Vertical Stiffness

What is Leg Spring Stiffness / Vertical Stiffness?

Leg spring stiffness in running describes how your legs behave like high-tech shock absorbers when your foot hits the ground. At each step, your muscles, tendons, and joints briefly store energy as they compress (like a spring being squeezed), then release it to help push you forward into the next stride. In biomechanics, it’s often expressed as the relationship between how much the leg “compresses” during ground contact and the peak force produced—stiffer springs deform less for a given force, while less-stiff springs deform more. 

Your analysis report will give you an indication of what an elite athlete would score if they ran at your analysis speed.  

Understanding leg spring stiffness matters because it influences running economy, speed, and injury risk. If stiffness is too low, you may waste energy with excessive sinking at contact, increasing muscle work and fatigue. If stiffness is too high, impact loads can increase, and certain tissues (such as the Achilles tendon, calf, or shin) may be stressed more. The “best” stiffness isn’t the same for everyone—it depends on factors such as pace, footwear, surface, strength, and technique. 

How to Improve Vertical Stiffness

Improving stiffness isn’t just about being “stiffer.” The goal is an appropriate, adaptable spring. Plyometrics, calf and hip strengthening, sprint drills, and gradual exposure to faster running can enhance elastic energy use, helping you run more efficiently and potentially reducing the risk of overuse injuries. 

Here are five strengthening-focused exercises that tend to improve the “spring” quality of the ankle–Achilles–knee–hip system (which is what drives leg spring stiffness). Do them 2–3×/week, leaving a day between harder sessions. 

1. Heavy standing calf raises (gastrocnemius) 

• 3–5 sets × 5–8 reps, slow lower (2–3s), full pause at bottom, hard squeeze at top 

• Load it (Smith machine, barbell, machine). This is your “stiffness engine” for push-off. 

1. Seated calf raises (soleus) 

• 3–4 sets × 8–12 reps, controlled tempo 

• The soleus is huge for running because it works a ton during mid-stance and supports elastic rebound. 

1. Single-leg Romanian deadlift (RDL) 

• 3–4 sets × 6–10 reps/side 

• Builds hip stability and posterior-chain strength so force goes into the ground efficiently instead of “leaking” at the pelvis/knee. 

1. Rear-foot elevated split squat (Bulgarian split squat) 

• 3–4 sets × 6–10 reps/side 

• Great for knee/hip strength through running-like ranges and for controlling collapse during stance. 

1. Isometric mid-thigh pull or isometric split squat hold 

• 3–5 × 20–45 seconds (hard effort), 1–2 min rest 

• Isometrics improve force transmission and tendon loading tolerance with low technique complexity. 

Quick add-on (if you want the “spring” specifically): after strength, do low-volume pogo jumps (2–3 × 20–30 contacts) with short ground contact—only if your calves/Achilles tolerate it. 

Happy Running