In some of the earliest low load studies, it was implied that a relatively slow movement speed was superior in regards to O2 drop. From the standpoint of injury rehab, we basically have little choice, and a slow movement speed is a safe method. However, there have been many opinions that in older populations is particular, high rate of force development is quite helpful. In aging there is a loss of type 2 fibers responsible for peak strength and fast contractions. By training at higher speed and force, this hopefully is mitigated. Explosive type training is also another heavily researched subject with data showing enhancement in peak force. In addition to fiber type change, optimal neurological firing strategy occurs with faster repetition speed. By training at slow speeds we may be neglecting this aspect.
Potential downsides to fast contractions are several. Obviously there is the potential for injury. In addition there may be some stress shielding both in the tendon and muscle. This concept is not often discussed. If there is a very rapid force change in the tendon muscle unit, the force is not necessarily transmitted equally along the tissue. So some areas get more or less strain. The higher strain area could become damaged, the lower strain area may not get loaded enough to stimulate positive cellular change. Coupled to this idea is the tension on the muscle through the range of motion. In a very fast contraction, the initial training force is highest at the beginning of the motion, but as you approach the end, you are minimally using the muscles as momentum has taken over. Another issue is the large metabolic consequence regarding the increase in work (physics wise) when you are doing faster contractions over the same time period. In other words the energy used is higher with faster rep speed over the same time duration (net more repetitions). We may not be able to sustain this amount of net work so a shorter training time may result but don't really want to short change the time the tissue is subjected to hypoxia. So there needs to be some compromise. Most studies doing comparisons in fast vs slow velocity try to equate work done. The last possible issue is what happens to intra muscular pressure during fast contractions(not much data on this). As mentioned above, it may be that not enough continuous compressive force is exerted on the muscle, sabotaging our low load theoretical rational (avoid cuffs by relying on intra muscular pressure to occlude vascular flow). So although the initial force is high, there could be a major intra muscular pressure reduction toward the middle to end of the given motion.
So the question is, what is the net effect of higher movement speed and what do the tracings look like. Is there some issue with blood inflow leakage during the set (preventing desat), is the higher work needed impossible to actually do?
The first attempt at doing this definitely showed me the difficulty in going from about 8 reps to 20. This is a tracing of 3 sets of chest press, sensor on the chest with a weight 15% below my usual low load. The first set the usual slow/moderate speed of about 8 reps. The next 2 sets were done fast on both the concentric and eccentric phases, with no pause at transition. Despite the O2 drop looking similar, the fast sets were very taxing.
Notice the red line, the total hemoglobin. The slow first set captures the variation in compressive force, but I suspect there would be more of this pattern in the fast sets if the time resolution was higher. The sensor only measures every 1 second. However despite this, the average hemoglobin drop was similar
Note as well the very typical O2 desat, no better nor worse.
In conclusion, with fast reps there is no penalty in regards to hemoglobin drop nor reduction in O2 desaturation. The only issue is the severe fatigue and possible over training if this is done too often. The other observation is the lack of lower O2 reached with much higher "work" done and presumably high lactate generation.
Another potential variation that may combine the benefits of rapid motion with less overall fatigue is doing a fast concentric phase then a slow eccentric (in order to limit the total reps over the same time).
Here are some examples:
This is a tracing of the pulldown, low load weight, 45 sec sets sensor on the lats.
The concentric phase is fast (<1 sec) but the eccentric is about 6 sec making the rep count in line with what has been done previously. Desaturation is also similar to previous protocols.
Here is the dip, sensor on chest, modified reverse drop set (start 15% below then set 2 and 3 at usual low load). The first set was done at the usual pace - about 7-8 reps over 45 sec.
There does not appear to be any deterioration of O2 drop with the change in contraction speed and fatigue was manageable.
Finally, this is the chest press with sensor on chest, modified reverse drop set (first set -15% then next two at usual low load). All three sets were done with fast concentric and slow eccentric. Excellent desat and hemoglobin drops.
Some concluding remarks:
- There is good study data that faster movement speed may be beneficial for various parameters.
- Doing more repetitions over the same time period will lead to much higher metabolic loading and fatigue. This could lead to over training.
- Despite data showing more reps under the same time time of tension generates higher lactate, this did not translate to lower O2 measured here. There may be some limit to muscle O2 drop that can be reached (as seen with cycling intervals).
- The effects on muscle O2 change and total hemoglobin are not much different comparing movement speed at the same load. So faster/slower movement velocity (within the limits tested here) does not seem to be a huge factor in the process of muscle hypoxia.
- There are also theoretical benefits to slower motion in regards to muscle/connective tissue effects (stress shielding and muscle strength uniformity through the range of motion).
- A fast concentric phase followed by a slow eccentric motion may be a fair compromise in balancing the benefits of motion speed with the metabolic demand.
- The variation of motion speed may be another way to train with low loads leading to beneficial adaptations.