Tracking Strength Training: What a Fitness Tracker Can – and Can't – Do

Anyone in the weight room usually has two devices: a smartphone for the training app and a wearable on the wrist. Both collect data – but not all of it is equally useful. A fitness tracker that tells you "412 kcal" and "heart rate 128" after leg day says little about whether you are getting stronger. What it does very well, however, is at the bottom of most rankings: showing how well you recover.

This article explains what you should really measure in strength training – and where technology helps. It belongs to the Muscle Building & Strength topic. As we make a tracker ourselves, the LEEK Pulse Pro, this article is marked as a disclosure piece; we name the limits of our product just as clearly as those of others.

Two levels: performance and recovery

Progress in strength training comes from two things: a rising training stimulus and sufficient recovery. Both can be measured – but with different tools.

The most common mix-up: people expect the tracker to measure performance and the log to do nothing. It is the other way round.

Level 1: performance – the training log

Only one thing shows you whether you are getting stronger: whether the numbers on the same exercise rise over weeks. After every working set, record:

• Exercise (always the same name, so comparisons are possible)
• Weight
• Reps
• Reserve – how many more clean reps would have been possible. How to estimate this is explained in RIR and RPE.
• Note if anything stands out: little sleep, pain, new technique.

From this data you can see immediately whether your progression is working. The system behind it is described in Progressive overload; how to keep a useful log in Keeping a training log.

What the tracker can (not) do when lifting

Optical heart rate sensors measure changes in blood flow under the skin. In strength training they reach their limits:

• Gripping and wrist flexion change blood flow at the sensor.
• Breath-holding and short, maximal effort make heart rate jump up and down.
• Arm movement shifts the sensor.

The result: heart rate curves in strength training are often delayed or inaccurate on the wrist. If you really want to know your heart rate during training, use a chest strap – more in Chest strap or wrist? and How accurately do fitness trackers measure heart rate?.

Automatic rep counting on some watches also works only moderately for compound lifts. And calorie burn in strength training is a rough estimate, because heart rate is only loosely related to energy use here.

Level 2: recovery – where the tracker shines

The most valuable data for strength athletes is generated not in the gym but at night:

• Sleep duration: lack of sleep measurably slows strength gains and muscle growth.
• Resting heart rate: a resting heart rate elevated over several days suggests incomplete recovery, stress or an incipient infection.
• Heart rate variability (HRV): if it drops below your normal range over several days, that is a signal to dose your load. Basics in Understanding HRV.

Together, these values form a picture that complements how you feel: are you ready for heavy sets today – or is a technique session wiser?

How to use recovery data in strength training

Important: this traffic light is a decision aid. How you feel and your performance in the warm-up are part of it. How to plan deloads with data specifically is explained in Deload with data: reading recovery values correctly.

Example: a week with data

Lena trains four times a week on an upper/lower split. Heavy lower-body training is planned for Tuesday. After a short night, her recovery score is clearly below her normal range and her resting heart rate five beats higher than usual. She swaps: moderate upper body on Tuesday, heavy legs on Wednesday, when her values are back in the normal range. Result: on Wednesday she manages one more squat rep than the previous week.

This simple rule – put the hardest session on the best day – brings many trainees more than any other use of data. Described in detail in Training with data.

Where the LEEK Pulse Pro fits – and where it doesn't

A good fit if you:

• do strength training and want to manage your recovery,
• want to wear your band at night without charging it every day,
• don't want a subscription or a cloud for your health data,
• use an Android smartphone.

A poorer fit if you:

• want live values and notifications on your wrist – the band deliberately has no display,
• want to record GPS routes when running or cycling,
• use an iPhone – the app is currently available for Android,
• need an exact heart rate in every set – a chest strap is the better choice for that.

All details are in LEEK Pulse Pro in detail, the comparison with other device classes in the big comparison.

Common tracking mistakes

Calorie values as a training goal. "600 kcal burned" is not a meaningful goal in strength training. Weight and reps are.

Overrating individual daily values. Recovery values fluctuate. Trends over three to seven days are what matter.

Taking the band off at night. Without overnight data, the most valuable information is missing.

Not recording performance. A tracker doesn't replace a training log.

Conclusion: two tools, one strategy

You don't track strength training with a single device but with a simple system: the training log shows whether you are getting stronger. The tracker shows whether you are recovering enough to keep getting stronger. If you bring both together, you won't necessarily train more, but you will train smarter. How to plan training in the long term is covered in Periodisation explained simply.