Muscle Shortening: What is it, physiology behind it, how to use it

INTRODUCTION

Short muscle length training (ST) is not a new concept in strength training. It was described in 1945 by Thomas Delorme, MD who performed ST with patients with femoral fractures. His progressive resistance exercise program was intended to increase muscle hypertrophy by activating the muscle maximally as the muscle approaches complete contraction. This was hypothesized to amplify muscle fiber recruitment, which would in turn increase contractile power of the muscle. Although ST exercises were included and described in Delorme’s strengthening protocols and reports (7) the term ST was not defined. For the purpose of this post, ST will be considered an isotonic or isometric contraction within 50% of the end of joint range of motion.
Contemporary strength training programs typically load muscles through their entire length, with maximal loading occurring at or around 50% joint ROM. (17). This approach will be referred to as long muscle length training (LT). Although LT is most common, ST has been applied by the strength and conditioning community as partial range of motion (ROM) training. It has also been employed as a strategy for working on weaknesses in ranges of maximal lifts called “sticking points” for decades (15). Although Delorme reported improved range of motion and soft tissue pliability with ST (7), little has been written about the physiological effects of ST compared to LT and no known reviews have been published on the potentially different outcomes that might be realized when training a muscle at short versus long length. The purpose of this review is to summarize the current evidence of ST and understand the implications for future clinical use.

HYPERTROPHY

Hypertrophy is the increase in cross sectional area of a muscle fiber after stress is loaded to a muscle. The result of hypertrophy is from a cross factor of the amount of mechanical and metabolic stress placed on the muscle (13). Mechanical stress on the muscle causes a stimulus for adaptation to arise. This adaptation occurs due to increased gene regulation and contractile protein synthesis. The rate of muscle protein synthesis depends on “…mechanical stress of the weight training workout, muscle cell hydration levels, and the anabolic hormonal and subsequent receptor response” (4). Thus, safely maximizing mechanical stress allows for the greatest hypertrophy gains.

A study by Kubo et al. (13) compared ST and LT in the knee extension exercise with nine healthy male subjects. Because of the larger amount of weight moved with LT, it could be assumed that LT would result in larger hypertrophic gains in the muscle compared to ST However, the results indicated no between-group difference in quadriceps muscle cross sectional area after 12 weeks of training. Perhaps this finding was influenced by the study design in which participants performed ST exercise with one leg and LT exercise with the other leg. This type of training may have triggered systemic growth hormone release, causing muscular gains in both lower extremities (13). Because of this, the results from this study must be taken with caution.

STRENGTH

Strength is the amount of force a muscle can produce with a single maximal effort and is measured during a muscular contraction. Muscle fiber hypertrophy, along with the number of muscle fibers and neuromuscular adaptation play a role in a muscle’s capacity to produce a force. Resistance training protocols and exercise strategies to improve strength are abundant in the scientific literature (4). Based on the principle of accentuation, partial ROM exercise has also been used by the strength and conditioning community fairly regularly in advanced athletes. Accentuation is purported to strengthen the muscle where maximal force production is required due to the non-optimal sarcomere length and moment arm (15).

Massey et al (16) studied untrained women performing a bench press 2 days per week for 10 weeks. Three groups were used: group one performed full ROM repetitions, group two performed partial ROM repetitions, and group three (quasi-control) performed a mix of two partial ROM sets and one full ROM set. Partial ROM was considered 2 to 5 inches from full elbow extension. In all three groups, strength increased significantly. Between group differences included a larger increase in strength in the full ROM group when compared to ST and ST plus added full ROM. The authors concluded that due to the lack of experience, full ROM might have been best for the group until experience was acquired in the lift. It also was noted that ST had a positive effect on strength throughout the entire range of motion in the lift (16).

Massey and colleagues (15) then applied that same protocol on untrained men. In this study ST, full ROM, and mixed groups increased bench press strength pre- to post test, but had no statistically significant difference between groups. The authors attributed this to the untrained background of the participants and the relatively short duration protocol. This would lead to a majority of strength gains through neuromuscular adaptation. When looking more closely at the results, a clinically meaningful difference may be gleaned. The full ROM group increased bench press by 25 lbs, ST group 24 lbs, and mixed group 16.5 lbs (15). This may indicate that in untrained individuals, more training may not substantially increase strength, initially. Current evidence shows that full ROM may be better for untrained individuals, and focusing on partial ROM would be beneficial for sport/activity specific gains. More research is required on ST and trained individuals compared to untrained with gender comparisons to make a more definitive statement on effectiveness of each method.

NEURAL ACTIVATION

Neural activation is the process by which the nervous system activates skeletal muscle. Resistance training has been shown to enhance neural activation and increase a muscles ability to generate tension of force (4, 12). Contractions at different muscle lengths have been shown to alter neural activation levels (15). Levels of neural activation seem to be unaffected by duration of the contraction, but change by the muscle length and angular velocity (3). When the muscle is in a shortened position, there is a non-optimal overlap of cross bridges and a shorter moment arm. Instead, in this end range position there is a decrease in the force of the contraction and joint torque (4).

