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Monthly Archives: November 2010

Juan Jose Gonzalez-Badillo and Mario C. Marques had a study published in the December issue of the Journal of Strength and Conditioning Research looking at the relationship between kinematic variables of jumping with vertical jumping height.

A lot of studies have looked at vertical jump, this study is interesting because of the population. The authors looked at 48 male track and field athletes (primarily jumpers and sprinters) of whom 25 were international athletes.

In this study, the athletes performed a counter-movement vertical jump in a Smith machine. The athlete held the empty barbell on the back of their shoulders and then did the vertical jump. They did three jumps on a force platform.

They divided the jump into three phases:
• Eccentric phase: beginning of the jump until maximum negative velocity occurred
• Transition phase: the moment after maximum negative velocity until velocity of the center of mass reaches 0 meters/second
• Concentric: End of the eccentric phase until maximal positive velocity was achieved

They ran correlations between a number of variables and jump height for each jump. The correlations between all of the variables are statistically significant and include time spent in the eccentric/concentric phases, impulse of eccentric/concentric/transition, force in all three phases, peak power in all three phases, average power in all three phases, and maximum negative velocity.

The majority of these correlations, while statistically significant, are very weak. For example, eccentric time explains between 8 and 11% of the variation in jump height.

Several of the variables explain almost 50% of the variation in jump height, these include:
• Force production in each phase
• Concentric peak power
• Concentric average power
• Maximum negative velocity

These variables have some important implications for a strength and conditioning professional. First, force production indicates the need to have a strong lower body to be a better jumper. Second, the relationship of concentric power indicates the importance of explosive training to be a better jumper. The negative velocity shows how a fast stretch can help result in the storage of elastic energy, resulting in a higher jump. This also indicates the importance of plyometrics and the Olympic lifts in the training of jumpers.

Gonzalez-Badillo, J.J. and Marques, M.C. (2010). “Relationship between kinematic factors and countermovement jump height in trained track and field athletes.” Journal of Strength and Conditioning Research, 24(12): 3443-3447.

The box squat is an exercise that is very popular in powerlifting and in the strength and conditioning of athletes. In theory, it requires the athlete to pause at the bottom of the squat, which takes the stretch-shortening cycle/elastic energy out of the lift and requires the athlete to become stronger to overcome the pause. It is also a great strengthening exercise for the trunk which is having to hold the weights longer in the box squat. The height of the box can be adjusted to train the athlete to become stronger during many parts of the lift.

McBride, et al in a 2010 article in the Journal of Strength and Conditioning Research compared the kinetic and muscle recruitment characteristics of the box squat and the squat.

They studied eight competitive powerlifters (mean body mass 108kg and mean 1-RM back squat 200kg). Their subjects first performed a 1-RM on the squat. Then, a week later performed back squats and box squats using 60, 70, and 80% of 1-RM for one rep each set.

The results are provided using graphs, so it is difficult to tease out the numbers. However, there are a few differences between the box squats and the squats:
• At 70% of 1-RM, the athletes produced more peak force during the concentric part of the box squat than the squat.
• At 80% of 1-RM, the athletes produced more peak power during the concentric part of the squat than the box squat.
• At 60% of 1-RM, the squat had a greater EMG measure of the biceps femoris than the box squat.
• At 70% of 1-RM, the box squat had a greater EMG measure at the vastus lateralis than the squat.
• At 80% of 1-RM, muscle activity at the vastus lateralis, vastus medialis, biceps femoris, and longissimus is higher in the squat compared to the box squat, but does not appear to be statistically significant.

In other words, this study is not reporting very many differences between the two exercises at 60, 70, and 80% of 1-RM. At first, this might seem to be a negative conclusion about the box squat, but it’s not. What it suggests is that the box squat is a substitute for the squat exercise, in other words performing it should not harm one’s performance on the back squat, lower body strength, or lower body power.

This has significant implications for advanced athletes. One of the challenges with designing strength and conditioning programs for elite athletes is that they have a great deal of experience with strength training and are unlikely to easily make gains. Most of the training variables (volume, load, intensity, rest, recovery, etc.) cannot easily be changed for an elite athlete – but the exercises that are being used can be changed. This study suggests that the box squat exercise can be substituted for the squat on a regular basis to expose the athlete to a new training stimulus.

As published, this study is not without limitations. The subjects were competitive powerlifters, but it’s unclear at what level they were competing at. Taking athletes with more or less experience with these lifts might see different results. Applying these results to non-powerlifters might be challenging. In addition, we don’t know the height of the box used in the exercises. The authors mention squatting to 70 degrees of knee flexion, but it’s unclear if this applies to the box squats. It’s possible that using a different height of the box would also see different results.

