Quadriceps Voluntary Activation Research Articles

Cross-education does not accelerate the rehabilitation of neuromuscular functions after ACL reconstruction: a randomized controlled clinical trial

PurposeCross-education reduces quadriceps weakness 8 weeks after anterior cruciate ligament (ACL) surgery, but the long-term effects are unknown. We investigated whether cross-education, as an adjuvant to the standard rehabilitation, would accelerate recovery of quadriceps strength and neuromuscular function up to 26 weeks post-surgery.MethodsGroup allocation was randomized. The experimental (n = 22) and control (n = 21) group received standard rehabilitation. In addition, the experimental group strength trained the quadriceps of the non-injured leg in weeks 1–12 post-surgery (i.e., cross-education). Primary and secondary outcomes were measured in both legs 29 ± 23 days prior to surgery and at 5, 12, and 26 weeks post-surgery.ResultsThe primary outcome showed time and cross-education effects. Maximal quadriceps strength in the reconstructed leg decreased 35% and 12% at, respectively, 5 and 12 weeks post-surgery and improved 11% at 26 weeks post-surgery, where strength of the non-injured leg showed a gradual increase post-surgery up to 14% (all p ≤ 0.015). Limb symmetry deteriorated 9–10% more for the experimental than control group at 5 and 12 weeks post-surgery (both p ≤ 0.030). One of 34 secondary outcomes revealed a cross-education effect: Voluntary quadriceps activation of the reconstructed leg was 6% reduced for the experimental vs. control group at 12 weeks post-surgery (p = 0.023). Both legs improved force control (22–34%) and dynamic balance (6–7%) at 26 weeks post-surgery (all p ≤ 0.043). Knee joint proprioception and static balance remained unchanged.ConclusionStandard rehabilitation improved maximal quadriceps strength, force control, and dynamic balance in both legs relative to pre-surgery but adding cross-education did not accelerate recovery following ACL reconstruction.

Greater intracortical inhibition associates with lower quadriceps voluntary activation in individuals with ACL reconstruction.

Decreased voluntary activation contributes to quadriceps weakness following anterior cruciate ligament reconstruction (ACLR). Alterations in neural excitability are likely responsible for reductions in quadriceps voluntary activation, and may be due to specific alterations in intracortical inhibition and facilitation. Therefore, we sought to determine if intracortical inhibition (SICI) and intracortical facilitation (ICF) associate with quadriceps voluntary activation in individuals with ACLR. Twenty-seven participants with a primary, unilateral ACLR were enrolled in this study. Bilateral central activation ratio (CAR) and paired-pulse transcranial magnetic stimulation were used to assess quadriceps voluntary activation, as well as SICI and ICF in the vastus medalis, respectively. Pearson Product Moment correlations were used to determine the association between CAR and (1) SICI, and (2) ICF in each limb. Lesser CAR associated with lesser SICI amplitude (r = 0.502, P = 0.008) in the ACLR limb. No associations in the contralateral limb were significant. Our results suggest greater intracortical inhibition associates with lesser voluntary activation in individuals with ACL. Implementing interventions that target intracortical inhibition may aid in restoring quadriceps voluntary activation following ACLR.

Persistent Muscle Inhibition after Anterior Cruciate Ligament Reconstruction: Role of Reflex Excitability.

Persistent voluntary quadriceps activation deficits are common after anterior cruciate ligament reconstruction (ACLR), but the direct causes are unclear. The primary purpose of this study was to determine whether spinal reflex excitability deficits are present in individuals with a history of ACLR, and secondarily to determine whether spinal reflex excitability predicts which individuals possess full voluntary quadriceps activation. One hundred and forty-seven individuals (74 healthy and 73 ACLR) participated in this cross-sectional case-control study. Quadriceps spinal reflex excitability was quantified using the Hoffmann reflex normalized to the maximal muscle response (H:M ratio). Voluntary quadriceps activation was evaluated with the burst superimposition technique and calculated via the central activation ratio (CAR). Separate 2 × 2 ANCOVA tests were used to compare between-limb and between-group differences for H:M ratio and CAR. A receiver operating characteristic curve was used to determine the accuracy of H:M ratio to predict if ACLR participants present with full voluntary activation (CAR ≥ 0.95). The ACLR H:M ratio was not different between limbs or compared with the healthy group (P > 0.05). Although ACLR CAR was lower bilaterally compared with the healthy group (P < 0.001), it did not differ between limbs. The H:M ratio has poor accuracy for predicting which individuals exhibit full voluntary activation (receiver operating characteristic area under the curve = 0.52, 95% CI = 0.37,0.66; odds ratio = 2.2, 95% CI = 0.8, 5.9). Spinal reflex excitability did not differ between limbs in individuals with ACLR or compared with healthy participants. The level of quadriceps spinal reflex excitability has poor accuracy at predicting which ACLR individuals would demonstrate full voluntary quadriceps activation.

