Determining the extent of neural activation during maximal effort

Effect of Using an 8-Figure Shoulder Brace on Posture and Muscle Activities during the Performance of Dental Hygiene Procedures

The incidence of work-related musculoskeletal disorders (MSDs) among dental workers has been increasing. Many ergonomic devices and accessories have been introduced. The aim of this study was to investigate the effects of an 8-figure shoulder brace on posture-related muscle activities in dental hygiene practitioners during scaling procedures. In this study, 33 participants (age: 21.9 ± 2.1 years, height: 162.0 ± 6.0 cm, weight: 55.8 ± 9.0 kg, body mass index: 21.2 ± 2.4 kg/m²) performed the scaling procedure with and without the 8-figure shoulder brace in a randomized order. The normalized electromyography activity in the amplitude probability distribution function and joint angles (cervical, thoracic, lumbar, and shoulder joints) were simultaneously recorded during scaling. A paired t test was used to compare the differences in muscle kinematics, with the alpha level set at 0.05. The dental hygienists who wore the 8-figure shoulder brace during scaling showed thoracic and lumbar extension, improved sitting postures, and reduced shoulder joint abduction. However, we also observed an unintended increase in internal rotation. Use of the 8-figure shoulder brace could prevent work-related MSDs in lumbar and thoracic regions by reducing the effort exerted by the upper trapezius and deltoid muscles, despite the increased muscular effort of the cervical erector spinae.

Neuromuscular rate of force development deficit in Parkinson disease

BACKGROUND

Bradykinesia and reduced neuromuscular force exist in Parkinson disease. The interpolated twitch technique has been used to evaluate central versus peripheral manifestations of neuromuscular strength in healthy, aging, and athletic populations, as well as moderate to advanced Parkinson disease, but this method has not been used in mild Parkinson disease. This study aimed to evaluate quadriceps femoris rate of force development and quantify potential central and peripheral activation deficits in individuals with Parkinson disease.

METHODS

Nine persons with mild Parkinson Disease (Hoehn & Yahr≤2, Unified Parkinson Disease Rating Scale total score=mean 19.1 (SD 5.0)) and eight age-matched controls were recruited in a cross-sectional investigation. Quadriceps femoris voluntary and stimulated maximal force and rate of force development were evaluated using the interpolated twitch technique.

FINDINGS

Thirteen participants satisfactorily completed the protocol. Individuals with early Parkinson disease (n=7) had significantly slower voluntary rate of force development (p=0.008; d=1.97) and rate of force development ratio (p=0.004; d=2.18) than controls (n=6). No significant differences were found between groups for all other variables.

INTERPRETATIONS

Persons with mild-to-moderate Parkinson disease display disparities in rate of force development, even without deficits in maximal force. The inability to produce force at a rate comparable to controls is likely a downstream effect of central dysfunction of the motor pathway in Parkinson disease.

Effects of Use and Disuse on Non-paralyzed and Paralyzed Skeletal Muscles

Skeletal muscle is an integral part of the somatic nervous system and plays a primary role in the performance of physical activities. Because physical activity is vital to countering the effects of aging and age related diseases and is a key component in the maintenance of healthy body composition it is important to understand the effects of use and disuse on skeletal muscle. While voluntary muscle activity provides optimal benefits to muscle and the maintenance of healthy body composition, neuromuscular electrical stimulation may be a viable alternative activity for individuals with paralysis. Body composition with a healthy muscle to fat ratio has been associated with healthy blood lipid and glucose profiles that may decrease the risk of cardiovascular and metabolic diseases.

Nonuniform muscle hypertrophy: its relation to muscle activation in training session

PURPOSE

Muscle hypertrophy in response to resistance training has been reported to occur nonuniformly along the length of the muscle. The purpose of the present study was to examine whether the regional difference in muscle hypertrophy induced by a training intervention corresponds to the regional difference in muscle activation in the training session.

METHODS

Twelve young men participated in a training intervention program for the elbow extensors with a multijoint resistance exercise for 12 wk (3 d · wk(-1)). Before and after the intervention, cross-sectional areas of the triceps brachii along its length were measured with magnetic resonance images. A series of transverse relaxation time (T2)-weighted magnetic resonance images was recorded before and immediately after the first session of training intervention. The T2 was calculated for each pixel within the triceps brachii. In the images recorded after the session, the number of pixels with a T2 greater than the threshold (mean + 1 SD of T2 before the session) was expressed as the ratio to the whole number of pixels within the muscle and used as an index of muscle activation (percent activated area).

