NEW TO F45, CROSSFIT OR HYROX CLASSES? WHY THE FIRST WEEKS HURT, AND HOW TO EASE IN

The first few weeks of a new group class like F45, CrossFit or a HYROX session tend to hurt in places you didn't know you had. Stairs get harder. Sitting down gets harder. That's common, it has a name, and the research reviews on it give consistent advice: introduce new kinds of exercise gradually. This post draws on three chapters of The Functional Fitness Protocol and the studies behind that advice.

Why a New Class Leaves You So Sore

The soreness that shows up a day or two after a new workout is called delayed-onset muscle soreness, or DOMS. A 2018 review by Hotfiel and colleagues describes it as a mild type of muscle injury caused by eccentric work (a muscle lengthening under load, like lowering into a lunge) or by exercise the body isn't used to. They report that soreness typically peaks 48 to 72 hours after the session, along with stiffness, reduced strength and sometimes swelling.

A 2003 review in Sports Medicine by Cheung and colleagues notes that DOMS is most common at the start of a season, after a stretch of reduced activity, and whenever people are first introduced to a new type of activity. A circuit class that mixes squats, lunges, jumps, sled pushes and carries is new to almost everyone the first time.

The Second Session Usually Hurts Less

Muscle adapts to this quickly, a pattern researchers call the repeated bout effect. A 2023 meta-analysis by Doma and colleagues pooled 20 studies of multi-joint exercises. Measured 24 and 48 hours later, soreness and creatine kinase (a blood marker of muscle damage) were significantly higher after the first session than after a second session of the same exercise. Strength and vertical jump performance were also lower after the first session than after the second. The authors conclude that a single session of these common exercises may protect against muscle damage in later sessions.

Put simply, the first class is often the roughest, as long as the next few don't jump sharply in load or volume.

Muscle Adapts Faster Than Tendon

This is the part that matters for joints. Muscle gets stronger and recovers relatively quickly. Tendons, which connect muscle to bone, adapt too, but on their own schedule.

A 2015 meta-analysis by Bohm, Mersmann and Arampatzis pooled 27 studies (37 training programs, 264 adults aged 18 to 50), each lasting at least eight weeks. Tendons became measurably stiffer with training, and the size of the load mattered more than the type of muscle contraction. Those changes came from programs lasting two months or longer, not from a handful of sessions.

A 2017 review by the same research group explains why that matters. Muscle and tendon differ in how quickly they adapt and in which kinds of loading they respond to. Jumping-type (plyometric) training, for example, appears to be a stronger stimulus for muscle than for tendon. The authors' concern is that muscle strength rising faster than tendon capacity puts extra strain on the tendon. That review focused on young athletes, so it describes a mechanism the authors propose rather than something measured in adult class-goers.

What the Injury Research on This Kind of Training Shows

I could not find a published study on F45 specifically. The research is on CrossFit, the most-studied format of this kind. A 2022 systematic review by Rodríguez and colleagues covered 25 studies and 12,079 CrossFit participants. The average reported injury prevalence was 35.3%, and injury rates ranged from 0.2 to 18.9 per 1,000 hours of training, similar to weightlifting and powerlifting, according to the authors. The shoulder (26%), spine (24%) and knee (18%) were the most commonly injured areas. The authors note the studies were of limited quality, so firm conclusions about risk factors weren't possible.

A 2023 review by Mehrab and colleagues included only prospective studies: three cohorts, 691 participants, 172 injuries. It found limited evidence that switching back and forth between prescribed and scaled loads during training was linked to more injuries, and that longer participation was protective. The authors suggest that newer participants, and anyone increasing their training load, get closer supervision from coaches.

How the Book Ramps Up

The Functional Fitness Protocol was written for endurance athletes over 50 moving into this style of training, so its plan is deliberately conservative. The first phase lasts four weeks, and its goal is technique, not fitness:

• Three sessions a week, about 45 minutes each, with real coaching where possible.
• Light loads and low volume. The only measure that counts is whether the movement looks right.
• Low-intensity versions of explosive moves first: a box step-up before a box jump, a light kettlebell swing before a heavy one.
• Load goes up in weeks three and four only where technique has held.
• Mobility work as the warm-up in every session.

Cheung's review gives similar practical advice: introduce new or eccentric-heavy exercise progressively over one to two weeks, and after a session that leaves you very sore, reduce intensity and duration for a day or two or train the body parts that are less affected. In a group class, that often means choosing the lighter weight and the lower-impact version of a movement for the first few weeks, even when the rest of the room is going all out.

Soreness or Something Else?

The book's rule of thumb: ordinary soreness from a new workout fades within 48 to 72 hours and doesn't get worse in the next light session. Pain that lingers, gets worse under load, or changes the way you move to avoid it is a different category, and the book's advice is to have someone look at it rather than keep guessing.

The book names the biggest risk in the first month as ego, not a lack of fitness: the urge to skip the beginner phase and match the person at the next station. Its plan treats the first four weeks as the foundation that everything after them is built on.

Sharp joint pain, swelling, numbness, or pain that keeps getting worse are reasons to see a doctor or physical therapist, not to push through. Muscle soreness is common after a new workout; this post can't assess anyone's joint pain.

Sources

1. Hotfiel T, Freiwald J, Hoppe MW, et al. Advances in delayed-onset muscle soreness (DOMS): Part I: pathogenesis and diagnostics. Sportverletzung Sportschaden. 2018;32(4):243–250. PMID 30537791
2. Cheung K, Hume P, Maxwell L. Delayed onset muscle soreness: treatment strategies and performance factors. Sports Medicine. 2003;33(2):145–164. PMID 12617692
3. Doma K, Matoso B, Protzen G, Singh U, Boullosa D. The repeated bout effect of multiarticular exercises on muscle damage markers and physical performances: a systematic review and meta-analyses. Journal of Strength and Conditioning Research. 2023;37(12):2504–2515. PMID 38015738
4. Bohm S, Mersmann F, Arampatzis A. Human tendon adaptation in response to mechanical loading: a systematic review and meta-analysis of exercise intervention studies on healthy adults. Sports Medicine – Open. 2015;1(1):7. PMID 27747846
5. Mersmann F, Bohm S, Arampatzis A. Imbalances in the development of muscle and tendon as risk factor for tendinopathies in youth athletes: a review of current evidence and concepts of prevention. Frontiers in Physiology. 2017;8:987. PMID 29249987
6. Ángel Rodríguez M, García-Calleja P, Terrados N, et al. Injury in CrossFit®: a systematic review of epidemiology and risk factors. The Physician and Sportsmedicine. 2022;50(1):3–10. PMID 33322981
7. Mehrab M, Wagner RK, Vuurberg G, et al. Risk factors for musculoskeletal injury in CrossFit: a systematic review. International Journal of Sports Medicine. 2023;44(4):247–257. PMID 36174660