Stretcher force and seat movement

There have been numerous BioRow clients who have asked the same question: “Does the BioRow system measure stretcher force?” The answer is yes, we can do it with a number of methods (RBN 2013/08, 2017/05), but the installation of stretcher force sensors is a very time-consuming and non-trivial task, because boats of various brands have a large variety of stretcher designs. Therefore, we do not measure the stretcher force directly in the BioRow standard testing routine (where installation time is very limited, from 20min for 1x to 70min for 8+), but can estimate it based on the seat movement. Here, relationships between the stretcher force and seat movements will be analysed.

Data was collected in a single equipped with the standard BioRowTel system (RBN 2009/10, 2017/12), augmented with the stretcher sensor measuring the horizontal force component at three points (RBN 2013/08, 2019/01). A male sculler (1.84cm, 85kg) performed the standard BioRow test protocol with an incrementally increasing stroke rate from 18 to 41 spm. The seat position was measured with the BioRow string sensor (2014/12) and was used for calculating the seat acceleration relative to the boat, which was summed up with the boat acceleration, so the seat acceleration relative the global frame of reference (the “Earth”) was derived (Fig.1):

M1x at 34spm

Fh_x + m_ra_r - Fs_x = 0                            (1)

where m_ra_r is an inertial force acting on the rower’s CM, which is a product of its mass m_r and acceleration a_r. So, the stretcher force could be derived as:

Fs_x = Fh_x + m_ra_r                                               (2)

In this study, we assume that the movement of the rower’s CM could be represented by seat. Of course, this is not completely accurate, because the upper body and oar could move significantly differently from the seat, which would affect both the moving rower’s mass m_r and its acceleration a_r. Also, a few other factors affect the balance of handle-stretcher forces, but are not included in Eq.1-2: 1) axial handle force, which is statically transferred to the stretcher (RBN 2019/02), 2) smaller friction force at the slides, 3) aerodynamical drag force acting on the rower’s body. All these assumptions are limitations of the accuracy of the study.

The stretcher force derived with Eq.2 during the stroke cycle Fs_x(d) was compared with the directly measured stretcher force Fs_x(m) (Fig.3). It was found that both forces were very similar during the recovery but quite different during the drive, which is a consequence of the mentioned limitations.

Importantly, both Fs_x(m) and Fs_x(d) curves cross zero at the same time before the catch (1): at this moment, the rower switches from pulling the stretcher (negative force) to pushing it (positive stretcher force), and this moment coincides with the negative maximum of the seat velocity during the recovery (2). Therefore, this study confirms that the seat movement can be used as an adequate indication of the stretcher force during the recovery.

In BioRow reports, the time at which there is a negative peak of seat velocity (2) is used as a starting point for the evaluation of rowers’ synchronisation in a crew (T1, RBN 2014/05, 2015/03). Therefore, the seat movement indicates synchronisation of rowers’ interactions through the stretcher before and at the catch, which is important for effective crew dynamics.

Fig.4 illustrates above point: better synchronisation of the seat movement in crew 1 before (T1) and at the catch (T2-T3) is related to the optimal pattern of boat acceleration (1) and more effective dynamics of the crew during the drive (2).

©2019 Dr. Valery Kleshnev www.biorow.com