noun

• (athletics, horse racing, car racing) Synonym of back straight (“straight part of a racetrack, running track, etc., opposite the finishing line”).
  "At the time appointed to start [2 1 2 P.M.], the horses made their appearance on the track, and were soon after called up by the judges for the race. They were startd at six minutes before three o'clock, the pacer having the pole, and leading round the first turn. On the backstretch he waited for the trotter, and let him take sides with him, it being apparent at this early stage that [George] Spicer did not intend to go any faster than the trotter would make him, at the same time keeping the trotter on the ouside all the way round, thereby making him go a greater distance in the race."
• (horse racing) An area next to a racetrack used to stable the racehorses and house employees.
  "Dominic Gatto liked to have someone at the track to represent him when one of his horses raced at Arlington Park or Belmont or Prescott Downs. Someone to relay a word to the jockey, keep a sharp eye on the trainer, distribute tips to the backstretch workers who were made to understand that Mr Dominic appreciated their services."
• (figuratively) The middle part of an event.
  "In some respects, it might be too faithful to the genre it savages; as in many real rom-coms, the energy flags during the plot’s backstretch, and audiences may share the drifting interest of Kyle (Bill Hader) and Karen (Ellie Kemper), who come to regret ever asking Joel and Molly to chronicle their meet-cute."
• (exercise) An exercise that stretches the muscles of the back.
  "I went over to the other bed and lay down. My knees were hanging over the side of the bed, providing me with a fairly decent back stretch. With my eyes closed, I could hear sleep calling."