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Hwang-Jae Lee, Won Hyuk Chang, Sun Hee Hwang, Byung-Ok Choi, Gyu-Ha Ryu and Yun-Hee Kim

The purpose of this study was to examine age-related gait characteristics and their associations with balance function in older adults. A total of 51 adult volunteers participated. All subjects underwent locomotion analysis using a 3D motion analysis and 12-channel dynamic electromyography system. Dynamic balance function was assessed by the Berg Balance Scale. Older adults showed a higher level of muscle activation than young adults, and there were significant positive correlations between increased age and activation of the trunk and thigh muscles in the stance and swing phase of the gait cycle. In particular, back extensor muscle activity was mostly correlated with the dynamic balance in older adults. Thus, back extensor muscle activity in walking may provide a clue for higher falling risk in older adults. This study demonstrates that the back extensor muscles play very important roles with potential for rehabilitation training to improve balance and gait in older adults.

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C. Buz Swanik, Scott M. Lephart, Frank P. Giannantonio and Freddie H. Fu

Anterior cruciate ligament (ACL) injury disrupts static and dynamic knee restraints, compromising functional stability. Deafferentation of ACL mechan-oreceptors alters the spinal reflex pathways to motor nerves and muscle spindles in addition to the cortical pathways for conscious and unconscious appreciation of proprioception and kinesthesia. These pathways are required by the feed-forward and feedback neuromuscular control systems to dynamically stabilize joints. Feed-forward motor control is responsible for preparatory muscle activity, while feedback motor control regulates reactive muscle activity. The level of muscle activation, preparatory or reactive, influences muscular stiffness, thereby providing dynamic restraint for the ACL-deficient athlete. Rehabilitation protocols should incorporate activities that enhance muscle stiffness while encouraging adaptations to peripheral afferents, spinal reflexes, and cortical motor patterns. Four elements crucial for reestablishing neuromuscular control and functional stability are proprioceptive and kinesthetic awareness, dynamic stability, preparatory and reactive muscle characteristics, and conscious and unconscious functional motor patterns.

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Angie Selseth, Marilyn Dayton, Mitchell L. Cordova, Christopher D. Ingersoll and Mark A. Merrick

Purpose:

To analyze vastus medialis obliquus (VMO) and vastus lateralis (VL) muscle activity during the concentric and eccentric phases of a lateral step-up exercise.

Design:

Repeated-measures. Dependent variable: the integrated electromyogram measured as a percentage of the maximal voluntary isometric contraction of the VMO and VL muscles. Independent variable: muscle contraction with 2 levels (concentric and eccentric).

Subjects:

Twenty-three volunteers with no previous history of knee surgery or anterior knee pain.

Methods:

Surface electrodes were positioned over the VMO and VL, and electromyographic data were collected during the exercise.

Results:

The 2 muscle phases of contraction were different when both dependent variables were considered simultaneously (F 2,7 = 33.2, P < .001). Concentric contractions produced greater muscle activity for VL (P < .05) and VMO (P < .05).

Conclusions:

Because concentric contractions produce greater activity than eccentric contractions do during the lateral step-up exercise, they provide a stronger stimulus for muscle activation, which might result in greater muscle strength gains.

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Nick Ball and Joanna Scurr

Electromyograms used to assess neuromuscular demand during high-velocity tasks require normalization to aid interpretation. This paper posits that, to date, methodological approaches to normalization have been ineffective and have limited the application of electromyography (EMG). There is minimal investigation seeking alternative normalization methods, which must be corrected to improve EMG application in sports. It is recognized that differing normalization methods will prevent cross-study comparisons. Users of EMG should aim to identify normalization methods that provide good reliability and a representative measure of muscle activation. The shortcomings of current normalization methods in high-velocity muscle actions assessment are evident. Advances in assessing alternate normalization methods have been done in cycling and sprinting. It is advised that when normalizing high-intensity muscle actions, isometric methods are used with caution and a dynamic alternative, where the muscle action is similar to that of the task is preferred. It is recognized that optimal normalization methods may be muscle and task dependent.

