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Daniel Gilfeather, Grant Norte, Christopher D. Ingersoll and Neal R. Glaviano

assessments alone. 11 , 12 One outcome that has been used to evaluate muscle function is the central activation ratio (CAR), assessed by the SIBT. CAR has previously been utilized in research as a way to measure the volitional activation of the quadriceps in a variety of pathologies. 13 – 15 The CAR is a

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Christopher Kuenze, Jay Hertel, Susan Saliba, David R. Diduch, Arthur Weltman and Joseph M. Hart

Context:

Normal, symmetrical quadriceps strength is a common clinical goal after anterior cruciate ligament reconstruction (ACLR). Currently, the clinical thresholds for acceptable unilateral quadriceps function and symmetry associated with positive outcomes after return to activity are unclear.

Objective:

To establish quadriceps-activation and knee-extension-torque cutoffs for clinical assessment after return to activity after ACLR.

Design:

Descriptive laboratory study.

Setting:

Laboratory.

Patients:

22 (10 female, 12 male; age = 22.5 ± 5.0 y, height = 172.9 ± 7.1 cm, mass = 74.1 ± 15.5 kg, months since surgery = 31.5 ± 23.5) recreationally active persons with a history of unilateral, primary ACLR at least 6 months prior and 24 (12 female/12 male, age = 21.7 ± 3.6 y, height = 168.0 ± 8.8 cm, mass = 69.3 ± 13.6 kg) recreationally active healthy participants.

Main Outcome Measures:

Patient-reported measures of pain, knee-related function, and physical activity level were recorded for all participants. Normalized knee-extension maximum-voluntary-isometric-contraction (MVIC) torque (Nm/kg) and quadriceps central-activation ratio (CAR, %) were measured bilaterally in all participants. Receiver-operator-characteristic (ROC) curves were used to establish thresholds for unilateral measures of normalized knee-extension MVIC torque and quadriceps CAR, as well as limb-symmetry indices (LSI). ROC curves then established clinical thresholds for normalized knee-extension MVIC torque and quadriceps CAR LSIs associated with healthy knee-related function.

Results:

Involved-quadriceps CAR above 89.3% was the strongest unilateral indicator of healthy-group membership, while quadriceps CAR LSI above 0.996 and knee-extension MVIC torque above 0.940 were the strongest overall indicators. Unilateral normalized knee-extension MVIC torque above 3.00 Nm/kg and quadriceps CAR LSI above 0.992 were the best indicators of good patient-reported knee-related outcomes.

Conclusions:

Threshold values established in this study may provide a guide for clinicians when making return-to-activity decisions after ACLR. Normalized knee-extension MVIC torque (>3.00 Nm/kg) and quadriceps CAR symmetry (>99.6%) are both strong indicators of good patient-reported outcomes after ACLR.

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Grant E. Norte, Jay N. Hertel, Susan A. Saliba, David R. Diduch and Joseph M. Hart

contraction (MVIC) torque, fatigue index (FI), central activation ratio (CAR), Hoffmann reflex (H-reflex), and active motor threshold (AMT). The International Knee Documentation Committee (IKDC) subjective knee evaluation form, 13 Knee Injury and Osteoarthritis Outcome Score, 14 and Western Ontario and

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Grant E. Norte, Katherine R. Knaus, Chris Kuenze, Geoffrey G. Handsfield, Craig H. Meyer, Silvia S. Blemker and Joseph M. Hart

postsurgery, and (3) percent change presurgery to postsurgery. Secondary outcome measures of quadriceps function included knee extension maximal voluntary isometric contraction torque (T MVIC ), quadriceps central activation ratio (CAR), and superimposed burst torque (T SIB ). Participants In total, 4

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Brandon Warner, Kyung-Min Kim, Joseph M. Hart and Susan Saliba

Context:

Quadriceps function improves after application of focal joint cooling or transcutaneous electrical nerve stimulation to the knee in patients with arthrogenic muscle inhibition (AMI), yet it is not known whether superficial heat is able to produce a similar effect.

Objective:

To determine quadriceps function after superficial heat to the knee joint in individuals with AMI.

Design:

Single blinded randomized crossover.

Setting:

Laboratory.

Patients:

12 subjects (4 female, 8 males; 25.6 ± 7.7 y, 177.2 ± 12.7 cm, 78.4 ± 18.2 kg) with a history of knee-joint pathology and AMI, determined with a quadriceps central activation ratio (CAR) of <90%.