The Effectiveness of Neuromuscular Electrical Stimulation in Improving Voluntary Activation of the Quadriceps: A Critically Appraised Topic.

Does NMES effectively enhance quadriceps voluntary activation in patients with orthopedic knee conditions? Summary of Key Findings: Four randomized controlled trials (RCTs) met the inclusion criteria and were included. Of those, 1 reported statistically significant improvements in quadriceps voluntary activation in the intervention group relative to a comparison group, but the statistical significance was not true for another study consisting of the same sample of participants with a different follow-up period. One study reported a trend in the NMES group, but the between-groups differences were not statistically significant in 3 of the 4 RCTs. Clinical Bottom Line: Current evidence does not support the use of NMES for the purpose of enhancing quadriceps voluntary activation in patients with orthopedic knee conditions. Strength of Recommendation: There is level B evidence that the use of NMES alone or in conjunction with therapeutic exercise does not enhance quadriceps voluntary activation in patients with orthopedic knee conditions (eg, anterior cruciate ligament injuries, osteoarthritis, total knee arthroplasty).

An anterior cruciate ligament injury does not affect the neuromuscular function of the non-injured leg except for dynamic balance and voluntary quadriceps activation

PurposeThe function of the anterior cruciate ligament (ACL) patients’ non-injured leg is relevant in light of the high incidence of secondary ACL injuries on the contralateral side. However, the non-injured leg’s function has only been examined for a selected number of neuromuscular outcomes and often without appropriate control groups. We measured a broad array of neuromuscular functions between legs of ACL patients and compared outcomes to age, sex, and physical activity matched controls.MethodsThirty-two ACL-deficient patients (208 ± 145 days post-injury) and active and less-active controls (N = 20 each) participated in the study. We measured single- and multi-joint neuromuscular function in both legs in each group and expressed the overall neuromuscular function in each leg by calculating a mean z-score across all neuromuscular measures. A group by leg MANOVA and ANOVA were performed to examine group and leg differences for the selected outcomes.ResultsAfter an ACL injury, duration (−4.3 h/week) and level (Tegner activity score of −3.9) of sports activity decreased and was comparable to less-active controls. ACL patients showed bilateral impairments in the star excursion balance test compared to both control groups (P ≤ 0.004) and for central activation ratio compared to active controls (P ≤ 0.002). There were between-leg differences within each group for maximal quadriceps and hamstring strength, voluntary quadriceps activation, star excursion balance test performance, and single-leg hop distance (all P < 0.05), but there were no significant differences in quadriceps force accuracy and variability, knee joint proprioception, and static balance. Overall neuromuscular function (mean z-score) did not differ between groups, but ACL patients’ non-injured leg displayed better neuromuscular function than the injured leg (P < 0.05).ConclusionsExcept for poorer dynamic balance and reduced quadriceps activation, ACL patients had no bilateral neuromuscular deficits despite reductions in physical activity after injury. Therapists can use the non-injured leg as a reference to assess the injured leg’s function for tasks measured in the present study, excluding dynamic balance and quadriceps activation. Rehabilitation after an ACL injury should be mainly focused on the injured leg.Level of evidenceIII.

Compression Garments, Muscle Contractile Function, and Economy in Trail Runners.

Physiological mechanisms behind the use of compression garments (CGs) during off-road running are unknown. To investigate the influence of wearing CGs vs conventional running clothing (CON) on muscle contractile function and running economy before and after short-distance trail running. Knee-extensor neuromuscular function and running economy assessed from two 5-min treadmill runs (11 and 14 km/h) were evaluated before and after an 18.6-km short-distance trail run in 12 trained athletes wearing either CGs (stocking + short-tight) or CON. Quadriceps neuromuscular function was assessed from mechanical and EMG recording after maximal percutaneous electrical femoral-nerve stimulations (single-twitch doublets at 10 [Db10] and 100 Hz [Db100] delivered at rest and during maximal quadriceps voluntary contraction [MVC]). Running economy (in mL O2 · km-1 · kg-1) increased after trail running independent of the clothing condition and treadmill speeds (P < .001). Similarly, MVC decreased after CON and CGs conditions (-11% and -13%, respectively, P < .001). For both clothing conditions, a significant decrease in quadriceps voluntary activation, Db10, Db100, and the low-to-high frequency doublet ratio were observed after trail running (time effect, all P < .01), without any changes in rectus femoris maximal M-wave. Wearing CGs does not reduce physiological alterations induced during short-distance trail running. Further studies should determine whether higher intensity of compression pressure during exercises of longer duration may be effective to induce any physiological benefits in experienced trail runners.