McBride, J.M., Skinner, J.W., Schafer, P.C., Haines, T.L., and Kirby, T.J. (2010). “Comparison of kinetic variables and muscle activity during a squat versus a back squat.” Journal of Strength and Conditioning Research, 24(12): 3195-3199.

Clark, et al had a study published in the December 2010 issue of the Journal of Strength and Conditioning Research looking at the effect of seven weeks of resisted sprint training on maximal velocity sprinting. The results are interesting and suggest that these tools aren’t really needed by certain groups of athletes.

First, they studied athletes. Their population consisted of Division III lacrosse athletes. These athletes were divided into one of three groups:
• Unresisted sprinting (UR): performed the sprinting protocol without resistance
• Weighted vest sprinting (WV): performed the sprinting protocol with 18.5% of body mass added via weighted vest.
• Weighted sled sprinting (WS): performed the sprinting protocol with 10% of body mass added via weighted sled.

All three groups performed the same sprinting protocol, it’s just that two groups had extra weight. The protocol involved sprinting 2x per week (except during week six, when only one session was performed) for seven weeks. The two workouts performed within each week were identical, it’s just that the repetitions were flipped. For example, on session one the short sprints were done first, then the medium sprints, then the long sprints. In session two, the long sprints would be performed first with the short ones performed last.

Before and after the seven weeks, subjects were tested on their 60 yard sprint time. The results are interesting:

Group Time % Change Average Velocity % Change
UR 1.97% 2.01%
WS 0.13% .09%
WV 1.2% 1.20%

The table above shows the percent improvement in the 60 yard sprint time and the percent improvement in average velocity during the 60 yard sprint. The unresisted group had improvements after seven weeks of training, the WV had smaller improvements, the WS essentially had none.

A 1.2-2.01% improvement probably isn’t statistically significant. But it would be significant in the real world of athletics or sprinting, especially over a seven week period.

Resisted sprinting is done to help athletes overcome Ozolin’s hypothetical “speed barrier.” His idea was that athletes “learn” to run at certain velocities and it is almost impossible to get them, under normal circumstances, to learn how to run at faster velocities. Thus, training modes like resisted and assisted sprinting is necessary to teach the athlete how to do this (and hopefully the athlete can transfer this to unresisted/unassisted sprinting).

The problem with this is that if the speed barrier exists, it may not exist for every athlete. Taking a world-class sprinter, who has been sprinting for 15-20 years and comparing them to a high school football player is comparing apples to oranges and things that are true with the sprinter won’t be true with the football player.

The fact that UR sprinting is effective isn’t surprising, the subject population isn’t at the point where they require the intervention of resisted sprinting. The fact that WV is more effective than WS is interesting, this may be due to how the athletes experience the loading. In WS training, the weight is behind the athlete and involves overcoming the friction of the sled on the ground.

It’s also very possible that WS and WV training are more effective at improving an athlete’s ability to accelerate. It would have been interesting for the authors to break out changes to the 5, 10, or 20 yard times and see how the training interventions affected that.

Clark, K.P., et al. (2010). “The longitudinal effects of resisted sprint training using weighted sleds versus weighted vests.” Journal of Strength and Conditioning Research, 24(12), 3287-3295.

One of the things that we “know” as strength and conditioning professionals is that strength and conditioning results in neural adaptations. This proceeds hypertrophy and explains why strength gains in beginners outpace increases in muscle size.

It is often preferable to hypertrophy in many athletes. For example, it doesn’t make sense to add 40-50 pounds of muscle mass to most athletes as they have to be able to run and jump with that extra mass. If we can make athletes faster and more explosive without all that extra muscle mass by training their nervous system this is often desirable. These neural adaptations also explain why weight class strength athletes (powerlifters and Olympic lifters) are able to be so stronger relative to their body mass.

The idea works like this, strength and conditioning trains the nervous system to do a number of things:
1. Recruit more muscle fibers: the more muscle fibers you can recruit, the stronger and more quickly you can contract the muscle.
2. Recruit muscle fibers more quickly: obviously something that we want athletes to be able to do.
3. Reduce the inhibition from antagonist muscles: in beginners, the antagonist muscles may interfere with movements. If these muscles can be quieted it can result in stronger and more explosive movements.
4. Bypass the size principle: it’s thought that we recruit muscle fibers according to a continuum, first we recruit the slower smaller muscle fibers and this gradually escalates until the larger faster fibers are recruited. A theory is that we learn to bypass these slower muscle fibers with training.

