—Dr Simmons raises two important questions: (1) Can contracting muscles develop force without shortening? (2) What is the precise mechanism for the increase in BP observed during isometric exercise?Before answering the first question, I want to make clear that the term shortening as used herein refers to the external length of the muscle because even isometric contraction involves internal shortening of the contractile element with simultaneous elongation of the stretched series elastic element.1 With this clarification in mind, the answer to the first question is yes. Muscles can develop force without external shortening.Consider the case of the isometric handgrip test. When the patient responds to our signal to begin exercise, two phases follow which will be analyzed in detail. The first phase is the short interval from the onset of contraction until the needle of the dynamometer reaches the desired level of relative tension. During
Stefadouros MA. Isometric Exercise Testing-Reply. Arch Intern Med. 1983;143(8):1632–1633. doi:10.1001/archinte.1983.00350080153040
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