Influence of fascicle length on twitch potentiation of the medial gastrocnemius across three ankle angles
Length dependence of post-activation potentiation (PAP) is a well-established phenomenon in animal models but less certain in intact whole human muscles. Recent advances in B-mode ultrasonography provide real-time imaging and evaluation of human muscle fascicles in vivo, thus removing the assumption that joint positioning alters fascicle length and influences the extent of PAP. The purpose of this study was to determine whether a conditioning maximal voluntary contraction (MVC) would influence the return of medial gastrocnemius (MG) fascicles to baseline length and alter the extent of twitch potentiation between three ankle positions.
Ultrasonography was used to measure MG fascicle length for baseline and potentiated twitches at angles of 10° dorsiflexion (DF), 0° neutral (NEU—tibia perpendicular to the sole of the foot), and 20° plantar flexion (PF). A MVC was used as a conditioning contraction and PAP determined for each ankle angle.
PAP of the plantar flexors was greater in PF (28.8 ± 2.6%) compared to NEU (19.8 ± 1.8%; p < 0.05) and DF (9.3 ± 2.8%; p < 0.0001). In PF, fascicle lengths (4.64 ± 0.17 cm) were shorter than both NEU (5.78 ± 0.15 cm; p < 0.0001) and DF (6.09 ± 0.15 cm; p < 0.0001). Fascicle lengths for the baseline twitches were longer (5.92 ± 0.11 cm) than the potentiated twitches (5.83 ± 0.10 cm; p < 0.01) at all joint angles.
Although PAP is greatest in PF compared to NEU and DF, the higher PAP in the PF joint angle cannot be attributed to fascicles remaining shortened following the MVC because across all joint positions, fascicles are similarly shortened following the MVC.
KeywordsUltrasound Maximal voluntary contraction Fascicle length Post-activation potentiation
Analysis of variance
- Avg. fall
- Avg. rise
Half relaxation time
Maximal voluntary contraction
Time to peak tension
The authors would like to acknowledge Noelannah Neubauer for her contribution to the subset of soleus data presented in this manuscript, and to the anonymous reviewers for their detailed feedback that assisted in improving the paper.
JMJ, RRS and SLK conceived and designed the study; SLK, RRS and AF collected and analyzed the data; JMJ, SLK, RRS and CLS interpreted the results and drafted the manuscript; JMJ edited the manuscript; JMJ, SLK and CLS edited and addressed reviewer comments; JMJ approved the final version.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- Sweeney HL, Stull JT (1990) Alteration of cross-bridge kinetics by myosin light chain phosphorylation in rabbit skeletal muscle: implications for regulation of actin-myosin interaction. Proc Natl Acad Sci USA 87:414–418. https://doi.org/10.1073/pnas.87.1.414 CrossRefPubMedCentralPubMedGoogle Scholar