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Yi-Ming Huang, Ya-Ju Chang, Miao-Ju Hsu, Chia-Ling Chen, Chia-Ying Fang and Alice May-Kuen Wong

The purpose of this study was to evaluate whether agonist muscle fatigue changed the coactivation time and the co-contraction magnitude of the agonist and antagonist muscle, and if the agonist muscle fatigue produced bias (constant error: CE) and inconsistency (variable error: VE) of the force. Subjects are 10 healthy people and one person with impaired proprioception. EMG and force for fast (0.19 ± 0.06 s) and slow (1.20 ± 0.44 s) targeted isometric dorsiflexions were recorded before and after fatigue of the dorsiflexors. The results revealed that the coactivation time increased after fatigue only in the slow contractions but the co-contraction magnitude did not change. The postfatigue increment of the CE was greater in the fast contractions than in the slow ones. We conclude that the postfatigue compensatory strategy can reduce the fatigue-induced bias. The change of muscles activation level after fatigue might be under the influence of the common drive. Impaired proprioception is a possible cause of the fatigue-related increase in bias and inconsistency.

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Alexandre Murian, Thibault Deschamps and Jean Jacques Temprado

The current study investigated the influence of resistance to motion and trial duration on the stability of bimanual coordination patterns and attentional demands. Seven participants performed in-phase and antiphase coordination patterns at a frequency of 1.5 Hz for 300 s. Resistance opposed to pronation–supination movements was manipulated. Attentional demands associated with the bimanual coordination patterns performance were measured using a probe reaction-time task. Results showed that variations in the level of resistance to motion, which induced corresponding variations in the amount of muscle activation during both the in-phase and the antiphase pattern, were associated with longer reaction time. Relative phase variability and attentional demands were higher for the antiphase pattern than for the in-phase pattern. Moreover, the attentional demands did not covary with the increase in the antiphase pattern over the trial duration. The in-phase pattern remained unaffected by resistance opposed to pronation–supination movement. The present findings and the time effect are discussed according to potential alterations localized in different sites at the cortical level.

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Richard E. Hughes, James C. Bean and Don B. Chaffin

Concurrent activation of muscles on opposite sides of joints is a common phenomenon. In simple planar mechanical systems, it is easy to identify such an electromyographic pattern as co-contraction of agonist and antagonist muscles. In complex 3-D systems such as the lumbar spine, it is more difficult to precisely identify whether EMG recordings represent co-contraction. Qualitative definitions of antagonist muscles emphasize that their actions wholly oppose the action of the prime movers. The qualitative definition of antagonist muscles was used to formulate a mathematical requirement for there to be co-contraction of agonists and antagonists. It was shown that the definition of co-contraction implies muscle activity beyond what is required to maintain equilibrium. The method was illustrated by classifying EMG recordings made of the lumbar region musculature during tasks involving combined torso extension and axial twisting loads. The method, which identified muscle activity in excess of that required to maintain static equilibrium, could be used to identify conditions in which muscle activation is required for something other than merely maintaining moment equilibrium.

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Priyanka Banerjee, Stephen H.M Brown, Samuel J. Howarth and Stuart M. McGill

The ProFitter 3-D Cross Trainer is a labile surface device used in the clinic and claimed to train spine stability. The purpose of this study was to quantify the spine mechanics (compression and shear forces and stability), together with muscle activation mechanics (surface electromyography) of the torso and hip, during three ProFitter exercises. Trunk muscle activity was relatively low while exercising on the device (<25%MVC). Gluteus medius activity was phasic with the horizontal sliding position, especially for an experienced participant. Sufficient spinal stability was achieved in all three exercise conditions. Peak spinal compression values were below 3400 N (maximum 3188 N) and peak shear values were correspondingly low (under 500 N). The exercises challenge whole-body dynamic balance while producing very conservative spine loads. The motion simultaneously integrates hip and torso muscles in a way that appears to ensure stabilizing motor patterns in the spine. This information will assist with clinical decision making about the utility of the device and exercise technique in rehabilitation and training programs.

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Tom G. Welter, Maarten F. Bobbert, Bauke M. van Bolhuis, Stan C.A.M. Gielen, Leonard A. Rozendaal and Dirkjan H.E.J. Veeger

We have investigated whether differences in EMG activity in mono- and bi-articuiar muscles for concentric and eccentric contractions (van Bolhuis, Gielen, & van Ingen Schenau, 1998) have to be attributed to a specific muscle coordination strategy or whether they are merely a demonstration of adaptations necessary to adjust for muscle contractile properties. Slow, multi-joint arm movements were studied in a horizontal plane with an external force applied at the wrist. Kinematics and electromyography data from 10 subjects were combined with data from a 3-D model of the arm and a Hill-type muscle model Data for both mono- and bi-articular muscles revealed a higher activation in concentric than in eccentric contractions. The model calculations indicated that the measured difference in activation (20%) was much larger than expected based on the force-velocity relationship (predicting changes of ~5%). Although these findings eliminate the force-velocity relationship as the main explanation for changes in EMG, it cannot be ruled out that other muscle contractile properties, such as history dependence of muscle force, determine muscle activation levels in the task that was studied.

