Central Activation Ratio Research Articles

Knee-Extension Torque Variability and Subjective Knee Function in Patients with a History of Anterior Cruciate Ligament Reconstruction.

When returning to physical activity, patients with a history of anterior cruciate ligament reconstruction (ACL-R) often experience limitations in knee-joint function that may be due to chronic impairments in quadriceps motor control. Assessment of knee-extension torque variability may demonstrate underlying impairments in quadriceps motor control in patients with a history of ACL-R. To identify differences in maximal isometric knee-extension torque variability between knees that have undergone ACL-R and healthy knees and to determine the relationship between knee-extension torque variability and self-reported knee function in patients with a history of ACL-R. Descriptive laboratory study. Laboratory. A total of 53 individuals with primary, unilateral ACL-R (age = 23.4 ± 4.9 years, height = 1.7 ± 0.1 m, mass = 74.6 ± 14.8 kg) and 50 individuals with no history of substantial lower extremity injury or surgery who served as controls (age = 23.3 ± 4.4 years, height = 1.7 ± 0.1 m, mass = 67.4 ± 13.2 kg). Torque variability, strength, and central activation ratio (CAR) were calculated from 3-second maximal knee-extension contraction trials (90° of flexion) with a superimposed electrical stimulus. All participants completed the International Knee Documentation Committee (IKDC) Subjective Knee Evaluation Form, and we determined the number of months after surgery. Group differences were assessed using independent-samples t tests. Correlation coefficients were calculated among torque variability, strength, CAR, months after surgery, and IKDC scores. Torque variability, strength, CAR, and months after surgery were regressed on IKDC scores using stepwise, multiple linear regression. Torque variability was greater and strength, CAR, and IKDC scores were lower in the ACL-R group than in the control group (P < .05). Torque variability and strength were correlated with IKDC scores (P < .05). Torque variability, strength, and CAR were correlated with each other (P < .05). Torque variability alone accounted for 14.3% of the variance in IKDC scores. The combination of torque variability and number of months after surgery accounted for 21% of the variance in IKDC scores. Strength and CAR were excluded from the regression model. Knee-extension torque variability was moderately associated with IKDC scores in patients with a history of ACL-R. Torque variability combined with months after surgery predicted 21% of the variance in IKDC scores in these patients.

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.

Reliability of the Superimposed-Burst Technique in Patients With Patellofemoral Pain: A Technical Report

The superimposed-burst (SIB) technique is commonly used to quantify central activation failure after knee-joint injury, but its reliability has not been established in pathologic cohorts. To assess within-session and between-sessions reliability of the SIB technique in patients with patellofemoral pain. Descriptive laboratory study. University laboratory. A total of 10 patients with self-reported patellofemoral pain (1 man, 9 women; age = 24.1 ± 3.8 years, height = 167.8 ± 15.2 cm, mass = 71.6 ± 17.5 kg) and 10 healthy control participants (3 men, 7 women; age = 27.4 ± 5.0 years, height = 173.5 ± 9.9 cm, mass = 78.2 ± 16.5 kg) volunteered. Participants were assessed at 6 intervals spanning 21 days. Intraclass correlation coefficients (ICCs [3,3]) were used to assess reliability. Quadriceps central activation ratio, knee-extension maximal voluntary isometric contraction force, and SIB force. The quadriceps central activation ratio was highly reliable within session (ICC [3,3] = 0.97) and between sessions through day 21 (ICC [3,3] = 0.90-0.95). Acceptable reliability of knee extension (ICC [3,3] = 0.75-0.91) and SIB force (ICC [3,3] = 0.77-0.89) was observed through day 21. The SIB technique was reliable for clinical research up to 21 days in patients with patellofemoral pain.

Effects of Neuromuscular Fatigue on Quadriceps Strength and Activation and Knee Biomechanics in Individuals Post-Anterior Cruciate Ligament Reconstruction and Healthy Adults.