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Shinya Fujii, Kazutoshi Kudo, Masahiro Shinya, Tatsuyuki Ohtsuki and Shingo Oda

This study investigated performance and wrist muscle activity during rapid-repetitive unimanual tapping with a drumstick in right-handed drummers and nondrummers. Analyses of performances revealed no difference in tapping frequency and peak tap force between drummers and nondrummers, although the drummers showed less variability in intertap interval than the nondrummers. Analyses of the electromyographic (EMG) data obtained by recording the activity of the flexor carpi ulnalis and the extensor carpi radialis muscles of the right wrist revealed several distinct differences between the two groups: the drummers showed a lower level of muscle cocontraction together with an earlier decline of wrist flexor muscle activity and a smaller variability of muscle activation time in the wrist flexors compared with the nondrummers. We suggest that these characteristics in wrist muscle activity in the drummers have been acquired following extensive practice for the efficient use of wrist muscles and stable drumming performance.

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Nicole D. Park, Robert D. Maresca, Kimberly I. McKibans, D. Reid Morgan, Timothy S. Allen and Gordon L. Warren

The study’s objective was to determine whether orally ingested caffeine could help overcome excitation-contraction-coupling failure, which has been suggested to explain part of the strength loss associated with eccentric-contraction-induced muscle injury. A sample of 13 college students (4 men and 9 women) was used in a double-blind, repeated-measures experimental design. Each participant performed 2 experimental trials, 1 with each leg, with each trial lasting 4 consecutive days. On a given day, each participant was randomly assigned to ingest a capsule containing 6 mg/kg of either caffeine or flour (placebo). On the day of and the first 2 days after a bout of 50 injurious eccentric contractions done by the knee extensors, the interpolated-twitch technique was used to assess electrically evoked strength, maximal voluntary isometric contraction (MVIC) strength, and percent muscle activation during MVIC both before and after capsule ingestion. These variables were also measured before and after capsule ingestion the day before the eccentric-contraction bout—when the muscle was uninjured. In injured muscle, caffeine had no effect on any variable. In uninjured muscle, caffeine also had no effect on electrically evoked strength but increased MVIC strength by 10.4% compared with placebo (p = .00002), and this was attributed to an increase in muscle activation (6.2%; p = .01). In conclusion, the data provide no evidence that caffeine ingestion can help overcome excitation-contraction-coupling failure, if it exists, in injured human muscle. The data do indicate that caffeine ingestion can increase MVIC strength and activation in uninjured muscle but not in injured muscle.

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Lindsey K. Lepley, Abbey C. Thomas, Scott G. McLean and Riann M. Palmieri-Smith

Context:

As individuals returning to activity after anterior cruciate ligament reconstruction (ACLr) likely experience fatigue, understanding how fatigue affects knee-muscle activation patterns during sport-like maneuvers is of clinical importance. Fatigue has been suggested to impair neuromuscular control strategies. As a result, fatigue may place ACLr patients at increased risk of developing posttraumatic osteoarthritis (OA).

Objective:

To determine the effects of fatigue on knee-muscle activity post-ACLr.

Design:

Case control.

Setting:

University laboratory.

Participants:

12 individuals 7–10 mo post-ACLr (7 male, 5 female; age 22.1 ± 4.7 y; 1.8 ± 0.1 m; mass 77.7 ± 11.9 kg) and 13 controls (4 male, 9 female; age 22.9 ± 4.3 y; 1.7 ± 0.1 m; mass 66.9 ± 9.8 kg).

Interventions:

Fatigue was induced via repetitive sets of double-leg squats (n = 8), which were interspersed with sets of single-leg landings (n = 3), until squats were no longer possible.

Main Outcome Measures:

2 × 2 repeated-measures ANOVA was used to detect the main effects of group (ACLr, control) and fatigue state (prefatigue, postfatigue) on quadriceps:hamstring cocontraction index (Q:H CCI).