Interestingly, research on neural adaptations is mixed and it is not a given that this in fact occurs. There is mixed research on whether the excitability of the corticospinal tract changes from training (See Caroll et al 2009 and Jensen et al 2005). There is mixed research about whether we can in fact recruit more muscle fibers as a result of training. Some authors feel that we already maximally recruit muscle fibers during movement (see Behm 1995, Knight and Kamen 2001). Other authors (Kubo et al 2010) find that training does result in an increased ability to active muscle fibers.

Research on this topic is really challenging to conduct, I’m not sure there is a way to directly measure the activity of the central nervous system. Certainly there’s not a way to do this that would be uninvasive. The other challenge is that a lot of literature is looking at untrained or elderly subjects, so just because an adaptation is occurring with them doesn’t necessarily mean that it translates to highly trained athletes.

Behm, D.G. (1995) ‘Neuromuscular implications and applications of resistance training’, Journal of Strength and Conditioning Research, 9(4): 264-274.

Carroll, T.J., Barton, J., Hsu, M. and Lee, M. (2009) ‘The effect of strength training on the force of twitches evoked by corticospinal stimulation in humans’, Acta Physiologica, 197: 161-173.

Jensen J.L., Marstrand, P.C.D. and Nielsen, J.B. (2005) ‘Motor skill training and strength training are associated with different plastic changes in the central nervous system’, Journal of Applied Physiology, 99: 1558-1568.

Knight, C.A. and Kamen, G. (2001) ‘Adaptations in muscular activation of the knee extensor muscles with strength training in young and older adults’, Journal of Electromyography and
Kinesiology, 11: 405-412.

Kubo, K., Ikebukuro, T., Yata, H. and Tsunoda, N. (2010) ‘Time course of changes in muscle and tendon properties during strength training and detraining’, Journal of Strength and Conditioning Research, 24(2): 322-331.

Turner and Jeffreys (2010) had a good literature review of the stretch shortening cycle along with thoughts on plyometrics in the August issue of Strength and Conditioning Journal.

 

Elastic energy, and its role in athletic performance, is an interesting debate to follow.  Turner and Jeffreys make the statement that the tendon is where elastic energy is stored during the stretch shortening cycle.  But they also acknowledge the controversy surrounding this concept, which was largely attributed to Bobbert and the late van Ingen Schenua, who argued that while there might be an increase in elastic energy it was at the expense of the muscle’s ability to form cross-bridges.  They felt that elastic energy, rather than boosting performance, resulted in better efficiency of movement (i.e. performance increased because the athlete wasted less energy, not because they produced more force).

 

The review covers the physiological and biomechanical underpinnings behind elastic energy and the stretch shortening cycle.

 

One of the topics they cover related to cross-bridge formation and how it relates to the amortization phase in a stretch-shortening cycle movement.  Essentially there point is that the amortization phase needs to be fast as the muscles don’t maintain cross-bridges very long.  I think this is a debatable point, I’m not aware that the ability exists to actually measure this in living people.  Their references for this point refer to a popular book by Kraemer and Fleck and Siff’s book, so they are not primary sources which makes this a somewhat weak statement.

 

Their suggestions for plyometrics training are logical.  The challenge with plyometrics is that we know, via research and practice, that they are effective.  There are research suggestions that strength levels and level of athletic ability positively influence the effectiveness of plyometrics.  But that’s about all we know.  We don’t know a lot about optimal volume, intensity, training frequency, progressions, etc. from a research standpoint – which means this becomes a matter solely of coaching practice.

 

Essentially they suggest a series of progressive steps:

  • Jump to a box: Develops jumping and landing mechanics.
  • Jump off a box and stick the landing
  • Short-response jumps: Essentially drop jumps.

 

The last progression is an interesting point.  Some literature contrasts between long-response jumps and short-response jumps.  The response referring to the time of the amortization and acceleration phases.  The thinking is that the short-response jumps (drop jumps) are more applicable to athletics than long-response jumps (for example, a counter-movement jump).

 

Turner, N. and Jeffreys, I.  (2010).  “The stretch-shortening cycle: Proposed mechanisms and methods for enhancement.”  Strength and Conditioning Journal, 32(4), 87-99.

Brad Schoenfeld had a literature review on hypertrophy in the October Journal of Strength and Conditioning Research (pg. 2857-2872). I found this to be an interesting article as I’ve been doing a similar review for the book that I’m working on.

Some interesting things that I took from his review:
• The role of satellite cells in hypertrophy: According to Schoenfeld, hypertrophy is thought to be mediated by the activity of satellite cells. This is done in several ways: first, they donate nuclei to muscle fibers; second they express a number of regulatory factors that aid in muscle growth.
• The role of Insulin-like growth factors (IGF): Schoenfeld calls IGF “…the most important mammalian anabolic hormone” (pg. 2859). He discusses its role in increasing the rate of protein synthesis, activation of satellite cells, and facilitation of the donating of myonuclei to the muscle fibers.
• The role of volume in hypertrophy training: Most studies recommend essentially 6-15 reps per set, with 2-4 sets resulting in the greatest gains from hypertrophy. This volume is also linked to the most potent growth hormone response from training.

