*4.2. Performance Features*

Over half of the included records (24/36—66.7%) used inertial sensors to monitor stroke quality. In rowing, the stroke is the most vital performance indicator and overall performance can be increased by either increasing the propulsive impulse or decreasing the drag impulse within a stroke cycle [7]. The ability to measure the quality of the stroke cycle with an abundance of metrics is thus of high interest to coaches and athletes alike. Stroke rate (cadence) was the most frequently extracted metric surrounding stroke quality (12/24—50.0%) [14,16,21,25,26,33–37]. Athletes can have an optimal stroke rate based on their physicality and thus it is an important metric to monitor. It can also be measured easily with correctly placed inertial sensors. Stroke variability was investigated by seven of the stroke quality concerned records (7/24—29.2%). Stroke by stroke variation in the inertial signals can indicate when athletes are performing well or have faults in their biomechanical processes, stroke variance is also highly investigated in swimming [5]. Three records (3/24—12.5%) addressed the different phases of a stroke, such as the recovery and drive and the ratio of these phases [21,31,32]. The drive/recovery phase ratio is generally used to describe an athlete's rhythm with beginners being advised to aim for 2:1 (drive: recovery). The rhythm of a rower can be directly impacted by increases and decreases in stroke rate; both of these parameters were measured simultaneously by Tessendorf et al. [21]. Stroke length was monitored in three of the included records (4/24—16.7%) [21,26,46,47]. Stroke length is measured in terms of the angle the oar sweeps from catch to finish position. By obtaining measures of stroke length, a coach can tailor training schedules to ensure athletes are entering catch and finish phase of their strokes optimally.

Over a third of the included records (13/36—36.1%) analysed metrics surrounding instrumented rowing materials. Six of the thirteen records investigating instrumented rowing materials (6/13—46.2%) measured boat velocity [25,26,34,35,39,43]. Boat velocity is the most important performance indicator in rowing. GPS is widely accepted as an accurate method to measure boat velocity in rowing; however, the signal is prone to drop outs. Thus, inertial sensors are an appealing method to obtain boat velocity or to work in conjunction with GPS to provide data during periods of drop out. Using inertial sensors to measure velocity can prove to be challenging due to gravitational o ffsets and sensor drift. Rowing, like swimming, consists of repetitive movements, relatively constant orientation and linear directional movement, providing advantages to the signal processing steps needed to measure velocity [5,50,51].

Five of the instrumented material records (5/13—38.5%) used inertial sensors to measure movement of the oar through the water [18,36,37,46,47]. This can provide coaches and athletes with a visual representation of oar's position through the stroke cycle. This gives potential for a golden standard template of a stroke which enters the di fferent stroke phases at the optimal times to be used as a standard that athletes with certain deficiencies try to replicate. Four of the records (4/13—30.1%) used inertial sensors to monitor the boats position [25,39,44,45], with one of these records investigating the seat position within the boat. Similar to velocity, position is often measured by GPS; nonetheless during drop outs, inertial sensors can assist in interpolation/extrapolation of the GPS data to continuously monitor boat position even when GPS signal drops out. It can also be used to measure stroke e fficiency in terms of the distance the boat travels per each stroke. This investigation used an inertial sensor to measure the position of a sliding rowing boat seat. This movement is a direct result of the rower's leg movement and thus is key for optimizing performance. Deficiencies in leg movement of a rower can significantly decrease the velocity of the boat.

Akin to boat position, three of the records (3/13—23.1%) used inertial sensors to monitor the orientation of the boat [18,35,44]. By fusing the sensors within an inertial sensor, typically an accelerometer and gyroscope sometimes accompanied by a magnetometer, an accurate estimation of orientation can be calculated. This is an important metric, as if the boat is deviating too much from its linear path then e fficiency is decreased. Thus, athletes can focus on stabilization in their training routines if this is an issue.

Oar stroke force was also an instrumented material metric of interest with three of the records (3/13—23.1%) investigating it. As mentioned earlier, performance in rowing is dependent on increasing propulsive impulse while decrease drag impulse in a stroke cycle [7]. The oar acts as the link between the forces developed by the rower to the blade and creates the propulsive force. Enabling coaches and athletes to measure accurately the oar stroke force is extremely advantageous as strength and conditioning programs can be prescribed to help increase the oar stroke force and in turn overall propulsion of the boat.

Seven of the reviewed papers (7/36—19.4%) investigated rowing athletes' physiological and physicality parameters using inertial sensors. Four of these records (5/7—71.4%) analysed crew synchronicity [12,14,20,23,32]. An elite rower has high special fitness; high coordination, motor control and functional strength [52]. As well as special fitness, team rowers also have to maintain boat stability while staying synchronised with their crew. High synchronicity has been related to increased performance as reflected by the average hull speed [23]. Thus, being able to use inertial sensor data as a measure of synchronicity between rowers can aid coaches in having a more informed understanding of their rowing crew's interactions during training and competition. Armstrong and Nokes [12] investigated synchronization through acceleration signatures and electromyography signals. This demonstrated the muscle recruitment requirements for different boat positions (stroke, bow). It was also clear to see in the EMG signal the difference between good rowing technique and rowing when 'shooting the slide', which is driving with the legs so the seat leads the back into the drive phase rather than leg and back drive acting as one phase. Two of the records (2/7—28.6%) measured athletes' power output. Estimates of the angular rotation of the oar shaft were obtained using a fitted an accelerometer; the inertial sensors were used in conjunction with force sensors and thus, with the shaft's radius, torque could be derived. Power is the product of torque and angular rotation and can be calculated using an inertial sensor and force sensor. Being able to quantify power output in training and rehabilitation means coaches can monitor their athletes more thoroughly and ensure that their program is achieving efficient results. Atallah et al. [17] (1/7—14.3%) used an earn worn sensor to classify activates and estimate energy output; this was a general study that incorporated rowing as one the activities. However, it does have the possibility to be used purely for rowing.

Three of the reviewed records (3/36—8.3%) provided general biofeedback measures such as tracking the athletes body during rowing, evaluating different methods of providing sonification feedback to rowers and activity classification [16,17,33].
