Before interviewing Heather Cassils I get an email: "In terms of pronouns I prefer he but I'm okay with she. I prefer to be called Cassils but don't mind if people call me Heather. I state it because I know it is not obvious."

This is not only considerate but also a fantastic understatement. From the high, soft Canadian voice and sleek, cropped hair to the rippling abs and occasional slick of scarlet lipstick, Cassils' identity is resolutely ambiguous.

By playing with body art, gay male aesthetics and extreme physical training – from weightlifting to martial arts – the artist has adapted his own female body into a series of powerful physical shapes that challenge any notion of binary gender.

"I resist the idea that you have to live as a man or as a woman," Cassils explains. "I didn't know any queer people until my 20s, let alone have any language for a trans identity. I just remember wanting to present my body in a more male fashion. The crux of my work is to create something that isn't so black-and-white."

Rough-hewn … Cassils performs Becoming an Image. Photograph: Heather Cassils and Manuel Vason, courtesy of Ronald Feldman Fine Art

Cassils' latest project, Becoming an Image, comes to Birmingham's Fierce festival on 5 October. It was originally conceived as a site-specific work for the ONE Archives in Los Angeles, the oldest active lesbian, gay, bisexual and transgender archive in the US. "I built this 2,000lb clay sculpture, moved the archive out, moved the audience in, then attacked the sculpture in the darkness," says Cassils.

The blows delivered during the 20-minute performance are illuminated only by intermittent flashes of a camera. The flash burns the image into the viewer's retina, creating a series of "live" photographs. "It represents senseless acts of violence against trans and queer bodies beyond the historical lens," says Cassils. The audience becomes both camera and witness to the beating, and the clay remains are then put on display.

"I wanted to draw attention to the fact that our genderqueer and trans brothers and sisters are so much more likely to experience physical violence: worldwide, transgender murders increased by 20% in 2012."

Audiences may see Cassils' muscle-heavy body as a way of challenging such violence, confronting bullying with strength. Through weight training, Muay Thai martial arts, traditional boxing, diet manipulation and supplements, the artist pushes the definition of a "biologically female body" to its visual extreme without taking testosterone or having surgery. Cassils is almost unfeasibly strong, almost entirely naturally.

"I started using weights when I was 16," says Cassils. "I had undiagnosed gall bladder disease in my teens, which may sound benign now, but back in 1987 it wasn't something doctors looked for in young girls."

And benign it certainly wasn't. "It got to the point where my insides were rotting, my bile ducts ruptured, my skin turned green and I went septic," says Cassils. "I was hospitalised, eventually. I had tubes in every orifice, two surgeries and a full blood transplant."

That's where Cassils' interest in physical discipline came from. "Training helped me feel strong again, and being ill made me want to articulate my own sense of wellness."

Claymaker … Cassils reshapes the 2,000lb clay sculpture. Photograph: Heather Cassils and Manuel Vason, courtesy of Ronald Feldman Fine Art

This desire sits uneasily with the artist's decision to take steroids for the 2011 piece Cuts: A Traditional Sculpture. "It was a real moral conundrum," says Cassils. "But Cuts is really a conversation with Carving a Traditional Sculpture by Eleanor Antin, where she crash-dieted for 45 days and documented her body wasting away. Just as Antin went through this sacrificial act of starving, I thought [taking steroids] was a necessary act."

It was, by all accounts, a deeply unpleasant process. "The project took over my life," says Cassils. "I had to force-feed myself to get enough nutrients, I couldn't leave the city because I'd vowed to take daily photographs, I needed to eat every three hours and train five times a week for at least two hours. My muscles became so tight my wife had to take my T-shirt off at night because I wasn't flexible enough to do it myself."

Cassils is very clear on the link between politics and performance art: "Our bodies are sculptures formed by society's expectations. I am a visual artist, and my body is my medium." And it is a medium that undergoes constant evolution. Does that make the struggle to be an artist even harder?

"It is difficult to be an artist," says Cassils. "Most people would probably prefer you not to be. But just hold true to your intentions. It's almost like artistic Darwinism – if you can keep going, eventually there will be a breaking point."