Babault et al. (3) used surface EMG and twitch interpolation during isometric quadriceps contractions to study ST and LT. The results showed an increase in neural activation in the ST position. This may indicate that there is a nervous system compensation for the decrease in power compared to optimal length contractions. This may also be a result of reduced alpha motor neuron inhibition by joint receptors, which respond to loading of the joint capsule and ligaments. This increase in neural activation was also found by Gandevia & Mackenzie who stated, “human muscles can be maximally activated by voluntary effort despite changes in their overall mechanical behavior” (9). These effects may be mediated at the level of the spinal cord, but more studies are required to better understand this phenomenon (3).

Patterns of neural activation also appear to change during isokinetic muscle contractions, compared to isometric. Performing an isokinetic contraction of 30 degrees per second showed no difference from isometric testing, but at 120 degrees per second there was a decrease in muscle twitch duration time at the shorter muscle length. This finding was proposed to be due to the need for higher rates of shorter duration muscle twitches to maximize activation (17). Recruiting additional motor units as well may regulate this mechanism. Kubo et al. describes a training effect during ST causing a reduction in muscle co-contraction, thus increasing efficiency at the end range of the movement (13). More research is required to substantiate these claims. Clinically, these results may show that ST increases muscle contraction efficiency in end range by reducing the antagonist co-contraction and increasing motor unit recruitment. This can be useful for task specific functions from activities of daily living to powerlifting for sport.

BLOOD FLOW AND MUSCLE OXYGEN CONSUMPTION

Exercise is known to increase blood flow and the delivery of oxygenated blood to the working muscles. With the increase in blood flow, there is an associated increase in blood perfusing the contracting muscles (6). Ameredes et al performed in situ experimentation on canine gastrocnemius and soleus via electrical stimulation. The results revealed an increasing reduction in force over time and reduced blood flow with LT. This was interpreted as a metabolically controlled decrement due to higher force of contraction and afterload during the LT. It may also be a result of increasing ATP requirements and total peripheral resistance during the more forceful contractions in the LT position. This would cause a reduction of blood flow to the area, but an increase in mVO2 due to more muscle mass being used at an increased intensity (2). The high tension and preload on the muscle at LT may also be significant factors decreasing the perfusion into the muscle and changing the total peripheral resistance (TPR) (6). Furthermore, during ST, blood flow into the muscle can be maximized, thus theoretically allowing for the greatest possible matching of oxygen perfusion and metabolite clearance. The different hypothesized metabolic mechanisms between ST and LT may be important to consider during resistance training when trying not to over-reach with clients who are beginning an exercise program and when trying to avoid overwork in patients with easily fatigable pathologies. Increasing blood flow to the area and keeping the TPR in the supplying arteries down can also mean less hemodynamic changes systemically. More studies on this factor are required to substantiate these claims.

TENDON STIFFNESS

Tendon stiffness is the mechanical property describing the relationship between the force applied to the muscle-tendon complex (MTC) and the change in the length of the unit. Therefore, if a greater degree of force is required to produce a certain amount of stretch, the MTC would be considered stiffer. Less force would mean that the MTC is more compliant. Muscle tendon stiffening may be viewed as a positive or negative effect of resistance exercise. If it is in conjunction with muscular stiffening and the surrounding fascia, this may cause detrimental effects to functional mobility and range of motion. On the other hand, stiffening of the tendon may allow for greater transmission of forces, and increased correction capabilities of the musculoskeletal system in balance strategies (14). In order to increase tendon stiffening, a certain threshold of resistance is required during exercise. This threshold was defined as somewhere between 60-100% max voluntary contraction in order to pass the Young’s modulus plastic zone, causing subsequent collagen synthesis for tendon stiffening (10).

Findings from Kubo et al. agree with these results that tendon stiffening requires a higher level of resistance. Low load resistance training (ST) did not change tendon stiffness significantly (+5%), whereas higher loads (LT) increased tendon stiffness significantly (+47%). This was attributed to the higher mechanical stress and torque placed on the muscle to move the joint through range (13). During LT, the tendon has a higher amount of preload from the stretch in it; it is then contracted, further increasing mechanical stresses. This is not the same in ST, where the tendon is on slack and will have a larger diameter in the center. In conclusion, it seems that in certain positions, ST may not allow for a high enough mechanical stress on the tendon to allow for stiffening.

If the mechanical stress is sufficient during ST but the tendon stiffness does not increase, the important question must be raised as to where these forces are transmitted. A hypothesis made by the authors is that it is directed radially out from the dilated muscle and absorbed by the nearby tubular layers of fascia and connective tissues. The fascia may accommodate for the mechanical stress on the tendon and allow for a smoother and more elastic contraction. This fascial force absorption in turn, may be attributing to the lack of stiffening of tendons during contractions at ST (8). These results may be dependent on the angle of muscular pennation, which may also change with joint angle (1). More research in this area is required to make conclusive statements on the rationale for lack of tendon stiffening during ST.

