Putting “Heavy” into Heavy Slow Resistance

Individual factors influence the ability to attain sufficient intensity in HSR training. The training background of the person is critical to understanding their RSLC. For example, total repetitions achieved on the leg press at 70% RM varied widely based on training history: individuals who trained for endurance completed 39.9 (± 17.6 reps) compared to those who trained for strength (17.9 ± 2.8 reps) [10]. The ability to complete repetitions at a given %RM has been termed “relative submaximal lifting capacity” (RSLC). This RSLC is influenced by a variety of other factors as well, such as the training status of the individual with trained individuals managing 9.67 (± 0.91) repetitions compared with untrained people (7.14 ± 0.74) when tested at 85% of 1RM in the back squat [11]. This is important to consider in tendon conditions, as some tendinopathies occur in highly trained people (e.g., patellar tendinopathy), while others can occur in more sedentary populations (e.g., gluteal tendinopathy).

In addition, the relationship of the RSLC to the 1RM varies across time in individuals. Braith et al. investigated 1RM and 7–10RM in a training group over time, and found that the % of RM increased more for the 7–10RM test than the max did [12]. The 7–10 repetition range increased by more than 10% (pre-training 68.4% ± 7.2 of RM baseline, post-training 79.1% ± 7.0 of RM). While the predictive ability of this 7–10RM test for 1RM was good (± 10%), it overestimated the RM in trained individuals, which suggests a more accurate estimating procedure that is based on training state may need to be developed. It can be concluded then that fluctuations in RSLC based on training type and status will influence the actual %RM intensity used even if repetition ranges are similar. Despite this variability, if 12 repetitions or more can be completed with a given load it is highly likely that this load will be less than the 70% RM intensity goal of tendon loading.

Of course, these studies have all involved uninjured individuals, and in a patient population the length of tendon symptoms and the resulting dysfunction may influence the relative intensity as well. Tendon pain typically results in a person unloading the affected area during tasks, which means most multi-joint and bilateral exercises find the individual using a movement strategy that decreases the load on the affected tendon while still accomplishing the task. This ability to offload the tendon can be controlled through the use of isolated exercises, as all movement solutions that accomplish the task also load the tendon (e.g., calf raises or leg extensions). In addition, isolated exercises early in rehabilitation require less skill on the part of the individual and make it easier for the clinician to apply the correct intensity. More complex movement patterns can be incorporated into the program once there is a base tendon capacity established that is sufficient to afford movement solutions that direct sufficient load through that tendon.

The second key variable that HSR addresses is the rate of loading, which can be visualized as the slope of the force–time curve during the time interval from onset of movement to peak force. If loaded too rapidly, the tendon responds elastically with less strain and will adapt differently than when loaded at a slower rate [7]. In HSR a slow lifting tempo is used as a surrogate for controlling this rate of force development. Repetition speed has a significant impact on RSLC and is not controlled for in most of the available RSLC research. To control this parameter when examining the 1RM prediction tests, Reynolds et al. used a metronome with a 1-s eccentric and 1-s concentric tempo, i.e., 30 repetitions per minute (r/m), to pace their exercise [13]. This tempo is much quicker than most HSR studies, which take 6 s per repetition (10 r/m) or 8 s per repetition (7.5 r/m) [1‐5].

The total time under tension (TUT) is defined as the time spent with the muscles under load and is determined by total repetitions and the speed of each repetition. However, TUT may be more of an artifact of the rate of movement used during a set than an important contributor to training effects. Instead, the duration of each repetition will affect the training intensity used if volume is held constant (i.e., lifting a maximum weight for the same number of repetitions but at different repetition durations). In addition, decreasing the speed of repetition (e.g., 4, 8, or 10 s) will significantly impact the number of repetitions that can be performed at the same load [14]. Wilk et al. found that exercise volume was lowest for sets using the slower repetition tempo, showing that even a small change in repetition tempo can significantly reduce the training volume. This was examined in the bench press at five different loads (40–80% 1RM) at tempos of approximately 2.8/1.4/1.0 s for the eccentric and concentric portion of the lift (5.6/2.8/1.9-s lifts) [15]. It was shown that slower repetition speeds drastically impact the RSLC with the weight lifted dropping below 70% RM at the sixth repetition for the slow group while the tempo was no longer maintained by the fourth repetition [15]. These data show that slower tempos at equivalent repetition zones reduce the intensity of the load that can be lifted. This reduction in RSLC appears to dip below the necessary 70% RM intensity threshold at around six repetitions when using a 6 s/repetition (s/rep) tempo. Since HSR protocols frequently use a 15/12/10/8 repetition approach at 6 s/rep, all of these repetition ranges exceed the probable maximum number that can be performed at the desired intensity. This strongly suggests that the intensity being used is not reaching the levels likely to elicit adaptations. Despite this, some good results have been seen with the HSR approach. This suggests that these outcomes are likely based on something besides adaptations to high load. It is interesting that Riel et al. were unable to replicate previous results for relief of plantar heel pain [5]. However, based on the very slow tempo (8 s/rep) used in their study, their discussion about under-dosage would make sense considering the impact tempo has on relative intensity at a set repetition range.