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Jun G. San Juan, Peter Kosek and Andrew R. Karduna

Subacromial impingement syndrome is the most common shoulder disorder. Abnormal superior translation of the humeral head is believed to be a major cause of this pathology. The first purpose of the study was to examine the effects of suprascapular nerve block on superior translation of the humeral head and scapular upward rotation during dynamic shoulder elevation. The secondary purpose was to assess muscle activation patterns during these motions. Twenty healthy subjects participated in the study. Using fluoroscopy and electromyography, humeral head translation and muscle activation were measured before and after a suprascapular nerve block. The humeral head was superiorly located at 60 degrees of humeral elevation, and the scapula was more upwardly rotated from 30 to 90 degrees of humeral elevation after the block. The differences were observed during midrange of motion. In addition, the deltoid muscle group demonstrated increased muscle activation after the nerve block. The study’s results showed a compensatory increase in humeral head translation, scapular upward rotation, and deltoid muscle activation due to the nerve block. These outcomes suggest that increasing muscular strength and endurance of the supraspinatus and infraspinatus muscles could prevent any increased superior humeral head translation. This may be beneficial in reducing shoulder impingement or rotator cuff tears over time.

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Alan R. Needle, Thomas W. Kaminski, Jochen Baumeister, Jill S. Higginson, William B. Farquhar and C. Buz Swanik

Context:

Rolling sensations at the ankle are common after injury and represent failure in neural regulation of joint stiffness. However, deficits after ankle injury are variable and strategies for optimizing stiffness may differ across patients.

Objective:

To determine if ankle stiffness and muscle activation differ between patients with varying history of ankle injury.

Patients:

Fifty-nine individuals were stratified into healthy (CON, n = 20), functionally unstable (UNS, n = 19), and coper (COP, n = 20) groups.

Main Outcome Measures:

A 20° supination perturbation was applied to the ankle as position and torque were synchronized with activity of tibialis anterior, peroneus longus, and soleus. Subjects were tested with muscles relaxed, while maintaining 30% muscle activation, and while directed to react and resist the perturbation.

Results:

No group differences existed for joint stiffness (F = 0.07, P = .993); however, the UNS group had higher soleus and less tibialis anterior activation than the CON group during passive trials (P < .05). In addition, greater early tibialis anterior activation generally predicted higher stiffness in the CON group (P ≤ .03), but greater soleus activity improved stiffness in the UNS group (P = .03).

Conclusion:

Although previous injury does not affect the ability to stiffen the joint under laboratory conditions, strategies appear to differ. Generally, the COP has decreased muscle activation, whereas the UNS uses greater plantar-flexor activity. The results of this study suggest that clinicians should emphasize correct preparatory muscle activation to improve joint stiffness in injury-rehabilitation efforts.

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Sheng Li, Jennifer A. Stevens, Derek G. Kamper and William Z. Rymer

The purpose of this study was to investigate the effect of motor imagery on the premotor time (PMT). Twelve healthy adults performed reaction time movements in response to external visual signals at rest, when holding an object (muscle activation), or performing different background imagined movements (motor imagery). When compared to rest, muscle activation reduced the PMT; imagined finger extension of the right hand and imagined finger flexion of the left hand elongated the PMT; imagined finger flexion of the right hand had no effect on the PMT. This movement-specific effect is interpreted as the sum of the excitatory effect caused by enhanced corticospinal excitability specifically for the primary mover of the imagined movement and an overall inhibition associated with increased task complexity during motor imagery. Our results clearly demonstrate that motor imagery has movement-specific effects on the PMT.

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Kevin R. Ford, Gregory D. Myer, Laura C. Schmitt, Timothy L. Uhl and Timothy E. Hewett

The purpose of this study was to identify alterations in preparatory muscle activation patterns across different drop heights in female athletes. Sixteen female high school volleyball players performed the drop vertical jump from three different drop heights. Surface electromyography of the quadriceps and hamstrings were collected during the movement trials. As the drop height increased, muscle activation of the quadriceps during preparatory phase also increased (p < .05). However, the hamstrings activation showed no similar increases relative to drop height. Female athletes appear to preferentially rely on increased quadriceps activation, without an increase in hamstrings activation, with increased plyometric intensity. The resultant decreased activation ratio of the hamstrings relative to quadriceps before landing may represent altered dynamic knee stability and may contribute to the increased risk of ACL injury in female athletes.

