Relay Exchange and Take-off Timing Analysis
1. Physics of the Relay Start A relay exchange involves a swimmer on the block reacting to the incoming swimmer touching the wall. The second swimmer can start their forward lean and arm swing while the first swimmer is still swimming the final meter. The rules state the second swimmer's feet must not lose contact with the block before the incoming swimmer touches the touchpad.
This timing is incredibly precise. The departing swimmer must anticipate the touch rather than react to it. If they wait until they see or hear the touch, they will lose 0.2 to 0.3 seconds of reaction time. Instead, they must observe the incoming swimmer's stroke rate and distance from the wall, initiating their dive sequence so that their feet leave the block at the exact millisecond the incoming swimmer makes contact.
Physically, this is an exercise in conservation of momentum and timing. The departing swimmer starts their arm swing and shifts their center of mass forward while still on the block. This allows them to enter the water with horizontal velocity and clean body alignment, maximizing their underwater glide phase.
2. Exchange Reaction Time Limits The official reaction time is defined as:
$$R_t = T_{\text{departure}} - T_{\text{touch}}$$
- Under FINA/USMS rules, a reaction time less than $-0.03$ seconds results in an automatic disqualification (DQ). - Perfect exchanges target a window between $+0.01$ and $+0.04$ seconds. - A conservative exchange of $+0.20$ seconds loses $0.15$ seconds of free time per exchange, totaling nearly half a second of lost speed across three exchanges.
In high-level competition, half a second is the difference between a gold medal and finishing off the podium. Therefore, teams spend significant training time refining their exchanges. The rules allow for a small tolerance (up to $-0.03$ seconds) to account for slight discrepancies in timing system calibration and the physics of the starting block sensor.
Starting blocks equipped with relay exchange sensors (such as the Omega OSB11) measure the force exerted by the swimmer. When the force drops to zero, the block registers the departure. This is compared directly to the touchpad signal. If the departure time is recorded as more than 30 milliseconds before the touch, the system flags a disqualification.
3. Timing Calibration and Touchpads Automatic officiating equipment uses dual-sensor inputs. The touchpad registers the incoming swimmer's impact, and the pressure-sensitive starting block registers the exact millisecond pressure drops to zero. Technical coaches should note that light touches from fatigued swimmers may delay touchpad registration, resulting in skewed exchange records.
The touchpad requires a certain threshold of force to close the internal switch and register the touch (typically around 1.5 to 2.5 kg of force). If a swimmer finishes poorly—gliding to the wall with their hand extended but not applying enough force immediately—the touch might not register until their body momentum carries them into the pad, creating a delay of 0.05 to 0.10 seconds.
This delay can result in a "false DQ." If the departing swimmer timed their start perfectly to the actual touch, but the touchpad registered the touch late, the official reaction time might calculate as $-0.05$ seconds, triggering a disqualification. To prevent this, major meets use high-speed backup cameras aligned with the finish line. Referees can review the video frame-by-frame to verify if the departing swimmer's feet left the block before the incoming swimmer's hand touched the wall, overriding the automatic timing system if a sensor error occurred.
4. Coaching Drills and Training To train reaction timing, coaches use visual cues rather than auditory ones. The departing swimmer watches the incoming swimmer's head break the water surface at the 5-meter mark, executing their windup to hit full extension at the touch.
One effective drill is the "5-Meter Flag Cue." The departing swimmer stands on the block and focuses on the 5-meter flags. As the incoming swimmer's head passes under the flags, the departing swimmer begins a mental count based on the swimmer's speed.
Another drill involves video feedback. Coaches use smartphones or specialized cameras to record exchanges at 120 or 240 frames per second. By reviewing the footage with the swimmers, they can point out exactly when the arm swing started, when the feet left the block, and when the touch occurred. This helps swimmers build a kinesthetic awareness of the correct timing, turning a high-risk maneuver into a reliable, consistent skill.