RESULTS

The percent activated area of the triceps brachii in the first session was significantly higher in the middle regions than that in the most proximal region. Similarly, the relative change in cross-sectional area induced by the training intervention was also significantly greater in the middle regions than the most proximal region.

CONCLUSION

The results suggest that nonuniform muscle hypertrophy after training intervention is due to the region-specific muscle activation during the training session.

Closed chain assessment of quadriceps activation using the superimposed burst technique

The superimposed burst technique is used to estimate quadriceps central activation ratio during a maximal voluntary isometric contraction, which is calculated from force data during an open-chain knee extension task. Assessing quadriceps activation in a closed-chain position would more closely simulate the action of the quadriceps during activity. Our aim was to determine the test-retest reliability of the quadriceps central activation ratio in the closed chain.

METHODS

Twenty-two healthy, active volunteers (13M/12F; age=23.8±3; height=72.7±14.5cm; mass=175.3±9.6kg) were recruited to participate. Knee extension MVIC torque and the peak torque during a superimposed electrical stimulus delivered to the quadriceps during an MVIC were measured to estimate quadriceps CAR. Interclass correlation coefficients were used to assess test-retest reliability between sessions, and Bland-Altman plots to graphically assess agreement between sessions.

RESULTS

Test-retest reliability was fair for CAR (ICC2,k=0.68; P=0.005), with a mean difference of -2.8±10.3%, and limits of agreement ranging -23.1-18.1%.

CONCLUSIONS

CAR calculated using the superimposed burst technique is moderately reliable in a closed-chain position using technique-based instruction. Although acceptable reliability was demonstrated, wide limits of agreement suggest high variability between sessions.

Changes in voluntary quadriceps activation predict changes in quadriceps strength after therapeutic exercise in patients with knee osteoarthritis

INTRODUCTION

Recent rehabilitation paradigms have suggested that improving voluntary muscle activation may optimize strength outcomes related to resistance training. The aim of this study was to determine if changes in voluntary quadriceps activation could predict changes in quadriceps strength following a 4 week therapeutic exercise regimen.

METHOD

Thirty-six participants with tibiofemoral osteoarthritis volunteered for this study, while 30 participants (14 males, 16 females, 58 ± 11.8 years, 172.2 ± 9.2 cm, 87.1 ± 18.5 kg) finished the 4 week supervised therapeutic exercise protocol and were used in the final analysis. Demographics, quadriceps strength and voluntary quadriceps activation using the burst superimposition technique were evaluated prior to the intervention. Following the therapeutic exercise program, quadriceps strength and voluntary activation were assessed. Simple correlations were performed to determine covariates in a multiple regression equation to evaluate if changes in voluntary quadriceps activation could predict changes in quadriceps strength.

RESULTS

There was a significant moderate simple correlation between participant height and change in MVIC (r=-0.44, P=0.01). Both height and changes in voluntary quadriceps activation significantly predicted changes in MVIC (R(2)=0.66, P<0.001). After controlling for patient height, a change in voluntary quadriceps activation produced a significant improvement in the prediction of a change in MVIC (ΔR(2)=0.47, P<0.001).

DISCUSSION

Changes in voluntary quadriceps activation predicted 47% of variance in the change in quadriceps strength. These results suggest that interventions aimed at manipulating quadriceps activation may be helpful for improving quadriceps strength in patients with tibiofemoral osteoarthritis.

Accessory muscle activation during the superimposed burst technique

Quadriceps muscle activation is assessed using the superimposed burst technique. This technique involves percutaneous muscle stimulation superimposed during maximal isometric volitional knee extension. It is unknown whether accessory muscle activation during maximal knee extension influences estimates of quadriceps muscle activation. Our aim was to compare accessory muscle activation while performing the superimposed burst technique using investigator delivered verbal instruction to constrain the system (CS) and a participant preferred (PP) technique. Twenty five healthy, active individuals (13M/12F, age=23.8 ± 3.35, height=72.73 ± 14.51 cm, and weight=175.29 ± 9.59 kg) were recruited for this study. All participants performed superimposed burst testing with (CS) and without (PP) verbal instruction to encourage isolated quadriceps activation during maximal isometric knee extension. The main outcome variables measured were knee extension torque, quadriceps central activation ratio and mean EMG of vastus lateralis, biceps femoris, and lumbar paraspinal muscles. There were significant differences in knee extension torque (CS=2.87 ± 0.93 Nm/kg, PP=3.40 ± 1.12 Nm/kg, p<0.001), superimposed burst torque (CS=3.40 ±0.98 Nm/kg, PP=3.75 ± 1.11 Nm/kg, p=0.002) and quadriceps CAR (CS=84.1 ± 12.0%, PP=90.2 ± 9.9%, p<0.001) between the techniques. There was also a significant difference in lumbar paraspinal EMG (CS=6.40 ± 8.52%, PP=11.86 ± 14.89%, p=0.043) between the techniques however vastus lateralis EMG was not significantly different. Patient instruction via verbal instruction to constrain proximal structures may help patient minimize confounders to knee extension torque generation while maximizing quadriceps activation.

