Garry J. Massey

Training-specific functional, neural, and hypertrophic adaptations to explosive- vs. sustained-contraction strength training

Training specificity is considered important for strength training, although the functional and underpinning physiological adaptations to different types of training, including brief explosive contractions, are poorly understood. This study compared the effects of 12 wk of explosive-contraction (ECT, n = 13) vs. sustained-contraction (SCT, n = 16) strength training vs. control (n = 14) on the functional, neural, hypertrophic, and intrinsic contractile characteristics of healthy young men. Training involved 40 isometric knee extension repetitions (3 times/wk): contracting as fast and hard as possible for ∼1 s (ECT) or gradually increasing to 75% of maximum voluntary torque (MVT) before holding for 3 s (SCT). Torque and electromyography during maximum and explosive contractions, torque during evoked octet contractions, and total quadriceps muscle volume (QUADSVOL) were quantified pre and post training. MVT increased more after SCT than ECT [23 vs. 17%; effect size (ES) = 0.69], with similar increases in neural drive, but greater QUADSVOL changes after SCT (8.1 vs. 2.6%; ES = 0.74). ECT improved explosive torque at all time points (17-34%; 0.54 ≤ ES ≤ 0.76) because of increased neural drive (17-28%), whereas only late-phase explosive torque (150 ms, 12%; ES = 1.48) and corresponding neural drive (18%) increased after SCT. Changes in evoked torque indicated slowing of the contractile properties of the muscle-tendon unit after both training interventions. These results showed training-specific functional changes that appeared to be due to distinct neural and hypertrophic adaptations. ECT produced a wider range of functional adaptations than SCT, and given the lesser demands of ECT, this type of training provides a highly efficient means of increasing function.

Biceps Femoris Aponeurosis Size: A Potential Risk Factor for Strain Injury?

PURPOSE A disproportionately small biceps femoris long head (BFlh) proximal aponeurosis has been suggested as a risk factor for hamstring strain injury by concentrating mechanical strain on the surrounding muscle tissue. However, the size of the BFlh aponeurosis relative to BFlh muscle size, or overall knee flexor strength, has not been investigated. This study aimed to examine the relationship of BFlh proximal aponeurosis area with muscle size (maximal anatomical cross-sectional area (ACSAmax)) and knee flexor strength (isometric and eccentric). METHODS Magnetic resonance images of the dominant thigh of 30 healthy young males were analyzed to measure BFlh proximal aponeurosis area and muscle ACSAmax. Participants performed maximum voluntary contractions to assess knee flexion maximal isometric and eccentric torque (at 50° s and 350° s). RESULTS BFlh proximal aponeurosis area varied considerably between participants (more than fourfold, range = 7.5-33.5 cm, mean = 20.4 ± 5.4 cm, coefficient of variation = 26.6%) and was not related to BFlh ACSAmax (r = 0.04, P = 0.83). Consequently, the aponeurosis/muscle area ratio (defined as BFlh proximal aponeurosis area divided by BFlh ACSAmax) exhibited sixfold variability, being 83% smaller in one individual than another (0.53 to 3.09, coefficient of variation = 32.5%). Moreover, aponeurosis size was not related to isometric (r = 0.28, P = 0.13) or eccentric knee flexion strength (r = 0.24, P ≥ 0.20). CONCLUSION BFlh proximal aponeurosis size exhibits high variability between healthy young men, and it was not related to BFlh muscle size or knee flexor strength. Individuals with a relatively small aponeurosis may be at increased risk of hamstring strain injury.

Influence of contractile force on the architecture and morphology of the quadriceps femoris

What is the central question of this study? Does contraction influence the fascicle length, pennation angle and effective physiological cross‐sectional area (effPCSA) of the quadriceps femoris muscle? Is there a stronger relationship between effPCSA and maximal strength if effPCSA is measured during maximal contraction rather than at rest? What is the main finding and its importance? Fascicle length decreased, pennation angle increased and effPCSA increased in a non‐linear manner with isometric torque. The effPCSA during maximal contraction and rest were correlated in a similar manner to maximal strength. The effPCSA at rest is sufficient to characterize the muscle size–strength relationship.

The functional significance of hamstrings composition: is it really a “fast” muscle group?

