The Science of Streamline Gliding After Entry

The 1.0–1.5 seconds from the moment of entry to the start of the first underwater dolphin kick is the fastest phase of the entire race. Entry speed can reach 6.5 m/s, far higher than the 2.0 m/s swimming speed underwater. But this speed decays rapidly, slowing by about 1.5–2.0 m/s per second. How to extend the effective glide distance of the streamline is one of the core competitive advantages of elite swimmers.

1. Fundamentals of Fluid Mechanics

The drag equation in water:

F_drag = 0.5 × ρ × v² × C_d × A

• ρ: density of water (1000 kg/m³)
• v: velocity
• C_d: drag coefficient (related to body posture)
• A: frontal projected area

The key lies in C_d and A—both are directly affected by body posture. A perfect streamline posture can reduce C_d by 30–40%.

2. Streamline Posture Checklist

Upper Body (from fingers to waist)

• [ ] Hands stacked: top hand wraps over the back of the bottom hand
• [ ] Thumbs hooked over the opposite index finger
• [ ] Arms fully extended forward, elbows locked
• [ ] Ears pressed tightly against the inner upper arms
• [ ] Chin slightly tucked, neck elongated
• [ ] Shoulder blades fully protracted

Torso

• [ ] Abdomen tightened (core engaged)
• [ ] Chest slightly concave
• [ ] Lower back neither sinking nor over-arched
• [ ] Hips, shoulders, and toes aligned in a straight line

Lower Body

• [ ] Legs fully together (thighs, knees, ankles)
• [ ] Toes pointed (plantar flexion)
• [ ] Feet together without crossing

3. Minimizing Frontal Projected Area

The frontal projected area of a good streamline (body silhouette viewed from directly ahead) is approximately 800–1000 cm², while swimmers with poor posture may reach as high as 1500 cm². The differences mainly come from:

• Hands not stacked: adds 100–200 cm²
• Legs separated: adds 200–400 cm²
• Head lifted: adds 150–300 cm²
• Body not straight: adds 100–200 cm²

4. Entry Technique

To establish a good streamline, it must begin from the moment of entry:

1. Concentrated entry point: hands, head, hips, and feet pass through the same point in sequence, like “drilling into the water”
2. Entry angle: 32–38 degrees is optimal
3. Entry depth: at the first full extension, the body should reach a depth of 0.5–0.8 meters
4. Streamline transition: maintain streamline after entry; do not immediately angle upward

5. Velocity Curve During the Glide Phase

According to FINA research:

Time	Velocity	Action
0 sec (moment of entry)	6.5 m/s	Full entry
0.3 sec	5.5 m/s	Stable glide begins
0.6 sec	4.5 m/s	Velocity begins to drop significantly
1.0 sec	3.5 m/s	Approaching underwater dolphin kick speed
1.5 sec	2.8 m/s	First dolphin kick initiated
2.5 sec	2.5 m/s	Steady underwater propulsion

6. When to Initiate the First Dolphin Kick?

Indicators for timing the first dolphin kick:

• Velocity decay point: when speed drops to approximately 2.8–3.0 m/s
• Time-based judgment: 1.0–1.5 seconds after entry
• Feel-based judgment: a noticeable slowdown in body speed

Kicking too early: wastes the high-speed phase of free gliding
Kicking too late: speed has already dropped too low, requiring more effort from the dolphin kick to recover

7. Training Methods

Training 1: Wall Push-Off Streamline Distance Measurement

• Push off the wall with both feet, full streamline
• Record glide distance (tests core stability)
• Men’s target: 8–10 meters
• Women’s target: 7–9 meters

Training 2: Eyes-Closed Streamline in Water

• Push off the wall and glide with eyes closed
• A partner observes whether you drift off course
• Trains proprioception

Training 3: Mirror Posture Check

• Simulate streamline standing up
• Check hand, ear, and abdominal positions in the mirror
• 3 minutes daily

Training 4: Video Analysis

• Use a GoPro to film underwater posture from the side
• Compare with elite swimmer footage to identify differences
• Once per month

8. Shoulder Mobility and Streamline

A good streamline requires full shoulder elevation. Test:

• Lie on your back on the floor
• Extend both arms overhead
• Can both arms lie completely flat on the floor?
• If not, shoulder stretching is needed

Improvement methods:

• Daily “Wall Slide” stretches, 3 sets
• Pectoral stretches
• Latissimus dorsi stretches

9. Quantitative Benchmarks

Indicators of an excellent streamline:

• 5-meter start time (from signal to 5 meters): men < 1.5 sec, women < 1.7 sec
• 15-meter underwater time: men < 5.8 sec, women < 6.4 sec
• Wall push-off 8-meter velocity: should be > 2.3 m/s

Conclusion

Streamline is the “simplest yet hardest to achieve” element of swimming technique. It requires no strength, no skill—only a fully extended body. But that extension demands exceptional proprioception and body control. It is worth every swimmer’s time to refine with precision.

Related Reading

• Underwater Dolphin Kick: The Speed Advantage Within the 15m Rule
• The Hidden Speed in Swimming Turns and Starts: Why Underwater Dolphin Kicking Is the Fastest Phase of the Entire Race
• Start Angle and Entry: How to Maximize Start Distance
• Underwater Dolphin Kick Distance After Entry: Rule Limits and Optimization Strategies