Many fitness experts recommend high-intensity interval training because it delivers impressive health benefits in a relatively short time.
These workouts alternate periods of very hard exercise with brief recovery breaks. HIIT can improve heart health, endurance, and metabolism, making it attractive for busy people who want effective workouts.
But not all HIIT workouts affect the body in the same way. According to new research published in ImmunoHorizons, running and cycling may produce very different immune responses, even when the workouts are equally challenging.
Researchers at the University of Delaware found that running HIIT triggered a stronger inflammatory response than cycling HIIT. The discovery could help athletes and everyday exercisers make smarter decisions about training and recovery.
Exercise places stress on the body, and some degree of stress is actually beneficial. When muscles are challenged, the body responds by repairing tissue and adapting to become stronger. This process involves the immune system, which releases signaling molecules that coordinate inflammation and recovery.
Scientists wanted to know whether different forms of HIIT create different patterns of immune activity. Although researchers have extensively studied endurance events such as marathons, much less attention has been given to the immune effects of HIIT workouts, despite their growing popularity.
To investigate, the research team recruited 26 healthy young adults. Participants completed a HIIT workout consisting of four high-intensity intervals lasting four minutes each, separated by three-minute recovery periods. Researchers carefully matched the running and cycling sessions for intensity, duration, and exercise cadence to ensure a fair comparison.
Blood samples were collected before exercise, immediately after the workout, one hour later, and 24 hours afterward. Scientists then analyzed the samples for cytokines, which are proteins that help regulate immune responses and inflammation.
One result stood out clearly. Running caused a significant increase in a cytokine called IL-8. Even a full day after the workout, IL-8 levels remained elevated. This response did not occur after cycling.
IL-8 is linked to the body’s response to small muscle injuries. During intense exercise, tiny tears develop in muscle fibers. The immune system responds by sending signals that help repair the damage. Higher IL-8 levels often indicate greater tissue stress and may contribute to delayed muscle soreness.
This finding helps explain a common experience among exercisers. People frequently report feeling more sore after intense running sessions than after cycling workouts. Running involves repeated impact forces as the feet strike the ground thousands of times. Cycling, on the other hand, supports body weight and reduces impact on muscles and joints.
The researchers also found encouraging similarities between the two exercise types. Both running and cycling increased levels of IL-6 immediately after exercise. This molecule helps regulate energy use during physical activity and may contribute to some of the health benefits associated with exercise.
Additionally, both forms of HIIT increased IL-10 levels after 24 hours. IL-10 is considered an anti-inflammatory cytokine because it helps shut down excessive inflammation and supports recovery. This suggests that both workouts ultimately produce beneficial immune responses.
The results indicate that cycling may offer a valuable alternative for people who want the health benefits of HIIT without as much muscle damage. This could be particularly useful for beginners, older adults, individuals carrying extra body weight, or those recovering from injuries.
One of the most important factors in long-term fitness success is consistency. Many people start exercise programs enthusiastically but stop because of soreness or discomfort. Lower-impact activities such as cycling may help reduce these barriers and encourage regular participation.
For athletes, the findings may also help guide training strategies. Coaches often look for ways to improve fitness while minimizing unnecessary stress. Incorporating more cycling-based HIIT sessions could provide cardiovascular benefits while reducing recovery demands.
The study has several strengths, including its direct comparison of running and cycling under controlled conditions and its detailed measurement of immune markers over time.
However, the research involved only young healthy adults, and the sample size was relatively small. Larger studies involving different age groups and fitness levels will be needed to confirm the findings.
The researchers plan to expand their work by examining additional HIIT protocols, resistance training programs, and other forms of exercise. They also hope to study older adults and people with health conditions to better understand how different populations respond.
Overall, the study suggests that both running and cycling HIIT can produce beneficial anti-inflammatory effects, but cycling appears to do so with less tissue damage and a milder inflammatory response. For people seeking an effective workout that may be easier on the body, cycling could be an attractive option.
The study was published in ImmunoHorizons.
Source: University of Delaware.
