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Adriana M. Holmes and David M. Andrews

The purpose of this research was to examine the effects of voluntarily manipulating muscle activation and localized muscle fatigue on tibial response parameters, including peak tibial acceleration, time to peak tibial acceleration, and the acceleration slope, measured at the knee during unshod heel impacts. A human pendulum delivered consistent impacts to 15 female and 15 male subjects. The tibialis anterior and lateral gastrocnemius were examined using electromyography, thus allowing voluntary contraction to various activation states (baseline, 15%, 30%, 45%, and 60% of the maximum activation state) and assessing localized muscle fatigue. A skin-mounted uniaxial accelerometer, preloaded medial to the tibial tuberosity, allowed tibial response parameter determination. There were significant decreases in peak acceleration during tibialis anterior fatigue, compared to baseline and all other activation states. In females, increased time to peak acceleration and decreased acceleration slope occurred during fatigue compared to 30% and 45%, and compared to 15% through 60% of the maximum activation state, respectively. Slight peak acceleration and acceleration slope increases, and decreased time to peak acceleration as activation state increased during tibialis anterior testing, were noted. When examining the lateral gastrocnemius, the time to peak acceleration was significantly higher across gender in the middle activation states than at the baseline and fatigue states. The acceleration slope decreased at all activation states above baseline in females, and decreased at 60% of the maximum activation state in males compared to the baseline and fatigue states. Findings agree with localized muscle fatigue literature, suggesting that with fatigue there is decreased impact transmission, which may protect the leg. The relative effects of leg stiffness and ankle angle on tibial response need to be verified.

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Mohammad H. Izadi Farhadi, Foad Seidi, Hooman Minoonejad and Abbey C. Thomas

-extremity malalignment in the frontal and horizontal planes can alter hip and knee muscle activation during functional activities. In this regard, numerous researchers 5 , 8 state that hip external rotator and abductor muscles can affect the control of malalignment in the frontal and horizontal planes. Vastus medialis

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Bente R. Jensen, Line Hovgaard-Hansen and Katrine L. Cappelen

Running on a lower-body positive-pressure (LBPP) treadmill allows effects of weight support on leg muscle activation to be assessed systematically, and has the potential to facilitate rehabilitation and prevent overloading. The aim was to study the effect of running with weight support on leg muscle activation and to estimate relative knee and ankle joint forces. Runners performed 6-min running sessions at 2.22 m/s and 3.33 m/s, at 100%, 80%, 60%, 40%, and 20% body weight (BW). Surface electromyography, ground reaction force, and running characteristics were measured. Relative knee and ankle joint forces were estimated. Leg muscles responded differently to unweighting during running, reflecting different relative contribution to propulsion and antigravity forces. At 20% BW, knee extensor EMGpeak decreased to 22% at 2.22 m/s and 28% at 3.33 m/s of 100% BW values. Plantar flexors decreased to 52% and 58% at 20% BW, while activity of biceps femoris muscle remained unchanged. Unweighting with LBPP reduced estimated joint force significantly although less than proportional to the degree of weight support (ankle).It was concluded that leg muscle activation adapted to the new biomechanical environment, and the effect of unweighting on estimated knee force was more pronounced than on ankle force.

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Jihong Park, W. Matt Denning, Jordan D. Pitt, Devin Francom, J. Ty Hopkins and Matthew K. Seeley

Context:

Although knee pain is common, some facets of this pain are unclear. The independent effects (ie, independent from other knee injury or pathology) of knee pain on neural activation of lower-extremity muscles during landing and jumping have not been observed.

Objective:

To investigate the independent effects of knee pain on lower-extremity muscle (gastrocnemius, vastus medialis, medial hamstrings, gluteus medius, and gluteus maximus) activation amplitude during landing and jumping, performed at 2 different intensities.

Design:

Laboratory-based, pretest, posttest, repeated-measures design, where all subjects performed both data-collection sessions.

Methods:

Thirteen able-bodied subjects performed 2 different land and jump tasks (forward and lateral) under 2 different conditions (control and pain), at 2 different intensities (high and low). For the pain condition, experimental knee pain was induced via a hypertonic saline injection into the right infrapatellar fat pad. Functional linear models were used to evaluate the influence of experimental knee pain on muscle-activation amplitude throughout the 2 land and jump tasks.

