(10) THE EFFECT OF A 12-WEEK HIIT PROTOCOL ON ISOKINETIC STRENGTH AND MUSCLE ACTIVATION IN OLDER MEN AND WOMEN

Purpose: The purpose of the current study was to evaluate the impact of high intensity interval training (HIIT) on lower body isokinetic muscle strength and muscle activation in older men and women. Methods: 15 older adult volunteers (Female: n=7, age=74.14±8.03 yrs, height=1.64±0.04 m, weight=74.82±13.38 kg; Male: n=8, Age=76.75±6.04, Height=1.77±0.07 m, Weight=84.94±14.55 kg) completed a 12-week total body HIIT program (resistance exercise, agility, circuit, and bike training) consisting of three sessions per week (35-40 min; nonconsecutive days). Work-to-rest ratios were increased from weeks 1-4 (20-sec:40-sec exercise to rest) to weeks 5-8 (30-sec:30-sec exercise to rest), to weeks 9-12 (40-sec:20-sec exercise to rest). Each participant was instructed to maintain 85-95% maximum heart rate throughout the study monitored by heart rate monitors. Isokinetic torque and muscle activation variables were assessed at Pre-, Mid- (week 6), and Post HIIT protocol. During the testing sessions, participants performed three maximal isokinetic knee extensions at 180°/s using a 90° range of motion. Surface electromyography (sEMG) of the vastus lateralis (VL) and rectus femoris (RF) were recorded and peak sEMG (pEMG; mV) was obtained for each contraction. The sEMG signal was normalized to pEMG and the rate of muscle activation (RER; % pEMG·s-1) was calculated as the highest 10 ms peak from the rectified, derivative signal after muscle activation onset for both RF (RFRER50) and VL (VLRER50). Participants were instructed to kick out “as hard and as fast as possible throughout the entire range of motion”. Peak isokinetic torque (pTQ) for each contraction was analyzed as the highest mean torque value for any 25ms epoch during the load range and averaged between contractions. Paired samples t-tests were conducted to examine pre- to post-intervention changes in peak torque and electromyographic (EMG) activity across multiple muscle-related variables. Results: The analysis revealed a non-significant increase in pTQ (t(9)=-1.76, p=0.101; Pre=103.46±27.70 Nm/s vs. Post=116.72±10.37 Nm/s), with a mean increase of 13.25 Nm (95% CI: -29.44, 2.93). VLpEMG activity showed no significant change (t(9)=-1.34, p=0.202; Pre=1.79±0.48 mV vs. Post=2.22±1.12 mV), with a mean increase of 0.43 mV (95% CI: -1.12, 0.26). However, significant increases were observed in RFRER50 (t(9)=-2.38, p=0.034; Pre=0.063±0.025 % pEMG·s-1 vs. Post=0.096±0.047 % pEMG·s-1) and RFpEMG (t(9)=-2.87, p=0.012; Pre=0.23±0.063 mV vs. Post=0.30±0.082 mV), with mean increases of 0.036 and 0.069, respectively. VLRER50 was not statistically significant (t(9)=-1.77, p=0.098, 95% CI: -0.041, 0.004; Pre=0.068±0.038 % pEMG·s-1 vs. Post=0.087±0.046 % pEMG·s-1). Conclusions: The data indicates increased neuromuscular activation following the intervention with confidence intervals suggesting a statistically reliable effect on rectus femoris EMG activity and activation (RFRER50). Although the VLRER50 was not significantly improved, the confidence interval suggests that the observed increase in neuromuscular recruitment may be meaningful but requires further investigation. Collectively, the study indicates potential improvements in neuromuscular function and muscle activation post-intervention. Practical Applications: Improved muscle activation patterns observed following the HIIT intervention may improve ability to complete activities of daily living. Acknowledgements: This study was funded by the Health Science Strategic Investment Fund – Creighton University