The resistance of a flywheel is nearly unlimited and self-adjusting, since the flywheels inertia opposes the force exerted. This suggests that all repetitions are performed maximally (especially without experienced use) which can pose positives and negatives depending on perspective and context of use. The athlete can train using the isoinertial device until exhaustion; despite force production decreases with fatigue and the flywheel can easily accommodate variations in force over an entire range of motion. This can be helpful since different joint angles can be trained and analyzed, athletes with altering biomechanical efficiency can still use the device with simple modifications.

The D-11 can test; power curve, test of power (single set of 10 repetitions at maximum exerted force), control test, test of balance, isometric test. For the last three tests mentioned, the inclusion of force plates are needed and the D-11 has enough ports for four force plates to be attached. The software offers real time; peak power values for both concentric and eccentric action displayed as repetitions or over continuous series. The system automatically counts sets, reps, recovery time, execution time and discs used. Users can also create a profile for individuals or teams, where all recordings are stored and a report can be printed at any time.

Much research has been conducted on the benefits of eccentric overload training in raising neural and muscular performance and in the rehabilitation and prevention of injuries. Some benefits include; higher forces are generated compared with traditional concentric action (LaStoya et al., 2003), unique neuromuscular activation which can be trained specifically (Enoka, 1996), stimulate specific micro-adaptations to regenerate a stronger muscle tissue (Brandenburg and Docherty. 2002), lower pulmonary stress compared to other forms of training (Abbott and Bigland. 1953), effective in injury rehabilitation (LaStoya et al., 2003; Lorenz and Reiman. 2011), increased hypertrophy adaptations when compared to other forms of training (Brandenburg and Docherty., 2002).

Injury Rehabilitation

Unaccustomed eccentric activity frequently results in muscle damage, which is expressed as pain, tenderness and strength loss (Howell et al. 1993). It is generally accepted that muscle injuries typically occur when a muscle under tension is overstretched beyond its limits, and Garrett (1990) has shown that high velocity eccentric action causes greater muscular damage than slow velocity (however this study looked at only untrained subjects). With the increased level of muscle soreness experienced with eccentric overload this would typically pose a problem implementing it into a training plan, however Nosaka et al. (2001) demonstrated the ‘repeated bout effect’. After full recovery has been achieved following the first eccentric overload bout, a repeated training results in minimal symptoms of muscle damage allowing eccentric overload to become a viable training means, especially when considered that the ‘repeated bout effect’ can last for several months (Nosaka et al. 2001). The exact mechanisms are not well defined but it seems to involve neural, mechanical and cellular adaptations (Nosaka et al. 2001).

Arguably one of the most effective techniques for muscular, tendon and ligament rehabilitation is the use of an eccentric focused protocol (Lorenz and Reiman, 2011). The effectiveness of eccentric training in rehabilitation is not new, Bigland and Woods (1976) found: less muscle activity is required to maintain the same force during eccentric action, less muscle fibers are needed to exert a specific force value and also a significant reduction in oxygen uptake during lengthening. Previous work made by Abbot and Bigland (1952) supports this theory and since then it has been common knowledge that eccentric execution results in greater force production with less energy expenditure and less oxygen consumption compared to concentric execution. Recently extensive research has confirmed the effectiveness of eccentric training in the rehabilitation of hamstring muscle strains, tendinopathy and anterior cruciate ligament (ACL) rehabilitation (Lorenz and Reiman, 2011)

The efficiency of using eccentric overload on a flywheel device has been conducted on healthy, elite sporting level subjects (Askling et al. 2003, Tous-Fajardo et al. 2006), injured highly trained athletes (Romero-Rodriguez et al. 2011) and recovering knee injury patients (Greenwood et al. 2007).

Greenwood et al. (2007) commented that the use of a non-gravity independent ergometer is as safe and as effective as standard training equipment for ACL deficient and ACL reconstruction patients. From the results it appears that the flywheel (FW) does offer superior eccentric resistance when compared to a traditional weight stack machine (WS), however this difference is not very big. It is discussed of the possibility of superior effects from the FW with a longer training period, since significant improvement from mid-test to post-test which may indicate performance was improving when the training was terminated. This makes logical sense as full ligament recovery typically takes longer than the study was performed.

Hamstring injuries are considered one of the most common and frustrating soft tissue injuries in elite athletes. An interesting study done by Askling et al. (2003) split 30 elite soccer players into two groups to examine the effectiveness of eccentric overload using an insoinertial leg curl device. The mechanism of hamstring injury is typically during the terminal swing phase during sprinting, just prior to foot contact, where the hamstrings are near maximal length and are eccentrically contracting to decelerate the leg and prepare for contact (Schache, 2009). At this ‘moment' there is a rapid, forceful contraction that is elongated over two joints with minimal sarcomere overlap (Jonhagen et al., 1996). One group of players performed 16 sessions of specific hamstring training (4 sets of 8 repetitions) using the flywheel leg-curl during preseason, with the control group performing no specific hamstring exercise. The training group clearly had a lower occurrence of hamstring injuries (3/15) compared to the control group (10/15) at the end of the season. It is also worth mentioning that there were significant (p<0.05) increases in strength (isokinetic concentric and eccentric muscle strength) and speed (30m sprint) shown in Table 1. A possible explanation for the reduction in injury occurrence may be explained via EMG results; higher ratio of ecc/conc of biceps femoris (BF) compared to semitendinosus (ST). Tous-Fajardo et al. (2006) proposes that BF plays the most critical role of decelerating compared to ST and BF is the most commonly injured muscle in the hamstrings complex. The flywheel appears to train this particular ‘window’ (30o knee flexion) much more effective than typical weight stack leg curl machines (Tous-Fajardo et al., 2006).