Stiffness and Running: Delaminated

Study Title:
Stiffness and Running Performance: From the Tissue to the Body: A Narrative Review

Authors:
Masson, L., Millet, GY., Kerhervé, HA.

Publication Information:
​Sports Medicine (2026)

​Background Information: Running economy (RE) is one of the biggest predictors of distance running performance, and gait mechanics — particularly what happens during ground contact — seem to hold the most potential for improving it. One concept that keeps coming up in conversation is “stiffness:” how the body resists deformation as it loads and unloads with each stride.

However, stiffness isn’t one clean, unified idea — it shows up at the whole-body level (spring-mass model), at the muscle-tendon level, and at the tissue level, each with its own definitions and measurement tools.

This review sets out to synthesize all three levels and clarify how they relate to RE, how fatigue affects them, and how they can be modulated to improve performance.

Study Methods: This was a narrative review. The authors searched scientific databases using combinations of terms like “spring-mass,” “stiffness,” “flexibility,” and “elastic” combined with “run,” limited to peer-reviewed English-language papers through March 2025. They excluded shoe-material/foot-specific stiffness studies, sprint-only research, animal studies, and pediatric/pathological populations. Reference lists were hand-searched for additional relevant papers, and the authors supplemented with material from their own database to fill definitional gaps.

Results:

• Quasi-stiffness (Kvert, Kleg, Kjoint): Meta-analyses show a negative correlation between whole body stiffness and RE overall — stiffer runners tend to be more economical — but individual studies are inconsistent, and one even found the opposite for ankle stiffness.
• Muscle-tendon unit stiffness: Greater passive stiffness and higher Achilles tendon stiffness are generally linked with better RE and performance, though results vary by sex, age, and which tendon is tested.
• Tissue stiffness: The least-studied level. Only one study has directly tied tissue stiffness to RE, and it found a link only with Achilles tendon stiffness — not muscle tissue.
• Fatigue effects: Quasi-stiffness tends to drop during higher-intensity efforts. For long/ultra events, tissue and tendon stiffness changes are inconsistent — sometimes increasing, sometimes decreasing, depending on distance, timing of measurement, and muscle group.
• Interventions: Plyometric warm-ups and strength training generally trend toward improved RE, often (but not always) tracking with stiffness changes. Footwear and surface effects on stiffness look more like short-term adaptive responses to novelty than a stable training effect.

Conclusions: There does seem to exist evidence linking stiffness and running economy, but the mechanisms driving it are still far from certain. The authors argue there’s likely an individually-optimal stiffness — not “stiffer is always better” — that maximizes energy transfer while minimizing injury risk, but this hasn’t been systematically tested. Big gaps remain around combining tissue-level and whole-body measures in the same study design.

My Thoughts: Let’s not blindly chase “more stiffness.” Many experience and astute coaches appreciate and utilize “rhythm” in their practices and combining both might be optimal. Listen to music while training. Listen to music while running. Watch the pros as they warm up and train - especially in track and field, especially in the Caribbean and in East Africa.

This review is a good reminder that stiffness is really a family of related-but-distinct concepts, and conflating them (as much of popular training conversation does) risks giving athletes bad advice.