Effects of the foot strike pattern on muscle activity and neuromuscular fatigue in downhill trail running

Minimizing musculo‐skeletal damage and fatigue is considered paramount for performance in trail running. Our purposes were to investigate the effects of the foot strike pattern and its variability on (a) muscle activity during a downhill trail run and (b) immediate and delayed neuromuscular fatigue. Twenty‐three runners performed a 6.5‐km run (1264 m of negative elevation change). Electromyographic activity of lower‐limb muscles was recorded continuously. Heel and metatarsal accelerations were recorded to identify the running technique. Peripheral and central fatigue was assessed in knee extensors (KE) and plantar flexors (PF) at Pre‐, Post‐, and 2 days post downhill run (Post2d). Anterior patterns were associated with (a) higher gastrocnemius lateralis activity and lower tibialis anterior and vastus lateralis activity during the run and (b) larger decreases in KE high‐frequency stimulus‐evoked torque Post and larger decrements in KE MVC Post2d. High patterns variability during the run was associated with (a) smaller decreases in KE Db100 Post and MVC Post2d and (b) smaller decreases in PF MVC Post and Post2d. Anterior patterns increase the severity of KE peripheral fatigue. However, high foot strike pattern variability during the run reduced acute and delayed neuromuscular fatigue in KE and PF.     

Contraction velocity influence the magnitude and etiology of neuromuscular fatigue during repeated maximal contractions

This study aimed to compare the magnitude and etiology of neuromuscular fatigue during maximal repeated contractions performed in two contraction modes (concentric vs isometric) and at two contraction velocities (30/s vs 240°/s). Eleven lower limb‐trained males performed 20 sets of maximal contractions at three different angular velocities: 0°/s (KE0), 30/s (KE30), and 240°/s (KE240). Cumulated work, number of contraction, duty cycle, and contraction time were controlled. Torque, superimposed and resting twitches, as well as gas exchange, were analyzed. Increasing contraction velocity was associated with greater maximal voluntary torque loss (KE0: ?9.8 ± 3.9%; KE30: ?16.4 ± 8.5%; KE240: ?32.6 ± 6.3%; P < 0.05). Interestingly, the torque decrease was similar for a given cumulated work. Compared with KE0, KE240 generated a greater evoked torque loss (Db₁₀₀: ?24.3 ± 5.3% vs ?5.9 ± 6.9%; P < 0.001), a higher O₂ consumption (23.7 ± 6.4 mL/min/kg vs 15.7 ± 3.8 mL/min/kg; P < 0.001), but a lower voluntary activation (VA) loss (?4.3 ± 1.6% vs ?11.2 ± 4.9%; P < 0.001). The neuromuscular perturbations were intermediate for KE30 (Db₁₀₀: ?10.0 ± 6.8%; VA: ?7.2 ± 2.8%). Although the amount of mechanical work cumulated strongly determined the magnitude of torque decrease, the contraction velocity and mode influenced the origin of the neuromuscular fatigue. The metabolic stress and peripheral fatigue increased but reduction of VA is attenuated when the contraction velocity increased from 0°/s to 240°/s.     

Central and peripheral fatigue in knee and elbow extensor muscles after a long‐distance cross‐country ski race

Although elbow extensors (EE) have a great role in cross‐country skiing (XC) propulsion, previous studies on neuromuscular fatigue in long‐distance XC have investigated only knee extensor (KE) muscles. In order to investigate the origin and effects of fatigue induced by long‐distance XC race, 16 well‐trained XC skiers were tested before and after a 56‐km classical technique race. Maximal voluntary isometric contraction (MVC) and rate of force development (RFD) were measured for both KE and EE. Furthermore, electrically evoked double twitch during MVC and at rest were measured. MVC decreased more in KE (?13%) than in EE (?6%, P = 0.016), whereas the peak RFD decreased only in EE (?26%, P = 0.02) but not in KE. The two muscles showed similar decrease in voluntary activation (KE ?5.0%, EE ?4.8%, P = 0.61) and of double twitch amplitude (KE ?5%, EE ?6%, P = 0.44). A long‐distance XC race differently affected the neuromuscular function of lower and upper limbs muscles. Specifically, although the strength loss was greater for lower limbs, the capacity to produce force in short time was more affected in the upper limbs. Nevertheless, both KE and EE showed central and peripheral fatigue, suggesting that the origins of the strength impairments were multifactorial for the two muscles.     

