Comparison of interpolation and central activation ratios as measures of muscle inactivation

Effect of home-based hot bathing on exercise-induced adaptations associated with short-term resistance exercise training in young men

This study investigated whether home-based bathing intervention (HBBI) improve muscle strength gain and protect cardiovascular function by short-term resistance training (RT). Thirty-one healthy young men measured the maximum voluntary isometric contraction (MVC) of knee extensor, electrically evoked knee extension torque, and mean arterial pressure (MAP). Then, participants were divided into three groups with matching MVC: shower without bathing (control, n = 10), thermoneutral bathing (36°C-bathing, n = 10), and hot bathing (40°C-bathing, n = 11), and conducted 2 weeks of HBBI. Following familiarization for HBBI, participants completed 2 weeks of HBBI and acute RT (five sessions of three sets of 10 isometric knee extension at 60% MVC). Baseline neuromuscular and cardiovascular function was assessed again following completion of the 2 weeks of intervention. MVC was non-significantly increased after the RT period in 40°C-bathing with large effect size (partial η2 = 0.450). The electrically evoked knee extension torque (10/100-Hz ratio) was significantly increased after the RT period in control (p = 0.020). MAP did not alter due to bathing intervention and RT (all p > 0.05). HBBI improved muscle strength without RT-induced alteration of peripheral muscle condition. Shower without bathing reduced muscle strength gain but increased peripheral muscle condition. Short-term RT does not adversely affect the cardiovascular function, regardless of HBBI.

Neuromuscular electrical stimulation during maximal voluntary contraction: a Delphi survey with expert consensus

PURPOSE

The use of electrical stimulation to assess voluntary activation of muscle/s is a popular method employed in numerous exercise science and health research settings. This Delphi study aimed to collate expert opinion and provide recommendations for best practice when using electrical stimulation during maximal voluntary contractions.

METHODS

A two-round Delphi study was undertaken with 30 experts who completed a 62-item questionnaire (Round 1) comprising of open- and closed-ended questions. Consensus was assumed if ≥ 70% of experts selected the same response; such questions were removed from the subsequent Round 2 questionnaire. Responses were also removed if they failed to meet a 15% threshold. Open-ended questions were analysed and converted into closed-ended questions for Round 2. It was assumed there was no clear consensus if a question failed to achieve a ≥ 70% response in Round 2.

RESULTS

A total of 16 out of 62 (25.8%) items reached consensus. Experts agreed that electrical stimulation provides a valid assessment of voluntary activation in specific circumstances, such as during maximal contraction, and this stimulation can be applied at either the muscle or the nerve. Experts recommended using doublet stimuli, self-adhesive electrodes, a familiarisation session, real-time visual or verbal feedback during the contraction, a minimum current increase of + 20% to ensure supramaximal stimulation, and manually triggering stimuli.

CONCLUSION

The results of this Delphi consensus study can help researchers make informed decisions when considering technical parameters when designing studies involving electrical stimulation for the assessment of voluntary activation.

Assessment of Fatigue and Recovery in Sport: Narrative Review

Fatigue is a phenomenon associated with decreases in both physical and cognitive performances and increases in injury occurrence. Competitive athletes are required to complete demanding training programs with high workloads to elicit the physiological and musculoskeletal adaptations plus skill acquisition necessary for performance. High workloads, especially sudden rapid increases in training loads, are associated with the occurrence of fatigue. At present, there is limited evidence elucidating the underlying mechanisms associating the fatigue generated by higher workloads and with an increase in injury risk. The multidimensional nature and manifestation of fatigue have led to differing definitions and dichotomies of the term. Consequently, a plethora of physiological, biochemical, psychological and performance markers have been proposed to measure fatigue and recovery. Those include self-reported scales, countermovement jump performance, heart rate variability, and saliva and serum biomarker analyses. The purpose of this review is to provide an overview of fatigue and recovery plus methods of assessments.

Estimates of voluntary activation in individuals with anterior cruciate ligament reconstruction: Effects of type of stimulator, number of stimuli, and quantification technique

BACKGROUND

Accurate quantification of voluntary activation is important for understanding the extent of quadriceps dysfunction in individuals with anterior cruciate ligament reconstruction (ACLR). Voluntary activation has been quantified using both percent activation derived from the interpolated twitch technique and central activation ratio (CAR) derived from the burst superimposition technique, as well as by using different types of electrical stimulators and pulse train conditions. However, it is unclear how these parameters affect voluntary activation estimates in individuals with ACLR. This study was performed to fill this important knowledge gap in the anterior cruciate ligament literature.

METHODS

Quadriceps strength and voluntary activation were examined in 18 ACLR participants (12 quadriceps/patellar tendon graft, 6 hamstring tendon graft; time since ACLR: 1.06 ± 0.82 years, mean ±  SD) at 90° of knee flexion using 2 stimulators (Digitimer and Grass) and pulse train conditions (3-pulse and 10-pulse). Voluntary activation was quantified by calculating both CAR and percent activation.

RESULTS

Results indicated that voluntary activation was significantly overestimated by CAR when compared with percent activation (p < 0.001). Voluntary activation estimates were not affected by pulse train conditions when using percent activation; however, 3-pulse stimuli resulted in greater overestimation than 10-pulse stimuli when using CAR (p = 0.003). Voluntary activation did not differ between stimulators (p > 0.05); however, the Digitimer evoked greater torque at rest than the Grass (p < 0.001).

CONCLUSION

These results indicate that percent activation derived from the interpolated twitch technique provides superior estimates of voluntary activation than CAR derived from burst superimposition and is less affected by pulse train conditions or stimulators in individuals with ACLR.

Sarcopenia and Neuroscience: Learning to Communicate

In the 1990s and early 2000s, the common definition for sarcopenia was age-related loss of skeletal muscle, and low levels of muscle mass were central to sarcopenia diagnosis. In more recent consensus definitions, however, low muscle strength displaces low muscle mass as a defining feature of sarcopenia. The change stems from growing evidence that muscle weakness is a better predictor of adverse health outcomes (e.g., mobility limitations) than muscle mass. This evidence accompanies an emerging recognition that central neural mechanisms are critical determinants of age-related changes in strength and mobility that can occur independently of variations in muscle mass. However, strikingly little practical attention is typically given to the potential role of the central nervous system in the aetiology or remediation of sarcopenia (i.e., low muscle function). In this article, we provide an overview of some mechanisms that mediate neural regulation of muscle contraction and control, and highlight the specific contributions of neural hypoexcitability, dopaminergic dysfunction, and degradation of functional and structural brain connectivity in relation to sarcopenia. We aim to enhance the lines of communication between the domains of sarcopenia and neuroscience. We believe that appreciation of the neural regulation of muscle contraction and control is fundamental to understanding sarcopenia and to developing targeted therapeutic strategies for its treatment.

Reduced Neural Excitability and Activation Contribute to Clinically Meaningful Weakness in Older Adults

BACKGROUND

Weakness is a risk factor for physical limitations and death in older adults (OAs). We sought to determine whether OAs with clinically meaningful leg extensor weakness exhibit differences in voluntary inactivation (VIA) and measures of corticospinal excitability when compared to young adults (YAs) and OAs without clinically meaningful weakness. We also sought to estimate the relative contribution of indices of neural excitability and thigh lean mass in explaining the between-subject variability in OAs leg extensor strength.

METHODS

In 66 OAs (75.1 ± 7.0 years) and 20 YAs (22.0 ± 1.9 years), we quantified leg extensor strength, thigh lean mass, VIA, and motor evoked potential (MEP) amplitude and silent period (SP) duration. OAs were classified into weakness groups based on previously established strength/body weight (BW) cut points (Weak, Modestly Weak, or Not Weak).

