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Thesis Format

Integrated Article

Degree

Master of Science

Program

Kinesiology

Supervisor

Dr. Charles L Rice

Abstract

Post-activation potentiation (PAP) is the acute enhancement of contractile properties following a short duration (<10s) high intensity contraction. Compared with isometric contractions, little is known about the PAP response induced by concentric conditioning contractions (CCs) and the effect of contractile speeds. In the dorsiflexors of 10 participants, twitch responses were measured following 5s of maximal effort concentric CCs at each of 10, 20 and 50°/s. Concentric PAP responses were compared to a maximal isometric voluntary contraction (MVC) matched for contraction time. Additionally, concentric CCs were compared to isometric CCs matched for mean torque, contraction area and time. The principal finding was that the PAP response following maximal concentric CCs was independent of contractile speed and, there was no difference in the PAP response between concentric CCs and an isometric MVC. Maximal contractions, regardless of contraction modality, likely produce sufficient Ca2+ to induce a full PAP response, and thus there was no difference between speeds or contraction type. Concentric CCs had significantly larger peak twitch torque than their isometric torque matches (49-58%), and faster maximal rates of torque development at the three speeds (62-77%). However, these responses are likely related to greater muscle activation (EMG), and not contraction modality per se. Thus, PAP responses following maximal concentric CCs are not affected by velocity and responses are not different from an isometric MVC. This indicates maximal CCs produce a full PAP response independent of contraction type (isometric vs concentric) or shortening velocity.

Summary for Lay Audience

Following repetitive high force contractions muscles can experience fatigue. However, following a high force contraction of short duration <10s muscle force can become enhanced for a short period ~30s-10min. This enhanced force response is called post-activation potentiation (PAP), and the PAP response is related to the force level of the previous contraction. PAP is a biochemical “warm-up” in which the previous contraction primes myosin, which is a key protein involved in muscle contraction. The contraction which creates this PAP response is well understood during an isometric contraction (no joint movement). However, isometric contractions have less applicability to daily dynamic movements. Thus, the purpose of the present study was to investigate the PAP response after concentric (muscle shortening) contractions at three joint speeds (10,20 and 50°/s) and compare the response to isometric contractions. In 10 participants (7 males and 3 females) the PAP response was measured after isometric and concentric contractions (10,20 and 50°/s) of the ankle joint muscles on the front of the leg. Following maximal isometric (no joint movement) and maximal concentric (muscle shortening) contractions at 10, 20 and 50°/s there was no difference in the PAP response. Because the force of the contraction to produce PAP is a determining factor, force between isometric and concentric contractions were matched. When matched for force, concentric contractions at all speeds produced a greater PAP response than isometric contractions. Additionally, there were no differences in the PAP response between concentric speeds. These findings show concentric contractions at slow and moderate speeds produce a similar PAP response as a maximal isometric contraction. However, when the force production was matched between contraction types (concentric and isometric) concentric contractions produced larger PAP responses regardless of the speed of contraction.

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