TEMPORAL ANALYSIS OF STROKE CYCLE IN ROWING

© 2007 Valery Kleshnev

Abstract

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.

INTRODUCTION

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:

• Support phase (drive, stroke, stride), when athletes have a contact with support substance (ground, water, snow, ice, etc.) and execute effort to propel themselves forward;
• Non-support phase (recovery), when resistance forces decrease speed of the athlete’s center of mass (CM).

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.

METHODS

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:

• Boat velocity (Vb) was measured using an electromagnetic impeller (Nielsen-Kellerman Co., accuracy ±1.0%).
• Boat shell acceleration (Ab) along horizontal axis was measured using an accelerometer (Analog Devices, accuracy ±1%).
• The oar angles in horizontal (?) and vertical (?) dimensions were measured using conductive-plastic potentiometers (Bourns, accuracy ±0.1%) connected to the oar shaft with a light arm and a bracket.
• The force applied to the oar handle (Fh) was measured using custom made strain-gauged transducer attached to the oar shaft (±0.5%). Each oar was dynamically calibrated before each session using a precision load cell (Applied Measurement) attached at the middle of the handle (0.15m for sweep oar, 0.06 for sculling oar). Also, more detailed measurements were conducted on eight male singles, where gate and stretcher forces (Fgate, Ffoot) were measured. This data is used here for illustration purpose only (Figure 1) and is not involved in definition of micro-phases.
• Seat position (Ls) was measured using spring loaded 10-turn potentiometer (Bourns) connected to the seat. Legs (seat) velocity Vleg was derived from Ls. Trunk position was measured on small boats; trunk and arms velocities (Vtrunk, Varm) were derived.

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:

Kdrag =(?Fbl·cos(?))/?Vb²(5)

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 / m_sys = Fsys / (m_b + m_row ) (7)

,where m_sy , m_b and m_row are masses of the system, boat and rower, correspondingly. The rowers’ centre of mass acceleration Arow was calculated as:

Arow = Frow / m_{row_a} (8)

,where m_{row_a} is actual mowing mass of the rower equal to rower’s mass m_row 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 * m_{b_a} (9)

,where boat acceleration Ab was measured, and m_{b_a} is actual boat mass equal to the boat mass m_b plus associated mass m_acc

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