RESULTS

The OAs had 63% less strength/BW when compared to YAs. Weak OAs exhibited higher levels of leg extensor VIA than Not Weak OAs (14.2 ± 7.5% vs 6.1 ± 7.5%). Weak OAs exhibited 24% longer SPs compared to Not Weak OAs, although this difference was insignificant (p = .06). The Weak OAs MEPs were half the amplitude of the Not Weak OAs. Regression analysis indicated that MEP amplitude, SP duration, and thigh lean mass explained ~62% of the variance in strength, with the neural excitability variables explaining ~33% of the variance and thigh lean mass explaining ~29%.

CONCLUSION

These findings suggest that neurotherapeutic interventions targeting excitability could be a viable approach to increase muscle strength in order to reduce the risk of physical impairments in late life.

Single bout of vibration-induced hamstrings fatigue reduces quadriceps inhibition and coactivation of knee muscles after anterior cruciate ligament (ACL) reconstruction

Persistent quadriceps strength deficits in individuals with anterior cruciate ligament reconstruction (ACLr) have been attributed to arthrogenic muscle inhibition (AMI). The purpose of the present study was to investigate the effect of vibration-induced hamstrings fatigue on AMI in patients with ACLr. Eight participants with unilateral ACLr (post-surgery time: M = 46.5, SD = 23.5 months; age: M = 21.4, SD = 1.4 years) and eight individuals with no previous history of knee injury (age: M = 22.5, SD = 2.5 years) were recruited. A fatigue protocol, consisting of 10 min of prolonged local hamstrings vibration, was applied to both the ACLr and control groups. The central activation ratio (CAR) of the quadriceps was measured with a superimposed burst of electrical stimulation, and hamstrings/quadriceps coactivation was assessed using electromyography (EMG) during isometric knee extension exercises, both before and after prolonged local vibration. For the ACLr group, the hamstrings strength, measured by a load cell on a purpose-built chair, was significantly (P = 0.016) reduced about 14.5%, indicating fatigue was actually induced in the hamstrings. At baseline, the ACLr group showed a trend (P = 0.051) toward a lower quadriceps CAR (M = 93.2%, SD = 6.2% versus M = 98.1%, SD = 1.1%) and significantly (P = 0.001) higher hamstrings/quadriceps coactivation (M = 15.1%, SD = 6.2% versus M = 7.5%, SD = 4.0%) during knee extension compared to the control group. The fatigue protocol significantly (P = 0.001) increased quadriceps CAR (from M = 93.2%, SD = 6.2% to M = 97.9%, SD = 2.8%) and significantly (P = 0.006) decreased hamstrings/quadriceps coactivation during knee extension (from M = 15.1%, SD = 6.2% to M = 9.5%, SD = 4.5%) in the ACLr group. In conclusion, vibration-induced hamstrings fatigue can alleviate AMI of the quadriceps in patients with ACLr. This finding has clinical implications in the management of recovery for ACLr patients with quadriceps strength deficits and dysfunction.

Neuromuscular fatigability amplitude and aetiology are interrelated across muscles

NEW FINDINGS

What is the central question of this study? Is neuromuscular fatigability interrelated between different muscle groups from the same individual during isometric all-out exercise? What is the main finding and its importance? Although the average decrease can vary between muscles, an individual demonstrates interrelated fatigability aetiology regardless of the muscle group tested. The inter-individual variability provides evidence of different profiles common between muscles, which can be regarded as an individual characteristic.

ABSTRACT

Neuromuscular fatigability is commonly attributed to central and peripheral origins. However, there is strong evidence of interactions between these two mechanisms. According to the idea that peripheral fatigability might be centrally regulated, one can hypothesize that neuromuscular fatigability would be correlated between different muscle groups at the individual level. Thirty-two healthy participants (16 women and 16 men) completed two 5 min fatiguing exercises [60 isometric maximal voluntary contractions (MVCs)] with finger flexors (FFs) and ankle plantar flexors (PFs) in two randomized sessions. Neuromuscular testing was conducted before, during (every six MVCs) and directly after the fatigue procedure. The force asymptote (F_A ) was calculated as the asymptote of the force-time relationship. Changes (post- vs. pre-fatigue) in the exercise-evoked force (ΔDb₁₀₀ ), voluntary activation (ΔVA) and central activation ratio (∆CAR) were also investigated. Significant correlations were found between FFs and PFs for F_A , ΔDb₁₀₀ and ΔVA (r = 0.65, r = 0.63 and r = 0.50, respectively). A significant negative correlation between ∆CAR and ∆Db₁₀₀ was evidenced for both PFs (r = -0.82) and FFs (r = -0.57). Neuromuscular fatigability is correlated between different muscle groups at the individual level. The results support the idea that a restrained motor drive prevents large peripheral perturbations and that individuals exhibit correlated fatigability aetiology regardless of the muscle group tested. Widely different central/peripheral profiles can be found amongst individuals, and a part of the fatigability aetiology can be regarded as an individual characteristic.

Central Activation Ratio Is a Reliable Measure for Gluteal Neuromuscular Function

CONTEXT

Central activation ratio (CAR) is a common outcome measure used to quantify gross neuromuscular function of the quadriceps using the superimposed burst technique, yet this outcome measure has not been validated in the gluteal musculature.

OBJECTIVE

To quantify gluteus medius (GMed) and gluteus maximus (GMax) CAR in a healthy population and evaluate its validity and reliability over a 1-week period.

DESIGN

Descriptive.

SETTING

Laboratory.

PATIENTS OR OTHER PARTICIPANTS

A total of 20 healthy participants (9 males and 11 females; age 22.2 [1.4] y, height 173.4 [11.1] cm, mass 84.8 [25.8] kg) were enrolled in this study.

INTERVENTIONS

Participants were assessed at 2 sessions, separated by 1 week. Progressive electrical stimuli (25%, 50%, 75%, and 100%) were delivered to the GMed and GMax at rest, and 100% stimuli were delivered during progressive hip abduction and extension contractions (25%, 50%, 75%, and 100% maximal voluntary isometric contraction).

MAIN OUTCOME MEASURES

GMed and GMax CAR, and hip abduction and hip extension maximal voluntary isometric contraction torque. Line of best fit and coefficient of determination (r2) were used to assess the relationship between torque output and CAR at varying levels of stimuli. Intraclass correlation coefficients, ICCs(3,k), were used to assess the between-session reliability.

RESULTS

GMed CAR was 96.1% (3.4%) and 96.6% (3.2%), on visits 1 and 2, respectively, whereas GMax CAR was 86.5% (7.5%) and 87.2% (10.7%) over the 2 sessions. A third-order polynomial demonstrated the best line of fit between varying superimposed burst intensities at rest for both GMed (r2 = .156) and GMax (r2 = .602). Linear relationships were observed in the CAR during progressive contractions with a maximal superimposed burst, GMed (r2 = .409) and GMax (r2 = .639). Between-session reliability was excellent for GMed CAR, ICC(3,k) = .911, and moderate for GMax CAR, ICC(3,k) = .704.

CONCLUSION

CAR appears to be an acceptable measure of GMed and GMax neuromuscular function in healthy individuals. Gluteal CAR measurements are reliable measures over a 1-week test period.