Why does knee extensor muscles torque decrease after eccentric-type exercise?

AIM: The purpose of this study was to re-examine central and peripheral origins of neuromuscular fatigue after a highly strenuous eccentric exercise of the knee extensor muscles (KE) using both voluntary/evoked contractions and electromyographic recordings (EMG). METHODS: Before, and 30 min after 15 min of intermittent one-logged downhill running, maximal percutaneous electrical stimulations (single twitch, 0.5 s tetanus at 20 Hz and 80 Hz) were applied to the femoral nerve of 10 male subjects. Electrically evoked superimposed twitches were delivered during isometric maximal voluntary contraction (MVC) to determine maximal voluntary activation (%VA). Vastus lateralis (VL), vastus medialis (VM) and biceps femoris (BF) EMG were recorded during MVC and quantified using the root mean square (RMS) value. M-wave characteristics were also determined. RESULTS: KE MVC and %VA decreased significantly with fatigue (-19.6+/-6.1%; P<0.001 and -7.8+/-6.6%; P<0.01, respectively). Peak tetanus tension at 20 and 80 Hz (P20 and P80, respectively) declined (P<0.001), concurrently with a decrement of the P20 x P80(-1) ratio (-37.3+/-16.6%; P<0.001). Antagonist muscle coactivation, RMS to M-wave peak-to-peak amplitude and MVC x P80(-1) ratios were unchanged after the fatiguing exercise. CONCLUSIONS: The results reveal that part of the large loss in MVC may have a central origin but most of the MVC decrement is due to the presence of low-frequency fatigue while possible contractile failure cannot be excluded.     

Isometric knee extensor fatigue following a Wingate test: peripheral and central mechanisms

Central and peripheral fatigue have been explored during and after running or cycling exercises. However, the fatigue mechanisms associated with a short maximal cycling exercise (30 s Wingate test) have not been investigated. In this study, 10 volunteer subjects performed several isometric voluntary contractions using the leg muscle extensors before and after two bouts of cycling at 25% of maximal power output and two bouts of Wingate tests. Transcranial magnetic stimulation (TMS) and electrical motor nerve stimulation (NM) were applied at rest and during the voluntary contractions. Maximal voluntary contraction (MVC), voluntary activation (VA), twitch amplitude evoked by electrical nerve stimulation, M wave and motor potential evoked by TMS (MEP) were recorded. MVC, VA and twitch amplitude evoked at rest by NM decreased significantly after the first and second Wingate tests, indicating central and peripheral fatigue. MVC and VA, but not the twitch amplitude evoked by NM, recovered before the second Wingate test. These results suggest that the Wingate test results in a decrease in MVC associated with peripheral and central fatigue. While the peripheral fatigue is associated with an intramuscular impairment, the central fatigue seems to be the main reason for the Wingate test‐induced impairment of MVC.     

