Running 40 kilometres this week sounds impressive. But if 35 of those kilometres were easy jogs at a heart rate barely above walking pace, your actual training stimulus was modest. Training volume (distance, time, reps) tells you what you did. Training load tells you how your body responded. Confusing the two is one of the most common reasons recreational athletes plateau — or get injured.
What Is the Difference Between Volume and Load?
Training volume is an external metric. It counts the output: kilometres run, minutes cycled, sets completed. It’s easy to measure and satisfying to track. But volume alone cannot tell you whether a session pushed your body toward adaptation or barely registered.
Training load is an internal metric. It measures the physiological cost of exercise — how hard your cardiovascular, muscular, and nervous systems actually worked. A landmark review in the British Journal of Sports Medicine confirmed that it is internal load, not external load, that drives the adaptations to training.
Two examples make this clear. A 10km run in cool weather at a comfortable pace might produce a moderate TRIMP score of 80. That same 10km in 35°C heat with humidity could push your heart rate 15 beats higher for the same pace, producing a TRIMP of 130. The volume was identical. The load — and the recovery needed afterward — was vastly different.
Similarly, a 45-minute strength session of heavy compounds at high effort produces a very different internal load than 45 minutes of light machine work, even though both log the same duration.
Why Does Internal Load Matter More for Results?
Your body adapts to stress, not to distance. When internal load is consistently too low, adaptation stalls. When it spikes too high relative to what you’ve built up over weeks, injury risk climbs sharply — this is precisely what the acute-to-chronic workload ratio monitors.
Research by Impellizzeri and colleagues argues that the relationship between external and internal load isn’t fixed. Factors like sleep quality, hydration, heat, altitude, stress, and accumulated fatigue all modulate how your body processes the same external work. On a day when your HRV is suppressed and your recovery score is low, the same workout creates a disproportionately higher internal load.
This is why elite coaches prescribe training by intensity targets (heart rate zones, power zones, RPE) rather than fixed volumes. The session adapts to the athlete’s readiness on that specific day.
How Can You Track Training Load Passively?
The simplest and most validated method is heart rate-based training load — specifically, TRIMP (Training Impulse). Any workout recorded with a heart rate monitor can produce a TRIMP score that quantifies internal load in a single number.
ScoreVitals calculates this automatically for every workout that syncs through Apple Health or Google Health Connect. Each session receives a score based on Banister’s TRIMP methodology, factoring in duration, heart rate intensity relative to your personal Karvonen zones, and recovery metrics.
Over weeks, your cumulative training load trend reveals whether you’re building fitness progressively or swinging between under-training and dangerous spikes.
Stop counting kilometres. Start measuring what those kilometres cost your body. Download ScoreVitals to see the internal load behind every workout.