Age-related Deficits in Voluntary Activation: A Systematic Review and Meta-analysis

Whether there are age-related differences in neural drive during maximal effort contractions is not clear. This review determined the effect of age on voluntary activation during maximal voluntary isometric contractions. The literature was systematically reviewed for studies reporting voluntary activation quantified with the interpolated twitch technique (ITT) or central activation ratio (CAR) during isometric contractions in young (18-35 yr) and old adults (>60 yr; mean, ≥65 yr). Of the 2697 articles identified, 54 were eligible for inclusion in the meta-analysis. Voluntary activation was assessed with electrical stimulation and transcranial magnetic stimulation on five different muscle groups. Random-effects meta-analysis revealed lower activation in old compared with young adults (d = -0.45; 95% confidence interval, -0.62 to -0.29; P < 0.001), with moderate heterogeneity (52.4%). To uncover the sources of heterogeneity, subgroup analyses were conducted for muscle group, calculation method (ITT or CAR), and stimulation type (electrical stimulation or transcranial magnetic stimulation) and number (single, paired, or train stimulations). The age-related reduction in voluntary activation occurred for all muscle groups investigated except the ankle dorsiflexors. Both ITT and CAR demonstrated an age-related reduction in voluntary activation of the elbow flexors, knee extensors, and plantar flexors. ITT performed with paired and train stimulations showed lower activation for old than young adults, with no age difference for the single electrical stimulation. Together, the meta-analysis revealed that healthy older adults have a reduced capacity to activate some upper and lower limb muscles during maximal voluntary isometric contractions; however, the effect was modest and best assessed with at least paired stimulations to detect the difference.

Peripheral Nociception Is Associated with Voluntary Activation Deficits and Quadriceps Weakness Following Total Knee Arthroplasty

BACKGROUND

Quadriceps weakness is a hallmark of total knee arthroplasty and is driven by reduced voluntary muscle activation following the surgical procedure. The mechanisms underlying postoperative activation deficits are not well established, although nociception has been implicated via both spinal reflex and supraspinal pathways. The purpose of this study was to assess the role of nociception in postoperative recovery of strength and activation.

METHODS

A total of 53 participants were assessed prior to total knee arthroplasty and at 6 weeks postoperatively. Quadriceps strength was measured by maximum voluntary isometric contraction, and activation was measured by the doublet interpolation technique. The pressure-pain threshold was used to measure local sensitization (at the knee joint) and systemic sensitization (at the forearm). Changes in outcomes (strength and activation) were regressed against pressure-pain threshold measurements. Mediation analyses were planned for significant associations to investigate whether deficits in voluntary activation were implicated on a causal pathway between pressure-pain threshold measures and postoperative strength loss.

RESULTS

Knee pressure-pain threshold measures were significantly associated with reduced voluntary quadriceps activation (beta = -0.04; p = 0.009) and diminished quadriceps strength after total knee arthroplasty (beta = -0.07; p = 0.001). There was also a mediation effect of voluntary activation on the relationship between the knee pressure-pain threshold and quadriceps strength. After correcting for multiple comparisons, relationships between the forearm pressure-pain threshold and strength and activation did not reach significance.

CONCLUSIONS

The measures of local nociceptor sensitization were related to reduced strength and activation following total knee arthroplasty. This is consistent with a causal pathway linking increased firing of knee joint nociceptors to reduced activation and reduced strength. Future randomized studies should investigate whether peripherally directed pain therapies reduce pain while also promoting the recovery of quadriceps strength via an improved capacity for voluntary activation.

LEVEL OF EVIDENCE

Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Benefits of direct patient discharge to outpatient physical therapy after total knee arthroplasty

Purpose: To investigate the effectiveness of home health physical therapy followed by outpatient physical therapy as compared to patients discharged directly to outpatient physical therapy in improving functional performance, strength/activation and residual knee pain outcomes among patients who received a total knee arthroplasty.Materials and methods: A secondary analysis of longitudinal data in which patients with total knee arthroplasty underwent home health physical therapy or were discharged directly to outpatient physical therapy. Main outcome measures included the stair climb test, timed up and go, 6-min walk test, quadriceps and hamstring strength, quadriceps activation and residual knee pain.Results: Patients referred to home health physical therapy prior to outpatient physical therapy demonstrated significantly greater declines in stair climb test (10.3; 95% CI [6.5, 14.1]; t = 5.41; p < 0.0001), timed up and go (2.0; 95% CI [1.0, 3.0]; t = 4.10; p < 0.0001), 6-min walk (53.8; 95% CI [29.4, 78.2]; t = 4.35; p < 0.0001), quadriceps strength (21.7%; 95% CI [19.3%, 24.9%]; t = 2.53; p = 0.01), hamstring strength (44.7%; 95% CI [43.4%, 45.7%], t = 3.17; p = 0.002) and higher residual knee pain (0.53; 95% CI [0.04, 1.03]; t = 2.17; p = 0.03) 1 month after total knee arthroplasty compared to those referred directly to outpatient physical therapy.Conclusions: These findings suggest that patients discharged directly to outpatient physical therapy had a more rapid recovery 1 month after total knee arthroplasty. Additional research is needed to investigate the potential causal relation between care pathways and clinical outcomes following total knee arthroplasty.Implications for rehabilitationTotal knee arthroplasty, typically performed to alleviate end-stage knee osteoarthritis, is the most commonly performed elective surgery in the United States.Despite improvement in pain, objective measurements of functional performance and strength often remain at preoperative levels one year after total knee arthroplasty.Patients discharged directly to higher intensity outpatient physical therapy have a more rapid recovery after total knee arthroplasty compared with those patients who received two weeks of home health prior to undergoing outpatient physical therapy.

Morphological and Mechanical Properties of the Human Patella Tendon in Adult Males With Achondroplasia

Achondroplasia is a genetic mutation of fibroblast growth factor receptor resulting in impaired growth plate development in long bones due to lower collagen turnover. Despite the characteristic shorter stature and lower strength in Achondroplasic groups, little is known of the tendon mechanical properties under loading. The aim of this study was therefore to conduct a between measure design of patella tendon (PT) mechanical properties (stress, strain, stiffness and Young's Modulus) in 10 men with Achondroplasia (22 ± 3 years) and 17 male controls (22 ± 2 years). PT mechanical properties were measured during isometric maximal voluntary contraction (iMVC) of the knee extensors using ultrasonography. The Achondroplasic group produced 54% less stress at iMVC than controls (29.4 ± 8.0 v 64.5 ± 14.0 MPa, P < 0.001, d = 3.12). Maximal excursion of the Achondroplasic PT was 22% less than controls at iMVC (7.4 ± 2.1 v 5.5 ± 1.7 mm, P < 0.001, d = 0.99), but there was no difference in strain between groups (13 ± 4 v 13 ± 3%, P > 0.05). Achondroplasic PT were 47% less stiff (748 ± 93 v 1418 ± 101 N·mm⁻¹, P < 0.001, d = 6.89) and had a 51% lower Young's modulus (0.39 ± 0.09 v 0.77 ± 0.14 GPa, P < 0.001, d = 3.46) than controls at iMVC. Achondroplasic PT are indeed more compliant than controls which may contribute to lower relative force production. The causes of higher Achondroplasic PT compliance are unclear but are likely due to the collagen related genetic mutation which causes Achondroplasia.

