthropometrical characteristics of runners and how it affects their performance.
- Effects of eccentric exercise on force steadiness and activation of the knee extensorsPublication . Vila-Chã, Carolina; Falla, Deborah; Farina, DarioAIM: The purpose of this study was to investigate the effects of eccentric exercise of the knee extensors on force steadiness and neural drive to the vasti muscles during submaximal isometric contractions. METHODS: Ten healthy subjects (age, mean ± SD, 24.9 ± 3.2 yrs) participated in the study. The subjects performed maximum voluntary contractions (MVC) of the knee extensors followed by isometric contractions at 2.5, 5, 10, 15, 20 and 30% MVC at baseline, immediately after and 24h after eccentric exercise of the quadriceps. During each contraction, force and surface EMG of the vastus medialis (VM), vastus lateralis (VL), biceps femoris (BF), and semitendinous (ST) muscles were recorded concurrently. Force steadiness was characterized by the coefficient of variation (CoV; SD divided by mean, %) of the force signal. RESULTS: The MVC force decreased from baseline (665.5 ± 256.9 N) to 24 h post exercise (596.2 ± 123.2 N, P<0.001) and from immediately post exercise (636.5 ± 131.8 N) to 24h post exercise (P<0.05). The CoV during the submaximal isometric contractions was greater immediately after eccentric exercise (up to 66% higher than baseline values, P<0.001) and remained higher 24h post exercise (up to 50% higher than baseline values, P<0.01). Reduced force steadiness was accompanied by increased EMG amplitude of the vasti muscles (P<0.01) whereas the EMG amplitude of the BF and ST did not change across conditions (P>0.05). CONCLUSION: Eccentric exercise of the quadriceps impairs knee extension force steadiness and is associated with reduced activation of the vasti muscles both immediately after and 24h after exercise. This reduction in force steadiness is not due to increased antagonist activity.