Results:

All subjects demonstrated higher Q:H CCI at prefatigue compared with postfatigue (F 1,23 = 66.949, P ≤ .001). Q:H CCI did not differ between groups (F 1,23 = 0.599, P = .447).

Conclusions:

The results indicate that regardless of fatigue state, ACLr individuals are capable of restoring muscle-activation patterns similar to those in healthy subjects. As a result, excessive muscle cocontraction, which has been hypothesized as a potential mechanism of posttraumatic OA, may not contribute to joint degeneration after ACLr.

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Fábio J. Lanferdini, Rodrigo R. Bini, Pedro Figueiredo, Fernando Diefenthaeler, Carlos B. Mota, Anton Arndt and Marco A. Vaz

Purpose:

To employ cluster analysis to assess if cyclists would opt for different strategies in terms of neuromuscular patterns when pedaling at the power output of their second ventilatory threshold (POVT2) compared with cycling at their maximal power output (POMAX).

Methods:

Twenty athletes performed an incremental cycling test to determine their power output (POMAX and POVT2; first session), and pedal forces, muscle activation, muscle–tendon unit length, and vastus lateralis architecture (fascicle length, pennation angle, and muscle thickness) were recorded (second session) in POMAX and POVT2. Athletes were assigned to 2 clusters based on the behavior of outcome variables at POVT2 and POMAX using cluster analysis.

Results:

Clusters 1 (n = 14) and 2 (n = 6) showed similar power output and oxygen uptake. Cluster 1 presented larger increases in pedal force and knee power than cluster 2, without differences for the index of effectiveness. Cluster 1 presented less variation in knee angle, muscle–tendon unit length, pennation angle, and tendon length than cluster 2. However, clusters 1 and 2 showed similar muscle thickness, fascicle length, and muscle activation. When cycling at POVT2 vs POMAX, cyclists could opt for keeping a constant knee power and pedal-force production, associated with an increase in tendon excursion and a constant fascicle length.

Conclusions:

Increases in power output lead to greater variations in knee angle, muscle–tendon unit length, tendon length, and pennation angle of vastus lateralis for a similar knee-extensor activation and smaller pedal-force changes in cyclists from cluster 2 than in cluster 1.

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Thomas J. Burkholder and T. Richard Nichols

Postural regulation is an important part of a variety of motor tasks, including quiet standing and locomotion. Muscle length feedback, both the autogenic length feedback arising from a muscle's own spindles, and heterogenic length feedback, arising from its agonists and antagonists, is a strong modulator of muscle force and well suited to postural maintenance. The effects of this reflex feedback on 3-D force generation and limb mechanics are not known. In this paper, we present a mechanical model for relating 3-D changes in cat hindlimb posture to changes in muscle lengths. These changes in muscle length are used to estimate changes in both intrinsic muscle force generation and muscle activation by length feedback pathways. Few muscles are found to have directly agonist mechanical actions, and most differ by more than 20°. Endpoint force fields are largely uniform across the space investigated. Both autogenic and heterogenic feedback contribute to whole limb resistance to perturbation, autogenic pathways being most dramatic. Length feedback strongly reinforced a restoring force in response to end-point displacement.

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Tom G. Welter and Maarten F. Bobbert

This study investigates the hypothesis that EMG measured from a muscle at a given force, length, and low-shortening velocity depends on the contraction history, specifically the distance over which the muscle has shortened. Slow linear horizontal wrist movements (3 cm/s) involving shoulder and elbow rotations towards a test position of 90° elbow flexion were performed. REMG was measured at the test position after wrist displacements over 6.5 and 13 cm. Muscle contraction speed was below 1% of maximum. A constant force (25 N) causing flexion torque in the elbow was exerted by the wrist. Inertial load was minimal. Two main elbow flexors (biceps caput longum and breve) showed significantly higher (14 and 24%) concentric REMG after 13-cm wrist movement than alter 6.5-cm. Eccentric EMG did not differ between the 6.5-and 13-cm conditions. It is concluded that adaptation of muscle activation is required to counteract the effects of contraction history on the force producing capacity of the muscle.