Laboratory-based experiment using a pretest/posttest design. To determine the effects of neuromuscular fatigue on quadriceps strength and activation and sagittal and frontal plane knee biomechanics during dynamic landing following anterior cruciate ligament reconstruction (ACLR). Impaired quadriceps central activation occurs post-ACLR, likely altering lower extremity biomechanics. Neuromuscular fatigue similarly reduces volitional muscle activation and impairs neuromuscular control. Upon return to full activity post-ACLR, individuals likely concurrently experience quadriceps central activation deficits and neuromuscular fatigue, though the effects of fatigue on muscle strength and activation and biomechanics post-ACLR are unknown. Seventeen individuals 7 to 10 months post-ACLR and 16 controls participated. Quadriceps strength and central activation ratio were recorded prefatigue and postfatigue, which was induced via sets of double-leg squats. Knee biomechanics were recorded during a dynamic landing activity prefatigue and postfatigue. Both groups demonstrated smaller knee flexion (initial contact, P = .017; peak, P = .004) and abduction (initial contact, P = .005; peak, P = .009) angles postfatigue. The ACLR group had smaller peak knee flexion angles (P<.001) prefatigue and postfatigue than controls. Knee flexion moment was smaller in those post-ACLR than controls prefatigue (P<.001), but not postfatigue (P = .103). Controls had smaller knee flexion moments postfatigue (P = .001). Knee abduction moment was smaller in both groups postfatigue (P = .003). All participants demonstrated significantly lower strength (P<.001) and activation (P = .003) postfatigue. Impaired strength, central activation, and biomechanics were present postfatigue in both groups, suggesting that neuromuscular fatigue may increase noncontact ACL injury risk. However, these changes were not exaggerated in those post-ACLR, likely because they already demonstrated a stiff-legged landing strategy prefatigue.

Effects of transcutaneous electrical nerve stimulation on quadriceps function in individuals with experimental knee pain.

Knee joint pain (KJP) is a cardinal symptom in knee pathologies, and quadriceps inhibition is commonly observed among KJP patients. Previously, KJP independently reduced quadriceps strength and activation. However, it remains unknown how disinhibitory transcutaneous electrical nerve stimulation (TENS) will affect inhibited quadriceps motor function. This study aimed at examining changes in quadriceps maximum voluntary contraction (MVC) and central activation ratio (CAR) before and after sensory TENS following experimental knee pain. Thirty healthy participants were assigned to either the TENS or placebo groups. All participants underwent three separate data collection sessions consisting of two saline infusions and one no infusion control in a crossover design. TENS or placebo treatment was administered to each group for 20 min. Quadriceps MVC and CAR were measured at baseline, infusion, treatment, and post-treatment. Perceived knee pain intensity was measured on a 100-mm visual analogue scale. Post-hoc analysis revealed that hypertonic saline infusion significantly reduced the quadriceps MVC and CAR compared with control sessions (P < 0.05). Sensory TENS, however, significantly restored inhibited quadriceps motor function compared with placebo treatment (P < 0.05). There was a negative correlation between changes in MVC and knee pain (r = 0.33, P < 0.001), and CAR and knee pain (r = 0.62, P < 0.001), respectively.

The Response to and Recovery From Maximum-Strength and -Power Training in Elite Track and Field Athletes.

There is a great deal of research on the responses to resistance training; however, information on the responses to strength and power training conducted by elite strength and power athletes is sparse. To establish the acute and 24-h neuromuscular and kinematic responses to Olympic-style barbell strength and power exercise in elite athletes. Ten elite track and field athletes completed a series of 3 back-squat exercises each consisting of 4 × 5 repetitions. These were done as either strength or power sessions on separate days. Surface electromyography (sEMG), bar velocity, and knee angle were monitored throughout these exercises and maximal voluntary contraction (MVC), jump height, central activation ratio (CAR), and lactate were measured pre, post, and 24 h thereafter. Repetition duration, impulse, and total work were greater (P < .01) during strength sessions, with mean power being greater (P < .01) after the power sessions. Lactate increased (P < .01) after strength but not power sessions. sEMG increased (P < .01) across sets for both sessions, with the strength session increasing at a faster rate (P < .01) and with greater activation (P < .01) by the end of the final set. MVC declined (P < .01) after the strength and not the power session, which remained suppressed (P < .05) 24 h later, whereas CAR and jump height remained unchanged. A greater neuromuscular and metabolic demand after the strength and not power session is evident in elite athletes, which impaired maximal-force production for up to 24 h. This is an important consideration for planning concurrent athlete training.