Role of carbohydrate in central fatigue: a systematic review

Carbohydrate (CHO) depletion is linked to neuromuscular fatigue during exercise. While its role at peripheral level is relatively well understood, less is known about its impact centrally. The aim of this systematic review was to critically analyze the effects of CHO on central fatigue (CF) assessed by various neurophysiological techniques. Four databases were searched using PRISMA guidelines through February 2016. The inclusion criteria were: CHO as intervention against a placebo control, fatigue induced by prolonged exercise and assessed using neurophysiological measures [voluntary activation (VA), superimposed twitch (SIT), M‐wave, electromyography], alongside maximal voluntary contraction (MVC). Seven papers were reviewed, where exercise duration lasted between 115 and 180 min. CHO improved exercise performance in three studies, whereby two of them attributed it to CF via attenuation of VA and SIT reductions, while the other indicated peripheral involvement via attenuation of M‐wave reduction. Although a few studies suggest that CHO attenuates CF, data on its direct effects on neurophysiological outcome measures are limited and mixed. Generally, measures employed in these studies were inadequate to conclude central contribution to fatigue. Factors including the techniques used and the lack of controls render additional confounding factors to make definitive deductions. Future studies should employ consistent techniques and appropriate neurophysiological controls to distinguish CHO effect at central level. The use of pharmacological intervention should be incorporated to elucidate involvement of central mechanisms.     

Endurance capacity and neuromuscular fatigue following high‐ vs moderate‐intensity endurance training: A randomized trial

High‐intensity exercise induces significant central and peripheral fatigue; however, the effect of endurance training on these mechanisms of fatigue is poorly understood. We compared the effect of cycling endurance training of disparate intensities on high‐intensity exercise endurance capacity and the associated limiting central and peripheral fatigue mechanisms. Twenty adults were randomly assigned to 6 weeks of either high‐intensity interval training (HIIT, 6‐8×5 minutes at halfway between lactate threshold and maximal oxygen uptake [50%Δ]) or volume‐matched moderate‐intensity continuous training (CONT, ~60‐80 minutes at 90% lactate threshold). Two time to exhaustion (TTE) trials at 50%Δ were completed pre‐ and post‐training to assess endurance capacity; the two post‐training trials were completed at the pretraining 50%Δ (same absolute intensity) and the “new” post‐training 50%Δ (same relative intensity). Pre‐ and post‐exercise responses to femoral nerve and motor cortex stimulation were examined to determine peripheral and central fatigue, respectively. HIIT resulted in greater increases in TTE at the same absolute and relative intensities as pre‐training (148% and 43%, respectively) compared with CONT (38% and ?4%, respectively) (P≤.019). Compared with pre‐training, HIIT increased the level of potentiated quadriceps twitch reduction (?34% vs ?43%, respectively, P=.023) and attenuated the level of voluntary activation reduction (?7% vs ?3%, respectively, P=.047) following the TTE trial at the same relative intensity. There were no other training effects on neuromuscular fatigue development. This suggests that central fatigue resistance contributes to enhanced high‐intensity exercise endurance capacity after HIIT by allowing greater performance to be extruded from the muscle.     

Fatigue and recovery after high-intensity exercise. Part II: Recovery interventions

The purpose of this study was to determine the effect of three types of recovery intervention to neuromuscular function after high-intensity uphill running exercise. The 20-min recovery interventions were (i) passive, (ii) active (running at 50 % of maximal aerobic speed), and (iii) low-frequency electromyostimulation. Evoked twitch and maximal voluntary contractions of knee extensor muscles (KE) and EMG of the vastus lateralis and vastus medialis were analysed immediately after the exercise, 10 min after the end of the recovery periods, and 65 min after the exercise (Post65). An all-out running test was also performed 80 min after the end of the fatiguing exercise. No significant differences were noted in any measured parameters but a tendency to a better performance during the all-out test was found after the electromyostimulation intervention (297.5 +/- 152.4 s vs. 253.6 +/- 117.1 s and 260.3 +/- 105.8 s after active and passive recovery, p = 0.13 and p = 0.12, respectively). At Post65, isometric maximal voluntary contraction torque did not return to the pre-exercise values (279.7 +/- 86.5 vs. 298.7 +/- 92.6 Nm, respectively; p < 0.05). During recovery, electrically evoked twitch was characterized by an increase of peak torque, maximal rate of force development and relaxation (+ 24 - 33 %; p < 0.001) but these values were still lower at Post65 than pre-exercise. Amplitude and surface of the M-wave decreased during recovery. These results show that the recovery of the voluntary force-generating capacity of KE after an intermittent high-intensity uphill running exercise do not depend on the type of recovery intervention tested here. It can also be concluded that the recovery of twitch contractile properties does not necessarily follow that of maximal muscle strength.     

