In his declining years, when he is confined to being a mostly silent, faltering man and a global icon, Muhammad Ali has become a diminished figure.

He is still revered, and comes as close as America can manage to a universally lauded black hero, but his legacy has been stripped of so much of its nuance and radical edge.

He turns 70 on Tuesday, and at the Muhammad Ali Centre in Louisville yesterday a party was held in his honour. Ali has devoted himself to the centre and its work, and he is depicted in its exhibits, photographs and writings as a man, a champion, fighting against racial segregation, war and poverty. Today, Ali is considered a brilliant fighter – among the greatest to have entered the ring – and also a symbol of empowerment and protest, a political and social crusader.

If he could, or if he was inclined, Ali might rail against his purification. Yet his image is ferociously managed, and if the old fires still burn then they are held deep within a body that has become an unreliable host to his spirit. Perhaps it is inevitable that the force of the 1960s and 1970s – the charisma and the fury, the lyrical and the brutal, the sheer captivating presence – must soften and compromise with age.

He has, in a sense, found peace, even if the tragedy he has to bear is that his voice and his vigour – the two weapons that he yielded with greater impact than his fists – are now impaired by Parkinson's disease.

Ali has remained a source of contention, though, as more critical assessments were made about his life outside the ring. In his 2001 book, Ghosts of Manila: The Fateful Blood Feud Between Muhammad Ali and Joe Frazier, Mark Kram, the late Sports Illustrated boxing writer, questioned the motives of Ali's political actions, painting a picture of him as an intellectual lightweight, a puppet of the Nation of Islam and a nasty, vindictive individual.

Even Thomas Hauser, Ali's official biographer, chastised him for declining to offer his opinion on al-Qaeda because he did not want to harm his business interests. Nobody comes closer to understanding, and explaining, Ali than Hauser, and he has criticised the commercialisation of the Ali brand, the way that he allowed himself to be adopted by corporate America as a bland but emotive symbol of hope.

This last incarnation has required Ali's past to be redrawn. He was enraged by the segregation of black people as he grew up in Louisville, but joined the Nation of Islam, a religious movement that preached black separatism and considered white people to be the creation of an evil scientist called Yakub. He chastised the mostly white sporting press, who initially lined up against him, and talked of his own beauty, which enthralled the black community, yet he was never more cruel and vindictive than in his taunting of the black boxers, Joe Frazier and Ernie Terrell.

Ali did not set out to be an activist, and even his refusal to be drafted to Vietnam was, at least at first, an act shaped by self-regard. He had previously been exempted because his IQ was too low, but then the rules were changed and he was called up. Ali had never campaigned against the war before, and it was only when he returned to the ring three years later that white, liberal America had shifted its opinion against the conflict, too, and so acclaimed him.

No man can be expected to stand for a single set of values, though. These complexities don't deserve to be abandoned in the cause of a bold but one-dimensional legacy. Ali was a contradictory man, capable of great warmth, humanity and wit, but also spite, hubris and selfishness. In the bid to depict a legend that fits certain corporate values, his true impact has been misplaced.

The power of Ali was not his message or his causes, but the way he affected individuals. Black Americans saw him as the embodiment of the identity they sought; white Americans embraced him as a symbol of their liberalism. In Africa, at a time when the colonialists were leaving, the native population considered Ali the representation of freedom. "He lived a lot of lives for a lot of people," said the American social activist Dick Gregory.

This is the real legacy of Ali: his spirit, his compelling force, the way that a man subject to the same conflicting values as everybody else, the weaknesses as well as the strengths, was able to make a difference by staying true to himself.