Fatigue diminishes motoneuronal excitability during cycling exercise.

Exercise-induced fatigue influences the excitability of the motor pathway during single-joint isometric contractions. This study sought to investigate the influence of fatigue on corticospinal excitability during cycling exercise. Eight men performed fatiguing constant-load (80% Wpeak; 241 ± 13 W) cycling to exhaustion during which the percent increase in quadriceps electromyography (ΔEMG; vastus lateralis and rectus femoris) was quantified. During a separate trial, subjects performed two brief (∼45 s) nonfatiguing cycling bouts (244 ± 15 and 331 ± 23W) individually chosen to match the ΔEMG across bouts to that observed during fatiguing cycling. Corticospinal excitability during exercise was quantified by transcranial magnetic, electric transmastoid, and femoral nerve stimulation to elicit motor-evoked potentials (MEP), cervicomedullary evoked potentials (CMEP), and M waves in the quadriceps. Peripheral and central fatigue were expressed as pre- to postexercise reductions in quadriceps twitch force (ΔQtw) and voluntary quadriceps activation (ΔVA). Whereas nonfatiguing cycling caused no measureable fatigue, fatiguing cycling resulted in significant peripheral (ΔQtw: 42 ± 6%) and central (ΔVA: 4 ± 1%) fatigue. During nonfatiguing cycling, the area of MEPs and CMEPs, normalized to M waves, similarly increased in the quadriceps (∼40%; P < 0.05). In contrast, there was no change in normalized MEPs or CMEPs during fatiguing cycling. As a consequence, the ratio of MEP to CMEP was unchanged during both trials (P > 0.5). Therefore, although increases in muscle activation promote corticospinal excitability via motoneuronal facilitation during nonfatiguing cycling, this effect is abolished during fatigue. We conclude that the unaltered excitability of the corticospinal pathway from start of intense cycling exercise to exhaustion is, in part, determined by inhibitory influences on spinal motoneurons obscuring the facilitating effects of muscle activation.

Intracortical Inhibition Associates with Quadriceps Voluntary Activation in Individuals with an Anterior Cruciate Ligament Reconstruction

Quadriceps strength is important for maintaining high physical function following anterior cruciate ligament reconstruction (ACLR), and deficits in voluntary activation (VA) lead to quadriceps weakness. ACLR individuals with deficits in quadriceps VA also demonstrate diminished corticomotor excitability, which reflects the balance of intracortical inhibitory and facilitatory activity. Therefore, specific alterations in intracortical inhibition and facilitation may be responsible for persistent deficits in quadriceps VA following ACLR. PURPOSE: To determine if short interval intracortical inhibition (SICI), intracortical facilitation (ICF), and the SICI: IFC ratio associate with quadriceps VA in individuals with ACLR. METHODS: Twenty-seven individuals (74% Female; 169±11cm; 73±19kg; 44±37 months post ACLR) with a history of a primary unilateral ACLR were enrolled into this cross-sectional study. All outcome measures were collected with the participant seated in an isokinetic dynamometer with the knee flexed at 90°. Paired pulse transcranial magnetic stimulation was used to assess SICI and ICF. SICI and ICF paradigms utilized two stimuli delivered at 80% and 120% of active motor threshold (AMT) at 3 and 15 msec apart, respectively. Motor evoked potentials (MEPs) elicited via SICI and ICF paradigms were then normalized to MEPs elicited at 120% of AMT. A reduction in SICI indicates greater inhibition, whereas an increase in ICF indicates greater facilitation. Quadriceps VA was evaluated with the superimposed burst technique and calculated via the central activation ratio (CAR). We used Spearman correlations (ρ) to evaluate associations between the non-normally distributed outcome measures (α = 0.05). RESULTS: Greater inhibition (i.e. lower SICI) was associated with lower VA (ρ = .502, P = 0.008). A greater SICI: ICF ratio, indicating more inhibition relative to facilitation, was associated with lower VA (ρ = -.530, P = 0.004). ICF was not associated with VA (ρ = -.089, P = 0.661). CONCLUSIONS: Reduced intracortical inhibition is associated with higher quadriceps VA in individuals with ACLR. Interventions that seek to reduce intracortical inhibition may improve VA in ACLR patients demonstrating persistent deficits in VA.

