A Gym vs Bike Training Comparison Coach’s Corner with Nick Kleban

Whether it’s off-season gains, correcting muscular imbalances, or mid-season maintenance, most cyclists will find themselves in the gym at times, incorporating a strength training element into their routine. We know it’s good for us, and we know it’s stressing the muscle groups we use on the bike, as we feel the fatigue on the post-gym session rides, but how much is it taking out of us exactly? 4iiii athlete and cycling coach, Nick Kleban, has shared a recent blog he penned, in which he discusses whether or not the efforts made in the weight room and those made on the bike can be equally quantified, using familiar cycling metrics like “TSS”. Don’t worry, we’ve broken down the array of acronyms thrown at you! Give it a read and see what this smart and speedy multidisciplinary racer has to say.

What Is TSS

Training Stress Score (TSS) is designed to estimate the internal metabolic load an athlete experiences during an endurance workout. As we cannot directly measure oxygen consumption, glycogen depletion, or metabolic disturbances during day-to-day training, TSS uses external data such as power, speed, or heart rate to approximate how hard the aerobic system is working.

The formula is: TSS = (Seconds × NP × IF) / (FTP × 3600) × 100

Duration (Seconds): The total time the aerobic system was active.

Normalized Power (NP): Estimates the metabolic cost of fluctuating intensities by weighting harder efforts more heavily.

Intensity Factor (IF = NP/FTP): Puts the metabolic cost in context by showing how close the athlete was working to their sustainable limit.

Functional Threshold Power (FTP): The highest power (in watts/W) a cyclist can sustain for about an hour, as a key metric for training, fitness assessment, and pacing

Combined, these variables produce a single value that reflects the accumulated aerobic stress of the session. The structure of the formula makes the purpose clear. TSS responds only to how long an athlete worked and how intense that effort was from a metabolic perspective. For example, if a rider with a 315W FTP were to log a 3hr ride (10,800 seconds), with a NP of 275W, the TSS would be calculated as such: (10,800 x 275 x 0.87) / (315 x 3600) = 2.28 TSS

Different Stressors

Strength training imposes a completely different physiological load than endurance exercise. Instead of prolonged oxidative metabolism, weight lifting creates brief episodes of high mechanical tension, large motor unit recruitment, and eccentric muscle strain. These stresses target tissues and the neuromuscular system, activating pathways such as mTOR (mammalian target of rapamycin) that stimulate repair and structural adaptation, not aerobic metabolism.

This is why a demanding strength session often shows little or no TSS. Heart rate and oxygen demand remain relatively low, yet the athlete may experience significant soreness, reduced force production, and central nervous system fatigue. These factors meaningfully affect readiness for endurance work, but none of them influence NP, IF, FTP, or duration, which are the only variables that determine TSS.

If strength training were to have a meaningful TSS-like score, it would need variables such as total mechanical load (sets × reps × weight), proximity to failure, eccentric loading, and velocity loss. As these factors are fundamentally different from the metabolic inputs used in endurance TSS, the two stress domains cannot be represented by the same scoring model without distorting both.

How to Track Gym Stress

Although TSS cannot represent strength-training stress, ignoring gym work and its effect on your training cycle creates real problems in load management. An athlete may appear fresh according to CTL (Chronic Training Load) and ATL (Acute Training Load) while still carrying substantial neuromuscular fatigue from a heavy lifting session. Conversely, converting gym work directly into TSS inflates CTL and creates a misleading picture of aerobic fitness.

Accurate tracking of gym stress requires metrics that reflect mechanical and neuromuscular load. Two practical approaches already commonly used are:

• Volume load (sets × reps × weight) captures mechanical work and helps track progression.

• Session RPE (Rating of Perceived Exertion) × duration reflects global internal strain across all training types.

My Take

Respect the distinct biological systems that are targeted by endurance and strength training. The TSS formula was built around aerobic physiology, and nothing in it reflects the biology of strength training. Treating these stressors as complementary but separate produces clearer data, better decisions, and more effective training prescriptions.

TSS should remain the tool for quantifying aerobic stress. Strength-training load should be captured separately using session RPE or volume load. These values should be displayed side by side in the training calendar so that coaches and athletes can understand the full picture of stress, but they should not be merged into a single chronic-load model.

Sources

Joe Friel – The Weightlifting PMC Part 1
Joe Friel – The Weightlifting PMC Part 2
TrainingPeaks – How Cyclists Can Calculate Total Load for Strength Training
TrainingPeaks – What Is TSS?
PubMed Central – Monitoring Training Load to Understand Fatigue in Athletes

Follow Nick here: @nick_kleban
Check out Nick’s Training Tools: here