The latest edition of the NSCA's Journal of Strength & Conditioning Research 
contained the following research paper on the effects of Swiss Ball training. 
 Note that, in this study, subjects sitting on the Ball produced very MUCH 
LESS force and EMG activity in leg extension movements, indicating that 
exercises done on the Ball, while possibly playing some role in seated 
balance training, should not be used as replacement for sport specific 
exercises on stable surfaces where the objective is to increase performance 
factors such as force, torque and power.  It will be interesting to see how 
other exercises fare in similar comparative studies.

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Muscle Force and Activation Under Stable and Unstable Conditions

David G. Behm, Kenneth Anderson & Robert S. Curnew

The Journal of Strength & Conditioning Research: Vol 16, No 3, pp 416–422, 
2002

ABSTRACT

The objective of this study was to determine differences in isometric force 
output, muscle activation (interpolated twitch technique), and electro
myographic activity of the quadriceps, plantar flexors (PF), and 
their antagonists under stable and unstable conditions. Instability in 
subjects was introduced by making them perform contractions while seated on a 
"Swiss ball."  Eight male subjects performed unilateral leg extensor (LE) and 
PF contractions while seated on a bench (LE), chair (PF), or a ball. 

Unstable LE (leg extension) and PF (plantarflexor) forces were 70.5 and 20.2% 
less than their stable counterparts, respectively. Unstable quadriceps and PF 
activation averaged 44.3 and 2.9% less than activation under stable 
conditions. 

Unstable antagonist/agonist ratios were 40.2 and 30.7% greater than stable 
ratios in the LE and PF protocols, respectively. The greater decrements with 
LE can be attributed to the instability of only 2 points of floor contact, 
rather than 3 points of floor contact as with the PF. 

Swiss balls may permit a strength training adaptation of the limbs, if 
instability is moderate, allowing the production of overload forces. 

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DISCUSSION

... The decreased activation under very unstable conditions could be ascribed 
to the excess stress associated with the increased postural demands. It could 
also be related to the dispersion of concentration (neural drive) in 
attempting to control 2 limbs with differing responsibilities (balance and 
force). In an attempt to maintain balance, synergistic and stabilizing 
muscles would play a greater role. Synergistic muscles have been shown to 
provide both inhibitory and facilitatory inputs to agonist muscle groups. 
Thus, the application of 2 major stressors to the central nervous system 
(attempting maximal force output while balancing on 2 points) in this study 
severely inhibited the ability to fully activate the quadriceps....

However, the activation of the PF, which experienced only a moderately 
unstable condition, was not significantly affected. Unstable PF activation 
was only 2.9% lower than stable PF activation. However, it must be emphasized 
that the PF condition had 3 points of contact, minimizing the stress on the 
equilibrium system. Secondly, the PF may be more amenable to complete 
activation in many individuals. Stable PF inactivation (1.8%) was 
significantly less than stable LE inactivation (18.6%). Under stable 
conditions, both McComas et al. (19) and Belanger and McComas (6) reported 
that half their subjects could fully activate their PF. 

Similarly, Behm and St-Pierre in 2 separate studies indicated that 10 of 12 
(3) and 11 of 16 (4) subjects could fully activate their PF during stable 
conditions. Because the PF posed a minimal challenge to the equilibrium of 
the body, may be accustomed to more chronic postural demands, and is a 
smaller muscle group than the quadriceps, which may be easier to fully 
activate, insignificant changes were experienced under this condition.

A question then arises as to why unstable PF forces were significantly less 
than stable PF forces, when there was no significant difference in muscle 
activation. Although not statistically significant, there was a trend for a 
greater antagonist/agonist ratio with the unstable PF condition. The unstable 
PF condition experienced 30.7% greater antagonist activity than the stable PF 
condition. 

Similarly, but in this instance statistically significant, unstable LE 
experienced 40.2% greater antagonist activity. The role of the antagonist in 
this case may be an attempt to control the position of the limb when 
producing force. Both De Luca and Mambrito (11) and Marsden et al. (18) 
reported that antagonist activity was greater when uncertainty existed in the 
required task. Increased antagonist activity may also be present to increase 
joint stiffness (16) to promote stability (14) . Whereas increased antagonist 
activity could be utilized to improve motor control and balance, it would 
also contribute to a greater decrement in force with the unstable 
conditions.....

Practical Applications Return to TOC

Unstable conditions can lead to decreases in the force output of the limb, 
muscle activation, and increases in antagonist activity. Greater degrees of 
instability exacerbate these changes. In the light of these findings, the use 
of Swiss or Physio balls as a resistance training modality for peripheral 
strength gains should be employed when the degree of instability is light to 
moderate, allowing an overload force or resistance to be developed. 

For example, if an individual is in a position whereby he or she cannot stay 
upright (attempting to stand or perform a squat maneuver on a Swiss ball), 
the amount of resistance that can be applied to the muscle will be negligible 
because all focus is on balance (extreme instability). 

On the other hand, performing contractions while seated on a Swiss ball, with 
1 or 2 feet on the floor (moderate-to-light instability), requires less focus 
to maintain balance, and hence more concentration and resources can be 
applied to moving greater resistances. However, whereas the resistive 
challenge to a limb under very unstable conditions may be less than that 
necessary to develop strength adaptations, the torso musculature may be under 
greater stress. With unstable conditions, a relatively small resistive torque 
on the distal portion of a limb can result in substantial motive torque by 
the torso. 

Perhaps, the greatest contribution of instability training may be to improve 
core stability rather than limb strength. In addition, the preliminary 
purpose of the stability ball need not be significant strength gains but an 
attempt to improve balance, stability, and proprioceptive capabilities. 
Further research is necessary to investigate the effects of instability 
training on torso strength and balance adaptations as well as the 
effectiveness of a prolonged resistance training program using both unstable 
and stable conditions...... 

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Dr Mel C Siff
Denver, USA
http://groups.yahoo.com/group/Supertraining/