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Mark D. Grabiner and Tammy M. Owings

For this study it was hypothesized that when participants intended to perform a maximum voluntary concentric (or eccentric) contraction but had an eccentric (or concentric) contraction imposed upon them, the initial EMG measured during the isometric phase preceding the onset of the dynamometer motion would reflect the intended contraction condition. The surface EMG of the vastus lateralis muscle was measured in 24 participants performing isokinetic concentric and eccentric maximum voluntary knee extensor contractions. The contractions were initiated from rest and from the same knee flexion angle and required the same level of external force to trigger the onset of dynamometer motion. Vastus lateralis EMG were quantified during the isometric phase preceding the onset of the dynamometer motion. When participants intended to perform a concentric contraction but had an eccentric contraction imposed upon them, the initial EMG resembled that of a concentric contraction. When they intended to perform an eccentric contraction but had a concentric contraction imposed upon them, the initial EMG resembled that of an eccentric contraction. Overall, the difference between concentric and eccentric contractions observed during the period of the initial muscle activation implies that descending signals include information that distinguishes between eccentric and concentric contractions.

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Saira Chaudhry, Dylan Morrissey, Roger C. Woledge, Dan L. Bader and Hazel R.C. Screen

Triceps surae eccentric exercise is more effective than concentric exercise for treating Achilles tendinopathy, however the mechanisms underpinning these effects are unclear. This study compared the biomechanical characteristics of eccentric and concentric exercises to identify differences in the tendon load response. Eleven healthy volunteers performed eccentric and concentric exercises on a force plate, with ultrasonography, motion tracking, and EMG applied to measure Achilles tendon force, lower limb movement, and leg muscle activation. Tendon length was ultrasonographically tracked and quantified using a novel algorithm. The Fourier transform of the ground reaction force was also calculated to investigate for tremor, or perturbations. Tendon stiffness and extension did not vary between exercise types (P = .43). However, tendon perturbations were significantly higher during eccentric than concentric exercises (25%–40% higher, P = .02). Furthermore, perturbations during eccentric exercises were found to be negatively correlated with the tendon stiffness (R 2 = .59). The particular efficacy of eccentric exercise does not appear to result from variation in tendon stiffness or extension within a given session. However, varied perturbation magnitude may have a role in mediating the observed clinical effects. This property is subject-specific, with the source and clinical timecourse of such perturbations requiring further research.

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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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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.

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Kristen E. Thomas and Leah R. Bent

The integration of vestibular and somatosensory information for the control of lower limb musculature remains elusive. To determine whether a subthreshold vestibular input influences the cutaneous evoked response, the isometric EMG activity in the posturally inactive soleus muscles of 13 healthy, seated subjects was collected. Vestibular afferents were activated using galvanic vestibular stimulation (GVS; 1.8–2.5mA, 500ms), while percutaneous electrical stimulation was delivered to the distal tibial nerve (11ms train of 3 × 1.0 ms pulses, 200Hz) to activate foot sole skin afferents. GVS elicited responses in soleus both independently and when combined with cutaneous stimulation. The responses to the combined sensory input showed an interaction between the two sensory modalities to influence muscle activation. Of note is the presence of significant muscle modulation in the combined condition, where subthreshold vestibular inputs altered the outcome of the cutaneous reflex response. This finding has implications for individuals with sensory deficiency. In the case of an absent or deficient sensory modality, balance protective reflexes to maintain postural equilibrium may be enhanced with targeted sensory augmentation.

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Hans H.C.M. Savelberg, Ingrid G.L. Van de Port and Paul J.B. Willems

By manipulating trunk angle in ergometer cycling, we studied the effect of body configuration on muscle recruitment and joint kinematics. Changing trunk angle affects the length of muscles that span the hip joint. It is hypothesized that this affects the recruitment of the muscles directly involved, and as a consequence of affected joint torque distributions, also influences the recruitment of more distal muscles and the kinematics of distal joints. It was found that changing the trunk from an upright position to approximately 20 deg forward or backward affected muscle activation patterns and kinematics in the entire lower limb. The knee joint was the only joint not affected by manipulation of the lengths of hip joint muscles. Changes in trunk angle affected ankle and hip joint kinematics and the orientation of the thigh. A similar pattern has been demonstrated for muscle activity: Both the muscles that span the hip joint and those acting on the ankle joint were affected with respect to timing and amplitude of EMG. Moreover, it was found that the association between muscle activity and muscle length was adapted to manipulation of trunk angle. In all three conditions, most of the muscles that were considered displayed some eccentric activity. The ratio of eccentric to concentric activity changed with trunk angle. The present study showed that trunk angle influences muscle recruitment and (inter)muscular dynamics in the entire limb. As this will have consequences for the efficiency of cycling, body configuration should be a factor in bicycle design.