Spinal Reflex Excitability does not Associate with Persistent Muscle Inhibition after Anterior Cruciate Ligament Reconstruction

Persistent deficits in quadriceps voluntary activation affect muscle function following anterior cruciate ligament reconstruction (ACLR). Previous research has attributed voluntary activation deficits to lower quadriceps spinal reflex excitability following acute knee effusion or injury. There is little evidence to indicate that spinal reflex excitability exists or influences persistent voluntary activation deficits in individuals with an ACLR. PURPOSE: To determine if quadriceps spinal reflex excitability in individuals with a unilateral ACLR differs between limb or compared to a control group. Additionally, we evaluated the ability of quadriceps spinal reflex excitability to predict full voluntary activation following ACLR. METHODS: One hundred and forty-seven individuals (74 ACLR and 73 controls) participated in this cross-sectional, case-control study. Quadriceps spinal reflex excitability was quantified using the Hoffmann reflex normalized to the maximal muscle response (H:M ratio). Quadriceps voluntary activation was evaluated with the burst superimposition technique and calculated via the central activation ratio (CAR). Separate 2x2 ANOVAs were used to compare differences between-limbs and between-groups for H:M ratio and CAR. From a receiver operating characteristic curve analysis, the area under the curve (AUC) was used to determine the accuracy of H:M ratio to predict full voluntary activation (CAR>0.95) in ACLR individuals. We determined the odds ratio (OR) of demonstrating full voluntary activation from spinal reflex excitability cutoff scores that maximized the sensitivity and specificity for predicting full activation. RESULTS: ACLR quadriceps H:M ratios were not different between limbs or compared to the control group (P>0.05). Quadriceps CAR was bilaterally lower in individuals with an ACLR compared to the control group (P<0.01), yet CAR did not differ between limbs. H:M ratio had poor accuracy for predicting ACLR participants exhibiting full voluntary activation (AUC=0.53, 95% CI=0.39,0.67; OR = 2.37, 95% CI=0.91,6.2). CONCLUSIONS: Spinal reflex excitability did not differ between limbs in individuals with ACLR or compared to controls. Quadriceps spinal reflex excitability has poor accuracy for predicting which ACLR individuals will demonstrate a CAR>0.95.

Peripheral and Central Fatigue Development during All-Out Repeated Cycling Sprints.

We investigated the development and recovery of peripheral and central fatigue during repeated cycling sprints and its influence on power output. On six separate days, 12 healthy males performed the following tests: 1, 4, 6, 8, and 10 × 10 s sprints with 30 s of passive recovery between sprints, as well as 8 × 10 s sprints with 10 s of passive recovery. Peripheral and central fatigue levels were quantified via changes in preexercise- to postexercise-potentiated quadriceps twitch force, as evoked by supramaximal electrical stimulation of the femoral nerve (30 s through 6 min recovery), and quadriceps voluntary activation (VA), respectively. Root mean square of the vastus lateralis and the vastus medialis electromyogram during sprints were normalized by maximal M wave amplitude (RMS·Mmax). From the first to the sixth sprint, we found significant and gradual reductions in power output (-25% ± 7%), RMS·Mmax (-7% ± 4%), twitch force (-47% ± 11%) and VA (-11% ± 6%). During the subsequent sprints, no additional reduction in power output, RMS·Mmax, twitch force or VA, was found. Reduction in between-sprints recovery duration led to a significant reduction in power output and RMS·Mmax but no change in peripheral and central fatigue. These findings are consistent with the hypothesis that central motor command and power output during all-out repeated sprints are limited in order to prevent excessive locomotor muscle fatigue. They also demonstrate that both the peripheral and central fatigue contribute significantly to the decline in power output elicited via repeated sprints.

