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Baker (2001) reported that peak power output during squat jumps was achieved at 46-51% of 1-RM in stronger rugby league athletes, compared to 58-69% 1-RM in the weaker rugby league athletes. Power output, however, was calculated from bar velocity and system mass using inverse dynamics which has been shown to result in an altered PMax load (Cormie et al., 2007a; Cormie et al., 2007c; Cormie et al., 2007b; Cormie et al., 2007e; McBride et al., 2011).

Stone et al. (2003a) investigated the effects of loading (10-90% 1-RM in 10% increments) on power output, calculated from bar velocity using inverse dynamics, during vertical jumps in trained subjects. Peak power output occurred in the 10% condition in both the squat jump (5113.07 ± 1482.17 W) and countermovement jump (5199.73 ± 1301.06 W), with a progressive decline in power output with an increase in load. When subjects were divided in to the weakest (n=5) and the strongest (n=5) peak power output occurred at 10% 1-RM whereas peak power occurred at 40% 1- RM in the strong group. The differences in optimal loading between the studies of Baker (2001) and Stone et al. (2003a) are likely due to the higher strength levels in the rugby league players in the earlier study, as both studies highlight that maximal strength can affect the load which elicits peak power during squat jumps. More recently, however, Lake et al. (2012) have suggested that barbell kinematics should not be used to assess power during squat jumps as it does not reflect displacement or velocity of the system CoM, and therefore leads to overestimation of velocity

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resulting in an elevated power values which was 18.7% greater than the power assessed via forward dynamics.

As previously mentioned, Cormie et al. (2007c) compared six different methods of assessing power during squats, squat jumps and power cleans across a spectrum of loads, in well trained males. Results demonstrated that one LPT plus barbell mass under-valued force and therefore power during the squat and jump squat, where as the one LPT and two LPT methods (including system mass) over-valued force and therefore power in line with the findings of Hori et al. (2007) during the hang power clean and squat jump. During the squat jump, the use of 1 LPT and mass resulted in a PMax load of 42% 1-RM (3379.56 ± 505.84 W), whereas all other methods identified body mass (no external load) as PMax load 6260.95-6496.95 W). The authors conclude that methods of assessing power need to be standardised to ensure that findings between studies are comparable. These findings and recommendations for the squat jump have been supported by a series of other studies published by these authors (Cormie et al., 2007a; Cormie et al., 2007b; Cormie et al., 2007e; McBride et al., 2011).

A recent study by Turner et al. (2012) found peak power output (calculated from the product of vertical ground reaction force and bar velocity) and peak bar velocity to occur at 20% 1-RM, in well trained rugby players, although this was not significantly (p>0.05) greater than the 30% 1-RM load; unfortunately they did not use loads <20% 1-RM. Unsurprisingly, peak vertical ground reaction force occurred at 100% 1-RM. Our work (Thomasson and Comfort, 2012; Comfort et al., 2013a) however, found that peak power output during squat jumps, calculated using forward dynamics, occurred at body mass (no external load) although this was not significantly different to the 10 and 20% loading conditions, in both well trained rugby league players and collegiate level athletes, in line with previous findings (Cormie et al., 2007c; Cormie et al., 2007b; Cormie et al., 2007e; Cormie et al., 2008; McBride et al., 2011).

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More recently, Pazin (2013) compared the peak power, peak force and peak velocity (of centre of mass) across loads (0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3 x body mass) during both squat jumps and countermovement jumps in male subjects (n=40). Loading was varied by the use of elastic resistance, to either add to or subtract load from body mass. Dependant variables were assessed with participants standing on a force plate with power calculated using forward dynamics. Results demonstrated that peak power was achieved at body mass, with no difference between strength trained athletes (n=10), speed trained athletes (n=10), physically active non-athletes (n=10) and sedentary individuals (n=10).

As with the findings for PMax loads during the squat, the mode of exercise (machine versus free weight) appears to influence the PMax load, with the use of a machine resulting in a increase in both 1-RM performance and PMax load (Harris et al., 2007; Harris et al., 2008). Harris et al. (2007) used forward dynamics to assess power output during loaded (10-100% 1-RM) squat jumps, performed on a modified hack squat machine, in national level rugby players. Results demonstrated that individual peak power output occurred at 39 ± 8.6% 1-RM. Interestingly, a change in load ±20% either side of the individuals PMax load resulted in only a small change (9.9 ± 2.4%) in peak power output. It is worth noting that these subjects were very strong (relative 1-RM 2.67 ± 0.46 kg/kg), which along with the use of a machine, may have resulted in the increased PMax load compared to previous studies (Stone et al., 2003a; Stone et al., 2003b; Cormie et al., 2011b; Cormie et al., 2011a; Thomasson and Comfort, 2012; Comfort et al., 2013a).

From the results of these studies it would appear that peak power output, during squat jumps, occurs at or around body mass (no external load) when assessed using forward dynamics (Cormie

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et al., 2007a; Cormie et al., 2007c; Cormie et al., 2007b; Cormie et al., 2007e; Cormie et al., 2008; Pazin et al., 2013), although there may be some variation between individuals, especially if very strong (Harris et al., 2007; Harris et al., 2008). It is also worth noting that during such exercises, performed at PMax loads, it is possible to perform a greater number of repetitions than those usually recommended (1-5 repetitions) for power development, with Thomasson and Comfort (2012) finding no decrease in performance during sets of 6 repetitions performed at body mass (PMax load), 20% and 40% 1-RM, with a more recent study finding no decrement in performance during 10 repetitions performed at body mass (PMax load), 10%, 20%, 30% and 40% 1-RM (Comfort et al., 2013a).