For many people, the main concern in a yoga class is whether they are breathing correctly or their legs are aligned. But for others, there are lingering doubts about whether they should be there at all, or whether they are betraying their religion.
Farida Hamza, a Muslim woman living in the US (pictured above), had been doing yoga for two or three years when she decided she wanted to teach it.
"When I told my family and a few friends, they did not react positively," she recalls. "They were very confused as to why I wanted to do it - that it might be going against Islam."
Their suspicions about yoga are shared by many Muslims, Christians and Jews around the world and relate to yoga's history as an ancient spiritual practice with connections to Hinduism and Buddhism.
Last year, a yoga class was banned from a church hall in the UK. "Yoga is a Hindu spiritual exercise," said the priest, Father John Chandler. "Being a Catholic church we have to promote the gospel, and that's what we use our premises for." Anglican churches in the UK have taken similar decisions at one time or another. In the US, prominent pastors have called yoga "demonic".
One answer to the question of whether yoga really is a religious activity will soon be given by the Supreme Court in the country of its birth, India.
Last month, a pro-yoga group petitioned the court to make it a compulsory part of the school syllabus on health grounds - but state schools in India are avowedly secular. The court said it was uncomfortable with the idea, and will gather the views of minority groups in the coming weeks.
So is yoga fundamentally a religious activity?
"Yoga is such a broad term - that's what causes a difficulty," says Rebecca Ffrench, the co-founder YogaLondon - a yoga teacher academy - and the philosophy tutor at the school.
There are different forms of yoga, she says, some of which are more overtly religious than others. Hare Krishna monks, for example, are adherents of bhakti yoga, the yoga of devotion. What most people in the West think of as yoga is properly known as hatha yoga - a path towards enlightenment that focuses on building physical and mental strength.
But what "enlightenment" means also depends on tradition. For some Hindus it is liberation from the cycle of reincarnation, but for many yoga practitioners it is a point where you achieve stillness in your mind, or understand the true nature of the world and your place in it.
Whether that is compatible with Christianity, Islam and other religions is debatable.
To those in the know, for example, the yogic asanas, or positions, retain elements of their earlier spiritual meanings - the Surya namaskar is a series of positions designed to greet Surya, the Hindu Sun God.
Image captionThe Surya Namaskar or Sun Salutations
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