Inertia and Cadence

In a recent Ask a Cycling Coach podcast from TrainerRoad they talk about trainer flywheel inertia – specifically on the Wahoo Kickr – as it pertains to training (26:45 into the audio). ie. using a high gear (low inertia) on the trainer will spin the flywheel faster and better imitate the ‘road feel’ of coasting, while using a low gear (high inertia) will better imitate trail riding and road hill climbing. I hadn’t considered this training aspect before so I decided to look into whether inertia in general has any effect on training efficacy or physiological adaptations.

Fair warning: this is a bit of a deep dive into a questionably-relevant-at-best topic!

A quick search on PubMed returned some relevant results. The most recent study on inertia was published in 2007 in the Journal of Sport Science and investigated the impact of high vs. low inertial loads in well trained cyclist (mean VO2max = 62.7 mL·min−1·kg−1) and found no significant effect on physiological or performance markers.

“There were no differences between inertial loads for mean peak torque, mean minimum torque, oxygen uptake, blood lactate concentration or perceived exertion… Crank inertial load has little direct effect on either physiology or propulsion biomechanics during steady-state cycling, at least when cadence is controlled.”

Edwards et al, 2007

Ok, so all else equal inertia shouldn’t affect training efficacy. But how does the body react to a change in inertia in an uncontrolled environment? ie. what happens when cadence isn’t controlled?

An earlier study from 2002 in the Journal of Biomechanics looked at how freely chosen cadence responded to crank inertial load. The authors hypothesized that cadence would increase as a response to higher inertia and pedal torque at a given submaximal power output.

 “Freely chosen pedal rate was higher at high compared with low crank inertial load.

“Peak crank torque was higher during cycling at 90rpm with high compared with low crank inertial load. Possibly, the subjects increased the pedal rate to compensate for the higher peak crank torque accompanying cycling with high compared with low crank inertial load.”

Hansen et al, 2002

Furthermore, the study highlighted the correlation of pedal rate (cadence) to gross (aerobic) efficiency, and found that gross efficiency therefore decreased as a response to higher inertia.

“gross efficiency at 250W [submaximal power output] was lower during cycling with high compared with low crank inertial load.”

Hansen et al, 2002

This has a few interesting implications. First is that higher inertia results in lower gross efficiency, ie. higher oxygen uptake. This could be interpreted to suggest that submaximal training done at a higher cadence would be more taxing on the aerobic system and therefore more effective en eliciting physiological adaptation.

But this isn’t any big revelation and isn’t even directly related to inertia; think about what your heart rate does in response to high cadence drills done at constant submaximal power output – your HR probably increases by a few bpm. This is a well known physiological phenomena and demonstrates the higher aerobic demand, ie. lower gross efficiency of pedaling at higher cadences.

Are Lower Cadences more Efficient?

The second implication is that higher cadence results in lower gross efficiency. This is counter to much of the traditional theory regarding cadence. The discussion of cadence optimization for energy efficiency is a broad topic and has been well discussed elsewhere, but to quickly summarize my understanding of the matter:

    • Cadences of 55-65 rpm are energetically optimal for oxygen uptake (Vercruyssen & Brisswalter, 2010)
    • Higher cadences (80-100 rpm) are typically self-selected for submaximal workloads for both untrained and elite cyclists, especially for durations under 15 minutes (Vercruyssen & Brisswalter, 2010)
    • Self-selected cadence decreases toward energetically optimal cadence beyond 15-30 minutes (Brisswalter et al, 2000) possibly reflecting a change in muscle recruitment or central fatigue onset
    • Self-selected cadence and cadence endurance (ability to maintain cadence) are related to (Brisswater et al, 2000):
      • Aerobic fitness; fatigue resistance at constant submaximal workload
      • Pedal torque; cadence will increase to maintain constant torque as inertia and/or power increases
      • Lower extremity joint moments; related to the question of crank length (future post!) cadence will adjust to minimize net joint moment, ie. foot speed around the pedal stroke (Marsh, Martin & Sanderson, 2000)
      • Neuromuscular fatigue; lower cadence requires greater muscular effort (I need to substantiate this) so RPE will be a factor of balancing aerobic efficiency with muscular efficiency
    • Insufficient evidence to claim cyclists should lower their self-selected cadence toward the energetically optimal 55-65 rpm. Basically, all this is academic and you probably don’t need to change what you’re currently doing!

Conclusion

The take-home message I would draw from this brief summary of the research is that inertia itself has no effect on training efficacy and only contributes by influencing cadence. However, cadence is too multi-factorial to try to optimize it based on one or two limited criteria like gross energy efficiency, alone. Instead, consider what cadence range you typically self-select during higher-intensity testing (eg. 20-min FTP test) or racing and aim to replicate that during training.

Specific drills done over a wide range of cadences throughout the training program will help expose your musculoskeletal & cardiovascular systems to a variety of metabolic demands and encourage further adaptations.

Limitations & Questions

    • This isn’t a comprehensive literature review by any means and I don’t even have access to the full texts of most of the studies referenced, so take any conclusions worth a grain of salt. But the goal is to estimate current best evidence and hypothesize any actionable effect on training. As always, n=1 is the most important n; consider what is best for your own training.
    • Why does cadence typically drop during sustained climbs at higher inertial loads? I suspect this is related to available gearing limiting self-selected cadence. eg. look at top pros climbing at 100+ rpm, where gearing is less of a limitation.

Resources

2 thoughts on “Inertia and Cadence

  1. Anecdotal observation:

    I can’t remember where I read this but it’s definitely true for me when talking about hill climbs. If your HR is redlining it’s better to shift up a gear and lower cadence for a given power level as it takes some stress off your aerobic system and shifts it to your muscles. The trade off is that you burn some matches that you won’t get back. Ease off the gas and your aerobic system will recover in 30 seconds whereas your muscles are fried.

    That’s why higher cadence is preferred especially by pros because over a long race/tour they’re limited by muscle endurance not aerobic capacity. Look at Chris Froome’s recently released power numbers. His heart rate is very de-coupled from his power. He also tends to spin >100rpm on hills. Some people think this is a smoking gun that he’s doping as Armstrong spun with a high cadence, sometimes 110rpm. HIgher aerobic capacity=higher cadence. Less efficient but you don’t fatigue your muscles as fast.

    Definitely has some implications for race tactics as is might be better to spin as long as possible then grind up the last hill to eke out a few more watts.

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  2. Great example, and I think you nailed it tying the cardiovascular/neuromuscular trade-off to burning matches.

    This goes to partially explaining why untrained/novice cyclists tend to pedal at low cadences and will naturally rise over time with increases in fitness – If the aerobic system is initially the rate limiter to power output, then greater demand is placed on the neuromuscular system. As aerobic/CV fitness improves the balance point between relative contributions will shift, ie. cadence will increase.

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