Central and peripheral fatigue kinetics during exhaustive constant‐load cycling

The kinetics of central and peripheral fatigue development during an intensive constant‐load cycling exercise was evaluated to better understand the mechanisms of task failure. Thirteen males cycled to exhaustion at 80% of maximal power output in intermittent bouts of 6 min of exercise with 4‐min break between bouts to assess quadriceps fatigue with maximal voluntary contractions and single (1 Hz), paired (10 and 100 Hz) potentiated and interpolated magnetic stimulations of the femoral nerve (TwQ). Surface electromyographic signals (EMG) of the quadriceps muscles were recorded during stimulations and cycling. Total cycling duration (TCD) was 27 min 38 s±7 min 48 s. The mechanical response evoked by magnetic stimulation decreased mostly during the first half of TCD (TwQ1 Hz reduction: ?34.4±12.2% at 40% TCD and ?44.8±9.2% at exhaustion; P<0.001), while a reduction in maximum voluntary activation was present toward the end of exercise only (?5.4±4.8% and ?6.4±5.6% at 80% TCD and exhaustion, respectively; P<0.01). The increase in quadriceps EMG during cycling was significantly correlated to the TwQ reduction for the rectus femoris (r²=0.20 at 1 Hz, r²=0.47 at 100 Hz, all P≤0.001). We conclude that peripheral fatigue develops early during constant‐load intense cycling and is compensated by additional motor drive, while central fatigue appears to be associated with task failure.     

Central and peripheral fatigue of the knee extensor muscles induced by electromyostimulation

The main purpose of this study was to characterise neuromuscular fatigue induced by 30 contractions of the knee extensor muscles evoked by electromyostimulation (EMS). Twelve healthy subjects were tested before and after a typical EMS session (frequency: 75 Hz, on-off ratio: 6.25 s on-20 s off) used for quadriceps femoris muscle strengthening. Surface electromyographic (EMG) activity and torque obtained during maximal voluntary and electrically evoked contractions were analysed to distinguish peripheral from central fatigue. Maximal voluntary torque of the knee extensor muscles decreased approximately 20 % (p < 0.001) following EMS. In the same way, peak torque associated to single (p < 0.05) and paired (p < 0.001) stimuli as well as M-wave amplitude (p < 0.05) significantly decreased as a result of EMS. The raw EMG activity of both vastus lateralis and rectus femoris muscle recorded during maximal voluntary isometric contraction significantly decreased after the session (-17.3 and -14.5 %, respectively) whereas no changes were observed when EMG signals were normalised to respective M-wave amplitudes. Similarly, voluntary activation estimated by using the twitch interpolation technique was unchanged following EMS. In conclusion, a typical session of EMS of the knee extensor muscles mainly induced neuromuscular propagation failure while excitation-contraction coupling and neural mechanisms were not significantly affected. It is recommended to interpret surface EMG data together with the corresponding M wave, at least for the knee extensor muscles, in order to distinguish peripheral from central causes of fatigue.     

Resistance training preserves skeletal muscle function during unloading in humans

