Bands produce a spring rate, where the load increases as you stretch it, where as the mass of weight is constant. A load that increases as you stretch it can be useful, but will have a negative effect on power production, i.e. how explosively fast you can move. Your muscle fibres will train to be strong and slow.
Exercises used to develop powerful, explosive speed must allow fast movement. The key is constant acceleration through the range of motion reaching maximum velocity at the point of release (Bompa). This is impossible if resistance increases throughout the movement, slowing your velocity as the distance travelled increases, which is the case with resistance bands.
For most applications of strength for sport performance, and specifically Thai boxing, power or speed of movement is the critical factor rather than pure strength. It’s important to train to optimise power for the specific combination of load and speed of movement required for your sport or activity. Striking sports require very fast movements, you usually have less than 300 milliseconds to generate as much force as possible. So you must train to generate a high force very quickly, specifically increasing your rate of force development (RFD) at 300 milliseconds or less.
It’s also important to consider how the bands affect the mechanics of your movement. For example, it’s not a good idea for fighters to extensively use resistance band punching or pressing actions as it adversely affects punching mechanics.
Variable resistance training is now popular with powerlifters aiming to get stronger for maximum strength single lifts by combining resistance bands or chains with free-weights. The purpose of this method is to counter act changes in force generating capabilities of the musculoskeletal system depending on changing joint angles throughout the lift.
There are three different strength curves. Single-joint movements (elbow flexion and extension, knee flexion and extension) generally have bell shaped strength curves were maximum strength is produced at the mid-phase of the lift. Multi-joint pulling movements (like pull ups and bent-over rows) have an descending strength curve where maximum strength is produced at the start of the lift. Lastly, ascending strength curve movements (like deadlifts, squats, bench press etc) produce the most force at the top of the lift.
Power lifters use bands or chains on these ascending strength curve lifts to add more load at the end of the lift were the strength is greatest, while reducing the load at the relatively weak bottom part of the lift. This way, maximum strength is developed throughout the whole range of motion. The most effective load ratio between weight plates and resistance bands or chains is still being experimented with.
Bands are also a very useful tool to either progress or regress bodyweight exercises. For example a wide-grip pull up can be assisted by stretching a band down from the overhead bar over a knee or foot provide a lifting force – allowing the target number of repetitions with bodyweight to be successfully completed if the trainee isn’t yet strong enough. It’s worth noting though that multi-joint pulling exercises exhibit a descending strength curve, where the maximum effort phase of the lift is toward the end – which is when the band is helping you the least as it’s less stretched!
This same process is also very useful for injury rehabilitation, allowing injured, detrained, weaker muscles to be worked over a range of motion that they couldn’t without assistance.
Bands are also a great way to create instability or perturbation. The directional force can be systematically applied to “feed a mistake” to help an athlete feel their mistake and learn resist an unwanted collapse. For example, a valgus (inward) knee flexion when squatting or lunging can be exaggerated with an inward pull of the band. This forces the trainee to externally rotate at the hip against the force of the band, creating kinesthetic awareness and switches on weak or underutilised stabilising muscles. Stability is very important to Thai boxers. We spend a great deal of time on one leg, and losing balance under impact gives your opponent scoring opportunities for free.
As with all the tools available in the gym, appropriate application is the key. Always select your training exercises and the tools used according to your specific training objectives for that block of training or meso cycle.