Neural adaptations to electrical stimulation strength training

This review provides evidence for the hypothesis that electrostimulation strength training (EST) increases the force of a maximal voluntary contraction (MVC) through neural adaptations in healthy skeletal muscle. Although electrical stimulation and voluntary effort activate muscle differently, there is substantial evidence to suggest that EST modifies the excitability of specific neural paths and such adaptations contribute to the increases in MVC force. Similar to strength training with voluntary contractions, EST increases MVC force after only a few sessions with some changes in muscle biochemistry but without overt muscle hypertrophy. There is some mixed evidence for spinal neural adaptations in the form of an increase in the amplitude of the interpolated twitch and in the amplitude of the volitional wave, with less evidence for changes in spinal excitability. Cross-sectional and exercise studies also suggest that the barrage of sensory and nociceptive inputs acts at the cortical level and can modify the motor cortical output and interhemispheric paths. The data suggest that neural adaptations mediate initial increases in MVC force after short-term EST.

Relationship between transcranial magnetic stimulation and percutaneous electrical stimulation in determining the quadriceps central activation ratio

OBJECTIVE

To determine the relationship between quadriceps central activation ratios (CARs) derived from a percutaneous electrical stimulation (CAR(SIB)) and a transcranial magnetic stimulation (CAR(TMS)) in healthy participants.

DESIGN

Nineteen healthy participants (5 men, 14 women, 23.7 ± 4.8 yrs, 66.8 ± 10.0 kg, and 170.1 ± 7.0 cm) qualified for this descriptive study. Muscle activation, using both methods (CAR(SIB) and CAR(TMS)), was measured at days 1, 14, and 28. All participants performed both methods in a counterbalanced order. Correlation coefficients and Bland-Altman plots were used to assess relationships and agreement between the two methods. For both methods, reliability was assessed at 14 and 28 days using Bland-Altman plots and intraclass correlation coefficients.

RESULTS

CAR(TMS) scores were higher than CAR(SIB) scores for all three sessions, with mean differences between CAR scores of -0.06 (95% confidence interval, -0.19-0.07), -0.03 (95% confidence interval, -0.14-0.08), and -0.03 (95% confidence interval, -0.11-0.05). There was a significant moderate positive correlation between CAR(SIB) and CAR(TMS) at 14 days from baseline (ρ = 0.45, P = 0.05). Intersession reliability was strong for CAR(SIB) at 14 and 28 days from baseline (intraclass correlation coefficients = 0.80 [P = 0.001] and 0.85 [P < 0.001], respectively). Intersession reliability for CAR(TMS) was moderate from baseline to 14 days (intraclass correlation coefficients = 0.68 [P = 0.01]).

CONCLUSIONS

It does not seem that the CAR(TMS) and CAR(SIB) methods are interchangeable measurements for evaluating volitional quadriceps activation; however, both measurements seem to have acceptable agreement at 14 and 28 days compared with day 1.

Neuromuscular contributions to age-related weakness

BACKGROUND

Declines in skeletal muscle mass and quality are important factors contributing to age-related weakness. Neural activation of agonist and antagonist muscles may also be important contributing factors.

METHODS

We conducted a review of the scientific literature on older adults to determine (a) methodologies used to quantify activation, (b) the potential role of agonist and antagonist activation on weakness, and (c) some possible neurophysiological mechanisms that may underlie impaired activation.

RESULTS

The cumulative evidence indicates that agonist activation is impaired in some, but not all, older adults and that this impairment contributes to age-related weakness. It is possible that antagonist coactivation also plays a role in age-related weakness, though a definitive link has not been established.

CONCLUSION

Future research should focus on improving quantitative measurement and mechanistic understanding of impaired activation with aging.

Quadriceps activation following knee injuries: a systematic review

CONTEXT

Arthrogenic muscle inhibition is an important underlying factor in persistent quadriceps muscle weakness after knee injury or surgery.

