100m Freestyle Pacing: The Science of Splitting Time Between the First and Second Half

Introduction

The 100m freestyle is known as swimming’s “golden distance”—it demands the explosive power of the 50m free while requiring the endurance management of the 200m free, making it the ultimate test of an athlete’s all-around ability. In Taiwan’s competitive swimming scene, from city and county meets to national championships, the 100m free is often the event with the most participants and the most engaged spectators. However, many swimmers fail not because they aren’t fast enough, but because of pacing errors: going out too hard in the first 50m and fading badly in the second 50m.

The Physiology of the First and Second Half

The 100m free relies primarily on the phosphocreatine (ATP-PCr) system and the glycolytic lactic acid system, with limited aerobic contribution. This means lactate begins to accumulate around 15 to 20 seconds into the race, and its concentration rises sharply after 40 to 50 seconds, causing muscle acidification—this is the main reason for the slowdown in the second 50m.

Ideal First-Half/Second-Half Split:

Elite swimmers typically swim the second 50m 1.5 to 2 seconds slower than the first 50m (in a long-course pool), a figure known as the “Split Difference.” A split difference that is too large indicates going out too hard in the first half; one that is too small suggests a conservative first half, leaving usable speed reserve on the table.

The First 50m: Strategy from the Start to the Turn

The rhythm of the first 50m is the key to success in the entire 100m free.

Start Segment (0–15m):
Underwater propulsion distance is recommended to be between 12 and 15 meters—underwater resistance is far lower than on the surface, so make the most of the starting momentum. However, spending too much time underwater can lead to hypoxia, affecting your rhythm later on.

Mid-Pool Swimming (15–45m):
This segment should reach and maintain maximum speed. A breathing rhythm of breathing every 2 strokes (2-2 breathing) is recommended; some swimmers use 1-2 or 2-3 breathing depending on personal preference. The head rotation angle should be small when breathing to avoid breaking the body’s streamline.

Turn (45–50m):
The turn is where the most speed is lost in the 100m free. A good turn can be 0.5 to 1 second faster than a poor one. Here are the key points for turn efficiency:

• Do not breathe on the last stroke before the turn; maintain speed as you glide into the flip turn
• After both feet touch the wall, bend the knees to approximately 90 degrees to maximize push-off power
• Maintain a streamline after pushing off the wall, and use dolphin kicks to accelerate

The Second 50m: The Wisdom of Fighting Lactate

The second 50m is the proving ground of the 100m free. By this point, lactate has accumulated heavily and the muscles begin to “burn.” The correct way to respond is:

Maintain Stroke Rate, Don’t Add Stroke Length:
When fatigue builds, many swimmers instinctively try to compensate for lost speed with bigger stroke movements, but this actually consumes more oxygen and accelerates the fade. The correct approach is to maintain stroke rate and accept that speed will naturally slow.

Keep Kicking:
The first thing to break down under fatigue is often the kick. Incorporate “fatigued-state kicking” drills into training so your body gets used to maintaining leg rhythm under lactate buildup.

Mental Management:
The final 25m is the ultimate test of willpower. Set a mental preset before the race—“no matter how painful, maintain the rate”—to avoid unconsciously slowing down in the midst of suffering.

Practical Advice

• Split Timing Training: Record your first and second 50m split times for every 100m free you swim in practice, and observe changes in your split difference over 2-week blocks
• Pace Swimming: Choose a target pace and simulate it repeatedly in training to help your body establish a sense of speed
• Timed Swims Under Fatigue: Immediately after your main set, do a timed 100m free to simulate racing under fatigue
• Race Warm-Up: The 100m free requires a more thorough warm-up; it is recommended to swim 400 to 600m before the race, finishing with a short sprint to wake up the nervous system

Conclusion

The art of pacing the 100m free lies in finding that first-half speed that sits “right on the edge”—fast enough to fully utilize your speed reserve, yet not exceeding the limit you can sustain in the second half. This boundary can only be precisely calibrated through extensive training and racing experience. Every timed practice is a pacing experiment, and every race is a calibration. Trust the data, trust your training—your personal best in the 100m free is waiting ahead.

Related Reading

• Pacing Analysis of the 100m Freestyle: Optimal First-Half/Second-Half Split Ratios and Their Impact on Performance
• Front and Back Half Pacing in the 100m Freestyle: Split Analysis of World Records
• Swimming Race Pacing Strategies: Front-to-Back Half Ratios for 50m, 100m, 200m, and 400m
• Split Strategy for the 50m Freestyle: Optimal Ratios for the First 15m, Middle Maintenance, and Final Sprint