Intervention:

3 treatment conditions for 15 min on separate days: superficial heat using a cervical moist-heat pack (77°C), sham using a cervical moist pack (room temperature at about 24°C), and control (no treatment). All subjects received all treatment conditions in a randomized order.

Main Outcome Measures:

Central activation ratio and knee-extension torque during maximal voluntary isometric contraction with the knee flexed to 60° were collected at pre, immediately post, 30 min post, and 45 min posttreatment. Skin temperature of the quadriceps and knee and room temperature were also recorded at the same time points.

Results:

Three (treatment conditions) by 4 (time) repeated ANOVAs found that there were no significant interactions or main effects in either CAR or knee-extension torque (all P > .05). Skin-temperature 1-way ANOVAs revealed that the skin temperature in the knee during superficial heat was significantly higher than other treatment conditions at all time points (P < .05).

Conclusions:

Superficial heat to the knee joint using a cervical moist-heat pack did not influence quadriceps function in individuals with AMI in the quadriceps.

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Albertas Skurvydas and Marius Brazaitis

The aim of the study was to evaluate the effect of plyometric training (PT) on central and peripheral (muscle) fatigue in prepubertal girls and boys. The boys (n = 13, age 10.3 ± 0.3 years) and girls (n = 13, age, 10.2 ± 0.3 years) performed continuous 2-min maximal voluntary contractions (MVCs) before and after 16 high-intensity PT sessions. PT comprised two training sessions per week of 30 jumps in each session with 20 s between jumps. The greatest effect of PT was on excitation–contraction coupling, (twitch force increased by 323% in boys and 21% in girls) and height of a counter–movement jump (increased by 37% in boys and 38% in girls). In contrast, the quadriceps voluntary activation index, central activation ratio, and MVC did not change significantly after PT. The thickness of the quadriceps muscle increased by 9% in boys and 14% in girls after PT. In conclusion, boys and girls demonstrated similar changes in indicators of central fatigue (50–60% decrease) and peripheral fatigue (45–55% decrease) after MVC before and after PT.

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Glyn Howatson, Raphael Brandon and Angus M. Hunter

There is a great deal of research on the responses to resistance training; however, information on the responses to strength and power training conducted by elite strength and power athletes is sparse.

Purpose:

To establish the acute and 24-h neuromuscular and kinematic responses to Olympic-style barbell strength and power exercise in elite athletes.

Methods:

Ten elite track and field athletes completed a series of 3 back-squat exercises each consisting of 4 × 5 repetitions. These were done as either strength or power sessions on separate days. Surface electromyography (sEMG), bar velocity, and knee angle were monitored throughout these exercises and maximal voluntary contraction (MVC), jump height, central activation ratio (CAR), and lactate were measured pre, post, and 24 h thereafter.

Results:

Repetition duration, impulse, and total work were greater (P < .01) during strength sessions, with mean power being greater (P < .01) after the power sessions. Lactate increased (P < .01) after strength but not power sessions. sEMG increased (P < .01) across sets for both sessions, with the strength session increasing at a faster rate (P < .01) and with greater activation (P < .01) by the end of the final set. MVC declined (P < .01) after the strength and not the power session, which remained suppressed (P < .05) 24 h later, whereas CAR and jump height remained unchanged.

Conclusion:

A greater neuromuscular and metabolic demand after the strength and not power session is evident in elite athletes, which impaired maximal-force production for up to 24 h. This is an important consideration for planning concurrent athlete training.

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Christopher Kuenze, Jay Hertel and Joseph M. Hart

Purpose:

Persistent quadriceps weakness due to arthrogenic muscle inhibition (AMI) has been reported after anterior cruciate ligament (ACL) reconstruction. Fatiguing exercise has been shown to alter lower extremity muscle function and gait mechanics, which may be related to injury risk. The effects of exercise on lower extremity function in the presence of AMI are not currently understood. The purpose of this study was to compare the effect of 30 min of exercise on quadriceps muscle function and soleus motoneuron-pool excitability in ACL-reconstructed participants and healthy controls.

Methods:

Twenty-six (13 women, 13 men) healthy and 26 (13 women, 13 men) ACL-reconstructed recreationally active volunteers were recruited for a case-control laboratory study. All participants completed 30 min of continuous exercise including alternating cycles of inclined-treadmill walking and bouts of squats and step-ups. Knee-extension torque, quadriceps central activation ratio (CAR), soleus H:M ratio, and soleus V:M ratio were measured before and after 30 min of exercise.