Voluntary activation of quadriceps femoris in patients with unilateral anterior cruciate ligament rupture within 6 months of injury: A cross-sectional observational study

BackgroundDeficits in quadriceps femoris strength and voluntary activation have been well documented in chronic anterior cruciate ligament (ACL) injuries, but less is known about the acute or early phase after injury. ObjectivesThe aim of this study was to evaluate and compare the levels of quadriceps voluntary activation (VA) and strength in both limbs of participants with unilateral ACL ruptures (complete tears) within 6 months of injury. DesignCross-sectional observational study. MethodSeventeen participants, 12 male, mean age 30 (17–45) years, performed maximal voluntary isometric contractions with the interpolated twitch technique. ResultsMean (SD) peak VA was significantly lower in the injured limb 76.5 (15.0) % than the uninjured limb 85.9 (6.7) % (p = 0.02). Mean (SD) peak torque in the injured limb was significantly lower 162.7 (74.1) Nm than the uninjured limb 240.5 (81.0) Nm (p < 0.01). ConclusionsThis between-limb difference in VA has not previously been observed in patients within 6 months of ACL rupture. Our findings suggest that early rehabilitation programs for adults with ACL rupture should focus on reducing VA deficits to facilitate improvement of the quadriceps femoris muscle strength in the injured limb to comparable values of the uninjured limb.

Is gamma loop dysfunction related to bilateral inhibition in anterior knee pain?

The potential relationship between bilateral quadriceps inhibition in individuals with unilateral anterior knee pain (AKP) and gamma loop dysfunction is examined in this study. Twelve individuals with unilateral AKP and 10 healthy controls were recruited. Quadriceps voluntary activation (%VA) was quantified using a triggered interpolated twitch technique. Gamma loop function was assessed through knee extension maximum voluntary isometric contractions before and after 20 minutes of 50-Hz patellar tendon vibration. The AKP group exhibited 5.7% lower %VA bilaterally compared with controls (P = 0.039, Cohen d = 0.79). After prolonged vibration, both groups exhibited an average 8.5% reduction in knee extension force in each limb (P < 0.001, Cohen d = 0.85). Significant force reductions after vibration suggest that gamma loop dysfunction was not seen in either control or AKP participants. Bilateral quadriceps inhibition in the AKP group does not appear to be associated with gamma loop dysfunction.

Measuring voluntary quadriceps activation: Effect of visual feedback and stimulus delivery

Introduction: Quadriceps voluntary activation, assessed via the superimposed burst technique, has been extensively studied in a variety of populations as a measure of quadriceps function. However, a variety of stimulus delivery techniques have been employed, which may influence the level of voluntary activation as calculated via the central activation ratio (CAR). The purpose was to determine the effect of visual feedback, stimulus delivery, and perceived discomfort on maximal voluntary isometric contraction (MVIC) peak torque and the CAR. Methods: Quadriceps CAR was assessed in 14 individuals on two days using three stimulus delivery methods; (1) manual without visual feedback, (2) manual with visual feedback, and (3) automated with visual feedback. Results: MVIC peak torque and the CAR were not different between the automated with visual feedback (MVIC=3.25, SE=0.14Nm/kg; CAR=88.63, SE=1.75%) and manual with visual feedback (MVIC=3.26, SE=0.13Nm/kg, P=0.859; CAR=89.06, SE=1.70%, P=0.39) stimulus delivery methods. MVIC (2.99, SE=0.12Nm/kg) and CAR (85.32, SE=2.10%) were significantly lower using manual without visual feedback compared to manual with visual feedback and automated with visual feedback (CAR P<0.001; MVIC P<0.001). Perceived discomfort was lower in the second session (P<0.05). Conclusion: Utilizing visual feedback ensures participant MVIC, and may provide a more accurate assessment of quadriceps voluntary activation.

CORR Insights(®): Does Tourniquet Use in TKA Affect Recovery of Lower Extremity Strength and Function? A Randomized Trial.

T ourniquet use during TKA [1] offers better visibility, decreases intraoperative bleeding, and helps in cement-bone interdigitation during surgery [4, 10]. Despite these benefits, the development of thromboembolic events like deep venous thrombosis [2, 13], pulmonary embolism [5], or multiple cerebral and cardiac microemboli [8, 9] all remain a concern when considering tourniquet use. Other complications include increased leg swelling and pain after release [11, 12], tourniquet-induced wound problems [7], peripheral nerve injury [6], or decreased knee ROM [3]. Although there are ongoing debates regarding the pros and cons of tourniquet use, no sufficient investigations have beenmade on its possible negative effect on the recovery of lower extremity strength and function, which are vital to postsurgical recovery. The current study by Dennis and colleagues adds to our understanding by finding greater postoperative quadriceps strength and lesser pain for patients in the nontourniquet cohort. In contrast, researchers found no differences in hamstring strength, quadriceps voluntary activation, or unilateral balance ability in patients who underwent bilateral simultaneous TKA. Additionally, the study results found more intraoperative blood loss in the nontourniquet group, but no differences in total blood loss or operative time when compared to the group undergoing same-day bilateral TKA.

