Even if you're not competing in HEMA, being fit helps you train effectively.
If you want to win in your fencing, it makes sense to be fit. Being able to move faster, cut harder, bind with more strength, and fence with more intensity for longer than your opponent are all unquestionable advantages. Yes, being more technical is also a massive advantage, before anyone objects, but over fifteen exchanges an out of condition and physically weak technical master might still struggle against a technically less advanced but athletic opponent.
What if you don’t care about winning? A lot of people aren’t interested in competitions for one reason or another - whether its personality, financial or life situation the reasons are many. Which is a shame, but some folks aren’t even interested in whether they’re winning practice sparring or not. “It’s all about the art and personal technical advancement, not about winning or losing” has some merit, undoubtedly. They should still get in shape though.
First of all, being in shape lets you train for longer. If you get exhausted in the warm up, your technical practice will be impacted if you can’t recover. If you get exhausted in technical practice, you can’t put in as many reps with such good form. If you get exhausted in sparring, you can fill in the consequences yourself...
EDIT: Since I first published this, a lot of people have been in touch to point out something else I missed. Strength and flexibility also reduce the chance of injury. I've heard of everything from shoulders dislocated swinging heavy sabres past the point of fatigue to chronic elbow tendinitis from supporting rapiers at extension with the common factor being less athletic starters who were pushed beyond their comfort zones in technical drills. If you're not up to the physical demands of your fencing style, then pushing yourself too hard to meet them will cause injury. Better to build and maintain the strength needed slowly and effectively in separate training, rather than thinking "just get better by doing" and permanently stop yourself from being able to fence.
Being fit - especially strength and flexibility - also allows you to train techniques better, or at all. There are techniques which require, or are substantially improved by, these attributes. A kickboxer who is inflexible cannot throw a question mark kick and a wrestler who is weak cannot execute a Karelin Lift. There are some HEMA techniques - the long, low rapier lunge comes to mind - which are similarly physically demanding.
It’s hard to put numbers on what the “historical” level of fitness is, especially without getting more specific on exactly which historical person you mean. I’ll take a moment to consider the case of the sort of person who might have fenced with a longsword in fifteenth century Bavaria according to the Ringeck glossa’s style. To steal an observation from Jack Gassmann, if you consider a 25 year old man at arms might have been training professionally to fight since he was 15 at the least, he has the equivalent of a post graduate education in fighting. He fights as a full time job, with a commensurate amount of physical activity. Jean de Maingre aka Boucicault was an early fifteenth century knight known to be able to climb up the underside of a ladder using only his hands in full harness. It seems reasonable to assume that such athletes could perform demanding technical movements in their fighting.
You might care to look to the Thirteenth Century “King’s Mirror” for the training that might be the main “diversion” of a professional fighter-courtier, amongst other sources:
in your knightly practices:
throwing апd pushing stones,
dancing апd jumping,
fencing апd wrestling,
Even the non-professional fighters who didn’t work as outright labourers, like an urban burgher with a skilled trade, had jobs which were much less sedentary than those done by most of the readership of this article.
“What’s the minimum acceptable level of fitness for the HEMA practitioner?” is similarly vague and hard to answer. But if you struggle to do your rapier lunges, or are done after one round of sparring, or find yourself developing injuries because moving the sabre is too much for your muscles and joints, then it’s clear that your fitness is holding you back from training effectively. Perhaps the question to ask is “is my lack of fitness interfering with my HEMA training?”
We don’t need to address these attributes in our HEMA training time. After all, most practitioners only get a few hours a week for all their technical training. General fitness training (i.e. lift, run, stretch) is going to address the majority of our fitness needs for HEMA, and if you’re interested in specific fitness work then you’re likely more competitive than those whom this article is addressing. Yet there are some niche examples of where fitness for historical martial arts has unusual requirements - like the supple wrist needed for sabre or cane work in many traditions, or the lower body strength and endurance needed to work in low Meyer longsword stances - in which case these need addressing if they hold you back.
What instructors can do, if they don’t want to devote significant time to physical training in class, is have classes which require fitness to keep up with. Working to an athletic pace and standard, not holding back to the comfort zone of the least fit, means that (as an MMA instructor of mine once put it) “you don’t have to be fit to start, but if you keep this up you’ll have to get fit”. As a bonus it also means the class itself will be more beneficial exercise for all involved and, since it will involve more repetitions and more demanding exercises, be more beneficial technical training.