High-intensity interval training, often called HIIT, has become one of the most popular forms of exercise in recent years. It involves short bursts of hard effort followed by brief recovery periods.
Many people choose HIIT because it can improve fitness, burn calories, and strengthen the heart in a relatively short amount of time. However, not all HIIT workouts affect the body in the same way.
A new study published in ImmunoHorizons has found that running-based HIIT causes a stronger inflammatory response than cycling-based HIIT, even when both workouts are matched for intensity, duration, and effort. The findings could help people choose exercise programs that provide health benefits while reducing soreness and tissue damage.
Researchers from the University of Delaware wanted to better understand how different types of HIIT affect the immune system. While scientists have spent decades studying the effects of long-distance running events such as marathons, much less is known about how HIIT influences inflammation and recovery.
Inflammation is a normal part of the body’s response to exercise. When people exercise, especially at high intensity, small amounts of damage occur in muscles and tissues.
The immune system responds to repair this damage, helping muscles adapt and become stronger. However, excessive inflammation can increase soreness, slow recovery, and make it harder for people to stick with an exercise program.
The research team recruited 26 healthy young adults. Each participant completed a HIIT workout that included four rounds of four minutes of high-intensity exercise followed by three-minute recovery periods. The workouts were carefully designed so that running and cycling sessions required similar levels of effort.
To understand how the body responded, researchers collected blood samples before exercise, immediately afterward, one hour later, and again 24 hours after the workout. They then measured several immune system signals known as cytokines. These molecules help regulate inflammation and recovery throughout the body.
One of the most important findings involved a cytokine called IL-8. Researchers found that IL-8 levels increased after running and remained elevated even 24 hours later. In contrast, cycling did not produce the same increase.
IL-8 is often released when the body responds to small muscle injuries caused by exercise. Higher levels may indicate more tissue damage and may help explain why people often feel more sore after running than after cycling. Because running involves repeated impact with the ground, it places greater stress on muscles, joints, and connective tissues than cycling, which is a lower-impact activity.
The researchers also examined other important inflammatory markers. Both running and cycling increased IL-6 immediately after exercise. Although IL-6 is often associated with inflammation, it also plays a positive role during exercise by helping the body manage energy and triggering beneficial biological responses.
Another cytokine called IL-10 increased about 24 hours after both types of exercise. IL-10 acts as a natural anti-inflammatory signal that helps calm the immune response and prevent excessive tissue damage. This suggests that both forms of HIIT eventually activate processes that support recovery and long-term health benefits.
The results indicate that cycling may provide many of the same beneficial anti-inflammatory effects as running while causing less short-term tissue stress. This could be especially important for people who are new to exercise, returning after a break, recovering from injury, or trying to maintain a consistent fitness routine.
Exercise adherence is one of the biggest challenges facing many people. Muscle soreness can discourage beginners from continuing their workouts. If a lower-impact activity such as cycling can provide similar health benefits with less discomfort, it may help more people stay active over the long term.
The findings may also be useful for recreational athletes who want to balance performance and recovery. Many athletes incorporate HIIT into their training programs because it improves cardiovascular fitness efficiently.
Choosing cycling-based HIIT sessions during certain training periods may allow athletes to gain fitness benefits while reducing accumulated stress on the body.
One of the strengths of this study is that the researchers directly compared running and cycling under carefully controlled conditions. However, the study included only a small number of healthy young adults, so the results may not apply to older adults, elite athletes, or people with medical conditions.
The researchers plan to continue investigating how different forms of HIIT affect inflammation and recovery. Future studies will examine other exercise styles, including resistance training, and will involve different age groups and clinical populations.
Overall, the findings suggest that while both running and cycling HIIT offer valuable health benefits, cycling may provide a gentler way to achieve similar anti-inflammatory effects with less muscle damage and soreness. For many people, that could make staying active easier and more enjoyable.
The study was published in ImmunoHorizons.
Source: University of Delaware.