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Harald Böhm, Gerald K. Cole, Gert-Peter Brüggemann and Hanns Ruder

The contribution of muscle in-series compliance on maximum performance of the muscle tendon complex was investigated using a forward dynamic computer simulation. The model of the human body contains 8 Hill-type muscles of the lower extremities. Muscle activation is optimized as a function of time, so that maximum drop jump height is achieved by the model. It is shown that the muscle series elastic energy stored in the downward phase provides a considerable contribution (32%) to the total muscle energy in the push-off phase. Furthermore, by the return of stored elastic energy all muscle contractile elements can reduce their shortening velocity up to 63% during push-off to develop a higher force due to their force velocity properties. The additional stretch taken up by the muscle series elastic element allows only m. rectus femoris to work closer to its optimal length, due to its force length properties. Therefore the contribution of the series elastic element to muscle performance in maximum height drop jumping is to store and return energy, and at the same time to increase the force producing ability of the contractile elements during push-off.

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Jwa-jun Kim, So-youn Ann and Se-yeon Park

Objectives:

The aim of this study was to investigate the effects of visual trace on shoulder muscle activation during diagonal pattern of exercises. Sixteen healthy male participants volunteered to participate.

Design:

Repeated-measure design.

Setting:

Laboratory setting.

Participants:

Sixteen physically active male participants volunteered to participate.

Intervention:

Five muscles of the shoulder were investigated during standing performance of diagonal shoulder exercises with and without visual trace. Two patterns of the diagonal exercises were used: diagonal 1 flexion (D1F) and diagonal 2 fexion (D2F). Two-way repeated measures analysis of variance was used, with factors being the presence of visual trace and exercise variations.

Main Outcome Measure:

Electromyography (EMG).

Results:

The average muscle activity values of the lower trapezius and anterior deltoid were higher with the D2F compared with the D1F (P < .05). The visual trace effect was observed within the serratus anterior, with values significantly greater in exercise with visual trace (P < .05). There was a significant increase of the lower trapezius during the exercise with the visual trace condition compared with the exercise without visual trace, which was only observed during D2F (P < .05).

Conclusions:

Present results suggest that the D2F exercise pattern is effective for activating lower trapezius and anterior deltoid muscles. The visual trace condition has the additional advantage of activating the scapulothoracic muscle activities depending on the specific pattern of diagonal shoulder exercise.

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Kristof Kipp, Ron Pfeiffer, Michelle Sabick, Chad Harris, Jeanie Sutter, Seth Kuhlman and Kevin Shea

The purpose of this study was to investigate muscle activation patterns during a landing task in boys and girls through the use of muscle synergies. Electromyographical data from six lower extremity muscles were collected from 11 boys and 16 girls while they performed single-leg drop-landings. Electromyographical data from six leg muscles were rectified, smoothed, and normalized to maximum dynamic muscle activity during landing. Data from 100 ms before to 100 ms after touchdown were submitted to factor analyses to extract muscle synergies along with the associated activation and weighing coefficients. Boys and girls both used three muscle synergies. The activation coefficients of these synergies captured muscle activity during the prelanding, touchdown, and postlanding phases of the single-leg drop-landing. Analysis of the weighing coefficients indicated that within the extracted muscle synergies the girls emphasized activation of the medial hamstring muscle during the prelanding and touchdown synergy whereas boys emphasized activation of the vastus medialis during the postlanding synergy. Although boys and girls use similar muscle synergies during single-leg drop-landings, they differed in which muscles were emphasized within these synergies. The observed differences in aspects related to the muscle synergies during landing may have implications with respect to knee injury risk.

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Barton E. Anderson and Kellie C. Huxel Bliven

Clinical Scenario:

Research has shown a link between poor core stability and chronic, nonspecific low back pain, with data to suggest that alterations in core muscle activation patterns, breathing patterns, lung function, and diaphragm mechanics may occur. Traditional treatment approaches for chronic, nonspecific low back pain focus on exercise and manual therapy interventions, however it is not clear whether breathing exercises are effective in treating back pain.

Focused Clinical Question:

In adults with chronic, nonspecific low back pain, are breathing exercises effective in reducing pain, improving respiratory function, and/or health related quality of life?

Summary of Key Findings:

Following a literature search, 3 studies were identified for inclusion in the review. All reviewed studies were critically appraised at level 2 evidence and reported improvements in either low back pain or quality of life following breathing program intervention.

Clinical Bottom Line:

Exercise programs were shown to be effective in improving lung function, reducing back pain, and improving quality of life. Breathing program frequencies ranged from daily to 2–3 times per week, with durations ranging from 4 to 8 weeks. Based on these results, athletic trainers and physical therapists caring for patients with chronic, nonspecific low back pain should consider the inclusion of breathing exercises for the treatment of back pain when such treatments align with the clinician’s own judgment and clinical expertise and the patient’s preferences and values.

Strength of Recommendation:

Grade B evidence exists to support the use of breathing exercises in the treatment of chronic, nonspecific low back pain.