FATIGUE

Fatigue is unrelenting weariness that is not relieved by rest or sleep. This type of physical exhaustion is often reported by persons with cancer, chronic fatigue syndrome and other medical conditions and can have a negative effect on physical ability, mood, motivation, and concentration. Researchers have investigated the effect of progressive resistive exercise (PRE) on fatigue (18, 19) although none have specifically tested ST or LT. Schmidt and colleagues (18) found that 12 weeks of biweekly resistance training using 60-80% of the repetition maximum did not improve reports of fatigue in women with breast cancer undergoing chemotherapy. Whereas, in a similar patient population, Travier and colleagues (19) found that a combination of aerobic exercise and PRE had positive effects on fatigue and strength. Results from studies of men with prostate cancer however, suggest that the type of PRE performed may result in different outcomes. In a study by Hanson and colleagues, only one of the 8 prescribed resistance exercises had maximal loading into the end ROM (11). The results included a 10% reduction in fatigue. In contrast, when 4 out of 6 exercises provided maximal loading into end range, fatigue decreased by 40% (20).

During ST, muscle may have an improved endurance capacity at the same MVC as LT. Ameredes et al. showed the ST position caused a decrease in passive tension by ~11% that of LT. This factor may have allowed an increase in perfusion through the muscle, thus increasing metabolite and CO2 clearance and a further increase in muscular work capacity. This may also cause decreased oxygen diffusion through the capillaries into the muscle and yet another reason for reduced performance in LT. In this study canine muscle did not fatigue significantly within 20 minutes during ST (9). These results were concurrent with literature on humans as well (5). This may suggest that ST can reduce the systemic fatigue of patients with pathologies causing decreased endurance such as fibromyalgia, cancer, and rheumatic disease.

SUMMARY AND CLINICAL APPLICATIONS

Muscle shortening exercise is a little known method of resistance training that has been used primarily for performance to increase strength within specific ranges of motion. ST has numerous benefits including: increased blood flow, increased endurance due to a possible muscle sparing effect, reduced fatigue, increased neuromuscular efficiency, and a positive effect on strength in untrained individuals.

A muscle sparing effect occurs specifically during shortened length contractions because the muscle cannot function at the same MVC it would in optimal range. It also receives an increase in blood perfusion to bring O2 and nutrients into the contracting muscle and remove CO2 and metabolic waste products. This increased perfusion may be taken advantage of during injury, periods of high delayed onset muscle soreness, and lighter training (or active rest) days for athletes. It also has implications in research of pathology. Specifically, those who are easily fatigued such as fibromyalgia, cancer, and rheumatic patients may benefit most. The decreased local and systemic fatigue along with blood perfusion into the muscle has numerous implications. More, high-level research is needed to better understand mechanisms and results from this information.
Currently, advanced athletes use ST to get over “sticking points” in the range of motion where there is the greatest challenge. This protocol has shown conflicting evidence to whether it will cause a lesser or equal in strength gains in untrained individuals. Despite this, it may still be used to strengthen particular portions of a range to maximize specificity of practice and beneficial neural adaptations. Besides what is included in this brief review of literature, current research on ST is still in its infancy and must be continued for the elucidation of benefits over LT and full ROM training.

Acknowledgements: Dr. Ellen Anderson of Rutgers School of Health Professions for introducing me to this work and being a mentor of mine throughout school. Your expertise and patience is invaluable and has left a lasting effect on many student’s lives.

Thanks for reading,

-Jared Burch, PT, DPT


References

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3. Babault, N, Pousson, M, Michaut, A, and Van Hoecke, J. Effect of quadriceps femoris muscle length on neural activation during isometric and concentric contractions. J Appl Physiol 94: 983-990, 2003.

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10. Grosset, JF, Breen, L, Stewart, CE, Burgess, KE, and Onambélé, GL. Influence of exercise intensity on training-induced tendon mechanical properties changes in older individuals. Age 36: 1433-1442, 2014.

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13. Kubo, K., Ohgo, K., Takeishi, R., Yoshinaga, K., Tsunoda, N., Kanehisa, H., & Fukunaga, T. (2006). Effects of isometric training at different knee angles on the muscle–tendon complex in vivo. Scand J Med Sci Spor 16(3), 159-167.

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16. Massey, CD, Vincent, J, Maneval, M, Moore, M, and Johnson, JT. Influence of range of motion in resistance training in women: early phase adaptations. J Strength Cond Res 19: 409-411, 2005.

17. Mookerjee, S, and Ratamess, N. Comparison of Strength Differences and Joint Action Durations Between Full and Partial Range-of-Motion Bench Press Exercise. J Strength Cond Res 13: 76-81, 1999.

18. Schmidt, ME, Wiskemann, J, Ambrust, P, Schneeweiss, A, Ulrich, CM, and Steindorf, K. Effects of resistance exercise on fatigue and quality of life in breast cancer patients undergoing adjuvant chemotherapy: A randomized controlled trial. Int J Cancer 137: 471–480, 2015.
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20. Winters-Stone, KM, Dobek, JC, Bennett, JA, Dieckmann, NF, Maddalozzo, GF, Ryan, CW and Beer, TM. Resistance training reduces disability in prostate cancer survivors on androgen deprivation therapy: evidence from a randomized controlled trial. Arch Phys Med Rehabil 96: 7-14, 2015.

 

 

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