def lactate_norm(series):
  """Calculates lactate norm of a series of data.

  Unlike a simple average, the lactate norm emphasizes higher values
  by raising all values to the 4th power, averaging them, then taking
  the 4th root. Emphasizing higher values is rooted in the fact that
  best-fit curves between VO2 (oxygen consumption) and lactate tend
  to follow a 4th-order relationship.

  """
  return (series ** 4).mean() ** 0.25