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HIIT - High-Intensity
Intermittent Training


Powerlifting Bench Press

Impact of Exercise Intensity on Body Fatness and Skeletal Muscle Metabolism

Tremblay et al (1) compared the effects  of a 15-week high-intensity, intermittent training (HIIT)  protocol and a 20-week endurance-training (ET) protocol  on body fat and skeletal muscle enzyme activity.  8  men and 9 women participated in ET, while 5 men and 5  women participated in HIIT.  Training was performed  on a cycle ergometer.  ET involved 30-45 minutes of  continuous cycling at 60-85% max HR reserve 4-5 times a  week.  HIIT involved bouts of short-interval work  and long-interval work;  short-interval work  consisted of 10-15 bouts of 15-30 seconds of cycling at  60% maximum 10-second power output.  Long-interval  work consisted of 4-5 bouts of 60-90 seconds of cycling  at 70% maximum 90-second work output.  Recovery  between bouts was sufficient to allow HR to return to 120-130  bpm.  Intensity was increased in both the short-interval  and long-interval protocols by 5% every 3 weeks.  Total  energy cost of the training sessions was estimated by  referring to VO2 max and maximal work-output measurements.   Body fat was measured before and after the ET and  HIIT protocols by the sum of 6 skinfolds.  Enzyme  activity of hexokinase (HK), phosphofructokinase (PFK),  malate dehydrogenase (MDH), and 3-hydroxyacyl coenzyme A  dehydrogenase (HADH) were measured before and after the  training protocols using muscle biopsies.  VO2 max  was measured before and after training using a  progressive cycle ergometer test to exhaustion, and  maximal cycle work output for 10, 30, and 90 seconds were  also measured.

FINDINGS:  Mean estimated total  energy cost of the ET program was 120.4 MJ, while that of  the HIIT program was 57.9 MJ.  Despite the lower  energy cost of exercise, subjects engaged in HIIT tended  to have greater reductions in skinfolds than the ET group.   When expressed on a per MJ basis (change in  skinfolds divided by energy cost of training program),  the HIIT group's reduction in body fat was 9 times higher  than the ET group (p < .01).

Both protocols resulted in significant  increases in MDH activity, while the HIIT program  significantly increased HK activity while the ET program  significantly reduced HK activity.  The HIIT program  also resulted in significant increases in PFK and HADH  activity, while the ET program did not significantly  change these measures.

Both programs resulted in a significant  increase in VO2 max, and the HIIT program resulted in a  significant increase in maximal cycle work output for 10,  30, and 90 seconds.

IMPLICATIONS:  This study gives more  evidence to support the idea that high-intensity interval  training is a more optimal program for fat loss than  moderate-intensity, continuous endurance exercise.  The  HIIT group in this study lost nine times more body fat  than the ET group for each MJ of energy expended during  training, indicating HIIT is much more efficient and  effective.  More evidence of the effectiveness of  the HIIT program is evident in the significant greater  increase in HADH activity as compared to the ET program;   HADH is a marker of the activity of beta oxidation.   Higher HADH activity means more body fat is being  used as a fuel source during rest.  This shows that  the reason why HIIT is so effective in reducing body fat  is the increase in fat metabolism during the rest periods  between training sessions;  basically, you lose more  fat while sitting around on the couch.

The increase in VO2 max seen with both  protocols indicates that both HIIT and ET can be an  effective methods for increasing VO2 max.  An  increase in VO2 max means the body can consume more  oxygen to produce energy and thus perform more work,  increasing performance on the track, the road, or other  athletic endeavor where oxygen uptake plays a major role  in performance.

1.  Tremblay, A., J. Simoneau, and C.  Bouchard.  Impact of Exercise Intensity on Body  Fatness and Skeletal Muscle Metabolism.  Metabolism.   43(7):814-818.  1994.

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