This prospective single-group case series will describe individual responses to a 9-week low-volume, high-intensity race-pace training block in approximately seven finswimmers from the Antioquia Underwater Activities League (LASA), Colombia. The intervention will be oriented towards race-specific swimming paces and will include repeated swimming efforts prescribed according to each athlete's main competitive event. External training load, heart rate, rating of perceived exertion, pace stability, and sport performance will be monitored throughout the block. The primary outcome will be the change in performance time in each athlete's main competitive event from baseline to post-intervention. The study will use an intra-individual descriptive approach and will not include randomisation, a control group, or causal inference.

Finswimming is a high-speed aquatic sport in which performance in short and middle-distance events depends on the ability to produce and sustain race-specific velocity. Traditional aquatic training models often emphasise high training volume; however, for events lasting from seconds to a few minutes, race-pace work, neuromuscular quality, and individualised load monitoring may be particularly relevant.

This study will prospectively follow approximately seven finswimmers from the Antioquia Underwater Activities League (LASA) during a 9-week low-volume, high-intensity training block. All participants will receive the same general training approach, but training distances, repetitions, target times, and race-pace references will be individualised according to each athlete's main competitive event.

Before the intervention, athletes will complete a basic characterisation and a baseline performance assessment using either an official competition result or a standardised control test. Training target times will be estimated from the baseline performance. During the 9-week block, the investigators will record external load variables, including distance, repetitions, weekly volume, target time, and actual time for each relevant repetition. Internal load will be monitored using heart rate and rating of perceived exertion. Pace stability will be estimated from repeated times in event-specific distances using descriptive indicators such as mean time, standard deviation, and coefficient of variation.

After the intervention, performance will be reassessed in the athlete's main event through an official competition or a comparable standardised control test. Data analysis will focus on individual responses, weekly trends, and descriptive changes from baseline to post-intervention. The study will not include a control group or randomisation; therefore, findings will be interpreted as exploratory and non-causal.

Inclusion Criteria:

• Finswimmers affiliated with the Antioquia Underwater Activities League (LASA).
• Regular participation in finswimming training.
• A defined main competitive event.
• Availability to complete the 9-week follow-up period.
• Ability to complete high-intensity swimming training according to the regular sport programme.
• Written informed consent from adult participants.
• For participants under 18 years of age, written informed consent from a parent or legal guardian and written assent from the participant.

Exclusion Criteria:

• Active musculoskeletal injury.
• Relevant pain or acute illness at baseline.
• Medical restriction for high-intensity swimming exercise.
• Inability to complete the planned follow-up.
• Withdrawal of consent or assent.
• Insufficient attendance or incomplete records for the main study variables.
• Any condition judged by the research team or coaching staff to compromise participant safety during high-intensity swimming.

Individual participant data will not be publicly shared because the study will include a small number of athletes from a specific sport league, which may increase the risk of re-identification even after anonymisation. De-identified aggregate data, data dictionaries, and analysis procedures may be made available upon reasonable request, subject to ethics approval and institutional data protection requirements.