The cross education of strength and skill following unilateral strength training in the upper and lower limbs

Cross education is the strength gain or skill improvement transferred to the contralateral limb following unilateral training or practice. The present study examined the transfer of both strength and skill following a strength training program. Forty participants (20M, 20F) completed a 6-wk unilateral training program of dominant wrist flexion or dorsiflexion. Strength, force variability, and muscle activity were assessed pretraining, posttraining, and following 6 wk of detraining (retention). Analyses of covariance compared the experimental limb (trained or untrained) to the control (dominant or nondominant). There were no sex differences in the training response. Cross education of strength at posttraining was 6% ( P < 0.01) in the untrained arm and 13% ( P < 0.01) in the untrained leg. Contralateral strength continued to increase following detraining to 15% in the arm ( P < 0.01) and 14% in the leg ( P < 0.01). There was no difference in strength gains between upper and lower limbs ( P > 0.05). Cross education of skill (force variability) demonstrated greater improvements in the untrained limbs compared with the control limbs during contractions performed without concurrent feedback. Significant increases in V-wave amplitude ( P = 0.02) and central activation ( P < 0.01) were highly correlated with contralateral strength gains. There was no change in agonist amplitude or motor unit firing rates in the untrained limbs ( P > 0.05). The neuromuscular mechanisms mirrored the force increases at posttraining and retention supporting central drive adaptations of cross education. The continued strength increases at retention identified the presence of motor learning in cross education, as confirmed by force variability.

NEW & NOTEWORTHY

We examined cross education of strength and skill following 6 wk of unilateral training and 6 wk of detraining. A novel finding was the continued increase in contralateral strength following both training and detraining. Neuromuscular adaptations were highly correlated with strength gains in the trained and contralateral limbs. Motor learning was evident in the trained and contralateral limbs during contractions performed without concurrent feedback.

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

PURPOSE

Cross-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.

METHODS

Group 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.

RESULTS

The 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.

CONCLUSION

Standard 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.

The cross education of strength and skill following unilateral strength training in the upper and lower limbs

Cross education is the strength gain or skill improvement transferred to the contralateral limb following unilateral training or practice. The present study examined the transfer of both strength and skill following a strength training program. Forty participants (20M, 20F) completed a 6-wk unilateral training program of dominant wrist flexion or dorsiflexion. Strength, force variability, and muscle activity were assessed pretraining, posttraining, and following 6 wk of detraining (retention). Analyses of covariance compared the experimental limb (trained or untrained) to the control (dominant or nondominant). There were no sex differences in the training response. Cross education of strength at posttraining was 6% ( P < 0.01) in the untrained arm and 13% ( P < 0.01) in the untrained leg. Contralateral strength continued to increase following detraining to 15% in the arm ( P < 0.01) and 14% in the leg ( P < 0.01). There was no difference in strength gains between upper and lower limbs ( P > 0.05). Cross education of skill (force variability) demonstrated greater improvements in the untrained limbs compared with the control limbs during contractions performed without concurrent feedback. Significant increases in V-wave amplitude ( P = 0.02) and central activation ( P < 0.01) were highly correlated with contralateral strength gains. There was no change in agonist amplitude or motor unit firing rates in the untrained limbs ( P > 0.05). The neuromuscular mechanisms mirrored the force increases at posttraining and retention supporting central drive adaptations of cross education. The continued strength increases at retention identified the presence of motor learning in cross education, as confirmed by force variability.

NEW & NOTEWORTHY

We examined cross education of strength and skill following 6 wk of unilateral training and 6 wk of detraining. A novel finding was the continued increase in contralateral strength following both training and detraining. Neuromuscular adaptations were highly correlated with strength gains in the trained and contralateral limbs. Motor learning was evident in the trained and contralateral limbs during contractions performed without concurrent feedback.

Carbohydrate Mouth Rinsing Does Not Prevent the Decline in Maximal Strength After Fatiguing Exercise

Black, CD, Schubert, DJ, Szczyglowski, MK, and Wren, JD. Carbohydrate mouth rinsing does not prevent the decline in maximal strength after fatiguing exercise. J Strength Cond Res 32(9): 2466-2473, 2018-Carbohydrate (CHO) rinsing has been shown to attenuate the decline of maximal voluntary contractions (MVCs) after fatiguing exercise-perhaps through a central mechanism. This study sought to determine the effect of a CHO rinse on MVC, voluntary activation, and contractile properties after fatiguing exercise. Thirteen adults participated in a double-blind, cross-over study. Maximal voluntary contraction of the dominant knee extensors was assessed, and voluntary activation (%VA) was determined using twitch interpolation. Participants then held 50% of MVC until volitional fatigue followed by a 20-second rinse with a solution of 8% maltodextrin (CHO) or placebo (PLA). Maximal voluntary contraction and %VA were reassessed immediately and 5 minutes after exercise. Maximal voluntary contraction did not differ between the CHO and PLA conditions initially (230 ± 90 vs. 232 ± 90 N·m; p = 0.69). Maximal voluntary contraction declined after exercise (p ≤ 0.01), but no differences were found between the CHO and PLA conditions (p ≥ 0.59). %VA did not differ between conditions (91.9 ± 2.9% vs. 91.5 ± 3.8%; p ≥ 0.11) nor did it change after exercise (p = 0.57). Twitch torque, rate of torque development, and rate of torque relaxation were reduced after exercise (p ≤ 0.05) but were unaffected by CHO rinsing (p > 0.05). Unlike a previous study, a CHO rinse did not preserve MVC after fatiguing exercise. This was likely due to a lack of central fatigue induced by the exercise protocol (as %VA was unaffected) as the CHO rinse is thought to work through a central mechanism.

Specific force of the vastus lateralis in adults with achondroplasia

Achondroplasia is a clinical condition defined by shorter stature and disproportionate limb length. Force production in able-bodied individuals (controls) is proportional to muscle size, but given the disproportionate nature of achondroplasia, normalizing to anatomical cross-sectional area (ACSA) is inappropriate. The aim of this study was to assess specific force of the vastus lateralis (VL) in 10 adults with achondroplasia (22 ± 3 yr) and 18 sex-matched controls (22 ± 2 yr). Isometric torque (iMVCτ) of the dominant knee extensors (KE) and in vivo measures of VL muscle architecture, volume, activation, and patella tendon moment arm were used to calculate VL physiological CSA (PCSA), fascicle force, and specific force in both groups. Achondroplasic muscle volume was 53% smaller than controls (284 ± 36 vs. 604 ± 102 cm³, P < 0.001). KE iMVCτ was 63% lower in achondroplasia compared with controls (95 ± 24 vs. 256 ± 47 N⋅m, P < 0.001). Activation and moment arm length were similar between groups ( P > 0.05), but coactivation of bicep femoris of achondroplasic subjects was 70% more than controls (43 ± 20 vs. 13 ± 5%, P < 0.001). Achondroplasic subjects had 58% less PCSA (43 ± 10 vs. 74.7 ± 14 cm², P < 0.001), 29% lower fascicle force (702 ± 235 vs. 1704 ± 303 N, P < 0.001), and 29% lower specific force than control subjects (17 ± 6 vs. 24 ± 6 N⋅cm^-2, P = 0.012). The smaller VL specific force in achondroplasia may be attributed to infiltration of fat and connective tissue, rather than to any difference in myofilament function. NEW & NOTEWORTHY The novel observation of this study was the measurement of normalized force production in a group of individuals with disproportionate limb length-to-torso ratios.

