Re-examining Exercise Repetition Speed

By Wayne L. Westcott

Movement speed has long been a controversial area in the field of strength training.  Although fast movement speeds may permit heavier weight loads, this is largely due to momentum.  For example, if Jim performs barbell curls with his back against the wall he can use an 80-pound weight load.  However, if Jim initiates each barbell curl by bending at the waist and rapidly driving his torso upwards, he can lift a 120-pound weight load.

Has Jim enhanced the training stimulus on his biceps muscles by performing fast, momentum-assisted repetitions?  Probably not.  To overcome the inertia of the 120-pound barbell, Jim incorporates the larger muscles of his hips and lower back.  Although Jim’s biceps assist in this powerful initial movement, once the barbell is in motion they basically go along for the ride.  Momentum-assisted lifts involve large acceleration forces at the beginning and large deceleration forces at the end of each repetition, but relatively low force during the middle range of movement.

Jim’s slower paced set of barbell curls with 80 pounds places more consistent stress on his biceps muscles throughout the exercise movement.  This increases the total muscle tension required of biceps and decreases the injury risk to the biceps and joint attachments.  It is therefore advisable for Jim to perform strict repetitions with the moderate weight load to safely and effectively stress his biceps muscles.

Other examples of momentum-assisted movements that are not recommended include bouncing the barbell off the chest during bench presses and bouncing in the bottom position of barbell squats.  Although these rebound actions allow the use of heavier weight loads, they increase the injury potential and should not be practiced.

Research Study One

Is there an ideal repetition speed?  To gain a better understanding of movement speed and strength development, we studied 163 men and women at three different repetition speeds.  As presented in Table 1, Group A trained at 4 seconds per repetition (2 seconds lifting and 2 seconds lowering).  Group B trained at 6 seconds per repetition (2 seconds lifting and 4 seconds lowering). Group C trained at 8 seconds per repetition (4 seconds lifting and 4 seconds lowering).

All of the research participants trained in a private facility with careful instruction and close supervision.  They averaged 10 repetitions per set for 13 different weight stack machine exercises.  The subjects trained 3 days per week for 8 weeks, with strength assessments taken at the end of the second and eighth weeks.  Overall strength was determined by calculating the average 10-repetition maximum weight load for the 13 different training exercises.

As shown in Table 1, all three training groups made statistically significant and similar improvements (22-23 pounds) in overall strength.  Based on these findings, it would appear that 4-second, 6-second, and 8-second repetitions performed on weight stack machines are equally effective for improving muscle strength.

It is noted that Group A subjects completed each exercise set in about 40 seconds (4 seconds x 10 repetitions), Group B subjects completed each exercise set in about 60 second (6 seconds x 10 repetitions), and

Group C subjects completed each exercise set in about 80 seconds (8 seconds x 10 repetitions).  That is, all three groups worked their muscles to fatigue within the anaerobic energy system (generally considered less than 90 seconds).

It is also noted that all three groups used moderate to slow training speeds.  That is, the lifting and lowering movements ranged from a moderate movement speed of 60 degrees per second to a relatively slow speed of 30 degrees per second for most of the exercises.

Table 1. Eight-week Changes in Muscle Strength at Different Exercise Speeds (N=163)

Training Protocol

(8 weeks)

Reps

Per Set

Time

Per Set

Mean Weight Load Increase 13 Weight Stack Machines
A.  4 secs/rep 10 reps 40 secs +22 lbs.*
B.  6 secs/rep 10 reps 60 secs +22 lbs.*
C.  8 secs/rep 10 reps 80 secs +23 lbs.*

*Statistically significant improvement (p<.01)

Research Study Two

In this study, 54 participants were randomly assigned to one of two training protocols, both of which performed 10 repetition sets at 6 seconds per repetition.  Half of the subjects trained with concentric muscle actions shorter than eccentric muscle actions (2 sec up, 4 sec down), and half trained with concentric muscle actions longer than eccentric muscle actions (4 sec up, 2 sec down).  That is, the first group spent about 33 percent of each repetition in concentric muscle action, whereas the second group spent approximately 66 percent of each repetition in concentric muscle action.While there were differences in the overall repetition speed among the three training groups in the first study (4, 6, or 8 seconds per rep), one aspect of each exercise protocol remained the same.  In all of these repetition speeds, the concentric (lifting) action was equal to or shorter than the eccentric (lowering) action (2 secs up, 2 secs down; 2 secs up, 4 secs down; 4 secs up, 4 secs down).  Because the concentric muscle action produces about 40 percent less muscle force output than the eccentric muscle action, it represents the weaker phase of each repetition.  We therefore decided to examine the effects of longer concentric than eccentric muscle actions on strength development.

All of the study participants were assessed on an independent measure of isometric muscle strength before and after the 10-week training program (MicroFit System).  Subjects who spent 33 percent of each exercise set performing concentric muscle actions experienced a 9-pound strength increase, whereas subjects who spent 66 percent of each exercise set in concentric muscle actions experienced a 12-pound strength increase.  The greater strength improvement (p<0.06) attained by the concentric emphasis group suggests that using a larger percentage of each repetition for the concentric muscle action may enhance the strength building stimulus.  The reason for this may be that more time is spent in the more challenging phase of each repetition, and less time is spent in the less challenging phase of each repetition.

It may be worth considering an occasional reversal of the standard 2 seconds up and 4 seconds down repetition protocol, at least as an alternative strategy in your strength training toolbox.  If clients are experiencing a strength plateau with the traditional training method, have them try a few weeks with 4 second lifting movements and 2 second lowering movements. Doing this may enhance the training stimulus and elicit further strength gains.

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