Results:

Experimental knee pain independently altered activation for all of the observed muscles during various parts of the 2 different land and jump tasks. These activation alterations were not consistently influenced by task intensity.

Conclusion:

Experimental knee pain alters activation amplitude of various lower-extremity muscles during landing and jumping. The nature of the alteration varies between muscles, intensities, and phases of the movement (ie, landing and jumping). Generally, experimental knee pain inhibits the gastrocnemius, medial hamstring, and gluteus medius during landing while independently increasing activation of the same muscles during jumping.

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James W. Youdas, Kady E. Adams, John E. Bertucci, Koel J. Brooks, Meghan M. Nelson and John H. Hollman

Context:

No published studies have compared muscle activation levels simultaneously for the gluteus maximus and medius muscles of stance and moving limbs during standing hip-joint strengthening while using elastic-tubing resistance.

Objective:

To quantify activation levels bilaterally of the gluteus maximus and medius during resisted lower-extremity standing exercises using elastic tubing for the cross-over, reverse cross-over, front-pull, and back-pull exercise conditions.

Design:

Repeated measures.

Setting:

Laboratory.

Participants:

26 active and healthy people, 13 men (25 ± 3 y) and 13 women (24 ± 1 y).

Intervention:

Subjects completed 3 consecutive repetitions of lower-extremity exercises in random order.

Main Outcome Measures:

Surface electromyographic (EMG) signals were normalized to peak activity in the maximum voluntary isometric contraction (MVIC) trial and expressed as a percentage. Magnitudes of EMG recruitment were analyzed with a 2 × 4 repeated-measures ANOVA for each muscle (α = .05).

Results:

For the gluteus maximus an interaction between exercise and limb factor was significant (F 3,75 = 21.5; P < .001). The moving-limb gluteus maximus was activated more than the stance limb's during the back-pull exercise (P < .001), and moving-limb gluteus maximus muscle recruitment was greater for the back-pull exercise than for the cross-over, reverse cross-over, and front-pull exercises (P < .001). For the gluteus medius an interaction between exercise and limb factor was significant (F 3,75 = 3.7; P < .03). Gluteus medius muscle recruitment (% MVIC) was greater in the stance limb than moving limb when performing the front-pull exercise (P < .001). Moving-limb gluteus medius muscle recruitment was greater for the reverse cross-over exercise than for the cross-over, front-pull, and back-pull exercises (P < .001).

Conclusions:

From a clinical standpoint there is no therapeutic benefit to selectively activate the gluteus maximus and gluteus medius muscles on the stance limb by resisting sagittal- and frontal-plane hip movements on the moving limb using resistance supplied by elastic tubing.

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Jeremy R. Dicus and Jeff G. Seegmiller

Few ankle inversion studies have taken anticipation bias into account or collected data with an experimental design that mimics actual injury mechanisms. Twenty-three participants performed randomized single-leg vertical drop landings from 20 cm. Subjects were blinded to the landing surface (a flat force plate or 30° inversion wedge on the force plate). After each trial, participants reported whether they anticipated the landing surface. Participant responses were validated with EMG data. The protocol was repeated until four anticipated and four unanticipated landings onto the inversion wedge were recorded. Results revealed a significant main effect for landing condition. Normalized vertical ground reaction force (% body weights), maximum ankle inversion (degrees), inversion velocity (degrees/second), and time from contact to peak muscle activation (seconds) were significantly greater in unanticipated landings, and the time from peak muscle activation to maximum VGRF (second) was shorter. Unanticipated landings presented different muscle activation patterns than landings onto anticipated surfaces, which calls into question the usefulness of clinical studies that have not controlled for anticipation bias.