A Combination of Amino Acids and Caffeine Enhances Sprint Running Capacity in a Hot, Hypoxic Environment.

Heat and hypoxia exacerbate central nervous system (CNS) fatigue. We therefore investigated whether essential amino acid (EAA) and caffeine ingestion attenuates CNS fatigue in a simulated team sport-specific running protocol in a hot, hypoxic environment. Subelite male team sport athletes (n = 8) performed a repeat sprint running protocol on a nonmotorized treadmill in an extreme environment on 4 separate occasions. Participants ingested one of four supplements: a double placebo, 3 mg.kg-1 body mass of caffeine + placebo, 2 x 7 g EAA (Musashi Create)+placebo, or caffeine + EAA before each exercise session using a randomized, double-blind crossover design. Electromyography (EMG) activity and quadriceps evoked responses to magnetic stimulation were assessed from the dominant leg at preexercise, halftime, and postexercise. Central activation ratio (CAR) was used to quantify completeness of quadriceps activation. Oxygenation of the prefrontal cortex was measured via near-infrared spectroscopy. Mean sprint work was higher (M = 174 J, 95% CI [23, 324], p < .05, d = 0.30; effect size, likely beneficial) in the caffeine + EAA condition versus EAAs alone. The decline in EMG activity was less (M = 13%, 95% CI [0, 26]; p < .01, d = 0.58, likely beneficial) in caffeine + EAA versus EAA alone. Similarly, the pre- to postexercise decrement in CAR was significantly less (M = -2.7%, 95% CI [0.4, 5.4]; p < .05, d = 0.50, likely beneficial) when caffeine + EAA were ingested compared with placebo. Cerebral oxygenation was lower (M = -5.6%, 95% CI [1.0, 10.1]; p < .01, d = 0.60, very likely beneficial) in the caffeine + EAA condition compared with LNAA alone. Co-ingestion of caffeine and EAA appears to maintain muscle activation and central drive, with a small improvement in running performance.

Increased spinal reflex excitability is associated with enhanced central activation during voluntary lengthening contractions in human spinal cord injury.

This study of chronic incomplete spinal cord injury (SCI) subjects investigated patterns of central motor drive (i.e., central activation) of the plantar flexors using interpolated twitches, and modulation of soleus H-reflexes during lengthening, isometric, and shortening muscle actions. In a recent study of the knee extensors, SCI subjects demonstrated greater central activation ratio (CAR) values during lengthening (i.e., eccentric) maximal voluntary contractions (MVCs), compared with during isometric or shortening (i.e., concentric) MVCs. In contrast, healthy controls demonstrated lower lengthening CAR values compared with their isometric and shortening CARs. For the present investigation, we hypothesized SCI subjects would again produce their highest CAR values during lengthening MVCs, and that these increases in central activation were partially attributable to greater efficacy of Ia-α motoneuron transmission during muscle lengthening following SCI. Results show SCI subjects produced higher CAR values during lengthening vs. isometric or shortening MVCs (all P < 0.001). H-reflex testing revealed normalized H-reflexes (maximal SOL H-reflex-to-maximal M-wave ratios) were greater for SCI than controls during passive (P = 0.023) and active (i.e., 75% MVC; P = 0.017) lengthening, suggesting facilitation of Ia transmission post-SCI. Additionally, measures of spinal reflex excitability (passive lengthening maximal SOL H-reflex-to-maximal M-wave ratio) in SCI were positively correlated with soleus electromyographic activity and CAR values during lengthening MVCs (both P < 0.05). The present study presents evidence that patterns of dynamic muscle activation are altered following SCI, and that greater central activation during lengthening contractions is partly due to enhanced efficacy of Ia-α motoneuron transmission.

