EXAMINING MOTOR UNIT POTENTIATION OF THE VASTUS LATERALIS DURING AN ISOMETRIC TRAPEZOID MUSLCE ACTION AT 50% OF MAXIMAL VOLUNTARY CONTRACTION

Jonathan D. Miller1, Omar M. Rehman1, Lauren M. Marquess1,Michael A. Trevino1, Eric M. Mosier1, and Trent J. Herda1. 1University of Kansas, Lawrence, KS; e-mail: [email protected] Analysis of motor unit action potential trains (MUAPT) during submaximal isometric trapezoid muscle actions in the first dorsal interosseous (FDI) and tibialis anterior (TA) muscles have indicated that motor unit (MU) firing rates are typically lower at derecruitment (DEREC) than at recruitment (REC). In addition, the MUs of these muscles tended to DEREC at higher forces than REC. This phenomenon is a result of increased MU twitch forces (potentiation). PURPOSE: The purpose of this study was to investigate potentiation in the VL. METHODS: Twenty-three healthy subjects (mean ± SD: age = 21±3 yrs; height = 174±10 cm; mass = 75±20 kg) participated in this study. Each subject completed a 22-s isometric trapezoid contraction of the leg extensors at 50% maximal voluntary contraction with electromyographic (EMG) signals recorded from the VL. The EMG signals were decomposed into individual MUAPTs. For each MU, 4 parameters were extracted from the firing rate data: (1) the REC threshold (expressed as a percentage of MVC [%MVC]), (2) the DEREC threshold (%MVC), (3) firing rate at REC (FRREC, pulses per second [pps]), and (4) firing rate at DEREC (FRDEREC, pps). Linear regressions were performed on the DEREC vs. REC relationships and the difference between FRREC and FRDEREC (FRDEREC – FRREC, [ΔFR]) vs. FRREC relationships for each subject. Slope, y-intercept, and r values were calculated for both relationships and each subject. RESULTS: For the DEREC vs. REC relationships, all relationships were significant (P \u3c 0.05, r = 0.57 – 0.99) and the mean (± SD) slope and y-intercept were 1.18 ± 0.43 DEREC/REC and -3.11 ± 10.9 DEREC, respectively. Thus, the lower threshold MUs were DEREC at lower forces than REC with the inverse being true for the highest threshold MUs. For the ΔFR vs. FRREC relationships, 15 of the 23 relationships were significant (R = 0.00 – -0.94) and the mean (± SD) slope and y-intercept were -0.80 ± 0.59 ΔFR/ FRREC and 7.67 ± 6.11 ΔFR, respectively. The majority of MUs decomposed in the present study had a positive ΔFR with this effect being more pronounced in higher threshold MUs. CONCLUSIONS: In the present study, MUs were reported to have lower forces at DEREC than at REC and have slightly higher firing rates at DEREC than REC. Thus, potentiation was limited at DEREC for the VL unlike for the FDI and TA muscles

THE EFFECTS OF PASSIVE STRETCHING PLUS VIBRATION ON VOLUNTARY INACTIVATION AND PEAK TORQUE OF THE PLANTAR FLEXORS AT A SHORT AND LONG MUSCLE LENGTH