OBJECTIVE

To determine the magnitude and prevalence of volitional quadriceps activation deficits after knee injury.

DATA SOURCES

Web of Science database.

STUDY SELECTION

Eligible studies involved human participants and measured quadriceps activation using either twitch interpolation or burst superimposition on patients with knee injuries or surgeries such as anterior cruciate ligament deficiency (ACLd), anterior cruciate ligament reconstruction (ACLr), and anterior knee pain (AKP).

DATA EXTRACTION

Means, measures of variability, and prevalence of quadriceps activation (QA) failure (<95%) were recorded for experiments involving ACLd (10), ACLr (5), and AKP (3).

DATA SYNTHESIS

A total of 21 data sets from 18 studies were initially identified. Data from 3 studies (1 paper reporting data for both ACLd and ACLr, 1 on AKP, and the postarthroscopy paper) were excluded from the primary analyses because only graphical data were reported. Of the remaining 17 data sets (from 15 studies), weighted mean QA in 352 ACLd patients was 87.3% on the involved side, 89.1% on the uninvolved side, and 91% in control participants. The QA failure prevalence ranged from 0% to 100%. Weighted mean QA in 99 total ACLr patients was 89.2% on the involved side, 84% on the uninvolved side, and 98.5% for the control group, with prevalence ranging from 0% to 71%. Thirty-eight patients with AKP averaged 78.6% on the involved side and 77.7% on the contralateral side. Bilateral QA failure was commonly reported in patients.

CONCLUSIONS

Quadriceps activation failure is common in patients with ACLd, ACLr, and AKP and is often observed bilaterally.

Quantification method affects estimates of voluntary quadriceps activation

The aim of this study was to investigate the effect of quantification method on estimates of voluntary quadriceps muscle activation. Twenty-two people with no history of serious lower extremity injuries underwent voluntary quadriceps activation testing at 60 degrees of knee flexion. Estimates of quadriceps activation were derived with: (1) a formula based on the interpolated twitch technique (ITT); (2) the central activation ratio (CAR); and (3) a modified central activation ratio. Predictive equations were developed that describe the relationships between the three methods. Significant differences (P < 0.001) were observed between the estimates of voluntary quadriceps muscle activation obtained using the three methods (ITT percent activation = 93.0 +/- 6.4%, CAR = 95.9 +/- 3.8%, modified CAR = 98.5 +/- 4.1%). Excellent correlation (r = 0.995) was observed between ITT-based percent activation and the CAR method. The associations between these methods and the modified CAR approach were weaker. Quantification method affects activation estimates. The equations developed will assist scientists in accurately comparing the results of studies that use different methods of quantifying activation.

Neuromuscular fatigue induced by an isotonic heavy-resistance loading protocol in knee extensors

The main aim of this study was to assess neuromuscular fatigue during a typical high-load, low-repetition loading protocol. Muscle stimulations were used to assess maximum voluntary contraction, resting single- and double-pulse twitch characteristics, and superimposed double-pulse twitch force (used to calculate voluntary activation) before and after an acute knee extension loading protocol. In our participants, who had previous resistance training experience, the mean voluntary activation level was 96.2% in an unfatigued state. Maximum voluntary contraction (-11.8%), resting double-pulse twitch force (-10.6%), and voluntary activation (-2.1%) were markedly decreased as a consequence of loading (P < 0.05). In addition, although potentiated twitch characteristics were observed during the loading protocol, this was short-lived, as fatigue surpassed the potentiation mechanisms. Our results show that both central and peripheral mechanisms contributed to neuromuscular fatigue during the present loading protocol.

The non-linear relationship between muscle voluntary activation level and voluntary force measured by the interpolated twitch technique

Interpolated twitch technique (ITT) is a non-invasive method for assessing the completeness of muscle activation in clinical settings. Voluntary activation level (VA), measured by ITT and estimated by a conventional linear model, was reported to have a non-linear relationship with true voluntary contraction force at higher activation levels. The relationship needs to be further clarified for the correct use by clinicians and researchers. This study was to established a modified voluntary activation (modified VA) and define a valid range by fitting a non-linear logistic growth model. Eight healthy male adults participated in this study. Each subject performed three sets of voluntary isometric ankle plantar flexions at 20, 40, 60, 80 and 100% maximal voluntary contraction (MVC) with real-time feedback on a computer screen. A supramaximal electrical stimulation was applied on tibia nerve at rest and during contractions. The estimated VA was calculated for each contraction. The relationship between the estimated VA and the actual voluntary contraction force was fitted by a logistic growth model. The result showed that according to the upper and lower limit points of the logistic curve, the valid range was between the 95.16% and 10.55% MVC. The modified VA estimated by this logistic growth model demonstrated less error than the conventional model. This study provided a transfer function for the voluntary activation level and defined the valid range which would provide useful information in clinical applications.

