If you’re interested in cutting-edge biomechanics, please sign up to the free newsletter using the form below and receive a single free study review every month!
Why study ballistic resistance-training?
Jump squats make it into almost every strength and conditioning program under the sun and every study involving rugby or football teams seems to include them. But not every researcher believes they are the best choice for power development. To understand why, it is necessary to delve into the biomechanics of how ballistic and non-ballistic movements differ and in what respects they are in fact very similar. In this article, Chris Beardsley (@SandCResearch) takes a detailed look at a very interesting study.
The study: A Comparison of Ballistic and Non-Ballistic Lower-Body Resistance Exercise and the Methods Used to Identify Their Positive Lifting Phases, by Lake, Lauder, Smith and Shorter, in Journal of Applied Biomechanics, 2011.
Ballistic resistance training involves releasing the external load into the air without decelerating. The most commonly used ballistic training exercise in athletic training is the jump squat, which many strength and conditioning professionals prefer to the heavy back squat for power development.
Researchers have also favored ballistic lifts for power development, noting that the mean acceleration, force and power achieved are higher, which they suggest should lead to greater adaptations. Moreover, there is no accompanying deceleration phase, as seen in concentric-only or stretch-shortening cycle movements. This means that there are no sections in the whole range of motion (ROM) where the muscles experience a reduced level of activity and therefore are less stimulated. Researchers have suggested that this leads to greater strength development at varying points in the joint angle ROM. For a detailed review of these three types of training, you can check out the great review article by Frost (2010).
The different lifting phases
Non-ballistic, explosive resistance exercises have two separate phases. The first phase is the propulsion phase. The propulsion phase comprises a period of acceleration and a point at which peak velocity is reached (which coincides with zero acceleration). The second phase is called the braking phase, which comprises a period of deceleration followed by a static state.
Comparing ballistic and non-ballistic lifts
There are at least two approaches for comparing the two lifting methods. The researchers describe two of these. Firstly, the peak bar displacement can be compared in both. In this instance, both lifts have an acceleration and a deceleration phase. The only difference is that the deceleration phase in the ballistic exercise occurs once the lifter has left the ground.
Secondly, purely the propulsion phases could be compared in each case. This would remove the effect of the deceleration phase in the non-ballistic movement but reduce its displacement.
Essentially, in both approaches, the ROM (or total bar displacement) of each exercise is not the same. However, in the first, the exercises are equalized insofar as both have an acceleration and a deceleration phase. In the second, both exercises only have an acceleration phase. As when comparing isoinertial (constant resistance) and variable resistance (i.e. bands and chains), there is no right or wrong way of comparing these different resistance-training methods. Each tells you about a different aspect of the training method. Only by comparing the methods in various ways can we build up a full picture of what is going on.
What did the researchers do?
The researchers recruited 30 men who all had some resistance training and were able to perform a modified back squat with a load equivalent to body mass. Firstly, the researchers tested the 1RM back squat of the subjects while taking measurements and then after seven days, the subjects returned to the laboratory so that they could perform traditional back squats and jump squats for reps. The subjects performed 3 sets of 3 reps with 45% 1RM in each exercise. The subjects rested for 1-3 minutes between each set.
The researchers used force plates to monitor ground reaction forces and reflective markers on key anatomical markers in conjunction with two digital cameras to measure displacement, velocity and acceleration.
Irrespective of the method used, the researchers found that the mean force in normal squats and jump squats was broadly comparable, even though the same weight was used for both. In normal training, lighter weights would typically be used for jump squats, which would reduce force. The chart below shows how similar the force is for ballistic and non-ballistic exercises.
As you can see from the chart above, the measurement method used led to significantly different mean force values to be recorded. The peak displacement method demonstrated much lower mean forces overall because it has a deceleration phase in which there is significantly reduced force. This drags down the average value compared to the peak impulse method, which only considers the propulsive phase.
The researchers found that the mean velocity for the ballistic exercise was significantly greater (c. 14%) than the mean velocity of the non-ballistic exercise, irrespective of which method of measurement was used, as can be seen from the chart below.
The mean velocity is higher during the ballistic exercise using both methods because in both cases it is measured over a longer displacement. It therefore has a longer period of time in which to build up speed.
What did the researchers conclude?
Overall, the researchers concluded that ballistic and non-ballistic lifts are underpinned by the same mechanical demands. They suggest that strength and conditioning coaches should review the common perception that ballistic lower-body resistance exercises such as jump squats are superior for developing lower-body power.
I think that this is welcome investigation, although a conclusion as bold as this is perhaps premature. What I like about it is that it is very easy to get tied up into a thought process that centers on the higher mean velocity that is seen in ballistic exercise, while forgetting that it is actually force and rate of force development that are more important.
This is one of the reasons that variable resistance (i.e. bands and chains) has become popular. Having said that, we must not forget that most sporting movements take place at fast speeds and therefore the ability to display higher velocity-specific strength must not be neglected in training.
The researchers note that there were two main limitations to this study, to which I have added several more:
- Only one load was considered, which at 45% was determined to be a compromise between the typical 30% loads used in jump squats and 60-70% loads used in traditional resistance training. It would have been very interesting to compare lighter jump squats and heavier traditional squats across force, velocity and power.
- The squats were performed to touch a marker, in order to ensure consistent depth, which may have altered the stretch-shortening cycle involvement.
- No measurements of time-to-peak velocity or rate of force development were taken, which would have been interesting to see, if only to confirm that they were in line with expectations. From previous research, we would not anticipate any significant variances in this regard, but even so…
- No comparisons of different types of squats were performed. Since Swinton (2012) showed that the rate of force development was significantly different between narrow, powerlifting and box squats, we must be more careful about which types of squats are performed and the biomechanical variations that they display. It would have been fascinating to see this experiment repeated with different squat styles.
- No EMG analysis of muscular activity was recorded. Since one of the fundamental rationales for performing ballistic movements is that there is a full ROM of muscular activity, this seems unfortunate. It would be great to see a comparison of EMG activity in ballistic and non-ballistic squats on a graph, with the level of activity plotted against hip flexion angle.