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Gloria M. Beim, Jorge L. Giraldo, Danny M. Pincivero, Matthew J. Borror and Freddie H. Fu

The purpose of this study was to compare electromyographic (EMG) activity of the abdominal muscles between the crunch exercise and five other popular abdominal exercises. Surface EMG recordings of four muscles (upper rectus, lower rectus, external oblique, and internal oblique) of the anterior abdominal wall were collected and analyzed on 20 healthy, male volunteers. EMG activity was recorded during execution of the abdominal crunch, the sit-up, and exercises performed with the Abflex machine, the AbRoller, the Nordic Track Ab Works, and the Nautilus crunch machine. The results indicate that the crunch exercise is comparable to the five other abdominal exercises with respect to muscle activation of the internal and external abdominal oblique muscles. Activation of the upper rectus abdominal muscles appears to be best achieved with the Abflex machine, whereas the crunch exercise is superior to the sit-up for activation of the upper and lower rectus abdominal muscles.

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Rupal Mehta, Marco Cannella, Sharon M. Henry, Susan Smith, Simon Giszter and Sheri P. Silfies

Trunk muscle timing impairment has been associated with nonspecific low back pain (NSLBP), but this finding has not been consistent. This study investigated trunk muscle timing in a subgroup of patients with NSLBP attributed to movement coordination impairment (MCI) and matched asymptomatic controls in response to a rapid arm-raising task. Twenty-one NSLBP subjects and 21 matched controls had arm motion and surface EMG data collected from seven bilateral trunk muscles. Muscle onset and offset relative to deltoid muscle activation and arm motion, duration of muscle burst and abdominal–extensor co-contraction time were derived. Trunk muscle onset and offset latencies, and burst and co-contraction durations were not different (p > .05) between groups. Patterns of trunk muscle activation and deactivation relative to arm motion were not different. Task performance was similar between groups. Trunk muscle timing does not appear to be an underlying impairment in the subgroup of NSLBP with MCI.

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Kieran O’Sullivan, Ellen Herbert, David Sainsbury, Karen McCreesh and Amanda Clifford

Context:

The gluteus medius (Gmed) is proposed to consist of 3 functional subdivisions (anterior, middle, and posterior). Gmed weakness and dysfunction have been implicated in numerous lower extremity disorders, including patellofemoral pain syndrome (PFPS). PFPS is a knee condition that frequently occurs in females and is associated with activities such as squatting and stair climbing. There is a lack of evidence for the role of the subdivisions of the Gmed in females with and without PFPS.

Objective:

To compare muscle activation in the 3 Gmed subdivisions during 4 weight-bearing exercises in women with and without PFPS.

Design:

Single-session, repeated-measures observational study.

Setting:

University research laboratory.

Participants:

Convenience sample of 12 women with PFPS and 12 age- and gender-matched asymptomatic controls.

Intervention:

Participants performed 4 weight-bearing exercises (wall press, pelvic drop, step-up-and-over, and unilateral squat) 3 times while surface electromyography (sEMG) activity of the Gmed segments was recorded.

Main Outcome Measures:

sEMG muscle activity for each functional subdivision of the Gmed during each weight-bearing exercise was analyzed using a mixed between–within-subjects ANOVA (post hoc Bonferroni).

Results:

No statistically significant differences in muscle activation were found between the PFPS and healthy participants (P = .97). Furthermore, there were no statistically significant differences between the exercises (P = .19) or muscle fibers (P = .36) independent of group analyzed. However, the activation of the subdivisions varied according to the exercise performed (P = .003).

