Many people assume that if their heart rate is elevated for long periods during an activity, they must be getting a strong “cardio” benefit.
This is especially common in sports and mixed-modal activities like:
• Brazilian Jiu-Jitsu (BJJ)
• CrossFit-style WODs
• Bootcamp classes
• Martial arts sparring
• Circuit-based functional training
These sessions can feel brutally hard. Heart rate is high, breathing is heavy, and fatigue is real. But high heart rate alone does not guarantee the same aerobic adaptations you’d get from steady, cyclic cardio.
How your heart rate gets elevated matters just as much as how high it goes.
Cyclic Cardio vs Mixed-Modal Effort
Traditional aerobic training — running, cycling, rowing, swimming — is cyclic and rhythmic.
Muscle contractions are:
• Repetitive
• Alternating
• Continuous
This allows blood flow and oxygen delivery to stay relatively consistent. The cardiovascular system adapts by improving:
• Stroke volume
• Mitochondrial density
• Capillary networks
• Overall aerobic efficiency
The heart gets better at delivering oxygen during sustained, steady demand.
Mixed-modal sports are different.
Heart Rate Is High — But for Different Reasons
In sports like BJJ or circuit-style training, heart rate often rises because of:
• Isometric muscle contractions (holding positions)
• Sudden bursts of force
• Short, intense efforts
• Pressure spikes from bracing and straining
• Emotional and competitive arousal
These factors increase heart rate, but not primarily through sustained aerobic demand. Instead, they involve:
• Increased blood pressure
• Mechanical compression of blood vessels
• Stress hormone surges
• Intermittent, uneven oxygen demand
So the cardiovascular response is driven by mixed stress signals, not smooth, rhythmic oxygen use.
Blood Flow Is Often Restricted
Many mixed-modal movements involve gripping, bracing, or holding tension.
This can:
• Partially restrict blood flow to working muscles
• Increase pressure load on the heart
• Limit continuous oxygen delivery
Because blood flow is repeatedly interrupted, the body cannot maintain the same steady-state aerobic conditions that drive classic endurance adaptations.
Even if heart rate looks similar on a monitor, the internal environment is different.
The Nervous System Plays a Bigger Role
Combat sports and high-skill activities add a strong nervous system component.
Adrenaline, focus, and emotional arousal elevate heart rate through sympathetic activation. This contributes to cardiovascular strain, but again, not in the same way as sustained aerobic work.
The heart is beating faster partly because the brain is in a heightened state, not only because muscles are demanding oxygen in a steady pattern.
Adaptations Tend to Be Mixed, Not Purely Aerobic
Mixed-modal training improves many valuable qualities:
• Muscular endurance
• Coordination
• Anaerobic capacity
• Skill under fatigue
• Resilience to intermittent effort
But it often produces less improvement in:
• Pure aerobic efficiency
• Stroke volume at submaximal effort
• Mitochondrial development
These adaptations are best stimulated by longer, steady efforts where heart rate is elevated due to continuous oxygen demand rather than intermittent tension and stress.
Why This Matters for Recovery and HRV
Because mixed-modal sessions include strength, anaerobic bursts, and nervous system stress, their recovery cost often resembles strength or high-intensity training more than easy cardio.
If someone relies only on these sessions for “cardio,” they may:
• Feel very fit
• Have high workout heart rates
• Still lack a strong aerobic base
That can show up as:
• Slower recovery between sessions
• More HRV volatility
• Difficulty maintaining low heart rates during steady efforts
The Big Takeaway
Activities like BJJ, CrossFit-style circuits, and bootcamps can drive heart rate high, but the reasons behind that elevation differ from steady aerobic exercise. Because blood flow, muscle contraction patterns, and nervous system stress are different, the aerobic adaptations are not the same — even at similar heart rates.
High heart rate does not automatically equal strong aerobic development.