The literature review that I’ve done pretty much confirms the things that Schoenfeld said (and it should be noted that he has 205 references in his article). There is some debate about the role of satellite cells and IGF, but overall it looks pretty good that these two factors are very important for hypertrophy. Among other things this suggests that the number of satellite cells might be a genetic limiting factor for hypertrophy.

Volume and intensity guidelines for hypertrophy training are really interesting, however. For obvious reasons, most of the literature deals with untrained subjects so applying these guidelines to trained athletes is problematic. In addition, most of the literature ultimately uses one of the Eastern European texts (primarily Harre’s textbook) for recommendations on volume/intensity for various goals. This may be 3rd or 4th hand, but that textbook is usually the ultimate source of a lot of these recommendations.

Schoenfeld, B.J. (2010). “The mechanisms of muscle hypertrophy and their application to resistance training.” Journal of Strength and Conditioning Research, 24(10), 2857-2872.

Stevenson and others (2010) published a study in the latest Journal of Strength and Conditioning Research (November 2010 issue, pgs. 2944-2954) examining the effects of elastic bands on a number of kinematic and kinetic variables related to back squat performance.

They studied recreational weight trainers with self-reported familiarity with the back squat exercise.  The study was organized so that subjects first performed a 1-RM on the back squat.  Then on subsequent days they performed 3×3 on the back squat either with 55% of 1-RM (NB) or with 55% of 1-RM plus an additional 20% of the 1-RM added using elastic bands (the 20% is exerted in the standing position).

The idea was to determine what effects the bands had on a number of kinematic and kinetic variables.

In theory, elastic bands (or what the literature calls variable resistance training (VRT)), provides a number of benefits:

  • First, since a lifter can lower more than they can lift, the desire of the bands to shorten quickly after being stretched at the top of the lift requires the lifter to control the barbell during the descent, increasing strength.
  • Second, as we are stronger at the top of a lift than at the sticking point, the bands make the lift more challenging as they are stretched out at the top of the lift.  Again, this could lead to enhanced strength.

Bands have not been well researched in the literature to date, despite the multitude of coaching endorsements.  Anderson et al (2008) and Bellar et al (2010) both found that using bands enhanced bench press and squat strength over 7-13 weeks of training compared to not using bands.

Having said that, the Stevenson et al (2010) study did not find beneficial results from using bands.  According to their study, using the bands in conjunction with 55% of 1-RM on the back squat had the following results:

Ascent Descent
Peak Velocity -3.8% +2.9%
Mean Velocity -2.5% -1.3%
Rate of Force Development +.2% -1.5%

In addition, peak force increased by 1.1% from using bands and peak power increased by .8% from using bands.

Some of these results are expected when you consider what the bands do.  It makes sense that the velocity during the ascent would decrease as a result of the bands, after all they are meant to slow the lifter down.  It also makes sense that, since they are shortening rapidly, the bands would lead to an increase in velocity during the descent – although it is interesting that this is only the peak velocity and not the mean.

The impact of the bands on rate of force development is almost negligible during the ascent, which is concerning if these are being used to improve power production.

There are a number of shortcomings with this study.  First, the subjects are not athletes.  This makes it difficult to apply the results to an athletic population.  Second, the subjects have self-reported experience with the back squat, there’s no objective criteria of this.  From the information presented, we have no idea how familiar the subjects actually are with the squat (for example, how strong are they?).  This is problematic because stronger, more experienced squatters may perform very differently during this experiment.

Bands seem promising from the standpoint of increasing performance on the bench press or squat.  What is not clear is whether they are promising from a standpoint of improving athletic power.

Anderson, C.E., Sforzo, G.A. and Sigg, J.A. (2008). “The effects of combining elastic and free weight resistance on strength and power in athletes.” Journal of Strength and Conditioning Research, 22(2): 567-574.

Bellar, D.M., Muller, M.D., Barkley, J.E., Kim, C-H., Ida, K., Ryan, E.J., Blis, M.V. and Glickman, E.L. (2010). “The effects of combined elastic- and free-weight tension versus free-weight tension on one-repetition maximum strength in the bench press.” Journal of Strength and Conditioning Research, 24: xx-xx.

Stevenson, M.W., et al. (2010). “Acute effects of elastic bands during the free-weight barbell back squat exercise on velocity, power, and force production.”  Journal of Strength and Conditioning Research, 24(11): 2944-2954.