PURPOSE: The intent of this investigation was to design and evaluate a low-volume, high-intensity resistance-training program to preserve knee extensor (KE) and plantar flexor (PF) size as measured by cross-sectional area (CSA), strength, and neuromuscular function (IEMG) with unloading. METHODS: Thirty-two men (age = 30 +/- 3 yr; weight = 80 +/- 4 kg; height = 181 +/- 2 cm) participated. Sixteen men underwent 21 d of unilateral lower-limb suspension (ULLS) and were assigned to control (ULLS-CON, N = 8) or countermeasures (ULLS-CM, N = 8). The remaining subjects were ambulatory for 21 d and were assigned to control (AMB-CON, N = 8) or countermeasures (AMB-CM, N = 8). Countermeasure subjects performed resistance training every third day during the 21-d period. RESULTS: KE and PF CSA decreased (P < 0.05) 7% in the ULLS-CON, whereas no changes occurred in ULLS-CM, AMB-CON, and AMB-CM. ULLS-CON maximal voluntary contraction (MVC) decreased 17% (P < 0.05) in the KE and PF. ULLS-CON torque-velocity characteristics (concentric and eccentric) decreased (P < 0.05), 22% to 12% and 20% to 14% (slow to fast) in the KE and PF, respectively. ULLS-CM PF increased (P < 0.05) in MVC and eccentric contractions, whereas no other changes occurred in MVC or torque-velocity characteristics in the KE or PF of the ULLS-CM, AMB-CON, and AMB-CM subjects. Submaximal IEMG increased (P < 0.05) whereas maximal IEMG decreased (P < 0.05) in the KE and PF of the ULLS-CON group. However, no change or slight improvements in IEMG activity were found in the KE and PF of the ULLS-CM, AMB-CON, and AMB-CM. CONCLUSION: These results indicate that a resistance-training paradigm employed every third day during 21 d of unloading was effective in maintaining skeletal muscle strength (static and dynamic) and size of the KE and PF.     

Acute neuromuscular and hormonal responses during contrast loading: Effect of 11 weeks of contrast training

The purpose of this study was to assess (1) acute neuromuscular and endocrine responses during a contrast loading protocol and (2) how these acute responses are possibly influenced by 11 weeks of contrast training. Contrast loading tests consisting of 4 sets of 80% 1 RM back squat and 4 sets of squat jump (SJ) were performed before and after training. Bilateral isometric leg extension (LE) assessed the impact of loading on isometric variables pre‐, mid‐, and post‐loading. Potentiated SJ performance was observed in set 2 (4.6%, P<0.05), before training only. Greater indications of fatigue were observed in SJ, isometric force, and vastus lateralis (VL) activation after training (P<0.05). Training‐induced improvements in SJ height, 80% 1 RM squat load, and maximum isometric LE force were observed (12%, 10%, and 7.7%, P<0.05). In conclusion, potentiated SJ performance occurred during a typical contrast loading protocol before the training period. However, potentiated SJ performance may alter through training, and therefore, the responsiveness of the individual should be periodically monitored and training protocols updated when necessary.     

Adaptations to unilateral lower limb suspension in humans.

This study examined the effect of unilateral lower limb suspension (ULLS) on neuromuscular function in humans. Eight subjects (31 +/- 4 years old) performed all ambulatory activity on crutches for 6 weeks while wearing a shoe with a 10-cm sole on the right foot to unweight the left lower limb. Knee extensor (KE) torque during eccentric, concentric, and isometric actions, and electromyography (EMG) of m. vastus lateralis (m. VL), m. gastrocnemius medialis (m. GM) and m. soleus (m. SL) during isometric actions were assessed pre-ULLS, post-ULLS, and after 4 d of recovery. Average muscle cross-sectional area (CSA) of the KE was measured pre- and post-ULLS and that of the ankle extensors (AE) post-ULLS using magnetic resonance imaging. Strength of the KE of the suspended left limb was reduced (p less than 0.05) 21 and 15%, respectively, after ULLS and 4 d later. Average muscle CSA of the left KE decreased (p less than 0.05) 16%. The KE of the non-suspended right limb showed no changes in muscle CSA. Thus, average muscle CSA of the KE of the suspended limb was 17% less (p less than 0.05) than that of the non-suspended limb. Average muscle CSA of the AE, likewise, was smaller (18%, p less than 0.05) in the left than right leg after ULLS. Maximal integrated EMG of VL and overall mean power frequency of GM and SL for submaximal isometric actions were both decreased (p less than 0.05) post-ULLS.(ABSTRACT TRUNCATED AT 250 WORDS)     