Does Tourniquet Use in TKA Affect Recovery of Lower Extremity Strength and Function? A Randomized Trial.

Tourniquet use during total knee arthroplasty (TKA) improves visibility and reduces intraoperative blood loss. However, tourniquet use may also have a negative impact on early recovery of muscle strength and lower extremity function after TKA. The purpose of this study was (1) to determine whether tourniquet use affects recovery of quadriceps strength (primary outcome) during the first 3 postoperative months; and (2) to examine the effects of tourniquet application on secondary outcomes: voluntary quadriceps activation, hamstring strength, unilateral limb balance as well as the effect on operative time and blood loss. Twenty-eight patients (mean age 62 ± 6 years; 16 men) undergoing same-day bilateral TKA (56 lower extremities) were enrolled in a prospective, randomized study. Subjects were randomized to receive a tourniquet-assisted knee arthroplasty on one lower extremity while the contralateral limb underwent knee arthroplasty without extended tourniquet use. In the former group, the tourniquet was inflated just before the incision was made and released after cementation; in the latter group, a tourniquet was not used (10 of 28 [36%]) or inflated only during component cementation (18 of 28 [64%]). The choice of no tourniquet or use just during cementation was based on surgeon choice, because some surgeons felt a tourniquet during cementation was necessary to achieve a dry surgical field to maximize cement fixation. A median parapatellar approach and the identical posterior-stabilized TKA design were used by all four fellowship-trained knee surgeons involved. Isometric quadriceps strength, hamstring strength, voluntary quadriceps activation, and unilateral balance were assessed preoperatively, 3 weeks, and 3 months after bilateral knee arthroplasty. Other factors, including pain, range of motion, and lower extremity girth, were assessed for descriptive purposes at each of these time points as well as on the second postoperative day. Quadriceps strength was slightly lower in the tourniquet group compared with the no-tourniquet group (group difference = 11.27 Nm [95% confidence interval {CI}, 2.33-20.20]; p = 0.01), and these differences persisted at 3 months after surgery (group difference = 9.48 Nm [95% CI, 0.43-18.54]; p = 0.03). Hamstring strength did not differ between groups at any time point nor did measures of quadriceps voluntary activation or measures of unilateral balance ability. There was less estimated intraoperative blood loss in the tourniquet group (84 ± 26 mL) than in the no-tourniquet group (156 ± 63 mL) (group difference = -74 mL [95% CI, -100 to -49]; p < 0.001). However, there was no difference in total blood loss between the groups (group difference = -136 mL [95% CI, -318 to 45]; p = 0.13). Patients who underwent TKA using a tourniquet had diminished quadriceps strength during the first 3 months after TKA, the clinical significance of which is unclear. Future studies may be warranted to examine the effects of tourniquet use on long-term strength and functional outcomes. Level I, therapeutic study.

Group III/IV Muscle Afferents Restrict Intramuscular Metabolic Perturbation In Exercising Humans

