External Load Volume and Variability Differentially Relate to Internal Load in Soccer

Imagine you are a coach trying to figure out how hard your athletes are really working. You have two ways to look at the game. First, there is the External Load: this is the objective, physical stuff you can measure with a stopwatch or a GPS watch. It's the total distance run, the number of times a player speeds up, or how many times they slam on the brakes. It's like counting the miles on a car's odometer. Second, there is the Internal Load: this is how the athlete's body and mind actually feel that work. It's the heart rate pumping, the sweat dripping, and the feeling of exhaustion when they say, "That was tough!" This is like the engine temperature gauge or the driver's feeling of fatigue.

For a long time, scientists and coaches have wondered: Does the total distance run perfectly match how tired the player feels? Or does the way the work is spread out matter more? For instance, is running 10 kilometers in one straight line just as tiring as running 10 kilometers with lots of stops, starts, and sharp turns? This question is crucial because if we get it wrong, we might push players too hard and get them injured, or not push them enough to get better. This study dives into that mystery, specifically looking at soccer players to see if the "shape" of their workload (how consistent or changeable it is) changes how their bodies and minds react.

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The Story of the Soccer Players and Their GPS Watches

In this study, researchers followed 35 semi-professional male soccer players over two summer seasons. They strapped these athletes with high-tech GPS watches that tracked every step, every sprint, and every heartbeat. The goal was to see how different ways of measuring the "External Load" (the physical work) connected to the "Internal Load" (the feeling of effort and the heart's reaction).

The team looked at the data in three different ways, like looking at a movie through three different lenses:

1. The Total Volume: How much total work was done in a week? (Think of this as the total miles driven).
2. The Monotony: Was the work the same every day, or did it vary? (Think of this as driving the exact same route every day vs. taking a different road each time).
3. The Percent Change: Did the workload jump up or drop down suddenly from one week to the next? (Think of this as suddenly hitting the gas pedal hard after a slow week).

They compared these three lenses against two types of "Internal Load" results:

• sRPE (Session Rating of Perceived Exertion): This is the player's own rating of how hard the session felt, multiplied by how long it lasted. It's the "feeling" score.
• TRIMP (Training Impulse): This is a math formula based on heart rate that calculates the physiological stress on the body. It's the "heart" score.

What They Found: The "Feeling" vs. The "Heart"

The results were surprisingly specific, showing that the "feeling" score and the "heart" score don't always react to the same things.

1. The "Feeling" Score (sRPE) Loves Volume and Sudden Changes
When the researchers looked at what made the players feel tired (sRPE), the biggest driver was simply Total Distance. The more kilometers a player ran in a week, the higher their "feeling" score went. It's like a car's fuel gauge: the more miles you drive, the lower the gas gets, regardless of how you drove.

However, the "feeling" score also reacted strongly to sudden changes. If a player had a week where they ran significantly more distance than the week before, their "feeling" score spiked. It's as if the body says, "Whoa, we just jumped up a gear!" Additionally, the "feeling" score went up if the players did a lot of decelerations (braking) and high-speed running in a very consistent, unchanging way throughout the week. If the workload was boringly the same every day (high monotony), the players reported feeling more exerted.

2. The "Heart" Score (TRIMP) Loves Distance and Braking Consistency
The "heart" score (TRIMP) was a bit different. It was definitely linked to Total Distance, just like the feeling score. But here is the twist: the "heart" score was not sensitive to sudden week-to-week changes. The heart didn't care as much about the jump in workload as the brain did.

Instead, the "heart" score cared deeply about braking. The study found that the number of times a player had to slow down (decelerate) and the consistency of those braking actions were huge factors. If a player had a week full of braking actions that were spread out evenly (consistent monotony), their heart rate stress went up. It seems the heart gets tired from the repeated mechanical stress of stopping and starting, even if the total distance isn't massive. Interestingly, the "heart" score didn't seem to care much about how many times players sped up (accelerations) or how much they ran at high speeds, unless it was part of that total distance.

What This Means for the Game

The paper suggests that we can't just look at one number to know how hard an athlete is working.

• If you want to know how mentally and physically drained a player feels, look at their total distance and watch out for sudden jumps in their weekly workload.
• If you want to know the physiological stress on their heart, look at their total distance but also pay close attention to how many times they braked and how consistent that braking was throughout the week.

The researchers noted that in this specific group of players, the workload was fairly stable week-to-week, which is why the "sudden change" factor wasn't as dominant for the heart as it was for the feeling. They also admitted that because they only studied one team, we can't be 100% sure this applies to every soccer player everywhere. However, the data clearly shows that deceleration (braking) is a unique player in the game: it's the only action that showed up as important for both the feeling of tiredness and the stress on the heart.

In short, running far makes you feel tired and strains your heart, but braking consistently makes your heart work harder and makes you feel like the week was a grind, especially if you don't get a break from it. Coaches need to track both the miles and the stops to keep their players healthy and performing at their best.