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Alycia Fong Yan, Richard Smith, Benedicte Vanwanseele and Claire Hiller

There has been little scientific investigation of the impact of dance shoes on foot motion or dance injuries. The pointed (plantar-flexed) foot is a fundamental component of both the technical requirements and the traditional aesthetic of ballet and jazz dancing. The aims of this study were to quantify the externally observed angle of plantar flexion in various jazz shoes compared with barefoot and to compare the sagittal plane bending stiffness of the various jazz shoes. Sixteen female recreational child dancers were recruited for 3D motion analysis of active plantar flexion. The jazz shoes tested were a split-sole jazz shoe, full-sole jazz shoe, and jazz sneaker. A shoe dynamometer measured the stiffness of the jazz shoes. The shoes had a significant effect on ankle plantar flexion. All jazz shoes significantly restricted the midfoot plantar flexion angle compared with the barefoot condition. The split-sole jazz shoe demonstrated the least restriction, whereas the full-sole jazz shoe the most midfoot restriction. A small restriction in metartarsophalangeal plantar flexion and a greater restriction at the midfoot joint were demonstrated when wearing stiff jazz shoes. These restrictions will decrease the aesthetic of the pointed foot, may encourage incorrect muscle activation, and have an impact on dance performance.

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Eadric Bressel, Gary D. Heise and Greg Bachman

The purpose of this study was to determine how muscle activity and oxygen consumption are influenced by reverse pedaling (RP) compared to forward pedaling (FP). Seventeen physically active males performed FP and RP at an external workrate of 157 W (80 rpm) while EMG data were collected from five muscles: rectus femoris (RF), biceps femoris (BF), gastrocnemius (GN), tibialis anterior (TA), and vastus medialis (VM). Oxygen consumption (V̇O2 L·min-1) data were collected. On-time durations and EMG amplitudes were quantified for each half-cycle (first 180° and second 180° of crank angle). V̇O2 was similar between pedaling conditions while muscles RF and BF exhibited phasic shifts in response to RP with no amplitude change. VM showed an increase and GN displayed a decrease in EMG amplitude from FP to RP. The phasic shifts in muscle activation seen in RP, particularly in RF and BF, may alter the sequence of the knee extensor–hip extensor joint moments during the first half-cycle of pedaling.

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Max R. Paquette, Audrey Zucker-Levin, Paul DeVita, Joseph Hoekstra and David Pearsall

The purpose of this study was to compare lower extremity joint angular position and muscle activity during elliptical exercise using different foot positions and also during exercise on a lateral elliptical trainer. Sixteen men exercised on a lateral elliptical and on a standard elliptical trainer using straight foot position, increased toe-out angle, and a wide step. Motion capture and electromyography systems were used to obtain 3D lower extremity joint kinematics and muscle activity, respectively. The lateral trainer produced greater sagittal and frontal plane knee range of motion (ROM), greater peak knee flexion and extension, and higher vastus medialis activation compared with other conditions (P < .05). Toe-out and wide step produced the greatest and smallest peak knee adduction angles, respectively (P < .05). The lateral trainer produced greater sagittal and frontal plane hip ROM and greater peak hip extension and flexion compared with all other conditions (P < .05). Toe-out angle produced the largest peak hip external rotation angle and lowest gluteus muscle activation (P < .05). Findings from this study indicate that standard elliptical exercise with wide step may place the knee joint in a desirable frontal plane angular position to reduce medial knee loads, and that lateral elliptical exercise could help improve quadriceps strength but could also lead to larger knee contact forces.

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Robert J. Delmore, Kevin G. Laudner and Michael R. Torry

Context:

Hip-adductor strains are among the most common lower-extremity injuries sustained in athletics. Treatment of these injuries involves a variety of exercises used to target the hip adductors.

Objective:

To identify the varying activation levels of the adductor longus during common hip-adductor exercises.

Design:

Descriptive study.

Setting:

Laboratory.

Participants:

24 physically active, college-age students.

Intervention:

None.

Main Measurement Outcomes:

Peak and average electromyographic (EMG) activity of the adductor longus muscle during the following 6 hip-adductor rehabilitation exercises: side-lying hip adduction, ball squeezes, rotational squats, sumo squats, standing hip adduction on a Swiss ball, and side lunges.

Results:

The side-lying hip-adduction exercise produced more peak and average activation than any other exercise (P < .001). Ball squeezes produced more peak and average activation than rotational squats, sumo squats, and standing adduction on a Swiss ball (P < .001). Ball squeezes had more average activation than side lunges (P = .001). All other variables for peak activation during the exercises were not statistically significant (P > .08). These results allowed the authors to provide an overall ranking system (highest to lowest muscle activation): side-lying hip adduction, ball squeezes, side lunges, standing adduction on a Swiss ball, rotational squats, and sumo squats.

Conclusion:

The study provides a ranking system on the activation levels of the adductor longus muscle for 6 common hip-adductor rehabilitation exercises, with the side-lying hip-adduction and ball-squeeze exercises displaying the highest overall activation.

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Samantha L. Winter and John H. Challis

The muscle fiber force–length relationship has been explained in terms of the cross-bridge theory at the sarcomere level. In vivo, for a physiologically realistic range of joint motion, and therefore range of muscle fiber lengths, only part of the force–length curve may be used; that is, the section of the force– length curve expressed can vary. The purpose of this study was to assess the accuracy of a method for determining the expressed section of the force– length curve for biarticular muscles. A muscle model was used to simulate the triceps surae muscle group. Three model formulations were used so that the gastrocnemius operated over different portions of the force–length curve: the ascending limb, the plateau region, and the descending limb. Joint moment data were generated for a range of joint configurations and from this simulated data the region of the force– length relationship that the gastrocnemius muscle operated over was successfully reconstructed using the algorithm of Herzog and ter Keurs (1988a). Further simulations showed that the correct region of the force–length curve was accurately reconstructed even in the presence of random and systematic noise generated to reflect the effects of sampling errors, and incomplete muscle activation.