|Temporal Analysis of Stroke Cycle in rowing|
TEMPORAL ANALYSIS OF STROKE CYCLE IN ROWING
© 2007 Valery Kleshnev
Analysis of the temporal structure of the cycle is versatile and valuable method in many cyclic sports. Biomechanical measurements were conducted in competitive rowing boats. Boat acceleration, velocity, handle force, oar angles and the segments velocities were measured. Accelerations of whole system and the rower’s CM were derived and used for definition of the temporal structure of the cycle. Six micro phases were derived during the drive and three during recovery phase. It was found that emphasis on acceleration of the boat and rower’s CM switches twice during the drive. Presence of the micro-phase D3 initial boat acceleration was defined as the most important indicator of efficiency of rowing technique. It creates faster moving platform on the stretcher for acceleration of the rower’s CM.
Temporal or phase analysis plays important role in modern sport biomechanics. It is the most versatile biomechanical method of analysis across different sports, because it is based on time only and can represent different motions as a sequence of phases and sub-phases. The phase analysis can play integrating role for other biomechanical methods, such as kinematics and kinetics analysis using video or instrumentation measurements. It can decrease complexity of many sporting techniques and helps their better understanding by the coaches and athletes, which essential for learning and improving of efficient technique (Bartlett, 1999). Each phase should have clearly defined biomechanical function and easily identified phase boundaries, often called key moments or key events. Phase analysis is well developed area in a number of cyclic sports. The most common is definition of two main phases of the cycle:
In many sports these phases divided on sub- or micro-phases. For example, support phase in running is divided on foot strike, mid-support and take-off. Recovery phase is divided on follow-through, forward swing and foot descent (James and Brubaker, 1973). Phase analysis in rowing is not as well developed as it is in other cyclic sports. The purpose of this study is to define sub-phases of the rowing cycle using acceleration patterns of two main masses in the rowing system: rowers and boat.
The main part of the measurements was conducted during the period 1999-2005 as a part of regular biomechanical service of elite athletes of Australian Institute of Sport and Australian National Team. Total number of 294 crews, both male and female, was measured in their own competitive boats. A radio telemetry system was used for data acquisition (12 bit, 25 Hz sampling frequency).
The following mechanical parameters were measured:
The data collected during one sample period was normalized, i.e. converted into a form, which represents one typical stroke cycle for this sample (Kleshnev, 1995, 2004).
The blade Fbl force was derived from measured handle force Fh and actual inboard Lin_a and outboard Lour_a length:
Fbl = Fh * (Lin_a / Lout_a) ( 1)
,where actual inboard Lin_a and outboard Lour_a were derived as:
Lin_a = Lin – Wh / 2 + Wg / 2 ( 2)
, where Wh is the handle width (0.12m for sculls and 0.30m for sweep oars, Wg = 0.04m is the gate width. Lout_a was calculated as:
Lout_a = (Loar – Lin) – Lbl / 2 – Wg / 2 ( 3)
,where Loar is the oar length, Lbl is the blade length.
The drag force Fdrag applied to the boat shell was derived as:
Fdrag = Kdrag * Vb2 (4)
,where Kdrag was calculated as a ratio of integrals of the blade propulsive force and square of boat speed during the stroke cycle:
Then the system propulsive force Fsys was defined as:
Fsys = Fbl * cos (?) – Fdrag ( 6)
The system centre of mass acceleration Asys was calculated as:
Asys = Fsys / msys = Fsys / (mb + mrow ) (7)
,where msy , mb and mrow are masses of the system, boat and rower, correspondingly. The rowers’ centre of mass acceleration Arow was calculated as:
Arow = Frow / mrow_a (8)
,where mrow_a is actual mowing mass of the rower equal to rower’s mass mrow minus a mass associated with the boat, which we assumed equal to 12% of the rower’s mass (feet 4% and shins 8%, Zatsiorsky and Yakunin, 1991). The force Frow applied to the rowers’ CM was derived as:
Frow = Fsys – Fb = Fsys –Ab * mb_a (9)
,where boat acceleration Ab was measured, and mb_a is actual boat mass equal to the boat mass mb plus associated mass macc
RESULTS AND DISCUSSION
We used the boat, rowers’ CM and the system CM accelerations as well as the oar and seat velocity for definition of the micro-phases of the stroke cycle, Figure 1 shows typical biomechanical parameters of a single sculler obtained during detailed measurements.
Figure 1. Typical biomechanical parameters and micro-phases of the stroke cycle (M1x, rate 32 str/min). Key events are marked with circles.
We defined six micro-phases of the drive phase D1 to D6 and three micro-phases of the recovery R1, R2, R3 (Table 1).
Table 1. Characteristics of micro-phases of the stroke cycle
Only D3 significantly increases its relative duration at higher stroke rate (Table 2). The trend of D3 time share is non-linear: it achieves its maximum at the stroke rates 32-36 and then decreases slightly.
Some inefficient crews don’t have D3 phase at all. The duration of the D3 must be optimal at the period of 0.08-0.12s. This means that the switching from push into the stretcher during D2 to pull the handle during D3 and back to push during D4 must be present, but it must be done quickly.
Table 2. Average ratio of each micro-phase to the drive time, its standard deviation, minimal and maximal values, and correlation with the stroke rate.
Coordination of the handle/gate and foot-stretcher forces during the drive phase was found quite complicated. More push (higher foot-stretcher force, legs work) means greater acceleration of the rower’s mass; more pull (higher handle/gate force, upper body work) means greater boat acceleration. The rower’s CM acceleration is the most important, which determines amount of kinetic energy accumulated during the drive and, hence, average speed of the rowers-boat system.
During micro-phase D3, “initial boat acceleration”, rowers accelerate the boat to create faster moving support on the foot-stretcher to further accelerate their bodies, which is extremely important for performing effective drive phase. Fast increasing of the handle force is the main condition of its presence.
During micro-phase D4, “rowers’ acceleration”, rowers push the stretcher again to accelerate themselves and accumulate the main part of kinetic energy. This push-pull-push-pull sequence during the drive requires significant coordination and “boat feel” from rowers.
1. Bartlett, R. (1999) Sport biomechanics: preventing injury and improving performance, Routledge, New York, USA, pp 276.
2. James, S.L. and Brubacker, C.E. (1973) Biomechanical and neuromuscular aspects of running, in Exercise and Sport Science Reviews – Volume 1 (ed. J.H. Wilmore), Academic Press, New York, USA, pp 189-216.
3. Kleshnev V., (2004) Technology for technique improvement. In: Rowing faster, ed. Nolte V., Human Kinetics, USA, pp. 209-228.
4. Zatsiorsky, V.M. and Yakunin. N. (1991). Mechanics and Biomechanics of Rowing – A Review, International Journal of Sports Biomechanics, 7, 229 – 281.