The Use of Hamstring Fatigue to Reduce Quadriceps Inhibition After Anterior Cruciate Ligament Reconstruction

Arthrogenic muscle inhibition, an inability to fully activate the quadriceps muscles, has been persistently observed after anterior cruciate ligament reconstruction (ACLr) surgery. Reductions in quadriceps activation may be partly due to the flexion reflex pathway, hamstrings activation, and reciprocal quadriceps inhibition. Since central fatigue has been shown to modify hamstring excitability and change the hamstring reflex response, hamstring fatigue might alleviate quadriceps muscle inhibition by counteracting the flexion reflex. In this study, nine young adult athletes (age: M = 19.9 years, SD = 1.7) with unilateral ACLr and nine control athletes (age: M = 24.0 years, SD = 2.4) with no previous history of knee injury performed tempo squats to induce fatigue. The ACLr group tended to use hamstrings for more hip flexion and trunk forward flexion than the control group. We assessed each participant's quadriceps inhibition through the central activation ratio (CAR), measured by twitch interpolation, before and after the induced fatigue. A mixed analysis of variance was used to examine the effect of fatigue on the CAR between pre- and post-fatigue and for both ACLr and control groups. The ACLr group showed significantly ( p = .010) greater CAR of the quadriceps post-fatigue ( M = 96.0%, SD = 7.6%) than pre-fatigue ( M = 81.2%, SD = 15.8%), while the control group showed no significant ( p = .969) pre-fatigue ( M = 96.9%, SD = 9.6%) and post-fatigue ( M = 97.0%, SD = 17.1%) differences. Thus, fatigue training may be used as a rehabilitation strategy to restore normal quadriceps function at the knee joint following ACL reconstruction by relaxing the hamstrings and overcoming quadriceps inhibition.

Magnitude of Deformity Correction May Influence Recovery of Quadriceps Strength After Total Knee Arthroplasty

BACKGROUND

Malalignment of the lower extremity is commonly seen in patients with severe osteoarthritis undergoing total knee arthroplasty (TKA) and is believed to play a role in quadriceps strength loss. Deformity correction is typically achieved through surgical techniques to provide appropriate ligamentous balancing. Therefore, this study examined the influence of change in lower extremity alignment on quadriceps strength outcomes after TKA.

METHODS

Seventy-three participants (36 male; mean age, 62 years; and mean body mass index, 29.7 kg/m2) undergoing primary unilateral TKA were used in this investigation. Before surgery and at 1 and 6 months after surgery, measures of isometric knee extensor strength, quadriceps activation, and long-standing plain films were collected. Using the films, measures of mechanical axis, distal femoral angle (DFA), proximal tibial angle, and patellofemoral angle were performed. Hierarchical linear regression was used to evaluate how change in alignment from baseline to 1 and 6 months influenced the change in quadriceps strength.

RESULTS

DFA was found to significantly contribute to changes in quadriceps strength at 1 and 6 months after TKA above those contributed by associated covariates. None of the other measures of lower extremity alignment were found to contribute to quadriceps strength in this sample.

CONCLUSION

Reductions in quadriceps strength experienced after TKA are likely to be influenced by changes in lower extremity alignment. Specifically, measures of DFA were found to significantly contribute to these changes. Future work is needed to prospectively examine measures of lower extremity alignment change and recovery after TKA.

Comparison of techniques to determine human skeletal muscle voluntary activation

Determining volitional activation (VA) can provide insights on the cause of muscle weakness in orthopedic and neurological populations. Two electrical stimulation techniques are traditionally used to quantify VA: interpolation (IT) and superimposition (CAR). IT allows for a more accurate VA estimation, however it requires individuals to be stimulated twice, compared to once for CAR, and thus increases stimulation associated discomfort. To date, there is no agreement on what is the best practical technique for calculating quadriceps VA. This paper aims to address this problem by determining what reference force (i.e., using either peak force or force at the time of stimulation) and type of stimulation (train of pulses (burst), doublet, and twitch) is the best technique to use. Our findings showed that the IT with the force at the time of stimulation as a reference should be used to determine VA and that when a burst was used, the VA ratio computations were more accurate. Additionally, using a twitch with a 2ms pulse duration produced reliable VA calculations and may be an acceptable alternative for pain-sensitive subjects. Accurate assessment of VA deficits can help clinicians design rehabilitation programs that are based on subject-specific strength impairments and are more effective.

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.

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

PURPOSE

The 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.

METHODS

Thirty-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.

RESULTS

After 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).

CONCLUSIONS

Except 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 EVIDENCE

III.

Men and Women Demonstrate Differences in Early Functional Recovery After Total Knee Arthroplasty

OBJECTIVE

To investigate whether sex affects the trajectory of functional recovery after total knee arthroplasty (TKA).

DESIGN

Retrospective analysis from a historical database containing data from 3 prospective clinical trials and a pilot study.

SETTING

Clinical laboratory setting.

PARTICIPANTS

Recruitment across studies was restricted to patients who underwent an elective unilateral TKA for the treatment of osteoarthritis and were between 50 and 85 years of age (N=301).

INTERVENTIONS

Across all 4 studies, patients received a TKA and physical therapy intervention. Measures of physical function and strength were assessed before TKA and 1, 3, and 6 months after TKA.

MAIN OUTCOME MEASURES

Using a repeated-measures maximum likelihood model, statistical inference was made to estimate the changes in outcomes from before surgery to 1, 3, and 6 months after TKA that were stratified by sex. Muscle strength was assessed during maximal isometric quadriceps and hamstrings contractions. Muscle activation was assessed in the quadriceps muscle. Physical function outcomes included timed Up and Go (TUG) test, stair climbing test, and 6-minute walk test (6MWT).

RESULTS

Women demonstrated less decline in quadriceps strength than did men at 1, 3, and 6 months after TKA (P<.04), whereas women demonstrated less decline in hamstrings strength 1 month after TKA (P<.0001). Women demonstrated a greater decline than did men on the TUG test (P=.001), stair climbing test (P=.004), and 6MWT (P=.001) 1 month after TKA. Sex differences in physical function did not persist at 3 and 6 months after TKA.

CONCLUSIONS

Sex affected early recovery of muscle and physical function in the first month after TKA. Women demonstrated better preservation of quadriceps strength but a greater decline on measures of physical function than did men.

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

BACKGROUND

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.

QUESTIONS/PURPOSES

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.

METHODS

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.

RESULTS

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).

CONCLUSIONS

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 OF EVIDENCE

Level I, therapeutic study.

ALONGAMENTO INTERMITENTE E CONTÍNUO AUMENTAM A AMPLITUDE DE MOVIMENTO E REDUZEM A FORÇA DOS FLEXORES DE PUNHO

Introdução: A manipulação do intervalo entre séries pode influenciar o desempenho de atividades subsequentes. Objetivo: Comparar o efeito do intervalo de alongamento estático de forma continua e intermitente no desempenho de flexores de punho. Métodos A amostra foi composta por 14 adultos jovens, hígidos, do gênero masculino (idade 31±9 anos; estatura 178±0,7 cm; massa 85±12 Kg). Foi avaliada a amplitude de movimento passivo (ADMP) em extensão de punho, a força isométrica máxima de flexão de punho associado a eletromiografia superficial antes e depois de dois protocolos de alongamento com diferentes intervalos entre séries. Para cada sujeito, um dos membros superiores foi alongado com o protocolo contínuo (CON) e o outro com o intermitente (INT), de forma aleatória. O protocolo CON consistiu na realização do alongamento durante 6 minutos ininterruptos, e o INT consistiu na realização de seis séries de 1 minuto por 20 segundos de intervalo entre as séries. A intensidade foi mantida a 70-90% da percepção subjetiva de desconforto. Resultados Os resultados de ADMP mostraram aumento significante entre as condições pré e pós-intervenção, em ambos os protocolos INT (81°±10 e 94°±10, P<0,001) e CON (87°±12 e 96°±11, P=0,004). Os resultados para o pico de força mostraram redução significante nas condições pós-intervenção para ambos os protocolos: INT (205±54 Kgf e 148±56 Kgf, P<0,001) e CON (211±39 Kgf e 144±36 Kgf, P<0,001). Os resultados para a taxa de produção de força mostraram aumento significante nas condições pré e pós-intervenção, para ambos os protocolos INT (0,52±0,29 Kgf/ms e 1,24±0,45 Kgf/ms, P<0,001) e CON (0,43±0,29 Kgf/ms e 1,11±0,34 Kgf/ms, P<0,001). Conclusão Ambos os protocolos aumentaram a amplitude passiva de movimento, reduziram a força pico e taxa de produção de força, sem modificações na ativação dos flexores de punho.

