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Shortening Contractions (shortening + contraction)

Distribution by Scientific Domains

Life Sciences	100%

Selected Abstracts

Neural control of shortening and lengthening contractions: influence of task constraints

THE JOURNAL OF PHYSIOLOGY, Issue 24 2008
Jacques Duchateau
Although the performance capabilities of muscle differ during shortening and lengthening contractions, realization of these differences during functional tasks depends on the characteristics of the activation signal discharged from the spinal cord. Fundamentally, the control strategy must differ during the two anisometric contractions due to the lesser force that each motor unit exerts during a shortening contraction and the greater difficulty associated with decreasing force to match a prescribed trajectory during a lengthening contraction. The activation characteristics of motor units during submaximal contractions depend on the details of the task being performed. Indexes of the strategy encoded in the descending command, such as coactivation of antagonist muscles and motor unit synchronization, indicate differences in cortical output for the two types of anisometric contractions. Furthermore, the augmented feedback from peripheral sensory receptors during lengthening contractions appears to be suppressed by centrally and peripherally mediated presynaptic inhibition of Ia afferents, which may also explain the depression of voluntary activation that occurs during maximal lengthening contractions. Although modulation of the activation during shortening and lengthening contractions involves both supraspinal and spinal mechanisms, the association with differences in performance cannot be determined without more careful attention to the details of the task. [source]

Limitations of relaxation kinetics on muscular work

ACTA PHYSIOLOGICA, Issue 2 2010
J. McDaniel
Abstract Aim:, Positive net work produced during cyclic contractions is partially limited by relaxation kinetics, which to date, have not been directly investigated. Therefore, the purpose of this investigation was to determine the influence of relaxation kinetics on cyclic work. Methods:, Soleus muscles of four cats were isolated and subjected to a series of work loops (0.5, 1, 1.5 and 2 Hz cycle frequencies) during which stimulation terminated prior to the end of the shortening phase to allow for complete muscle relaxation and matched discrete sinusoidal shortening contractions during which stimulation remained on until the completion of the shortening phase. Muscle length changes during these protocols were centred on optimum length and were performed across muscle lengths that represented walking gait. Results:, When muscle excursions were centred on Lo relaxation kinetics decreased muscular work by 2.8 ± 0.8%, 12.1 ± 4.1%, 27.9 ± 4.5% and 40.1 ± 5.9% for 0.5, 1, 1.5 and 2 Hz respectively. However, relaxation kinetics did not influence muscular work when muscle excursions represented walking gait. In addition, muscular work produced at muscle lengths associated with walking gait was less than the work produced across Lo (55.7 ± 20.0%, 53.5 ± 21.0%, and 50.1 ± 22.0% for 0.5, 1 and 1.5 Hz respectively). Conclusion:, These results imply that relaxation kinetics are an important factor that limit the ability of muscle to produce work; however, the influence of relaxation kinetics on physiological function may depend on the relation between the optimum length and natural excursion of a muscle. [source]

Metabolic cost of lengthening, isometric and shortening contractions in maximally stimulated rat skeletal muscle

ACTA PHYSIOLOGICA, Issue 2 2004
J. G. M. Beltman
Abstract Aim:, The present study investigated the energy cost of lengthening, isometric and shortening contractions in rat muscle (n = 19). Methods:, With electrical stimulation the rat medial gastrocnemius muscle was maximally stimulated to perform 10 lengthening, isometric and shortening contractions (velocity 25 mm s,1) under experimental conditions (e.g. temperature, movement velocity) that resemble conditions in human movement. Results:, Mean ± SD force,time-integral of the first contraction was significantly different between the three protocols, 2.4 ± 0.2, 1.7 ± 0.2 and 1.0 ± 0.2 N s, respectively (P < 0.05). High-energy phosphate consumption was not significantly different between the three modes of exercise but a trend could be observed from lengthening (7.7 ± 2.7 ,mol , P muscle,1) to isometric (8.9 ± 2.2 ,mol , P muscle,1) to shortening contractions (10.4 ± 1.6 ,mol , P muscle,1). The ratio of high-energy phosphate consumption to force,time-integral was significantly lower for lengthening [0.3 ± 0.1 ,mol , P (N s),1] and isometric [0.6 ± 0.2 ,mol , P (N s),1] contractions compared with shortening [1.2 ± 0.2 ,mol , P (N s),1] contractions (P < 0.05). Conclusion:, The present results of maximally stimulated muscles are comparable with data in the literature for voluntary human exercise showing that the energy cost of force production during lengthening exercise is ,30% of that in shortening exercise. The present study suggests that this finding in humans probably does reflect intrinsic muscle properties rather than effects of differential recruitment and/or coactivation. [source]