Repeated maximal volitional effort contractions in human spinal cord injury: initial torque increases and reduced fatigue

BACKGROUND

Substantial data indicate greater muscle fatigue in individuals with spinal cord injury (SCI) compared with healthy control subjects when tested by using electrical stimulation protocols. Few studies have investigated the extent of volitional fatigue in motor incomplete SCI.

METHODS

Repeated, maximal volitional effort (MVE) isometric contractions of the knee extensors (KE) were performed in 14 subjects with a motor incomplete SCI and in 10 intact subjects. Subjects performed 20 repeated, intermittent MVEs (5 seconds contraction/5 seconds rest) with KE torques and thigh electromyographic (EMG) activity recorded.

RESULTS

Peak KE torques declined to 64% of baseline MVEs with repeated efforts in control subjects. Conversely, subjects with SCI increased peak torques during the first 5 contractions by 15%, with little evidence of fatigue after 20 repeated efforts. Increases in peak KE torques and the rate of torque increase during the first 5 contractions were attributed primarily to increases in quadriceps EMG activity, but not to decreased knee flexor co-activation. The observed initial increases in peak torque were dependent on the subject's volitional activation and were consistent on the same or different days, indicating little contribution of learning or accommodation to the testing conditions. Sustained MVEs did not elicit substantial increases in peak KE torques as compared to repeated intermittent efforts.

CONCLUSIONS

These data revealed a marked divergence from expected results of increased fatigability in subjects with SCI, and may be a result of complex interactions between mechanisms underlying spastic motor activity and changes in intrinsic motoneuron properties.

Relationships between voluntary activation and motor unit firing rate during maximal voluntary contractions in young and older adults

The extent to which an individual can voluntarily produce maximal muscular force can be estimated using the interpolated twitch technique. Incompleteness of activation is typically attributed to either incomplete recruitment, suboptimal firing rates or both of these mechanisms. The purpose of this study was to assess the relationship between muscle activation and maximal motor unit firing rates. Measures of muscle activation and motor unit firing rates during maximal effort contractions were obtained from 15 subjects (8 young, 7 older) throughout a 6-week strength training program for the knee extensors. High resolution interpolated twitch responses were obtained using a circuit that removes the force level of the maximal voluntary contraction prior to amplification of the additional evoked force. Maximal firing rates were obtained using a four-wire needle electrode. Electrical stimulation of the knee extensors during maximal effort contractions evoked a transient increase in the force and the amplitude of this additional force was correlated with maximal firing rates at r = -0.62 (p < 0.05). Central activation ratio and activation level, two indexes of activation, were correlated with maximal firing rates at r = 0.58 (p < 0.05) and r = 0.68 (p < 0.05), respectively. The training program elicited parallel increases in muscular activation and maximal firing rates. These results provide direct support for maximal firing rate as a significant factor limiting maximal force production.

Phenotypic plasticity and functional asymmetry in response to grip forces exerted by intercollegiate tennis players

This study examined phenotypic and functional responses to extreme asymmetry in hand use, and recorded the grip forces that occur during the tennis serve to assess the forces that contribute to asymmetry. Compressive grip performance was measured for Division I collegiate tennis players (24 female, 24 male) and nonathlete college students (18 females, 17 males) during three experiments: single-repetition maximum voluntary contraction, 30 consecutive repetitions, and a 30-s static hold. Tennis players had significant asymmetry in both forearm circumference and grip strength. The dominant hand of female tennis players produced 25% more force than the opposite hand, while the difference between hands of male tennis players was 18%. However, endurance over 30-repetitions and during the 30-s hold did not significantly differ between the limbs of tennis players. No significant asymmetry in forearm measurements, grip strength, or endurance was detected between the limbs of nonathletes. Grip forces generated during the tennis serve were recorded for 4 male tennis players, using a racket with a dynamometer incorporated into the handle. Peak handgrip forces generated during the serve (150-250 N) averaged 31-44% of the maximum grip force of the participant. Tennis players have increased muscle mass and strength of the dominant limb. Moderate forces, occurring hundreds of times each match and practice, are likely the primary forces contributing to this morphological and functional asymmetry.