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Mikael Scohier, Dominique De Jaeger and Benedicte Schepens

The purpose of this study was to mechanically evoke a triceps surae stretch reflex during the swing phase of running, to study its within-the-step phase dependency. Seven participants ran on a treadmill at 2.8 m·s−1 wearing an exoskeleton capable of evoking a sudden ankle dorsiflexion. We measured the electromyographic activity of the soleus, medial and lateral gastrocnemii just after the perturbation to evaluate the triceps surae stretch reflex. Similar perturbations were also delivered at rest. Our results showed that the stretch reflex was suppressed during the swing phase of running, except in late swing where a late reflex response was observed. At rest, all triceps surae muscles showed an early reflex response to stretch. Our findings suggest that the triceps surae short/medium-latency stretch reflex cannot be evoked during swing phase and thus cannot contribute to the control of the locomotor pattern after aperturbation during this phase.

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Allyson M. Carter, Stephen J. Kinzey, Linda F. Chitwood and Judith L Cole

Context:

Proprioceptive neuromuscular facilitation (PNF) is commonly used before competition to increase range of motion. It is not known how it changes muscle response to rapid length changes.

Objective:

To determine whether PNF alters hamstring muscle activity during response to rapid elongation.

Design:

2 X 2 factorial.

Setting:

Laboratory.

Participants:

Twenty-four women; means: 167.27 cm, 58.92 kg, 21.42 y, 18.41% body fat, 21.06 kg/m2 BMI.

intervention:

Measurements before and after either rest or PNF were compared.

Main Outcome Measures:

Average muscle activity immediately after a rapid and unexpected stretch, 3 times pretreatment and posttreatment, averaged into 2 pre-and post- measures.

Results:

PNF caused decreased activity in the biceps femoris during response to a sudden stretch (P = .04). No differences were found in semitendinosus activity (P = .35).

Conclusions:

Decreased muscle activity likely results from acute desensitization of the muscle spindle, which might increase risk of muscle and tendon injury.

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Nicole D. Harshbarger, Bradly L. Eppelheimer, Tamara C. Valovich McLeod and Cailee Welch McCarty

Clinical Scenario:

It has been suggested that posterior shoulder tightness is a common contributor to shoulder impingement in overhead-throwing athletes. The incidence of shoulder pain in the general population has been reported to be as high as 27%, and as many as 74% of the patients who were seen for shoulder issues had signs of impingement. Particularly regarding physically active adults, shoulder impingement is frequent among overhead-throwing athletes and may lead to lost participation in sport, as well as other injuries including labral pathologies. Therefore, finding an effective mechanism to reduce posterior shoulder tightness in overhead athletes is important and may help prevent impingement-type injuries. Typically, posterior shoulder tightness is identified by measuring horizontal humeral adduction; although another clinical measure that is commonly used is the bilateral measurement of glenohumeral internal-rotation (IR) range of motion (ROM). It is important to note, however, that the measurement of glenohumeral IR ROM specifically aims to identify glenohumeral IR ROM deficits (GIRD). Although GIRD is believed to be a leading contributor to posterior shoulder tightness, this measure alone may not capture the full spectrum of posterior shoulder tightness. While treatment interventions to correct any ROM deficits typically include a stretching protocol to help increase IR, joint mobilizations have been found to produce greater mobility of soft tissue and capsular joints. However, it is unclear whether the combination of both joint mobilizations and a stretching protocol will produce even larger gains of ROM that will have greater longevity for the patient suffering from posterior shoulder tightness.

Focused Clinical Question:

Does the use of joint mobilizations combined with a stretching protocol more effectively increase glenohumeral IR ROM in adult physically active individuals who participate in overhead sports and are suffering from posterior shoulder tightness, compared with a stretching protocol alone?

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Gerrit Jan van Ingen Schenau, Maarten F. Bobbert and Arnold de Haan

This target article addresses the role of storage and reutilization of elastic energy in stretch-shortening cycles. It is argued that for discrete movements such as the vertical jump, elastic energy does not explain the work enhancement due to the prestretch. This enhancement seems to occur because the prestretch allows muscles to develop a high level of active state and force before starting to shorten. For cyclic movements in which stretch-shortening cycles occur repetitively, some authors have claimed that elastic energy enhances mechanical efficiency. In the current article it is demonstrated that this claim is often based on disputable concepts such as the efficiency of positive work or absolute work, and it is argued that elastic energy cannot affect mechanical efficiency simply because this energy is not related to the conversion of metabolic energy into mechanical energy. A comparison of work and efficiency measures obtained at different levels of organization reveals that there is in fact no decisive evidence to either support or reject the claim that the stretch-shortening cycle enhances muscle efficiency. These explorations lead to the conclusion that the body of knowledge about the mechanics and energetics of the stretch-shortening cycle is in fact quite lean. A major challenge is to bridge the gap between knowledge obtained at different levels of organization, with the ultimate purpose of understanding how the intrinsic properties of muscles manifest themselves under in-vivo-like conditions and how they are exploited in whole-body activities such as running. To achieve this purpose, a close cooperation is required between muscle physiologists and human movement scientists performing inverse and forward dynamic simulation studies of whole-body exercises.

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Yoshinobu Ohira, Tomoo Yoshinaga, Wataru Yasui, Makoto Ohara and Takato Tanaka

The effects of chronic stretching or shortening of the soleus muscle of adult rats during hindlimb suspension on muscle mass and contractile properties were studied. Rats suspended with the ankle joint immobilized in either a dorsiflexed (Susp-DF. soleus stretched), a plantarflexed position (Susp-PF, soleus shortened), or without immobilization (Susp-Free. soleus shortened) were compared with cage control rats. Suspension-related muscle atrophy was prevented in Susp-DF. The relative muscle weight in Susp-PF was also less than in cage control and Susp-DF. Both isometric maximum twitch tension (Pt) and maximum tetanic tension (Po) in the Susp-Free and Susp-PF were less than control. Both Pt and Po in Susp-DF were normal. The twitch time-to-peak tension and one-half relaxation time tended to be reduced by chronic shortening of the muscle. The rate of tension development during a twitch (dp/dt), expressed as g/s, of Susp-Free group was decreased, but that expressed as g/s/g Pt was greater than controls. That in Susp-DF was subnormal. The fatigue resistance in Susp-Free was normal but was reduced in Susp-DF and Susp-PF. These data suggest that the decreases in the rat soleus mass and maximum tension production and the shift toward a fast-twitch type following hindlimb suspension are prevented by chronic stretching of muscle, although detrimental effect was induced for the fatigue resistance.