Browsing by Author "Johnston, Kaleena"

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    An Examination of Sarcomere Length Non-uniformities in Actively Stretched Muscle Myofibrils
    (2015-04-30) Johnston, Kaleena; Herzog, Walter
    Residual force enhancement (RFE) is a characteristic of skeletal muscle describing the increase in isometric steady-state force following an active stretch, compared to the force of an isometric contraction at the same final length. It has been argued that RFE is a result of unstable sarcomeres on the descending limb of the force-length relationship, causing long, weak sarcomeres to lengthen more than short, strong sarcomeres when a myofibril is actively stretched, as described by the Sarcomere Length Non-uniformity Theory (SLNT). While the SLNT is currently the most popular explanation for RFE, its primary predictions have never been experimentally tested. In this research we performed experiments on rabbit psoas muscle myofibrils, comparing isometric contractions to isometric contractions following active stretch in order to examine the predictions of the SLNT. The results suggest that, while sarcomere length non-uniformities may play a role, the SLNT does not fully capture the mechanism of RFE.
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    A new paradigm for muscle contraction
    (Frontiers in Physiology, 2015-06-10) Herzog, Walter; Powers, Krysta; Johnston, Kaleena; Duvall, Mike
    Muscle contraction has fascinated lay people and scientists for centuries. However, a good understanding of how muscle contraction occurs seemed only possible once microscopy techniques had evolved to a level where basic structural features, such as the regular cross striation patterns of fibers, could be observed in the late 19th century. In the early 20th century, a stimulated muscle was simply considered a new elastic body (Gasser and Hill,1924). Shortening and work production took place with a fixed amount of energy tha twas stored in this body and evolved elastically through stimulation. However, this notion was proven false when Wallace Fenn demonstrated that muscle produced an increasing amount of total energy when increasing its mechanical work output; an observation that was in contradiction with Hill’s elastic body theory (Fenn, 1923,1924). Specifically, Fenn, who worked in the laboratory of Hill and measured heat and work production in frog muscles, found that a muscle allowed to shorten liberated more energy than a muscle held isometrically or a muscle that was stretched. This has become known as the Fenn effect in muscle physiology.
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    On sarcomere length stability during isometric contractions before and after active stretching
    (2019-11-27) Johnston, Kaleena; Moo, Engkuan; Jinha, Azim; Herzog, Walter
    Sarcomere length (SL) instability and SL non-uniformity have been used to explain fundamental properties of skeletal muscles, such as creep, force depression following active muscle shortening and residual force enhancement following active stretching of muscles. Regarding residual force enhancement, it has been argued that active muscle stretching causes SL instability, thereby increasing SL non-uniformity. However, we recently showed that SL non-uniformity is not increased by active muscle stretching, but it remains unclear if SL stability is affected by active stretching. Here, we used single myofibrils of rabbit psoas muscle and measured SL non-uniformity and SL instability during isometric contractions and for isometric contractions following active stretching at average SLs corresponding to the descending limb of the force-length relationship. We defined isometric contractions as contractions during which mean SL remained constant. SL instability was quantified by the rate of change of individual SLs over the course of steady-state isometric force and SL non-uniformity was defined as deviations of SLs from the mean SL at an instant of time. We found that whereas the mean SL remained constant during isometric contraction, by definition, individual SLs did not. SLs were more stable in the force-enhanced, isometric state following active stretching compared with the isometric reference state. We also found that SL instability was not correlated with the rate of change of SL non-uniformity. Also, SL non-uniformity was not different in the isometric and the post-stretch isometric contractions. We conclude that since SL is more stable but similarly non-uniform in the force-enhanced compared with the corresponding isometric reference contraction, it appears unlikely that either SL instability or SL non-uniformity contribute to the residual force enhancement property of skeletal muscle.