Isokinetic training has been a popular form of resistance training since the late 1960s (Hislop and Perrine, 1967; Thistle et al., 1967). Isokinetic dynamometers provide the trainee with the ability to contract skeletal muscles with near-maximal or maximal effort at controlled velocities. Several studies have shown that isokinetic resistance training at a spectrum of joint angular velocities increases muscle strength (Adeyanju et al., 1983; Cadore et al., 2014; Coyle et al., 1981; Kelly et al., 2007; Lesmes et al., 1978) and power and rate of force development (Cadore et al., 2014; Kanehisa and Miyashita, 1983). In addition, isokinetic resistance training has been shown to increase knee extension rate of velocity development (Brown and Whitehurst, 2003; Murray et al., 2007), and knee flexion and extension muscle endurance (Adeyanju et al., 1983; Lesmes et al., 1978). However, studies investigating the carryover effects of isokinetic training to dynamic maximal muscular strength and endurance performances are few (Pipes and Wilmore, 1975). Although isokinetic peak concentric (CON) and eccentric (ECC) torques at various angular velocities have been shown to correlate to several measures of performance such as vertical jump (Bosco et al., 1983), 40-yd dash sprint speed (Anderson et al., 1991), agility (i.e. figure 8 drill) performance (Anderson et al., 1991), 6-sec maximal stationary cycling performance (Wilson et al., 1997), kicking velocity (Masuda et al., 2005), and throwing velocity (Pedegana et al., 1982), part of the issue is that the vast majority of studies have only used single-joint isokinetic measures. Single-joint exercises isolate one major joint action and/or major muscle group in an open kinetic chain and may lack external validity when used to assess athletic performance (Wilson et al., 1997). The majority of athletic skills involve multiple-joint actions recruiting large muscle mass. Wilson et al. (1997) showed stronger relationships between 6-sec maximal stationary cycling performance and multiple-joint isokinetic squat performance (r = 0.57 to 0.65) than 6-sec maximal cycling performance and single-joint knee extension isokinetic peak torque (r = 0.45 to 0.51). Thus, multiple-joint isokinetic measures may serve as a more valid isokinetic testing and training modality for athletic purposes.
The concept of multiple-joint isokinetic exercise dates back to the 1970s (Pipes and Wilmore, 1975) although relatively few studies have examined its utility. Several researchers have used multiple-joint isokinetic assessment protocols for exercises such as the leg press (Dvir 1996; Engle, 1983; Levine et al., 1991), squat (Fry et al, 2000; 2003; Hortobagyi and Katch, 1990; Weiss and Relyea, 1997; Weiss et al., 1996; 1998; Wilson et al., 1997), and bench press/chest press (Hoffman et al., 2011; Hortobagyi and Katch, 1990; Miyaguchi and Demura, 2012) and have generally shown high testing reliability (Fry et al, 2000; 2003; Weiss et al., 1996; Wilson et al., 1997). Weiss and Relyea (1997) showed that multiple-joint isokinetic squat testing across a spectrum of linear velocities produced similar force-velocity and power-velocity curves to single-joint isokinetic assessments.
However, only few studies have investigated strength and local muscular endurance increases during multiple-joint isokinetic resistance training. In particular, the potential for multiple-joint isokinetic resistance training to elicit transfer of training effects to non-isokinetic measures of muscle strength and endurance performance remains under studied. Pipes and Wilmore (1975) showed that isokinetic resistance training (including three multiple-joint exercises) significantly increased isokinetic (elbow flexion, extension, shoulder extension, bench press at 24 and 136[degrees]/sec), isometric (bench press, elbow flexion, extension, knee extension at 90[degrees] and 135[degrees]), and dynamic leg press, bench press, arm curl, and bent-over row 1RM strength. Sharp et al. (1982) showed that 4 weeks of training on a quasi-isokinetic dynamometer designed to replicate a swim stroke increased arm power by 18.7% and swim performance by 3.8%. Papadopoulos et al. (2014) investigated 8 weeks (2 times per week) of ECC isokinetic leg press training and reported significant increases (13 to 26%) in drop jump height and maximal power in addition to large increases in maximal ECC (65%) and CON (32%) leg press force. It appears from the results of only few studies that multiple-joint isokinetic resistance training may enhance non-isokinetic performance. However, potential transfer effects to dynamic maximal strength and endurance performance remain largely unknown despite increasing popularity of new isokinetic dynamometers developed during the past 10 years.
Therefore, the purpose of the present study was to investigate the magnitude of isokinetic and dynamic strength and muscular endurance increases using a recently-developed dynamometer. It was our hypothesis that 6 weeks of multiple-joint isokinetic resistance training alone would significantly increase free-weight (bench press and bent-over row) maximal strength and upper-body local muscular endurance in women. This study design enabled us to investigate the potential for multiple-joint isokinetic resistance training to increase isokinetic and dynamic muscular strength and endurance performance. Specifically, the primary objective of the present study was to investigate potential transfer of training effects of multiple-joint isokinetic training to dynamic 1RM strength and local muscle endurance (modified push-up) performance and not to compare multiple-joint isokinetic resistance training to other resistance training modalities. Thus, only one isokinetic training group was examined for this purpose.
Seventeen healthy women agreed to participate in the present study (Table 1). None of the women were actively participating in resistance training prior to the study. Seven of the subjects had no resistance training experience whereas 10 subjects had previous experience but had not trained within a 6-month period prior to initiating the present study. Subjects underwent one week of familiarization (2-3 sessions) with study procedures prior to testing and refrained from all other exercise throughout the experimental period. Familiarization focused on subjects' ability to perform the testing exercises and accustom them to the isokinetic dynamometer. During this time, height was measured using a wall-mounted stadiometer and body mass was measured using an electronic scale. Percent body fat was estimated via a three-site skinfold test. The sites measured were the triceps, suprailiac, and thigh skinfolds for women using methodology previously described (Jackson and Pollock, 1980). Body density was calculated using the equation of Jackson and Pollock (1980) and percent body fat was calculated using the equation of Siri (1956). The same research assistant performed all skinfold assessments. This study was approved by the college's Institutional Review Board and conformed to the policy statement with respect to the Declaration of Helsinki. Each subject was informed of the study requirements, criteria, and risks and subsequently signed an informed consent document prior to participation. No subject had any physiological or orthopedic limitations that could have affected exercise performance as determined by completion of a health history questionnaire.
In order to examine the primary hypothesis of the present investigation, subjects were randomly divided into an isokinetic resistance training (IRT) group or a non-exercising control (CTL) group. The IRT group under-went 6 weeks of training (2 days per week on nonconsecutive days) consisting of 5 sets of 6-10 repetitions at 7585% of subjects' peak CON and ECC strength for the isokinetic chest press and seated row exercises at an average linear velocity of 0.15 m x [s.sup.-1] (3-sec CON and 3-sec ECC phases). Peak CON and ECC force during the chest press and seated row, 1RM bench press and bent-over row, and maximum number of push-ups performed were assessed pre and post training.
Free-weight strength testing
One-repetition maximum strength was assessed for the...