Pygame animating image by transforming it

The simplest solution would be to call extend or contract once per frame, but then the animation would be frame rate bound.

import pygame


class Muscle(pygame.sprite.Sprite):
    def __init__(self, screen, posX=200, posY=200, contraction=0.5,
                 extension=1.5, length=40, thickness=20, min_len=0.2,max_len=1.5, power=20):
        super(Muscle, self).__init__()
        self.screen = screen
        self.contracting = False  # To check if the muscle is contracting or extending.
        self.contraction = contraction
        self.relaxation = 1
        self.length = length
        self.thickness = thickness
        if power < 1:
            self.power = 1
            power = 1
        else:
            self.power = power
        self.image = pygame.Surface((length, thickness))
        color_index = 5 * power
        self.image.fill((255, color_index, color_index))
        self.rect = self.image.get_rect()
        self.rect.x = posX
        self.rect.y = posY

    # This method gets called every frame.
    def update(self):
        if self.contracting:
            self.contract()
        else:
            self.extend()

    def render(self):
        self.screen.blit(self.image, (self.rect.x, self.rect.y))

    def contract(self):
        expected_width = self.length * self.contraction
        counter = 0
        if self.image.get_width() > expected_width:
            self.image = pygame.transform.scale(self.image, (self.rect.width, self.image.get_height()))
            self.rect.x += 1
            self.rect.width -= 2
            self.render()
            counter += 1

    def extend(self):
        expected_width = self.length * self.relaxation
        counter = 0
        if self.image.get_width() < expected_width:
            self.image = pygame.transform.scale(self.image, (self.rect.width, self.image.get_height()))
            self.rect.x -= 1
            self.rect.width += 2
            self.render()
            counter += 1


def main():
    screen = pygame.display.set_mode((640, 480))
    clock = pygame.time.Clock()
    muscle = Muscle(screen)
    all_sprites = pygame.sprite.Group(muscle)

    done = False

    while not done:
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                done = True
            # Change the state of the muscle.
            elif event.type == pygame.KEYDOWN:
                if event.key == pygame.K_c:
                    muscle.contracting = True
            elif event.type == pygame.KEYUP:
                if event.key == pygame.K_c:
                    muscle.contracting = False

        all_sprites.update()
        screen.fill((30, 30, 30))
        all_sprites.draw(screen)

        pygame.display.flip()
        clock.tick(30)


if __name__ == '__main__':
    pygame.init()
    main()
    pygame.quit()

Alternatively, you can call extend or contract after some time interval. Check out these answers to see how you can implement a timer.