Omar M. Rehman1, Jonathan D. Miller1, Lauren M. Marquess1, Jeremy D. Lippman1, Eric M. Mosier1, Michael A. Trevino1, Trent J. Herda1 1University of Kansas, Lawrence, Kansas; e-mail: [email protected] PURPOSE: Passive stretching (PS) has been reported to reduce peak torque (PT) due to increases in percent voluntary inactivation (%VI) and mechanical mechanisms. Conversely, prolonged vibration (VIB) solely reduces PT as a result of %VI. It remains unclear the influence PS may have on PT and %VI at short and long muscle lengths. Therefore, this study examined the effects of PS, in conjunction with VIB, on PT and %VI of the plantar flexors (PF) at a short and long muscle length. METHODS: 14 healthy men (Age = 21.9 ± 3.5 yrs) volunteered for this study. Subjects completed 1 familiarization and 2 randomized visits (control [CON] and VIB). The CON visit consisted of 8, 30-s PS of the PF, whereas the VIB visit consisted of PS with VIB during the post-testing. VIB was applied to the Achilles tendon and 55 Hz 2-m before post-testing and continued during the remainder of testing. Subjects completed maximal voluntary contractions (MVC) of the right PF at an ankle joint of 70° plantarflexion (short muscle length [PF]) and 105° dorsiflexion (long muscle length, [DF]) in random order pre- and post-PS. During MVCs, an evoked doublet stimulus was applied to the tibial nerve to calculate %VI. PT was determined from a 0.25-s epoch during the MVC (Nm) prior to stimulation. Two separate 3-way ANOVAs (treatment [CON vs. VIB] x time [Pre vs. Post] x muscle length [PF vs. DF]) were used to examine differences in PT and %VI. RESULTS: For %VI, there were no significant 3- or 2-way interactions (p \u3e 0.05). There were significant main effects for time (p = 0.003) and length (p = 0.007). %VI was greater post- (10.2 ± 9.7%) than pre-PS (5.5 ± 5.5%) and at the DF (10.9 ± 8.7%) than PF (4.8 ± 7.7%). For PT, there was no 3-way interaction. There was a significant 2-way interaction (time x treatment; p = 0.008). PT decreased pre- to post-CON (p = 0.016, pre = 109.8 ± 27.9 Nm, post = 101.7 ± 28.0 Nm) and -VIB (p \u3c 0.001, pre = 113.3 ± 33.8 Nm, post = 92.8 ± 31.9 Nm). In addition, PT was greater at the DF (122.4 ± 37.8 Nm) than PF (86.4 ± 20.9 Nm). CONCLUSION: No further decreases in PT or increases %VI occurred with VIB following PS and, therefore, suggested that strength losses following PS is primarily neural. In addition, muscle activation deficiencies were present at the longer muscle length despite greater PT in comparison to the shorter muscle length

ACUTE EFFECTS OF PASSIVE STRETCHING ON THE ELECTROMECHANICAL DELAY AT SHORT AND LONG MUSCLE LENGTHS

Lauren M. Marquess1, Omar M. Rehman1, Jonathan D. Miller1, Jeremy Lippman1, Eric M. Mosier1, Michael A. Trevino1, and Trent J. Herda1; e-mail: [email protected]; 1University of Kansas, Lawrence, Kansas Electromechanical delay (EMD) is a measure of the time lag between the initial electrical stimulus and the onset of torque production. The EMD has been reported to increase following 20 mins of passive stretching (PS) of the plantarflexors at a 90° degree joint angle (neutral), which is believed to be the result of a decrease in muscle stiffness. However, it is unknown the influence that PS of the plantarflexors has on the EMD at short and long muscle lengths. PURPOSE: The purpose of this study was to examine the effects of 4 minutes of PS on the EMD at a short and long muscle length before (pre-) and after (post-) PS. METHODS: Thirteen healthy men (age = 21.54 ± 2.67 yrs;) performed the EMD assessments before and after 8 passive stretches that lasted 30 seconds each for a total of 4 minutes of PS. To measure EMD (ms), a single electrical stimulus was applied to the tibial nerve. The resulting twitch torque was measured with the torque signal from the isokinetic dynamometer. An electromyographic (EMG) sensor was placed on the soleus to measure the M-wave from the electrical stimulus. The short muscle length was 20° plantarflexion (PF) from neutral, whereas, the long muscle length was 15° dorsisflexion (DF) from neutral. The difference in time from the start of the M-wave to the onset of torque production was calculated manually by an experienced investigator (LM) with a custom written LabVIEW (v 11) software program. For the statistical analysis, a 2-way repeated-measures ANOVA (time [pre-PS vs. post-PS] x muscle length [PF vs. DF]) was used to analyze possible differences in the EMD as a result of PS. RESULTS: There was no 2-way interaction (p = 0.089) and no main effect for time (p = 0.239), however, there was a main effect for length (p \u3c 0.001). The EMD for the long muscle length (6.64 ± 0.94 ms) was greater than the short muscle length (4.67 ± 1.07 ms). There were not significant differences in EMD pre- to post-PS. CONCLUSION: The lack of a signicant increase in EMG following PS may be the result of a relatively short duration of PS (i.e., 20 vs 4 mins). In addition, the transmission of force at DF is an longer absolute distance than PF and, therefore, EMD duration was greater