Group III/IV muscle afferent feedback limits the development of peripheral fatigue during intense endurance exercise to a critical threshold, by restricting central motor drive (CMD). Pharmacological blockade of these sensory neurons inhibits this regulatory mechanism and causes this threshold to be exceeded with the consequence of excessive end-exercise fatigue. However, the intramuscular metabolic perturbation associated with this altered level of peripheral fatigue is unknown. PURPOSE: To investigate the role of group III/IV locomotor muscle afferents in limiting intramuscular metabolic perturbation during intense cycling exercise. METHODS: Eight subjects performed a 5 km cycling time trial under control conditions (CTRL) and with lumbar intrathecal fentanyl impairing feedback from the μ-opioid receptor-sensitive lower limb muscle afferents (FENT). Vastus lateralis muscle biopsies were obtained before and immediately after exercise. Exercise-induced changes in intramuscular metabolites were determined using liquid chromatography (mass spectrometry). Quadriceps fatigue was quantified via pre- to post-exercise changes in potentiated quadriceps twitch force (ΔQtw) evoked by electrical femoral nerve stimulation. Central fatigue was measured by comparing pre- to post-exercise changes in quadriceps voluntary activation (ΔVA). RESULTS: Although power output during the first half of the time trial with FENT was 10 ± 1% higher compared to CTRL, it was 11 ± 4% lower during the second half (P < 0.01). ΔQtw was greater following FENT than CTRL (−52 ± 2 vs. −31 ± 1%, P < 0.001); ΔVA was also more pronounced in FENT than CTRL (-11 ± 3% vs. -6 ± 1%, P < 0.001). The pre- to post-exercise-induced increase in intramuscular inorganic phosphate, adenosine diphosphate, and lactate was 133%, 190% and 129% greater in FENT than CTRL (P < 0.001), respectively. Phosphocreatine depletion was 47% greater in FENT compared to CTRL (P < 0.01). CONCLUSIONS: Group III/IV muscle afferents limit peripheral fatigue and intramuscular metabolic perturbation during intense endurance exercise. Therefore, the intramuscular metabolic milieu, representing a key stimulus for group III/IV afferent feedback which is in turn associated with reductions in CMD, could be highly regulated to prevent the development of excessive peripheral fatigue during whole body endurance exercise.

Peripheral And Central Fatigue Development And Short-term Recovery During All-out Repeated Sprints

Recent evidence suggests that power output during repeated sprints of short duration is adjusted by the central nervous system to restrain the development of locomotor muscle fatigue to a critical threshold. This adjustment of power output might involve a complex interplay between peripheral and central determinants of muscle fatigue. The rate of peripheral and/or central fatigue development and its influence on the pattern of power output during repeated sprints remain however largely unknown. PURPOSE:We aimed to determine the contribution of peripheral and central fatigue to power output reduction during repeated sprints and whether power output levels off when a critical threshold of peripheral fatigue is reached. METHODS: On separate days, twelve healthy subjects performed the following tests: 1, 4, 6, 8 and 10, 10 s all-out sprints, each with 30 s of passive recovery between sprints, as well as 8, 10 s sprints, each with 10 s of passive recovery to test the influence of varying the work / recovery ratio on our hypotheses. Peripheral fatigue was quantified via changes in pre- to post-exercise potentiated quadriceps twitch force evoked by supramaximal electrical stimulation of the femoral nerve (ΔQtw, 0.5 through 6 min recovery). Central fatigue was estimated via changes in pre- to post-exercise voluntary quadriceps activation (ΔVA). EMG during sprints was normalized by maximal M-wave amplitude (ΔRMS.Mmax-1). RESULTS: From the 1st to the 6th sprint, we found a significant and gradual reduction in ΔQtw (- 47 ± 3 %), ΔRMS.Mmax-1 (- 7 ± 1 %) and power output (- 25 ± 2 %). During the 4 subsequent sprints, no additional reduction of quadriceps fatigue, RMS.Mmax-1 or power output was found but VA was significantly reduced compared to baseline (- 11 ± 2 %). Recovery of Qtw, post-sprints (0.5 through 6 min), was reduced progressively following sprint #1 through #6, with no further reduction thereafter (through sprint #10). CONCLUSIONS: Both peripheral and central fatigue contribute to the reduction of power output during repeated sprints. The leveling off of power output and RMS.Max-1 when the same degree of peripheral fatigue was reached, independently of the work / recovery duration ratio, suggest that central motor drive was restricted to limit excessive development of peripheral muscle fatigue.

Neural Excitability Alterations After Anterior Cruciate Ligament Reconstruction.