Dynamic muscle quality of the plantar flexors is impaired in claudicant patients with peripheral arterial disease and associated with poorer walking endurance

OBJECTIVE

Peripheral arterial disease and intermittent claudication (PAD-IC) negatively affects physical activity and function. There is evidence for plantarflexor muscle dysfunction and weakness; however, the extent to which this dysfunction can be attributed to reduced muscle size or quality, or both, is not yet known. This study investigated whether in vivo plantarflexor muscle quality during static and dynamic contractions is altered by PAD-IC and whether such changes are associated with impaired walking endurance according to initial and absolute claudication distances.

METHODS

The study recruited 22 participants, consisting of 10 healthy controls and 12 claudicant patients with occlusion of the superficial femoral artery (seven unilateral and five bilateral). Muscle quality of the combined gastrocnemius muscles during static contractions was calculated by normalizing the estimated maximal potential muscle force to the physiological cross-sectional area of the lateral and medial gastrocnemius. Muscle quality during dynamic contractions of the combined plantarflexor muscles was calculated as the ratio of peak voluntary concentric plantarflexor power and the summed volume of lateral and medial gastrocnemius.

RESULTS

Dynamic muscle quality was 24% lower in the claudicating-limb and asymptomatic-limb groups compared with controls (P = .017 and P = .023). The differences were most apparent at the highest contraction velocity (180°/s). Dynamic muscle quality was associated with reduced walking endurance (R = 0.689, P = .006 and R = 0.550, P = .042 for initial and absolute claudication distance, respectively). The claudicating-limb group demonstrated a trend toward reduced static muscle quality compared with controls (22%, P = .084). The relative contribution of the soleus muscle to plantarflexion maximum voluntary contraction was significantly higher in the claudicating-limb and asymptomatic-limb groups than in controls (P = .012 and P = .018).

CONCLUSIONS

The muscle strength of the plantarflexors in those with PAD-IC appears to be impaired at high contraction velocities. This may be explained by some reduction in gastrocnemii muscle quality and a greater reliance on the prominently type I-fibered soleus muscle. The reduced dynamic capability of the plantarflexor muscles was associated with disease severity and walking ability; therefore, efforts to improve plantarflexor power through dynamic exercise intervention are vital to maintain functional performance.

Corticospinal and intracortical excitability of the quadriceps in patients with knee osteoarthritis

Deficits in voluntary activation of the quadriceps muscle are characteristic of knee osteoarthritis (OA), contributing to the quadriceps weakness that is also a hallmark of the disease. The mechanisms underlying this central activation deficit (CAD) are unknown, although cortical mechanisms may be involved. Here, we utilize transcranial magnetic stimulation (TMS) to assess corticospinal and intracortical excitability in patients with knee OA and in a comparably aged group of healthy older adults, to quantify group differences, and to examine associations between TMS measures and pain, quadriceps strength, and CAD. Seventeen patients with knee OA and 20 healthy controls completed testing. Motor evoked potentials were measured at the quadriceps by superficial electromyographic recordings. Corticospinal excitability was assessed by measuring resting motor threshold (RMT) to TMS stimulation of the quadriceps representation at primary motor cortex, and intracortical excitability was assessed via paired-pulse paradigms for short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF). No statistically significant differences between patients with knee OA and healthy controls were found for RMT, SICI or ICF measures (p > 0.05). For patients with knee OA, there were significant associations observed between pain and RMT, as well as between pain and ICF. No associations were observed between CAD and measures of corticospinal or intracortical excitability. These data suggest against direct involvement of corticospinal or intracortical pathways within primary motor cortex in the mechanisms of CAD. However, pain is implicated in the neural mechanisms of quadriceps motor control in patients with knee OA.

The reliability of the interpolated twitch technique during submaximal and maximal isometric muscle actions

The reliability of the interpolated twitch technique during submaximal and maximal isometric muscle actions. J Strength Cond Res 27(10): 2909-2913, 2013-The purpose of this study was to examine the test-retest reliability of the percent voluntary activation (%VA) vs. force relationships. Fourteen healthy men (mean ± SD age = 21 ± 2.6 years) and 8 women (age = 21 ± 1.8 years) completed 4 maximal voluntary contractions (MVCs) and 9 randomly ordered submaximal isometric plantar flexions from 10 to 100% of the MVC. Transcutaneous electrical stimuli were delivered to the tibial nerve using a high-voltage constant-current stimulator (DS7AH; Digitimer, Herthfordshire, United Kingdom). The %VA was calculated for each maximal and submaximal MVC. Paired-samples t-tests were used to quantify systematic variability, whereas the intraclass correlation coefficients (ICCs), standard error of the mean (%SEM), and minimum differences (%MD; expressed as a percentage of the means) were used for test-retest reliability. Systematic variability was not present at any of the contraction intensities (p > 0.05). The ICCs ranged from 0.52 to 0.84, whereas the %SEM ranged from 6.75 to 38.45%, and the %MD ranged from 18.71 to 106.58%. The ICCs were ≥0.74 at contraction intensities ranging from 40 to 100% MVC (6.75-16.78% SEM), whereas the ICCs were ≤0.65 (20.95-38.45% SEM) for the contraction intensities ≤30% MVC. Although not statistically tested, the ICCs tended to be higher, whereas the %SEMs lower for contractions ≥40% MVC. Future research using %VA during submaximal contraction intensities to predict a true maximal force may want to exclude contraction intensities <40% MVC. In addition, caution is warranted when interpreting the changes in the %VA during MVCs after an experimental intervention.

The effects of a doublet stimulus and force level on the electromechanical delay

The purpose of this study was to examine the effects of a doublet stimulus evoked during isometric submaximal contractions on the electromechanical delay (EMD). Twenty-three healthy individuals performed isometric step contractions ranging from 10 to 70% maximal voluntary contraction (MVC) with a doublet stimulus applied during the contractions. A 2-way repeated-measures analysis of variance (gender [male vs. female] × contraction intensity [10% vs. 20% vs. 30% vs. 40% vs. 50% vs. 60% vs. 70% MVC]) was used to analyze the EMD. The EMD at 10% was significantly less than at 30% (p = 0.022), 40% (p = 0.044), 50% (p = 0.012), 60% (p = 0.041), and 70% MVC (p < 0.001). In addition, the EMD at 70% MVC was significantly higher than at 20% (p = 0.002), 30% (p = 0.006), 40% (p = 0.009), and 50% MVC (p = 0.021). The EMD is the time difference between muscle activation and the onset of muscle force production, with the stretching of the series elastic component (SEC) as the major contributor. A doublet stimulus applied during an isometric contraction minimizes the effects of the SEC during the interpolated twitch technique. Therefore, when the SEC is minimized, the EMD may be lengthened because of mechanisms associated with the excitation-contraction coupling at higher contraction intensities. The EMD measurement, produced with a doublet stimulus, could potentially be a useful tool to examine the effects of fatigue, exercise, stretching, and so on, on the excitation-contraction coupling during various force levels. However, future research is needed to further elucidate the mechanisms that result in a lengthening of the EMD under these conditions.

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.