Ultrasonic Assessment of Quadriceps Muscle Quality Does Not Reflect Muscle Function in Healthy Individuals

Muscle quality assessed by ultrasound imaging represents the composition of the muscle. Poor muscle quality (i.e. greater echo intensity) indicates greater intramuscular fat and connective tissue, and is associated with lesser functional ability and strength in older adults. If validated as a robust indicator of muscle function, this measure could be used to evaluate patient populations who experience muscle dysfunction and are unable to produce maximal muscle contraction due to pain and inhibition (e.g. knee osteoarthritis, anterior cruciate ligament injury). However, the relationships between muscle quality and muscle function are unknown. PURPOSE: To evaluate relationships between quadriceps muscle quality and isometric peak torque, isokinetic peak torque, and voluntary activation. METHODS: Quadriceps (vastus lateralis) muscle quality was assessed from the gray-scale echo intensity obtained from transverse plane ultrasound images. Quadriceps function was assessed via maximal isometric peak torque, isokinetic peak torque at 60 and 240 °/s, and central activation ratio (CAR) in 23 healthy individuals (20 females, 3 males; Age = 21±1 yr; Height = 1.70±0.01m; Mass = 65.6±9.8kg). CAR refers to the ratio of peak voluntary torque to the torque resulting from an electrical stimulus superimposed on the quadriceps during maximal isometric contraction. Relationships between muscle quality and each index of muscle function were calculated via Pearson correlations. RESULTS: Quadriceps muscle quality was not significantly correlated with isometric peak torque (r = 0.324, p = 0.152), isokinetic peak torque at 60°/s (r = 0.013, p = 0.955) or 240°/s (r = −0.071, p = 0.749), or CAR (r = 0.366, p = 0.103). CONCLUSION: Muscle quality does not appear to be an indicator of muscle function in healthy individuals. These findings suggest that muscle quality is not likely a surrogate for muscle function in individuals with joint pathologies who experience muscle dysfunction. The disagreement between our results in healthy, young individuals and those from previous investigations involving older adults may suggest that muscle quality reflects undefined factors associated with sarcopenia. Future research is necessary to evaluate relationships between muscle quality and function in pathological populations.

Quadriceps Strength Asymmetry After Anterior Cruciate Ligament Reconstruction Alters Knee Joint Biomechanics and Functional Performance at Time of Return to Activity

Background: Quadriceps strength deficits are observed clinically after anterior cruciate ligament (ACL) injury and reconstruction and are often not overcome despite rehabilitation. Given that quadriceps strength may be important for achieving symmetrical joint biomechanics and promoting long-term joint health, determining the magnitude of strength deficits that lead to altered mechanics is critical. Purpose: To determine if the magnitude of quadriceps strength asymmetry alters knee and hip biomechanical symmetry as well as functional performance and self-reported function. Study Design: Cross-sectional study; Level of evidence, 3. Methods: A total of 73 patients were tested at the time they were cleared for return to activity after ACL reconstruction. Quadriceps strength and activation, scores on the International Knee Documentation Committee form, the hop for distance test, and sagittal plane lower extremity biomechanics were recorded while patients completed a single-legged hop. Results: Patients with high and moderate quadriceps strength symmetry had larger central activation ratios as well as greater limb symmetry indices on the hop for distance compared with patients with low quadriceps strength symmetry (P < .05). Similarly, knee flexion angle and external moment symmetry were higher in the patients with high and moderate quadriceps symmetry compared with those with low symmetry (P < .05). Quadriceps strength was found to be associated with sagittal plane knee angle and moment symmetry (P < .05). Conclusion: Patients with low quadriceps strength displayed greater movement asymmetries at the knee in the sagittal plane. Quadriceps strength was related to movement asymmetries and functional performance. Rehabilitation after ACL reconstruction needs to focus on maximizing quadriceps strength, which likely will lead to more symmetrical knee biomechanics.

