**1. Introduction**

Intracycle velocity variation (IVV) is a biomechanical variable that reflects the velocity fluctuation within a swimming cycle and was one of the first swimming-related research topics [1,2] aiming to better understand performance evolution constraints. IVV depends on the interaction between propulsive and resistive forces for each upper limb cycle, with the interaction between these accelerations and decelerations considered an efficiency estimator [3,4]. The first attempt to evaluate this variable was made for the backstroke, breaststroke and front crawl [1], and concluded that common stopwatches could not adequately assess swimming velocity (changes were observed within an s or an m). Velocity was measured with a natograph (recording the distance travelled every 1/5 of an s), and its variation was observed in each studied swimming technique (with front crawl being the fastest due to its smoothness). At that time, swimming was associated with motor cars' mechanics since, if driving with a variable speed would be wasteful, the same should occur in the human machine. This study provided important insights and investigation lines for the current topic.

Afterwards, the natograph was improved [2,5–7], with several mechanical devices beginning to be used (cable speedometers [8,9], accelerometers [10], and other gadgets [11]),

**Citation:** Fernandes, A.; Afonso, J.; Noronha, F.; Mezêncio, B.; Vilas-Boas, J.P.; Fernandes, R.J. Intracycle Velocity Variation in Swimming: A Systematic Scoping Review. *Bioengineering* **2023**, *10*, 308. https://doi.org/10.3390/ bioengineering10030308

Academic Editors: Christina Zong-Hao Ma, Zhengrong Li and Chen He

Received: 31 December 2022 Revised: 20 February 2023 Accepted: 24 February 2023 Published: 28 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

all characterized by a mechanical connection to a swimmer's anatomical point. Despite the incapacity to monitor the swimmer's bodily inertia due to the constant change in the position of the centre of mass, these methods were very interactive and relevant to training due to the immediate output availability. Cinematography was also very common for evaluating IVV [12–14], qualitatively and quantitatively assessing the movements in a three-dimensional nature with (at least) two cameras. These image-based methods, usually involving the digitisation of film or video images, presented similar issues related to the body inertia capture, as well as image distortions, water bobbles and waves, parallax, digitising and calibration errors, and reduced interactivity (due to the delay between data collection and the swimmer feedback as a result of image processing).

Methods dealing with the centre of mass motion have the abovementioned problems but are even more time-consuming and complex. Nowadays, depending on the aims of IVV investigation, researchers are divided between using an anatomical fixed point or the centre of mass [15–17]. Considering the accessibility of mechanical methods, the agreement between these measures was evaluated, but the centre of mass reference was constantly overestimated, and it is axiomatically considered a gold standard in those comparisons [17–19]. Due to the current approach to this issue, forward hip movements were considered a good estimate of the swimmers' horizontal velocity and displacement, being relevant for diagnostic purposes but not representing the movement of the centre of mass [15,16,20]. Hip error magnitude should also be considered because it overestimates swimming velocity and, consequently, the IVV of the four conventional swimming techniques [17–19].

Despite the above-referenced methodological concerns, the association between swimming IVV and performance continues to be investigated even though the findings are quite divergent. Increases in velocity were associated with lower [3,21], stable [22–33] and higher IVV [34,35] in different swimming techniques. Better propulsive continuity in front crawl and lower swimming economy in breaststroke and butterfly (due to elevated resistive forces and amount of work) are the suggested explanations. In addition, when comparing competitive swimming levels for the same pace and swimming technique, better swimmers were observed to have higher [36,37], lower [10,21,23,33,34,38,39] or similar IVV [40,41] values compared to their counterparts. Regarding conventional swimming techniques, breaststroke presents the highest IVV values, followed by butterfly, backstroke and front crawl [3], although alternative techniques' scores are very similar [42].

Considering the IVV research background and its significance to assess biomechanical development in swimming, the aim of the current study wa to accomplish a systematic scoping review of IVV in competitive swimming regarding the four conventional techniques, assessment and quantification methods, participants' information (sex, competitive level and age category), protocols, and association with swimming economy and hydrodynamic drag. The closest work to a review about IVV is a book chapter [43] addressing it as a relevant variable to assess swimming biomechanical and coordinative development, as well as its association with swimmers' technique, exercise intensity, economy and fatigue.

#### **2. Materials and Methods**

The current systematic scoping review protocol was designed according to PRISMA 2020 [44] and Prisma-ScR guidelines [45], as well as Cochrane recommendations [46]. The protocol was created and pre-registered as an OSF project on 6 July 2022 (https://osf.io/m43pj, accessed on 23 December 2022).

#### *2.1. Eligibility Criteria*

Original peer-reviewed articles and texts from the Proceedings Book of the Biomechanics and Medicine in Swimming, published in any language or date, were included in the current study. Letters, editorials, meetings abstracts, commentaries, and reviews were excluded. The eligibility criteria were defined by the Population, Exposition, Comparator, Outcomes and Study (PECOS) design model, in accordance with PRISMA guidelines:

(i) population (competitive swimmers of any sex, with no injuries, excluding triathletes, divers and Paralympic athletes and artistic and open-water swimmers); (ii) exposure (IVV assessments at any swimming distance, pace, technique and protocol); (iii) comparison (not mandatory if intervention was performed); (iv) outcome (IVV was the primary outcome, with the secondary outcomes being described in the 2.6. data items subsection and not used as inclusion/exclusion criteria) and (v) study design (no limitations for the study strategy).