Quadriceps EMG/force relationship in knee extension and leg press

PURPOSE: This study compared the relationship between surface electromyographic (EMG) activity and isometric force of m. quadriceps femoris (QF) in the single-joint knee extension (KE) and the multi-joint leg press (LP) exercises. METHODS: Nine healthy men performed unilateral actions at a knee angle of 90 degrees at 20, 40, 60, 80, and 100% of maximal voluntary contraction (MVC). EMG was measured from m. vastus lateralis (VL), m. vastus medialis (VM), m. rectus femoris (RF), and m. biceps femoris (BF). RESULTS: There were no differences in maximum EMG activity of individual muscles between KE and LP. The QF EMG/force relationship was nonlinear in each exercise modality. VL showed no deviation from linearity in neither exercise, whereas VM and RF did. BF activity increased linearly with increased loads. CONCLUSIONS: The EMG/force relationship of all quadricep muscles studied appears to be similar in isometric multi-joint LP and single-joint KE actions at a knee angle of 90 degrees. This would indicate the strategy of reciprocal force increment among muscles involved is comparable in the two models. Furthermore, these data suggest a nonuniform recruitment pattern among the three superficial QF muscles and surface EMG recordings from VL to be most reliable in predicting force output.     

Vitamin and mineral supplementation and neuromuscular recovery after a running race

PURPOSE: This double-blind study investigated the effects of vitamin and mineral complex supplementation on the neuromuscular function of the knee-extensor muscles after a prolonged trail running race. METHODS: Twenty-two well-trained endurance runners took either placebo (Pl group) or vitamins and minerals (Vm group) for 21 d before the race and for 2 d after the race. Maximal voluntary contractions (MVC) and surface EMG activity of the vastus lateralis (VL) muscle were recorded before (pre) and 1 h (post), 24 h (post 24) and 48 h (post 48) after the race. Central activation ratio (CAR), neural (M-wave), and contractile (muscular twitch) properties of the quadriceps muscles were analyzed using electrical stimulation techniques. RESULTS: The knee-extensor MVC was significantly (P < 0.01) reduced after exercise for both groups (Vm: 36.5 +/- 3.0 %; Pl: 36.9 +/- 2.1%), but MVC recovery was greater for Vm than Pl after 48 h (11%, P < 0.05). The reduced MVC after exercise was associated with a significant reduction in maximal EMG normalized to the M-wave in VL muscle and in CAR for both groups. Characteristics of the muscular twitch were not significantly altered for either groups, whereas M-wave duration increased significantly (P < 0.05) after exercise. CONCLUSIONS: The reduction of MVC immediately after the race appeared to result from peripheral mechanisms such as a failure in muscle membrane excitation and, to a lesser extent, from reduced central activation. The cause of the depressed MVC 24 h after the race seemed to be located within the muscle itself. A dietary supplementation of a vitamin and mineral complex does not attenuate the loss of contractile function immediately after the running exercise, and it may accelerate the recovery of maximal force capacity.     

Acute fatigue-induced changes in muscle mechanical properties and neuromuscular activity in elite handball players following a handball match