Relationships between skinfold thickness and electromyographic and mechanomyographic amplitude recorded during voluntary and non-voluntary muscle actions

INTRODUCTION

The purpose of this study was to examine possible correlations between skinfold thicknesses and the a terms from the log-transformed electromyographic (EMGRMS) and mechanomyographic amplitude (MMGRMS)-force relationships, EMG M-Waves, and MMG gross lateral movements (GLM).

METHODS

Forty healthy subjects performed a 6-s isometric ramp contraction from 5% to 85% of their maximal voluntary contraction with EMG and MMG sensors placed on the vastus lateralis (VL) and rectus femoris (RF). A single electrical stimulus was applied to the femoral nerve to record the EMG M-waves and MMG GLMs. Skinfold thickness was assessed at the site of each electrode. Pearson's product correlation coefficients were calculated comparing skinfold thicknesses with the a terms from the log-transformed EMGRMS-and MMGRMS-force relationships, EMG M-waves, and MMG GLMs.

RESULTS

There were no significant cor1relations (p>0.05) between the a terms and skinfold thicknesses for the RF and VL from the EMGRMS and MMGRMS-force relationships. However, there were significant correlations (p<0.05) between skinfold thicknesses and the EMG M-waves and MMG GLMs for the RF (r=-0.521, -0.376) and VL (r=-0.479, -0.484).

DISCUSSION

Relationships were only present between skinfold thickness and the amplitudes of the EMG and MMG signals during the non-voluntary muscle actions.

Comparing the reliability of voluntary and evoked muscle actions

PURPOSE

The purpose of this exploratory study was to quantify the test-retest reliability, intertrial variability and correlations between variables calculated during voluntary and evoked muscle actions.

METHODS

During three separate trials of isometric leg extension muscle actions with 14 men [mean age (± SD) = 21.9 (± 3.2) years; height = 179.2 (± 8.0) cm; mass = 77.7 (± 10.9) kg], peak torque (PT(V)), time to peak torque (TPT(V)), rate of torque development (RTD(V)), time to peak rate of torque development (TRTD(V)), electromechanical delay (EMD(V)) and EMG amplitude (EMG(RMS)) were quantified for voluntary muscle actions. Peak twitch torque (PTE ), time to peak twitch torque (TPT(E)), rate of torque development (RTD(E)), time to peak rate of torque development (TRTD(E)), electromechanical delay (EMD(E)), peak-to-peak M-wave (Mp-p) and M-wave area (M(area)) were calculated for evoked twitches. All electromyographic measurements were recorded over the vastus lateralis.

RESULTS

Voluntary intraclass correlation coefficients (ICC) were 0.84-0.96, except for TRTD(V) (0.30) and EMD(V) (0.74). Evoked ICC were 0.80-0.95, except for EMD(E) (0.52). Relative standard errors of measurement for TPT and TRT(D) were greater for voluntary than evoked, whereas the value for PTE was greater than PT(V). There were low to moderate correlations (r = -0.41-0.51) between common voluntary and evoked variables.

CONCLUSIONS

Evoked variables showed lower intertrial variability than voluntary. Overall, voluntary and evoked muscle actions provide unique, complimentary information regarding neuromuscular function that cannot be used interchangeably.

Variability and distribution of muscle strength and its determinants in humans

INTRODUCTION

Inter-individual variability in measurements of muscle strength and its determinants was identified to: (1) produce a normative data set describing the normal range and (2) determine whether some measurements are more informative than others when evaluating inter-individual differences.

METHODS

Functional and morphological characteristics of the vastus lateralis were measured in 73 healthy, untrained adult men.

RESULTS

Inter-individual variability (coefficient of variation) was greater for isometric maximal voluntary contraction (MVC) torque (18.9%) compared with fascicle force (14.6%; P=0.025) and physiological cross-sectional area (PCSA; 17.2%) compared with anatomical cross-sectional area (ACSA, 13.0%; P<0.0005). The relationship between ACSA and isometric MVC torque (r(2)  =0.56) was weaker than that between PCSA and fascicle force (r(2)  =0.68).

CONCLUSIONS

These results provide a normative data set on inter-individual variability in a variety of muscle strength-related measurements and illustrate the benefit of using more stringent measures of muscle properties. Muscle Nerve 49: 879-886, 2014.

Relationships between the mechanomyographic amplitude patterns of response and concentric isokinetic fatiguing tasks of the leg extensors

The purpose of the present study was to examine possible correlations between the b terms (slopes) form the log-transformed mechanomyographic amplitude (MMGRMS)-force relationships and the fatigue index calculated from 50 maximal concentric contractions. Forty healthy subjects (age = 21 ± 2 yr) performed isometric ramp contractions from 5% to 85% of their maximal voluntary contraction followed by a 50-repetition concentric fatigue protocol of the leg extensors, fatigue index (%) was calculated from the 50-repetitions. MMG was recorded during the ramp contractions from the vastus lateralis (VL) and rectus femoris (RF). The b terms (slopes) were calculated from the log-transformed MMGRMS-force relationships. Correlations were performed comparing the b terms from the MMGRMS-force relationships for the VL and RF with the fatigue index. Significant positive correlations were found among the b terms from the MMGRMS-force relationships for the VL (p = 0.007, r = 0.417) and RF (p = 0.014, r = 0.386) with the fatigue index. The b terms from the log-transformed MMGRMS-force relationships for the VL and RF may have reflected muscle fiber type composition and, thus, correlated with the fatigue index. This adds further support that the MMGRMS-force relationships may reflect muscle fiber type composition.

Muscle volume as a predictor of maximum force generating ability in the plantar flexors post-stroke

INTRODUCTION

Post-stroke muscle weakness is commonly thought to be the result of a combination of decreased voluntary activation and decreased maximum force generating ability (MFGA). We assessed the ability of muscle volumes obtained using MRI to estimate the MFGA of the plantar flexor muscle group in individuals post-stroke.

METHODS

MRI was used to measure muscle volume of the plantar flexor muscle group in 17 individuals with post-stroke hemiparesis. A modified burst superimposition test was used to measure force of volitional contraction and predict the MFGA of the plantar flexors.

RESULTS

While muscle volume obtained by means of MRI provided information on the overall size of muscle, it overestimated the force generating ability of the paretic plantar flexors.

CONCLUSIONS

Results suggest that MRI-derived muscle volume underestimates the functional impairment in individuals post-stroke. Interestingly, the central activation ratio had a strong relationship with the maximum volitional force of contraction.

Minimally invasive total knee arthroplasty improves early knee strength but not functional performance: a randomized controlled trial

A prospective, randomized investigation compared early clinical outcomes of total knee arthroplasty (TKA) using conventional or minimally invasive surgical (MIS) approaches (n = 44). Outcome measures included isometric quadriceps and hamstrings strength, quadriceps activation, functional performance, knee pain, active knee range of motion, muscle mass, the Short Form-36, and Western Ontario and McMaster University Osteoarthritis Index, assessed preoperatively and 4 and 12 weeks after TKA. Four weeks after TKA, the MIS group had greater hamstring strength (P = .02) and quadriceps strength (P = .07), which did not translate to differences in other outcomes. At 12 weeks, there were no clinically meaningful differences between groups on any measure. Although MIS may lead to faster recovery of strength in patients undergoing TKA, there was no benefit on longer-term recovery of strength or functional performance.

Relationship between intensity of quadriceps muscle neuromuscular electrical stimulation and strength recovery after total knee arthroplasty

BACKGROUND

Neuromuscular electrical stimulation (NMES) can facilitate the recovery of quadriceps muscle strength after total knee arthroplasty (TKA), yet the optimal intensity (dosage) of NMES and its effect on strength after TKA have yet to be determined.

