Recovery phase and boat speed efficiency
Discussions about the rhythm
of the recovery phase are quite often seen in rowing literature and rowing
forums. It was argued that “proper” movements of the rower during the recovery
phase could make the boat velocity less variable and increase its efficiency
and performance. A theory behind this idea originates from a 1986 article by
Sanderson and Martindale (1), which concluded: “…the maximum kinetic energy of the rower and boat relative to the
rower-boat centre of mass can be reduced by speeding up the initial part of the
recovery, slowing the middle part of the recovery, and speeding up the final
part of the recovery.” This hypothesis has not yet been proved experimentally.
Definitions of the shell
velocity efficiency Es was discussed in RBN 2015/01. The analysis of a large data
sample measured in singles (n=2760) revealed that Es is highly dependent on
the stroke rate (y=-0.00028x+0.988, r=-0.63), which explains 40% of its
variation. So, a simple correlation between variables may reflect their mutual
dependence on the stroke rate, but not relationship between them. Therefore,
linear trends were built on rate-dependant variables and their deviations from
the trend were analysed, and were called normalized variables (e.g. nEs).
To follow the hypothesis
above, the shapes of the handle and rower’s segment velocities during recovery
should be more rectangular, i.e. the ratio of its average velocity to the
maximal (R = Vav/Vmax, which was analysed in the
previous Newsletter) should be higher. However, the correlations between nEs
and R
of velocities during the recovery were insignificant (r<0.1), which means various rhythms of recovery appear to be
ineffective on the variation of the boat speed and shell efficiency and
disproves the above hypothesis.
To find factors affecting the
shell velocity efficiency, all 166 biomechanical variables were correlated with
nEs
and the factors were ranked. The highest magnitude of correlation was found in
the length of the leg drive (r=-0.45, y=-0.0218x+0.011). This means, a longer seat displacement increases the boat
velocity variation and energy losses, but
the effect is very small: typically, every 1cm of longer seat travel decreases
efficiency by 0.02% - less than 0.1s over 2km race.
To illustrate this finding, two
single scullers of a similar level, but with different rowing styles were
compared at 34spm (Fig.1). Sculler 1 (1.93m, 98kg) has a much longer seat
travel of 56cm (Table 1) and his technique could be classified as the ‘Adam’
style (RBN 2006/03). Sculler 2 (1.96m, 95kg) represents an exaggerated ‘DDR’
style and has only a 35cm legs drive at a similar stroke length. The comparison
of the main biomechanical variables (Fig.2) shows that Sculler 2 also has a
better finish technique: he returns the trunk earlier (a), at the end of the
drive (“Finish through the handle” RBN 2006/10), which creates less boat
acceleration at the beginning of the recovery (b).
The highest peak of the boat acceleration (c) and velocity (d) during recovery were related to much higher legs (seat) velocity (e) in Sculler 1 due to his longer seat travel. This increases variation of the boat speed and reduces the shell efficiency. As a result, Sculler 2 had a 4.4% lower variation of boat velocity, so his shell efficiency was 1.2% higher, which means a gain of about 5s at 2km race. In fact, the speed of the sculler 2 was 18s faster, (albeit at different weather conditions.) Only the stroke rate and length of seat travel affect the boat speed variation and efficiency; other characteristics of the recovery phase appeared to have no effect on it.
The above finding confirms
Volker Nolte’s (2) principle N.4: “The
displacement of the centre of gravity (i.e., the seat displacement - VK)
in the horizontal plane should be minimised without losing length in the stroke
and there should be no lost time with stops or pauses”.
Does this mean that half slide
rowing is preferable? Of course not, because this would reduce power production
and average boat speed, but the gain is very small. However, the length of legs drive should be optimal
and excessive seat travel could be counter- productive.
References
1.
Sanderson B., Martindale W.
1986. Towards optimizing rowing technique. Med. Sci- Sporls Exerc, Vol. 18, No.
4, pp. 454-468.
2. Nolte V. 1991. Introduction to the Biomechanics of Rowing. In: FISA Coaching Development Programme Course -
Level III. p. 89