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.

Quadriceps neural alterations in anterior cruciate ligament reconstructed patients: A 6-month longitudinal investigation.

The purpose of this investigation was to evaluate differences in quadriceps corticospinal excitability, spinal-reflexive excitability, strength, and voluntary activation before, 2 weeks post and 6 months post-anterior cruciate ligament reconstruction (ACLr). This longitudinal, case-control investigation examined 20 patients scheduled for ACLr (11 females, 9 males; age: 20.9 ± 4.4 years; height:172.4 ± 7.5 cm; weight:76.2 ± 11.8 kg) and 20 healthy controls (11 females, 9 males; age:21.7 ± 3.7 years; height: 173.7 ± 9.9 cm; weight: 76.1 ± 19.7 kg). Maximal voluntary isometric contractions (MVIC), central activation ratio (CAR), normalized Hoffmann spinal reflexes, active motor threshold (AMT), and normalized motor-evoked potential (MEP) amplitudes at 120% of AMT were measured in the quadriceps muscle at the specific time points. ACLr patients demonstrated bilateral reductions in spinal-reflexive excitability compared with controls before surgery (P = 0.02) and 2 weeks post-surgery (P ≤ 0.001). ACLr patients demonstrated higher AMT at 6 months post-surgery (P ≤ 0.001) in both limbs. No MEP differences were detected. Quadriceps MVIC and CAR were lower in both limbs of the ACLr group before surgery and 6 months post-surgery (P ≤ 0.05) compared with controls. Diminished excitability of spinal-reflexive and corticospinal pathways are present at different times following ACLr and occur in combination with clinical deficits in quadriceps strength and activation. Early rehabilitation strategies targeting spinal-reflexive excitability may help improve postoperative outcomes, while later-stage rehabilitation may benefit from therapeutic techniques aimed at improving corticospinal excitability.

Persistent neuromuscular and corticomotor quadriceps asymmetry after anterior cruciate ligament reconstruction.

Return to activity in the presence of quadriceps dysfunction may predispose individuals with anterior cruciate ligament reconstruction (ACLR) to long-term joint degeneration. Asymmetry may manifest during movement and result in altered knee-joint-loading patterns; however, the underlying neurophysiologic mechanisms remain unclear. To compare limb symmetry of quadriceps neuromuscular function between participants with ACLR and participants serving as healthy controls. Descriptive laboratory study. Research laboratory. A total of 22 individuals with ACLR (12 men, 10 women) and 24 individuals serving as healthy controls (12 men, 12 women). Normalized knee-extension maximal voluntary isometric contraction (MVIC) torque (Nm/kg), quadriceps central activation ratio (CAR) (%), quadriceps motor-neuron-pool excitability (Hoffmann reflex to motor wave ratio), and quadriceps active motor threshold (AMT) (% 2.0 T) were measured bilaterally and used to calculate limb symmetry indices for comparison between groups. We used analyses of variance to compare quadriceps Hoffmann reflex to motor wave ratio, normalized knee-extension MVIC torque, quadriceps CAR, and quadriceps AMT between groups and limbs. The ACLR group exhibited greater asymmetry in knee-extension MVIC torque (ACLR group = 0.85 ± 0.21, healthy group = 0.97 ± 0.14; t44 = 2.26, P = .03), quadriceps CAR (ACLR group = 0.94 ± 0.11, healthy group = 1.00 ± 0.08; t44 = 2.22, P = .04), and quadriceps AMT (ACLR group = 1.13 ± 0.18, healthy group = 1.02 ± 0.11; t34 = -2.46, P = .04) than the healthy control group. Asymmetries in measures of quadriceps function and cortical excitability were present in patients with ACLR. Asymmetry in quadriceps strength, activation, and cortical excitability persisted in individuals with ACLR beyond return to recreational activity. Measuring the magnitude of asymmetry after ACLR represents an important step in understanding long-term reductions in self-reported function and increased rate of subsequent joint injury in otherwise healthy, active individuals after ACLR.