OBJECTIVE

The primary objective of this study was to determine whether the intensity of NMES application was related to the recovery of quadriceps muscle strength early after TKA. A secondary objective was to quantify quadriceps muscle fatigue and activation immediately after NMES to guide decisions about the timing of NMES during rehabilitation sessions.

DESIGN

This study was an observational experimental investigation.

METHODS

Data were collected from 30 people who were 50 to 85 years of age and who received NMES after TKA. These people participated in a randomized controlled trial in which they received either standard rehabilitation or standard rehabilitation plus NMES to the quadriceps muscle to mitigate strength loss. For the NMES intervention group, NMES was applied 2 times per day at the maximal tolerable intensity for 15 contractions beginning 48 hours after surgery over the first 6 weeks after TKA. Neuromuscular electrical stimulation training intensity and quadriceps muscle strength and activation were assessed before surgery and 3.5 and 6.5 weeks after TKA.

RESULTS

At 3.5 weeks, there was a significant association between NMES training intensity and a change in quadriceps muscle strength (R(2)=.68) and activation (R(2)=.22). At 6.5 weeks, NMES training intensity was related to a change in strength (R(2)=.25) but not to a change in activation (R(2)=.00). Furthermore, quadriceps muscle fatigue occurred during NMES sessions at 3.5 and 6.5 weeks, whereas quadriceps muscle activation did not change.

LIMITATIONS

Some participants reached the maximal stimulator output during at least 1 treatment session and might have tolerated more stimulation.

CONCLUSIONS

Higher NMES training intensities were associated with greater quadriceps muscle strength and activation after TKA.

Age-related changes in motor cortical properties and voluntary activation of skeletal muscle

Aging is associated with dramatic reductions in muscle strength and motor control, and many of these agerelated changes in muscle function result from adaptations in the central nervous system. Aging is associated with widespread qualitative and quantitative changes of the motor cortex. For example, advancing age has been suggested to result in cortical atrophy, reduced cortical excitability, reduced cortical plasticity, as well as neurochemical abnormalities.The associated functional effects of these changes likely influence numerous aspects of muscle performance such as muscle strength and motor control. For example, there is evidence to suggest that the muscle weakness associated with aging is partially due to impairments in the nervous system's ability to fully activate motor neurons- particularly in the larger proximal muscle groups. In this review article we discuss age-related changes in the motor cortex, as well as the abilityor lack thereof- of older adults to voluntarily activate skeletal muscle. We also provide perspectives on scientific and clinical questions that need to be addressed in the near future.

Evolving concepts on the age-related changes in "muscle quality"

The deterioration of skeletal muscle with advancing age has long been anecdotally recognized and has been of scientific interest for more than 150 years. Over the past several decades, the scientific and medical communities have recognized that skeletal muscle dysfunction (e.g., muscle weakness, poor muscle coordination, etc.) is a debilitating and life-threatening condition in the elderly. For example, the age-associated loss of muscle strength is highly associated with both mortality and physical disability. It is well-accepted that voluntary muscle force production is not solely dependent upon muscle size, but rather results from a combination of neurologic and skeletal muscle factors, and that biologic properties of both of these systems are altered with aging. Accordingly, numerous scientists and clinicians have used the term "muscle quality" to describe the relationship between voluntary muscle strength and muscle size. In this review article, we discuss the age-associated changes in the neuromuscular system-starting at the level of the brain and proceeding down to the subcellular level of individual muscle fibers-that are potentially influential in the etiology of dynapenia (age-related loss of muscle strength and power).

Quadriceps neuromuscular function and self-reported functional ability in knee osteoarthritis

The purposes of this study were to determine 1) the relationships of self-reported function scores in patients with knee osteoarthritis (OA) to both maximal isometric torque and to isotonic power at a variety of loads, and 2) the degree to which muscle volume (MV) or voluntary activation (VA) are associated with torque and power measures in this population. Isometric maximal voluntary contraction (MVC) torque and isotonic power [performed at loads corresponding to 10, 20, 30, 40, and 50% MVC, and a minimal load ("Zero Load")] were measured in 40 participants with knee OA. Functional ability was measured with the Western Ontario and McMaster Osteoarthritis Index (WOMAC) function subscale. MV was determined with magnetic resonance imaging, and VA was measured with the interpolated twitch technique. In general, power measured at lower loads (Zero Load and 10-30% MVC, r(2) = 0.21-0.28, P < 0.05) predicted a greater proportion of the variance in function than MVC torque (r(2) = 0.18, P < 0.05), with power measured at Zero Load showing the strongest association (r(2) = 0. 28, P < 0.05). MV was the strongest predictor of MVC torque and power measures in multiple regression models (r(2) = 0.42-0.72). VA explained only 6% of the variance in MVC torque and was not significantly associated with power at any load (P > 0.05). Quadriceps MVC torque and power are associated with self-reported function in knee OA, but muscle power at lower loads is more predictive of function than MVC torque. The variance in MVC torque and power between participants is due predominantly to differences in MV and has little to do with deficits in VA.

Using submaximal contractions to predict the maximum force-generating ability of muscles

INTRODUCTION

Muscle weakness can be caused by decreases in either the maximum force-generating ability of a muscle (MFGA) or neural drive from the nervous system (e.g., after a stroke). Presently, there is no agreed-upon practical method for calculating the MFGA in individuals with central nervous system pathology. The purpose of this study was to identify the best method for determining MFGA.

METHODS

The predicted and estimated MFGA of the muscles of 23 non-neurologically impaired subjects (13 males, 21.9 ± 1.9 years) were compared using the burst superimposition, twitch interpolation, doublet interpolation, twitch-to-tetanus ratio, and the adjusted burst superimposition methods.

RESULTS

The adjusted burst superimposition test was the most accurate predictor of MFGA.

CONCLUSIONS

Further testing is needed to validate the use of the adjusted burst superimposition test in a neurologically impaired population.

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.

Early neuromuscular electrical stimulation to improve quadriceps muscle strength after total knee arthroplasty: a randomized controlled trial

BACKGROUND

The recovery of quadriceps muscle force and function after total knee arthroplasty (TKA) is suboptimal, which predisposes patients to disability with increasing age.

OBJECTIVE

The purpose of this investigation was to evaluate the efficacy of quadriceps muscle neuromuscular electrical stimulation (NMES), initiated 48 hours after TKA, as an adjunct to standard rehabilitation.

DESIGN

This was a prospective, longitudinal randomized controlled trial.

METHODS

Sixty-six patients, aged 50 to 85 years and planning a primary unilateral TKA, were randomly assigned to receive either standard rehabilitation (control) or standard rehabilitation plus NMES applied to the quadriceps muscle (initiated 48 hours after surgery). The NMES was applied twice per day at the maximum tolerable intensity for 15 contractions. Data for muscle strength, functional performance, and self-report measures were obtained before surgery and 3.5, 6.5, 13, 26, and 52 weeks after TKA.

RESULTS

At 3.5 weeks after TKA, significant improvements with NMES were found for quadriceps and hamstring muscle strength, functional performance, and knee extension active range of motion. At 52 weeks, the differences between groups were attenuated, but improvements with NMES were still significant for quadriceps and hamstring muscle strength, functional performance, and some self-report measures.

LIMITATIONS

Treatment volume was not matched for both study arms; NMES was added to the standard of care treatment. Furthermore, testers were not blinded during testing, but used standardized scripts to avoid bias. Finally, some patients reached the maximum stimulator output during at least one treatment session and may have tolerated more stimulation.

CONCLUSIONS

The early addition of NMES effectively attenuated loss of quadriceps muscle strength and improved functional performance following TKA. The effects were most pronounced and clinically meaningful within the first month after surgery, but persisted through 1 year after surgery.