Conclusions:

Similar levels of muscle activation were recorded in the Gmed subdivisions of the PFPS and healthy participants during the different exercises. This is the first study to examine all 3 Gmed subdivisions in PFPS. Future studies using larger sample sizes should also investigate onset and duration of muscle activation in all Gmed subdivisions in both healthy individuals and those with PFPS.

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Jeni R. McNeal, William A. Sands and Michael H. Stone

Purpose:

The aim of this study was to investigate the effects of a maximal repeated-jumps task on force production, muscle activation and kinematics, and to determine if changes in performance were dependent on gender.

Methods:

Eleven male and nine female athletes performed continuous countermovement jumps for 60 s on a force platform while muscle activation was assessed using surface electromyography. Performances were videotaped and digitized (60 Hz). Data were averaged across three jumps in 10-s intervals from the initial jump to the final 10 s of the test.

Results:

No interaction between time and gender was evident for any variable; therefore, all results represent data collapsed across gender. Preactivation magnitude decreased across time periods for anterior tibialis (AT, P < .001), gastrocnemius (GAS, P < .001) and biceps femoris (BF, P = .03), but not for vastus lateralis (VL, P = .16). Muscle activation during ground contact did not change across time for BF; however, VL, G, and AT showed significant reductions (all P < .001). Peak force was reduced at 40 s compared with the initial jumps, and continued to be reduced at 50 and 60 s (all P < .05). The time from peak force to takeoff was greater at 50 and 60 s compared with the initial jumps (P < .05). Both knee fexion and ankle dorsifexion were reduced across time (both P < .001), whereas no change in relative hip angle was evident (P = .10). Absolute angle of the trunk increased with time (P < .001), whereas the absolute angle of the shank decreased (P < .001).

Conclusions:

In response to the fatiguing task, subjects reduced muscle activation and force production and altered jumping technique; however, these changes were not dependent on gender.

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Simon Wang and Stuart M. McGill

Spine stability is ensured through isometric coactivation of the torso muscles; however, these same muscles are used cyclically to assist ventilation. Our objective was to investigate this apparent paradoxical role (isometric contraction for stability or rhythmic contraction for ventilation) of some selected torso muscles that are involved in both ventilation and support of the spine. Eight, asymptomatic, male subjects provided data on low back moments, motion, muscle activation, and hand force. These data were input to an anatomically detailed, biologically driven model from which spine load and a lumbar spine stability index was obtained. Results revealed that subjects entrained their torso stabilization muscles to breathe during demanding ventilation tasks. Increases in lung volume and back extensor muscle activation coincided with increases in spine stability, whereas declines in spine stability were observed during periods of low lung inflation volume and simultaneously low levels of torso muscle activation. As a case study, aberrant ventilation motor patterns (poor muscle entrainment), seen in one subject, compromised spine stability. Those interested in rehabilitation of patients with lung compromise and concomitant back troubles would be assisted with knowledge of the mechanical links between ventilation during tasks that impose spine loading.

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Jill L. McNitt-Gray

The target article, thoughtfully constructed by Dr. Prilutsky, effectively synthesizes available data on multijoint movements regarding coordination patterns of major two- and one-joint muscles, provides evidence for an optimization criterion that predicts critical features of muscle activation patterns, and explores the functional consequences of muscle coordination. This work also provides a clear set of definitions and an organizational framework that is currently needed for a productive interdisciplinary discussion regarding the underlying control mechanisms used during realistic multijoint movements. Although identification of an optimization criterion that predicts muscle recruitment strategies would greatly simplify control logic required for rehabilitation and musculoskeletal modeling, our experimental data during landings indicate more than one criterion may exist. Preliminary review of our experimental landing data suggests the rules identified by Prilutsky apparently hold for some subjects during portions of the landing movements. The presence of more than one muscle activation pattern used to achieve the same NJMs demonstrates there may be more than one optimization criterion that predicts critical features of muscle activation patterns. The functional consequences of more than one control criterion may also prove to be an asset, particularly when adapting to different environmental constraints.