Muscle atrophy contributes to quadriceps weakness after anterior cruciate ligament reconstruction

Quadriceps weakness persists after anterior cruciate ligament reconstruction. Muscle atrophy and activation failure may contribute. This study examined the roles of atrophy and activation failure in quadriceps weakness after anterior cruciate ligament reconstruction. Case series. Twenty patients six months post-anterior cruciate ligament reconstruction participated. Atrophy was determined as peak quadriceps cross sectional area from magnetic resonance images. Quadriceps activation was quantified via the central activation ratio, while muscle strength was measured isometrically. All testing was performed bilaterally. Hierarchical linear regression and one-way ANOVAs were performed to examine the relation of muscle strength with activation and atrophy. Cross sectional area (R(2)=0.307; p=0.011) explained more of the variance in quadriceps strength than central activation ratio (R(2)<0.001; p=0.987). Strength and cross sectional area were lower in the injured (strength: 2.03±0.51Nm/kg; cross sectional area: 68.81±17.80cm(2)) versus uninjured limb (strength: 2.89±0.81Nm/kg; cross sectional area: 81.10±21.58cm(2); p<0.001). There were no side-to-side differences in central activation ratio; however, quadriceps activation failure was present bilaterally (injured: 0.87±0.12; uninjured: 0.85±0.14; p=0.571). Quadriceps cross sectional area was strongly related to muscle strength six months after anterior cruciate ligament reconstruction and substantial injured versus uninjured limb deficits were demonstrated for strength and cross sectional area. Patients may benefit from exercises aimed at improving quadriceps cross sectional area post-operatively.

Combination of eccentric exercise and neuromuscular electrical stimulation to improve quadriceps function post-ACL reconstruction

BackgroundNeuromuscular electrical stimulation (NMES) has been shown to reduce quadriceps activation failure (QAF), and eccentric exercise has been shown to lessen muscle atrophy post-ACL reconstruction. Given that these are two critical components of quadriceps strength, intervention combining these therapies may be effective at reinstituting quadriceps function post-reconstruction. Thus, the aim of this study was to evaluate the effectiveness of a combined NMES and eccentric exercise intervention to improve the recovery of quadriceps activation and strength post-reconstruction. MethodsThirty-six individuals post-injury were placed into four treatment groups (N&E, NMES and eccentrics; E-only, eccentrics only; N-only, NMES-only; and STND, standard of care) and ten healthy controls participated. N&E and N-only received the NMES protocol 2× per week for the first 6weeks post-reconstruction. N&E and E-only received the eccentric exercise protocol 2× per week beginning 6weeks post-reconstruction. Quadriceps activation was assessed via the superimposed burst technique and quantified via the central activation ratio. Quadriceps strength was assessed via maximal voluntary isomeric contractions (Nm/kg). Data was gathered on three occasions: pre-operative, 12-weeks-post-surgery and at return-to-play. ResultsNo differences in pre-operative measures existed (P>0.05). E-only recovered quadriceps activation better than N-only or STND (P<0.05). N&E and E-only recovered strength better than N-only or the STND (P<0.05) and had strength values that were similar to healthy at return-to-play (P>0.05). ConclusionEccentric exercise was capable of restoring levels of quadriceps activation and strength that were similar to those of healthy adults and better than NMES alone.Level of evidence: Level 3, Parallel longitudinal study

Quadriceps muscle function after exercise in men and women with a history of anterior cruciate ligament reconstruction.

