Calves, tendons, and hamstrings for runners

The short answer

Heavy calf work makes the Achilles tendon stiffer, and in 1 trial using heavy calf holds it also made running cheaper in oxygen. The Nordic hamstring curl about halves hamstring injuries in team and sprint sports, and no trial in distance runners turned up in the research behind this plan. Injury trials in runners are mixed, and the clearest running rule comes from a large cohort, which linked single runs more than 10% longer than your recent longest to more overuse injuries.

What makes a tendon adapt

Tendons adapt to the load placed on them. A meta-analysis of 27 studies in healthy adults aged 18 to 50 found that tendon stiffness rose with training, and the size of the gain depended on how heavy the load was. Programs above 70% of max gave a large effect, and lighter programs gave almost none. The type of contraction made no difference, and programs of 12 weeks or more showed a somewhat larger effect, though the difference was not significant (Bohm 2015).

In 1 trial, each person trained 1 leg at high tendon strain and the other at low strain for 14 weeks, and only the high-strain leg's Achilles tendon grew stiffer and thicker (Arampatzis 2007). The light leg got about the same stimulus as daily life.

Coaches turn this into calf raises with a weight you can lift for about 6 to 10 reps, moved slowly. Light, bouncy raises for high reps stay well under that load.

Calves and running economy

In 1 trial, 14 weeks of heavy isometric calf training raised calf strength about 7% and Achilles tendon stiffness about 16%, and running economy improved about 4% at the same speeds (Albracht and Arampatzis 2013). The authors linked the gain to the stiffer tendon storing and returning more energy with each stride. Running economy is the oxygen it costs to hold a given pace, so better economy means the same pace for less effort.

The calves matter more with age. In a survey of 2,886 relay runners, runners 40 and over reported more calf, Achilles, and hamstring injuries than younger runners, which the authors thought may reflect differences in training (McKean 2006). A survey shows association, not cause.

The Nordic curl, and where its evidence comes from

The Nordic hamstring curl is a kneeling exercise where you lower your body forward slowly while a partner or a bar holds your ankles. Across 15 studies and 8,459 athletes, programs that included it had about half the rate of hamstring injuries (van Dyk 2019).

Those studies come from team and sprint sports. No trial in distance runners was found, so using the Nordic for a runner is coaching practice built on that evidence.

In 20 recreationally active men, a low-volume and a high-volume Nordic program raised hamstring fascicle length and lowering strength by similar amounts over 6 weeks. The fascicle gains faded within 2 weeks of stopping, while the strength held for at least 4 weeks (Presland 2018). That suggests a small weekly dose is enough to build the change, and that it fades without regular work.

Injury research in runners is mixed

Across many sports, strength training cut injuries to under 1/3, and overuse injuries could be almost halved (Lauersen 2014). Runners have not shown the same clear result.

A meta-analysis of 9 trials and 1,904 runners found that exercise programs did not clearly lower injury risk overall. Supervised programs did lower it, which the authors linked to people doing more of the sessions (Wu 2024). In 325 novice runners, all supervised by a physiotherapist, a hip and core program lowered leg injuries and cut weekly overuse injuries by 39% compared with stretching. A foot and ankle program in the same trial did not lower overuse injuries and had more acute injuries than stretching (Leppänen 2024).

In runners, strength work builds the capacities linked to fewer injuries, and it seems to help most when people do it consistently, which is why Durability sessions stay short.

The single-run rule

The old rule says to add no more than 10% a week. A 2025 study of 5,205 runners, mean age 45.8, followed 588,071 sessions over 18 months. A single run more than 10% longer than the longest run of the previous 30 days went with a higher rate of overuse injury. Runs 10 to 30% longer had a 64% higher rate, runs 30 to 100% longer a 52% higher rate, and runs more than twice as long a 128% higher rate. The week-to-week change in distance showed no link (Frandsen 2025).

This is an observational study, so it shows association, not cause. It is a large study, and its practical step is to let your longest run grow a little at a time, measured against the longest run of the last month.

In 5,205 runners, a single run more than 10% longer than the longest of the last 30 days went with a higher overuse injury rate.

How Peak Hybrid uses this

Durability runs 2 days a week, about 35 minutes each, legs and trunk only. Calf raises and single-leg lifts are heavy and slow, with 2 reps left, and Day A includes a Nordic ladder. In weeks 1 to 4, and for 2 weeks after any harder move, the session lands at least 2 days before a hard run. On 5K / 10K, your long run grows up to 10% past your longest run of the last 30 days, and every run stays within that limit. The details are on the plans page.