Results:

There was a significant group × time interaction for knee-extension torque (P = .002), quadriceps CAR (P = .03), and soleus V:M ratio (P = .03). The effect of exercise was smaller for the ACL-R group than for matched controls for knee-extension torque (ACL-R: %Δ = −4.2 [−8.7, 0.3]; healthy: %Δ = −14.2 [−18.2, −10.2]), quadriceps CAR (ACL-R: %Δ = −5.1 [−8.0, −2.1]; healthy: %Δ = −10.0 [−13.3, −6.7]), and soleus V:M ratio (ACL-R: %Δ = 37.6 [2.1, 73.0]; healthy: %Δ = −24.9 [−38.6, −11.3]).

Conclusion:

Declines in quadriceps and soleus volitional muscle function were of lower magnitude in ACL-R subjects than in healthy matched controls. This response suggests an adaptation experienced by patients with quadriceps AMI that may act to maintain lower extremity function during prolonged exercise.

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Conrad M. Gabler, Adam S. Lepley, Tim L. Uhl and Carl G. Mattacola

Clinical Scenario:

Proper neuromuscular activation of the quadriceps muscle is essential for maintaining quadriceps (quad) strength and lower-extremity function. Quad activation (QA) failure is a common characteristic observed in patients with knee pathologies, defined as an inability to voluntarily activate the entire alpha-motor-neuron pool innervating the quad. One of the more popular techniques used to assess QA is the superimposed burst (SIB) technique, a force-based technique that uses a supramaximal, percutaneous electrical stimulation to activate all of the motor units in the quad during a maximal, voluntary isometric contraction. Central activation ratio (CAR) is the formula used to calculate QA level (CAR = voluntary force/SIB force) with the SIB technique. People who can voluntarily activate 95% or more (CAR = 0.95–1.0) of their motor units are defined as being fully activated. Therapeutic exercises aimed at improving quad strength in patients with knee pathologies are limited in their effectiveness due to a failure to fully activate the muscle. Within the past decade, several disinhibitory interventions have been introduced to treat QA failure in patients with knee pathologies. Transcutaneous electrical nerve stimulation (TENS) and cryotherapy are sensory-targeted modalities traditionally used to treat pain, but they have been shown to be 2 of the most successful treatments for increasing QA levels in patients with QA failure. Both modalities are hypothesized to positively affect voluntary QA by disinhibiting the motor-neuron pool of the quad. In essence, these modalities provide excitatory afferent stimuli to the spinal cord, which thereby overrides the inhibitory afferent signaling that arises from the involved joint. However, it remains unknown whether 1 is more effective than the other for restoring QA levels in patients with knee pathologies. By knowing the capabilities of each disinhibitory modality, clinicians can tailor treatments based on the rehabilitation goals of their patients.

Focused Clinical Question:

Is TENS or cryotherapy the more effective disinhibitory modality for treating QA failure (quantified via CAR) in patients with knee pathologies?

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Jihong Park, Terry L. Grindstaff, Joe M. Hart, Jay N. Hertel and Christopher D. Ingersoll

Context:

Weight-bearing (WB) and non-weight-bearing (NWB) exercises are commonly used in rehabilitation programs for patients with anterior knee pain (AKP).

Objective:

To determine the immediate effects of isolated WB or NWB knee-extension exercises on quadriceps torque output and activation in individuals with AKP.

Design:

A single-blind randomized controlled trial.

Setting:

Laboratory.

Participants:

30 subjects with self-reported AKP.

Interventions:

Subjects performed a maximal voluntary isometric contraction (MVIC) of the quadriceps (knee at 90°). Maximal voluntary quadriceps activation was quantified using the central activation ratio (CAR): CAR = MVIC/(MVIC + superimposed burst torque). After baseline testing, subjects were randomized to 1 of 3 intervention groups: WB knee extension, NWB knee extension, or control. WB knee-extension exercise was performed as a sling-based exercise, and NWB knee-extension exercise was performed on the Biodex dynamometer. Exercises were performed in 3 sets of 5 repetitions at approximately 55% MVIC. Measurements were obtained at 4 times: baseline and immediately and 15 and 30 min postexercise.

Main Outcome Measures:

Quadriceps torque output (MVIC: N·m/Kg) and quadriceps activation (CAR).

Results:

No significant differences in the maximal voluntary quadriceps torque output (F 2,27 = 0.592, P = .56) or activation (F 2,27 = 0.069, P = .93) were observed among the 3 treatment groups.

Conclusions:

WB and NWB knee-extension exercises did not acutely change quadriceps torque output or activation. It may be necessary to perform exercises over a number of sessions and incorporate other disinhibitory interventions (eg, cryotherapy) to observe acute changes in quadriceps torque and activation.