Sex differences in lower extremity neuromuscular function have been reported after anterior cruciate ligament reconstruction (ACLR). Research evidence supports different levels of fatigability in men and women and between patients with ACLR and healthy controls. The influence of sex on the response to continuous exercise in patients with ACLR is not clear. To compare quadriceps neuromuscular function after exercise between men and women with ACLR. Descriptive laboratory study. Laboratory. Twenty-six active volunteers (13 men [50%]: age = 24.1 ± 4.4 years, height = 179.1 ± 9.8 cm, mass = 80.1 ± 9.4 kg, months since surgery = 43.5 ± 37.0; 13 women [50%]: age = 24.2 ± 5.6 years, height = 163.0 ± 5.9 cm, mass = 62.3 ± 8.3 kg, months since surgery = 45.8 ± 42.7) with a history of unilateral primary ACLR at least 6 months earlier. Thirty minutes of continuous exercise comprising 5 separate 6-minute cycles, including 5 minutes of uphill walking and 1 minute of body-weight squatting and step-ups. Normalized knee-extension maximal voluntary isometric contraction torque, quadriceps superimposed-burst torque, and quadriceps central activation ratio before and after exercise. We performed separate 2 (sex: men, women) × 2 (time: preexercise, postexercise) repeated-measures analyses of variance for the 3 variables. Separate, independent-samples t tests were calculated to compare preexercise with postexercise change in all dependent variables between sexes. A significant group-by-time interaction was present for knee-extension torque (P = .04). The percentage reduction in knee-extension maximal voluntary isometric contraction torque (men = 1.94%, women = -10.32%; P = .02) and quadriceps central activation ratio (men = -1.45%, women = -8.69%; P = .03) experienced by men was less than that observed in women. In the presence of quadriceps dysfunction, female participants experienced greater-magnitude reductions in quadriceps function after 30 minutes of exercise than male participants. This indicates a reduced ability to absorb knee-joint loads, which may have significant implications for reinjury and joint osteoarthritis in women after ACLR.

Muscle activation varies with contraction mode in human spinal cord injury.

To better understand volitional force generation after chronic incomplete spinal cord injury (SCI), we examined muscle activation during single and repeated isometric, concentric, and eccentric knee extensor (KE) maximal voluntary contractions (MVCs). Torque and electromyographic (EMG) activity were recorded during single and repeated isometric and dynamic KE MVCs in 11 SCI subjects. Central activation ratios (CARs) were calculated for all contraction modes in SCI subjects and 11 healthy controls. SCI subjects generated greater torque, KE EMG, and CARs during single eccentric vs. isometric and concentric MVCs (all P < 0.05). Torque and EMG remained similar during repeated eccentric MVCs; however, both increased during repeated isometric (>25%) and concentric (>30%) MVCs. SCI subjects demonstrated greater muscle activation during eccentric MVCs vs. isometric and concentric MVCs. This pattern of activation contrasts with the decreased eccentric activation demonstrated by healthy controls. Such information may aid development of novel rehabilitation interventions.

Pre-operative quadriceps activation is related to post-operative activation, not strength, in patients post-ACL reconstruction.

Quadriceps activation failure is considered to contribute to the weakness that lingers following anterior cruciate ligament (ACL) reconstruction. Importantly, the impact of pre-operative quadriceps activation on post-operative quadriceps function is unknown. Understanding this relationship is clinically important; as the counteractive approach clinicians should employ pre-operatively to mitigate post-operative quadriceps weakness is unclear. Accordingly, the primary purpose of this study was to investigate the relationship between pre-operative quadriceps activation and post-operative quadriceps strength and activation. Fifty-four individuals post-ACL injury reported for testing on two occasions: prior to surgery and post-surgery once they returned to activity. Quadriceps activation was assessed using the burst superimposition technique and quantified using the central activation ratio. Quadriceps strength was assessed using isometric contractions that were performed at 90° of knee flexion. Multiple linear regressions were utilized to detect the relationships between pre-operative activation and strength and post-operative activation and strength. Pre-operative activation was not associated with post-operative strength (R(2) = 0.064, P = 0.186). Pre-operative quadriceps activation and strength were associated with post-operative activation (R(2) = 0.383, P ≤ 0.001) and strength (R(2) = 0.465, P ≤ 0.001), respectively. Individuals with better pre-operative quadriceps activation demonstrated greater post-operative activation. Similarly, individuals with better pre-operative strength demonstrated better post-operative strength. Pre-operative quadriceps activation was not a predictor of post-operative strength. From a clinical perspective, our work indicates that clinicians should utilize therapies targeting both quadriceps activation and strength prior to ACL reconstruction in order to maximize these factors post-ACL reconstruction, as pre-operative activation and strength are related to post-operative activation and strength, respectively at return-to-activity.