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Martin H. Rose, Annemette Løkkegaard, Stig Sonne-Holm and Bente R. Jensen

We investigated lower-extremity isometric tremor Approximate Entropy (irregularity), torque steadiness and rate of force development (RFD) and their associations to muscle activation strategy during isometric knee extensions in patients with Parkinson’s disease (PD). Thirteen male patients with idiopathic PD and 15 neurologically healthy matched controls performed isometric maximal contractions (extension/flexion) as well as steady submaximal and powerful isometric knee extensions. The patients with PD showed decreased isometric tremor irregularity. Torque steadiness was reduced in PD and the patients had increased muscle coactivation. A markedly lower RFD was found in PD and the decreased RFD correlated with reduced agonist muscle activation. Furthermore, patient RFD correlated with the Movement-Disorder-Society-Unified-Parkinson’s-Disease-Rating-Scale 3 (motor part) scores. We concluded that both knee isometric tremor Approximate Entropy and torque steadiness clearly differentiate between patients with PD and healthy controls. Furthermore, severely compromised RFD was found in patients with PD and was associated with decreased agonist muscle activation.

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Marco Arkesteijn, Simon Jobson, James Hopker and Louis Passfield

Background:

Previous research has shown that cycling in a standing position reduces cycling economy compared with seated cycling. It is unknown whether the cycling intensity moderates the reduction in cycling economy while standing.

Purpose:

The aim was to determine whether the negative effect of standing on cycling economy would be decreased at a higher intensity.

Methods:

Ten cyclists cycled in 8 different conditions. Each condition was either at an intensity of 50% or 70% of maximal aerobic power at a gradient of 4% or 8% and in the seated or standing cycling position. Cycling economy and muscle activation level of 8 leg muscles were recorded.

Results:

There was an interaction between cycling intensity and position for cycling economy (P = .03), the overall activation of the leg muscles (P = .02), and the activation of the lower leg muscles (P = .05). The interaction showed decreased cycling economy when standing compared with seated cycling, but the difference was reduced at higher intensity. The overall activation of the leg muscles and the lower leg muscles, respectively, increased and decreased, but the differences between standing and seated cycling were reduced at higher intensity.

Conclusions:

Cycling economy was lower during standing cycling than seated cycling, but the difference in economy diminishes when cycling intensity increases. Activation of the lower leg muscles did not explain the lower cycling economy while standing. The increased overall activation, therefore, suggests that increased activation of the upper leg muscles explains part of the lower cycling economy while standing.

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Mikko Virmavirta and Paavo V. Komi

Electromyographic (EMG) activities of gluteus maximus (GL), vastus later-alis (VL), vastus medialis (VM), tibialis anterior (TA), and gastrocnemius (GA) were measured telemetrically from four world-class athletes during the entire ski jumping performance. Integrated electromyographic activities (IEMG) were calculated from the different phases of jump. TA and GA showed alternate activation during the curve, suggesting that maintenance of the inrun position is a process requiring continuous active control. VL and VM were observed to contribute mostly to the entire takeoff phase whereas GL became strongly active within the last 4 meters of the takeoff. GA was slightly but continuously active during the inrun and showed only a small increase during takeoff. The quick lifting of the skis, as evidenced by the activation of TA, does not seem to allow effective use of GA at the end of the takeoff. Strong continuous activity of the knee extensors and TA dominated the midflight phase whereas the activation of GL and GA increased toward the end of the flight.

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Justin M. Stanek, Todd A. McLoda, Val J. Csiszer and A.J. Hansen

Context:

Selected muscles in the kinetic chain may help explain the body’s ability to avert injury during unexpected perturbation.

Objective:

To determine the activation of the ipsilateral rectus femoris (RF), gluteus maximus (MA), gluteus medius (ME), and contralateral external obliques (EO) during normal and perturbed gait.

Design:

Single-factor, repeated measures.

Setting:

University research laboratory.

Participants:

32 physically active, college-age subjects.

Intervention:

Subjects walked a total of 20 trials the length of a 6.1-m custom runway capable of releasing either side into 30° of unexpected inversion. During 5 trials, the platform released into inversion.

Main Outcome Measures:

Average, peak, and time to peak EMG were analyzed across the 4 muscles, and comparisons were made between the walking trials and perturbed trials.

Results:

Significantly higher average and peak muscle activity were noted for the perturbed condition for RF, MA, and EO. Time to peak muscle activity was faster during the perturbed condition for the EO.

Conclusion:

Rapid contractions of selected postural muscles in the kinetic chain help explain the body’s reaction to unexpected perturbation.