The purpose of the present study was to determine the acute fatigue development in muscle mechanical properties and neuromuscular activity in response to handball match play. Male elite handball players ( n = 10) were tested before and after a simulated handball match for maximal isometric strength [maximal voluntary contraction (MVC)] and rate of force development (RFD) with synchronous electromyography (EMG) recording, while maximal vertical jump parameters were assessed using force plate analysis. Quadriceps and hamstrings MVC and RFD decreased significantly post-match (∼10%, P <0.05 and ∼16–21%, P <0.05, respectively). During quadriceps, MVC mean EMG amplitude [mean average voltage (MAV)] decreased for the vastus lateralis (VL) and rectus femoris (RF) (21–42%, P ≤0.05), while MAV also decreased in the antagonist biceps femoris (BF) muscle (48–55%, P <0.01). During hamstring MVC, MAV was reduced in BF (31%, P <0.01). Maximum EMG amplitude during quadriceps MVC was reduced for the VL (28%, P <0.01) and the RF (5%, P <0.05). During hamstring MVC, maximum EMG was reduced for BF (21%, P <0.01). Post-match maximal jump height was reduced (5.2%, P <0.01), as was also work (6.8%, P <0.01), velocity of center of mass (2.4–4.0%, P <0.01) and RFD (∼30%, P <0.05). In conclusion, maximal (MVC) and rapid muscle force characteristics (RFD, impulse) were acutely affected concurrently with marked reductions in muscle EMG following handball match play, which may potentially lead to impaired functional performance.     

Neuromuscular fatigue during sustained contractions performed in short-term hypoxia

PURPOSE: Hypoxia is known to change neuronal activity in vitro and to impair performance in vivo. The present study was designed to study neuromuscular fatigue in acute hypoxia, and we hypothesized that hypoxia results in additional fatigue during sustained contractions, presumably because of increased central fatigue. METHODS: Twelve healthy subjects participated in a normoxic (NX) and hypoxic (HX) experiment performed on separate days. Hypoxia was induced by breathing an HX air mixture containing 12% oxygen. Before, during, and after a 90-s sustained voluntary maximal contraction (MVC) of the first dorsal interosseus muscle, we measured force, voluntary activation (VA), and parameters of motor cortical excitability (motor-evoked potentials (MEP) and silent periods (SP)). Measures of peripheral nerve and muscle function, compound motor action potential (M-wave), and muscle twitch forces were also taken. RESULTS: During the MVC, force declined similarly during both HX and NX. VA decreased throughout the contraction in HX, but, surprisingly, this decrease in VA in HX did not exceed that observed in NX. Also, motor cortical excitability changed to a similar degree in HX and NX; that is, MEP amplitude and SP duration increased. M-wave amplitude decreased significantly during the sustained MVC in NX and HX. The only difference observed between NX and HX was the quicker recovery of the muscle twitch in HX, which was even potentiated after 5 min of recovery. CONCLUSION: The present results show that peripheral and central neuromuscular adaptations during a sustained fatiguing contraction are similar in NX and HX. The quicker recovery and potentiation of twitch forces in HX suggest alterations in myosin phosphorylation, which may enhance contractile force.     

Heavy‐resistance exercise‐induced increases in jump performance are not explained by changes in neuromuscular function

Post‐activation potentiation (PAP) is the increased involuntary muscle twitch response to stimulation following strong contraction. The enhancement to whole‐body explosive muscular performance (PE) after heavy‐resistance exercise is often attributed to modulations in neuromuscular function that are proposed to reflect PAP, but the evidence to support this is equivocal. We assessed the neuromuscular basis of PE using transcranial magnetic stimulation (TMS) of the primary motor cortex, and electrical stimulation of the femoral nerve. Eleven male athletes performed heavy‐resistance exercise with measures of countermovement jump (CMJ) pre‐ and 8 min post‐exercise. Pre‐exercise and after the final CMJ, single‐ and paired‐pulse TMS were delivered during submaximal isometric knee‐extensor contractions to measure corticospinal excitability, short‐interval intracortical inhibition (SICI), and intracortical facilitation (ICF), with motor evoked potentials recorded from rectus femoris. Twitch responses to motor nerve stimulation during and post maximum‐knee‐extensor contractions were studied to quantify voluntary activation (VA) and potentiated twitch (Q_tw,pot). The experimental protocol successfully induced PE (+4 ± 1% change in CMJ, P = 0.01), but no changes were observed for maximum voluntary force, VA, corticospinal excitability, SICI or ICF (all P > 0.05), and Q_tw,pot declined (P < 0.001). An enhancement of muscular performance after heavy‐resistance exercise was not accompanied by PAP, or changes in measures of neuromuscular function.     

