Effects of Instability on Force and Velocity

The following is an exclusive excerpt from the book Developing the Core, published by Human Kinetics. All text and images provided by Human Kinetics.

The effect of instability exercises, such as sit-ups or squats, on the ability to exert force or generate high velocity is quite controversial in the literature. Siff (1991) observed that the wider range of movement that is available with the use of a ball is preferable to similar actions performed in most circuit training gyms because it provides resistance through a greater range of motion (better flexibility). Additionally, stability balls are often advocated to promote proper posture while seated in order to prevent low back pain (Norris 2000).

However, instability deficits have been reported and include the depression of force or power output with instability. For example, the use of a stability ball resulted in decreased force output during knee extension (↓70 percent) (Behm, Anderson, and Curnew 2002), plantar flexion (↓20 percent) (Behm, Anderson, and Curnew 2002), and isometric chest press (↓60 percent) (Anderson and Behm 2004). Similarly, Kornecki and Zschorlich (1994) demonstrated 20 to 40 percent decreases in muscular power when utilizing an unstable pendulum-like device during pushing movements. Muscle contributions to stability increased on average by 40 percent when a handle was changed from stable to unstable during pushing movements (Kornecki, Kebel, and Siemienski 2001). While isometric force appears to be reduced, 1RM isokinetic barbell bench press strength on the stability ball compared to a stable flat bench was reported to be similar (Cowley, Swensen, and Sforzo 2007; Goodman et al. 2008). These two studies utilized untrained women and recreationally active people, respectively, so it is not known whether elite lifters could also maintain their high forces on an unstable base.

Koshida et al. (2008) suggested that the small decrements in force, power, and velocity (6 to 10 percent) with a dynamic bench press performed on a stability ball may not compromise the training effect. However, because they implemented a 50 percent of 1RM resistance, the possible beneficial training effects may be more applicable to localized muscular endurance rather than maximal and hypertrophic strength training. These studies imply that the type of muscle action performed affects strength on unstable platforms.

Furthermore, an increase in the stiffness of the joints due to instability can limit force, power, and performance. A stiffening strategy is adopted when people are presented with a threat of instability (e.g., walking on a balance beam, stepping on ice, or standing on an unstable platform) (Carpenter et al. 2001). This type of stiffening strategy can adversely affect the amount and velocity of voluntary movements (Adkin et al. 2002). New movement patterns, especially those performed when unstable, are generally learned at a low velocity. However, most sports are performed at high velocities, resulting in a contradiction of training specificity (Behm 1995; Behm and Sale 1993).

Drinkwater, Pritchett, and Behm (2007) had participants perform the barbell back squat with varying resistance on a stable floor, foam pads, or a BOSU ball. There were significant instability-induced decrements in power, force, and velocity as well as in range of motion. The deficits were generally greater as the resistance increased. Similarly, McBride, Cormie, and Deane (2006) reported reductions in peak force, rate of force development, and agonist muscle activity when performing squats on a rubber disc versus a stable force platform. These findings suggest that squats performed under increasingly unstable conditions may not provide an optimal environment for strength and power training.

Sport-specific practice may be sufficient to improve balance and performance when unstable (Willardson 2004). For example, triathletes have been reported to be more stable and less dependent on vision for postural control than untrained or recreationally active trained individuals (Nagy et al. 2004). Gymnasts were reported to be more efficient at integrating and responding to changes in balance (Vuillerme, Teasdale, and Nougier 2001). Highly trained athletes may not benefit from instability training to the same extent as less experienced people. Wahl and Behm (Wahl and Behm 2008) illustrated that highly resistance-trained athletes did not experience significantly greater muscle activation when exercises were performed on moderately unstable devices (e.g., inflatable discs such as DynaDisc and BOSU ball). Thus not all segments of the population may derive the greatest results from instability training.

Similarly, training with particular sport equipment that increases stability during practice can impair proprioception (position sense). This response was evident with national-level skiers who performed more poorly than their regional-level counterparts when tested for balance on a force platform without their ski boots on (Noe and Paillard 2005). The authors speculated that the inferior performance of the national-level skiers could be a long-term effect of wearing ski boots, which restrict range of motion, lending further support to the training specificity model.

Moreover, while younger hockey players demonstrated a significant correlation between static balance and skating speed, more experienced hockey players did not. Since static balance is not as essential as dynamic balance for hockey, this suggests that sport-specific practice is an ample stimulus for dynamic stability and skating-speed training adaptations (Behm, Wahl, et al. 2005). Unfortunately, training in the same environment as the sport or activity is not always possible. For example, some outdoor sports (such as football and baseball) can’t be practiced on the playing field during the winter season in northern climates, nor can sports that utilize ice surfaces be practiced normally when the arenas are closed in the warmer seasons. Thus alternative challenges to the athlete’s balance may be necessary. These could include static balance activities such as standing on one leg or with eyes closed on wobble boards and inflated discs. However, in accordance with the concept of training specificity, dynamic balance activities such as jumping, landing, running, or changing direction using unstable surfaces would provide a more specific transfer of balance and stability skills to the actual sport movement.

Exercises that require balance should also be incorporated into youth resistance training programs (Behm et al. 2008) because balance is essential for optimal sports performance and the prevention of athletic injuries (Verhagen et al. 2005). Given that balance and coordination are not fully developed in children (Payne and Isaacs 2005), balance training may be particularly beneficial for reducing the risk of injury while performing resistance training, particularly to the lower back. Adult studies have demonstrated increased trunk muscle activation from performing activities on an unstable versus a stable surface (Behm et al. 2010a; Behm et al. 2010b); the advantage of training on an unstable surface is that high muscle activity can be achieved without imposing heavy weights (Behm et al. 2010a, 2010b). When incorporating balance training into a child’s resistance training program, exercises should progress from simple static balance activities on stable surfaces to more complex static instability training using devices such as wobble boards, BOSU balls, and stability balls (Behm and Anderson 2006; Behm et al. 2008). Over time, the program can be made more challenging by changing the base of support, the moment or lever arm of the body segment, the movement pattern, or the speed of motion.

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