The objectives of the study were to determine the validity and
reliability of peak velocity (PV), average velocity (AV), peak
power (PP) and average power (AP) measurements were made
using a linear position transducer. Validity was assessed by
comparing measurements simultaneously obtained using the
Tendo Weightlifting Analyzer System[R] and T-Force Dynamic
Measurement System[R] (Ergotech, Murcia, Spain) during two
resistance exercises, bench press (BP) and full back squat (BS),
performed by 71 trained male subjects. For the reliability study,
a further 32 men completed both lifts using the Tendo Weightlifting
Analyzer System[R] in two identical testing sessions one
week apart (session 1 vs. session 2). Intraclass correlation coefficients
(ICCs) indicating the validity of the Tendo Weightlifting
Analyzer System[R] were high, with values ranging from 0.853 to
0.989. Systematic biases and random errors were low to moderate
for almost all variables, being higher in the case of PP (bias
[+ or -] 157.56 W; error [+ or -] 131.84 W). Proportional biases were identified
for almost all variables. Test-retest reliability was strong
with ICCs ranging from 0.922 to 0.988. Reliability results also
showed minimal systematic biases and random errors, which
were only significant for PP (bias -19.19 W; error [+ or -] 67.57 W).
Only PV recorded in the BS showed no significant proportional
bias. The Tendo Weightlifting Analyzer System[R] emerged as a
reliable system for measuring movement velocity and estimating
power in resistance exercises. The low biases and random errors
observed here (mainly AV, AP) make this device a useful tool
for monitoring resistance training.
Key words: Back squat, bench press, concentric phase, weight
Validity, precision and reliability in power, strength and movement velocity measurements are essential for assessing performance in many sport disciplines. The main variables traditionally considered in strength training for stimulus control have been the type and order of exercises, the number of sets and repetitions, the workload or training intensity and the recovery time (Kraemer and Ratamess, 2004). Movement velocity is another factor that may be useful for assessing and monitoring resistance training (Izquierdo et al., 2006; Pereira and Gomes, 2003) characterizing the type of neuromuscular response and the subsequent adaptations. The use of workloads providing different levels of resistance (light, medium or heavy) will affect movement velocity (high, moderate or low, respectively) (Sanchez-Medina and Gonzalez-Badillo, 2011) and by varying this velocity power levels could be improved (McBride et al., 2002). For example, light and medium load intensities are best at increasing average power (Baker et al., 2001; Cronin and Sleivert, 2005). Some studies have shown minimal changes in peak power across a spectrum of loads (Kawamori et al., 2005; Kilduff et al., 2007). In addition, heavy loads do not seem suitable to optimize power levels in exercise like bench press (BP) and bench pull (Sanchez-Medina et al., 2013), although they have been shown to improve peak force (Kawamori et al., 2005), power levels and rate of force development in Oympic lifts and their derivatives exercises (i.e., mid-thigh clean pull, back squat) (Comfort et al. 2012; Cormie et al., 2011). These variations demonstrate that force and power levels depend on the type of exercise.
The direct acquisition of this kinetic data requires the use of a force platform. Considered as the "gold standard" for measuring variables associated with strength and power (Nigg and Herzog, 1994), this method is not always practical and cost-effective, and generally limited to laboratory-based settings (Walsh et al., 2006). However, other kinematic systems like the linear position transducer are becoming popular tools for estimating power output, strength and movement velocity in resistance exercises (Cormie et al., 2007a; 2007b; Hori et al., 2007). Linear transducers use a cord attached to a subject or equipment to obtain displacement, movement velocity and acceleration data. These data are then used to estimate strength and power when the mass of the load and/or subject are taken into account. Several studies (Cronin et al., 2004; Drinkwater et al., 2007) have demonstrated the validity of this system for estimating strength and power (r = 0.86-1.00).
Recently, linear position transducer systems like the Tendo FiTROdyne and the Tendo Weightlifting Analyzer System[R] (TWAS) (TENDO Sports Machines; Trencin, Slovak Republic), have been used to assess peak power (Jones et al., 2008). Other studies have revealed their moderate to high test-retest reliability when measuring peak power and peak movement velocity using different loads in various exercises (back squat, bench press and free weight biceps curl) (Jennings et al., 2005; Stock et al., 2011).
The aim of this study was to assess the validity and reliability of the TWAS to measure movement velocity and power (average and peak) in full back squat and bench press exercises. We speculated that the results of both studies (validity and reliability) would show strong correlation, and that average values obtained using the TWAS would not differ from those of the criterion instrument (validity). Further, we expect similar average values between both testing sessions, indicating a high reliability of the TWAS in all the tests. The presence of systematic and proportional biases for the different variables studied was also addressed.
The study protocol was approved by the Review Board of the Department of Physical Activity and Sports Science (Universidad Alfonso X El Sabio, Madrid, Spain). Procedures were in accordance with ethical standards (Harriss and Atkinson, 2011). Seventy-one men (age: 21.6 [+ or -] 2.1 years; weight: 71.9 [+ or -] 10.7 kg; height: 1.76 [+ or -] 0.08 m) volunteered to participate in the validity study and 32 male volunteers (age: 20.7 [+ or -] 3.4 years; weight: 76.3 [+ or -] 8.5 kg; height: 1.72 [+ or -] 0.05 m) were recruited for the reliability study. All participants were healthy students from the Faculty of Physical Activity and Sports Science that engaged in physical activity at least three times per week and were experienced in weight training (bodybuilding and recreation) (years of experience: 3.2 [+ or -] 1.72). Participants were informed of the experimental procedures and purpose of the study and signed an informed consent document before the commencement of testing. Furthermore, participants were asked to refrain from strenuous physical effort 48 hours before each testing session. All tests were carried out at the same time and under similar environmental conditions (21[degrees]C-24[degrees]C and 60%-70% humidity).
Validity was assessed by comparing measurements simultaneously obtained using the TWAS and T-Force Dynamic Measurement System (TFDMS) during two resistance exercises BP and full BS. The reliability of the TWAS was assessed by comparing measurements made in 2 different sessions 1 week apart
Both studies followed the same protocol for measuring movement velocity and power. Two days before the test was carried out, subjects performed a short practice test consisting of a few exercise sets using light and medium loads. As the subjects were experienced, only two practice sessions were completed to avoid learning effects. The protocols followed similar guidelines to those prescribed by Mate et al. (2014). On the test day, participants performed a general warm up followed by a specific warm up (Table 1) to become familiar with the lift being performed. The warm-up consisted of 5 minutes of gentle running and 5 minutes of stretching and joint mobility exercises of the upper and lower extremities. Three exercise sets were performed: the first consisted of 8 repetitions of bar exercises at a moderate execution speed, the second of 6 repetitions lifting a 20 kg load at submaximal speed and the third of 4 repetitions lifting a 30 kg load at maximal speed. The recovery time between each set was always 1 minute. After a rest period of 3 min, participants initiated the protocol for measuring movement velocity and power in the BS. It consisted of 4 sets: 4 repetitions lifting a 40 kg load, 3 repetitions lifting a 50 kg load, 2 repetitions lifting a 60 kg load and, finally, a set in which subjects performed as many repetitions as possible using a load of 85% relative to the one repetition maximum (1RM), or maximal strength (estimated in the previous set according to the movement velocity obtained with the T-Force Dynamic Measurement System[R]) (Sanchez-Medina et al., 2010). If 5 to 6 repetitions were carried out, the 85% of 1RM was taken as valid given the direct relationship between the number of repetitions that can be executed at 85% and 1RM (Baechle and Earle, 2008).
Three minutes of...