Neuromuscular dysfunction is common after anterior cruciate ligament reconstruction (ACL-R). However, little is known about quadriceps spinal-reflex and descending corticomotor excitability after ACL-R. Understanding the effects of ACL-R on spinal-reflex and corticomotor excitability will help elucidate the origins of neuromuscular dysfunction. To determine whether spinal-reflex excitability and corticomotor excitability differed between the injured and uninjured limbs of patients with unilateral ACL-R and between these limbs and the matched limbs of healthy participants. Case-control study. Laboratory. A total of 28 patients with unilateral ACL-R (9 men, 19 women; age = 21.28 ± 3.79 years, height = 170.95 ± 10.04 cm, mass = 73.18 ± 18.02 kg, time after surgery = 48.10 ± 36.17 months) and 29 participants serving as healthy controls (9 men, 20 women; age = 21.55 ± 2.70 years, height = 170.59 ± 8.93 cm, mass = 71.89 ± 12.70 kg) volunteered. Active motor thresholds (AMTs) were collected from the vastus medialis (VM) using transcranial magnetic stimulation. We evaluated VM spinal reflexes using the Hoffmann reflex normalized to maximal muscle responses (H : M ratio). Voluntary quadriceps activation was measured with the superimposed-burst technique and calculated using the central activation ratio (CAR). We also evaluated whether ACL-R patients with high or low voluntary activation had different outcomes. The AMT was higher in the injured than in the uninjured limb in the ACL-R group (t27 = 3.32, P = .003) and in the matched limb of the control group (t55 = 2.05, P = .04). The H : M ratio was bilaterally higher in the ACL-R than the control group (F1,55 = 5.17, P = .03). The quadriceps CAR was bilaterally lower in the ACL-R compared with the control group (F1,55 = 10.5, P = .002). The ACL-R group with low voluntary activation (CAR < 0.95) had higher AMT than the control group (P = .02), whereas the ACL-R group with high voluntary activation (CAR ≥ 0.95) demonstrated higher H : M ratios than the control group (P = .05). The higher VM AMT in the injured limbs of ACL-R patients suggested that corticomotor deficits were present after surgery. Higher bilateral H : M ratios in ACL-R patients may be a strategy to reflexively increase excitability to maintain voluntary activation.

Changes in voluntary quadriceps activation predict changes in muscle strength and gait biomechanics following knee joint effusion

BackgroundIt has been hypothesized that arthrogenic muscle inhibition is responsible for altering physical function following knee injury. The association between the onset of arthrogenic muscle inhibition, measured using voluntary quadriceps activation, and changes in muscle strength and gait biomechanics are unknown. MethodsOutcomes were collected before and following a 60ml experimental knee effusion in eighteen healthy participants. Voluntary quadriceps activation was the predictor variable, while the criterion variable included, maximal voluntary isometric strength, peak knee flexion angle, peak internal knee extension moment, and peak vertical ground reaction forces during the first half of stance phase upon stair descent. Percent change scores (Δ) were imputed into linear regression equations to determine associations between predictor and criterion variables. FindingsThe variance in Δ voluntary quadriceps activation significantly predicted 87% the variance in the Δ strength (R2=0.87, P<0.001; Δ strength=−2.15+1.77Δ voluntary quadriceps activation) and 25% of the Δ vertical ground reaction force following effusion (R2=0.25, P=0.04; Δ vertical ground reaction force=−6.1+0.57 Δ voluntary quadriceps activation). After accounting for Δ knee flexion angle, Δ voluntary quadriceps activation predicted an additional 29% (Δ R2=0.29, P=0.007) of the variance in the Δ knee extension moment (R2=0.54, P=0.003, Δ knee extension moment=−10.79+0.74Δ knee flexion angle+1.64Δ voluntary quadriceps activation) following knee effusion. InterpretationImmediate quadriceps activation deficits following joint effusion result in immediate alterations in muscle strength, knee extensor moment and vertical ground reaction force during gait.

Contributions of neural excitability and voluntary activation to quadriceps muscle strength following anterior cruciate ligament reconstruction

BackgroundPersistent quadriceps weakness is common following anterior cruciate ligament reconstruction (ACLr). Alterations in spinal-reflexive excitability, corticospinal excitability and voluntary activation have been hypothesized as underlying mechanisms contributing to quadriceps weakness. The aim of this study was to evaluate the predictive capabilities of spinal-reflexive excitability, corticospinal excitability and voluntary activation on quadriceps strength in healthy and ACLr participants. MethodsQuadriceps strength was measured using maximal voluntary isometric contractions (MVIC). Voluntary activation was quantified via the central activation ratio (CAR). Corticospinal and spinal-reflexive excitability were measured using active motor thresholds (AMT) and Hoffmann reflexes normalized to maximal muscle responses (H:M), respectively. ACLr individuals were also split into high and low strength subsets based on MVIC. ResultsCAR was the only significant predictor in the healthy group. In the ACLr group, CAR and H:M significantly predicted 47% of the variance in MVIC. ACLr individuals in the high strength subset demonstrated significantly higher CAR and H:M than those in the low strength subset. ConclusionIncreased quadriceps voluntary activation, spinal-reflexive excitability and corticospinal excitability relates to increased quadriceps strength in participants following ACLr. Clinical relevanceRehabilitation strategies used to target neural alterations may be beneficial for the restoration of muscle strength following ACLr.