Athletes with an ACL reconstruction show a different neuromuscular response to environmental challenges compared to uninjured athletes

BackgroundEvidence suggests that neuromuscular alterations in patients with an anterior cruciate ligament reconstruction (ACLR) are rooted in neurocognitive and proprioceptive deficits. The aim of this study was to assess neuromuscular control of athletes with ACLR under increased cognitive and environmental challenges.Research questionDo athletes with ACLR show a different neuromuscular response to cognitive and environmental challenges relative to controls?MethodsCross-sectional study. Twenty athletes who had an ACLR (age: 23.7 ± 4.3 years, 14 males, time post-surgery: 258.6 ± 54 days) and twenty uninjured controls (age: 21.4 ± 1.5 years, 14 males) performed a stepping down-task in four environmental conditions: no additional challenges, while performing a cognitive dual-task, while undergoing an unpredictable support surface perturbation, and with the cognitive dual-task and unpredictable perturbation combined. Muscle activations of the vastus medialis (VM), vastus lateralis, hamstrings medialis (HM), hamstrings lateralis (HL), gastrocnemius medialis, gastrocnemius lateralis (GL) and gluteus medius were recorded with surface EMG. A three-way ANOVA with main effects for group, dual-task and perturbation was used to compare muscle activations.ResultsAthletes with ACLR show larger HM (ES = 0.45) and HL activation (ES = 1.32) and lower VM activation (ES = 0.72), compared to controls.Athletes with ACLR show a significantly smaller increase in VM (ES = 0.69), VL (ES = 0.53) and GL activation (ES = 0.52) between perturbed and unperturbed tasks compared to controls. Furthermore, under cognitive loading a significantly larger decrease in HM activation (ES = 0.40) and (medial) co-contraction (ES = 0.75) was found in athletes with ACLR compared to controls.SignificanceAthletes with ACLR show an altered neuromuscular response which might represent an arthrogenic muscle response. They show less additional adaptation to perturbed tasks compared to controls, potentially as result of altered proprioceptive input. Furthermore a larger influence of increased cognitive loading on the neuromuscular control was found in athletes with ACLR, indicating that also neurocognitive limitations may contribute to altered neuromuscular control.     

Localization of muscle damage within the quadriceps femoris induced by different types of eccentric exercises

This study examined localization of muscle damage within the quadriceps femoris induced by different types of eccentric exercises by using transverse relaxation time (T ₂)‐weighted magnetic resonance imaging (MRI ). Thirty‐three young males performed either of the following three exercises: single‐joint eccentric contraction of the knee extensors (KE ), eccentric squat (S), or downhill walking (DW ) (n=11/exercise). KE and S consisted of 5‐set×10‐lowering of 90% one‐repetition maximum load. DW was performed for 60 minutes with −10% slope, 6 km/h velocity, and 20% body mass load carried. At pre‐ and 24‐, 48‐, and 72‐hours post‐exercise, T ₂‐MRI was scanned and T ₂ values for the rectus femoris (RF ), vastus intermedius (VI ), vastus lateralis (VL ), and vastus medialis (VM ) at proximal, middle, and distal sites were calculated. Additionally, soreness felt when static pressure was applied to these sites and maximal isometric knee extension torque were measured. Maximal torque significantly (P <.05) decreased (7%‐15%) at 24‐48 hours after all exercises. T ₂ significantly increased (3%‐9%) at 24‐72 hours after all exercises, with heterogeneities within the muscles found in each exercise. Effect size and peak change of T ₂, as well as soreness, overall indicated that the proximal RF after KE and middle VM after S and DW were most affected by these exercises. The VL did not show any significant T ₂ increase after all exercises. These results suggest that muscle damage specifically localizes at the proximal RF by KE and at the middle VM by S and DW , while the VL is least damaged regardless of the exercises.