Does a Mental Training Session Induce Neuromuscular Fatigue?

Mental training, as physical training, enhances muscle strength. Whereas the repetition of maximal voluntary contractions (MVC) induces neuromuscular fatigue, the effect of maximal imagined contractions (MIC) on neuromuscular fatigue remains unknown. Here, we investigated neuromuscular alterations after a mental training session including MIC, a physical training session including MVC, and a combined training session including both MIC and MVC of the elbow flexor muscles. Ten participants performed 80 MIC (duty cycle, 5-s MIC and 10-s rest), 80 MVC (identical duty cycle), or 80 MVC and 80 MIC (5-s MVC, 2-s rest, 5-s MIC, and 3-s rest) in three separate sessions. MVC torque was assessed five times over the course of the training and 10 min after the end of the training in the three protocols. Central activation ratio (CARc), reflecting central fatigue, and corticospinal excitability, at rest and during MIC, were estimated using transcranial magnetic stimulation. Both the physical training and the combined training induced an approximately 40% drop of MVC torque, accompanied with an approximately 10% decrease of CARc without significant difference between the two sessions. On the contrary, the repetition of MIC did not reduce maximal force production capacity and did not alter CARc. Corticospinal excitability was always facilitated during MIC compared with that during rest, ensuring that the participants imagined the desired movement. These results suggested that one session of mental training alone or combined with physical training do not induce (additional) neuromuscular fatigue despite the repetitive activation of the corticospinal track. Motor imagery may be added to physical practice to increase the total workload without exacerbating neuromuscular fatigue.

Cross-education strength and activation after eccentric exercise.

After injury, eccentric exercise of the injured limb is often contraindicated. Cross-education training, whereby the uninvolved limb is exercised, is an alternative that may improve quadriceps muscle strength and activation in the unexercised limb. To determine the effect of eccentric exercise on quadriceps strength and activation gains in the unexercised limb. Eighteen healthy individuals were randomly assigned to an eccentric training group or a control group. Quadriceps strength and activation measures were collected at preintervention, midintervention, and postintervention. Eccentric training participants exercised their dominant limb with a dynamometer in eccentric mode at 60°/s, 3 times per week for 8 weeks. Quadriceps strength was quantified at 30° and 60°/s in concentric and eccentric modes. Quadriceps activation was assessed using the burst superimposition technique and quantified via the central activation ratio. A 2 × 3 repeated-measures analysis of variance was used to detect the effects of group and testing session on quadriceps strength and activation. Where appropriate, post hoc Bonferroni multiple-comparisons procedures were used. We found greater eccentric strength in the unexercised limbs of eccentric training participants between preintervention and midintervention and between preintervention and postintervention (preintervention to midintervention: 30°/s P = .05; preintervention to postintervention: 30°/s P = .02, 60°/s P = .02). No differences were noted in concentric strength (P > .05). An overall trend toward greater quadriceps activation in the unexercised knee was detected between preintervention and postintervention (P = .063), with the eccentric training group demonstrating a strong effect (Cohen d = 0.83). Control strength did not change (P > .05). Exercising with eccentric actions resulted in mode-specific and velocity-specific gains in quadriceps strength in the unexercised limb. A trend toward greater quadriceps activation in the unexercised knee was noted, suggesting that strength gains may have occurred because of enhanced neural activity. This type of therapy may be a useful addition to rehabilitation programs designed to improve quadriceps strength.