Hamstrings muscle fiber composition may be predominantly fast‐twitch and could explain the high incidence of hamstrings strain injuries. However, hamstrings muscle composition in vivo, and its influence on knee flexor muscle function, remains unknown. We investigated biceps femoris long head (BFlh) myosin heavy chain (MHC) composition from biopsy samples, and the association of hamstrings composition and hamstrings muscle volume (using MRI) with knee flexor maximal and explosive strength. Thirty‐one young men performed maximal (concentric, eccentric, isometric) and explosive (isometric) contractions. BFlh exhibited a balanced MHC distribution [mean ± SD (min‐max); 47.1 ± 9.1% (32.6–71.0%) MHC‐I, 35.5 ± 8.5% (21.5–60.0%) MHC‐IIA, 17.4 ± 9.1% (0.0–30.9%) MHC‐IIX]. Muscle volume was correlated with knee flexor maximal strength at all velocities and contraction modes (r = 0.62–0.76, P < 0.01), but only associated with late phase explosive strength (time to 90 Nm; r = −0.53, P < 0.05). In contrast, BFlh muscle composition was not related to any maximal or explosive strength measure. BFlh MHC composition was not found to be “fast”, and therefore composition does not appear to explain the high incidence of hamstrings strain injury. Hamstrings muscle volume explained 38–58% of the inter‐individual differences in knee flexor maximum strength at a range of velocities and contraction modes, while BFlh muscle composition was not associated with maximal or explosive strength.

Muscle size and strength: debunking the “completely separate phenomena” suggestion

This is a post-peer-review, pre-copyedit version of an article published in European Journal of Applied Physiology. The final authenticated version is available online at: http://dx.doi.org/10.1007/s00421-017-3616-y

Does normalization of voluntary EMG amplitude to MMAX account for the influence of electrode location and adiposity

Voluntary surface electromyography (sEMG) amplitude is known to be influenced by both electrode position and subcutaneous adipose tissue thickness, and these factors likely compromise both between‐ and within‐individual comparisons. Normalization of voluntary sEMG amplitude to evoked maximum M‐wave parameters (MMAX peak‐to‐peak [P‐P] and Area) may remove the influence of electrode position and subcutaneous tissue thickness. The purpose of this study was to: (a) assess the influence of electrode position on voluntary, evoked (MMAX P‐P and Area), and normalized sEMG measurements across the surface of the vastus lateralis (VL; experiment 1: n = 10); and (b) investigate if MMAX normalization removes the confounding influence of subcutaneous tissue thickness [muscle‐electrode distance (MED) from ultrasound imaging] on sEMG amplitude (experiment 2; n = 41). Healthy young men performed maximum voluntary contractions (MVCs) and evoked twitch contractions during both experiments. Experiment 1: voluntary sEMG during MVCs was influenced by electrode location (P ≤ 0.046, ES≥1.49 “large”), but when normalized to MMAX P‐P showed no differences between VL sites (P = 0.929) which was not the case when normalized to MMAX Area (P < 0.004). Experiment 2: voluntary sEMG amplitude was related to MED, which explained 31%‐38% of the variance. Normalization of voluntary sEMG amplitude to MMAX P‐P or MMAX Area reduced but did not consistently remove the influence of MED which still explained up to 16% (MMAX P‐P) and 23% (MMAX Area) of the variance. In conclusion, MMAX P‐P was the better normalization parameter for removing the influence of electrode location and substantially reduced but did not consistently remove the influence of subcutaneous adiposity.

Tendinous tissue properties after short and long-term functional overload: Differences between controls, 12 weeks and 4 years of resistance training.

The potential for tendinous tissues to adapt to functional overload, especially after several years of exposure to heavy‐resistance training, is largely unexplored. This study compared the morphological and mechanical characteristics of the patellar tendon and knee extensor tendon‐aponeurosis complex between young men exposed to long‐term (4 years; n = 16), short‐term (12 weeks; n = 15) and no (untrained controls; n = 39) functional overload in the form of heavy‐resistance training.

The influence of patellar tendon and muscle–tendon unit stiffness on quadriceps explosive strength in man

What is the central question of this study? Do tendon and/or muscle–tendon unit stiffness influence rate of torque development? What is the main finding and its importance? In our experimental conditions, some measures of relative (to maximal voluntary torque and tissue length) muscle–tendon unit stiffness had small correlations with voluntary/evoked rate of torque development over matching torque increments. However, absolute and relative tendon stiffness were unrelated to voluntary and evoked rate of torque development. Therefore, the muscle aponeurosis but not free tendon influences the relative rate of torque development. Factors other than tissue stiffness more strongly determine the absolute rate of torque development.

IS BICEPS FEMORIS LONG HEAD APONEUROSIS SIZE A RISK FACTOR FOR HAMSTRING STRAIN INJURY

Background Biceps femoris long head (BFlh) proximal aponeurosis (PA) size appears to be highly variable between individuals (Handsfield et al., 2010) and may not be proportional to muscle size and strength. Individuals with a relatively small aponeurosis may be at higher risk of strain injury (Fiorentino et al., 2012), and this could be an important intrinsic risk factor. Objective To examine the relationship of BFlh PA area with BFlh muscle size (volume and maximal anatomical cross-sectional area, ACSAmax) and hamstrings voluntary maximal isometric strength (MIS). Design Cross-sectional study. Setting Laboratory. Participants Thirty healthy, recreationally active participants (age: 20.7±2.6 yrs, height: 179.3±7.0 cm, body mass: 72.2±7.2 kg, mean±s) with no history of hamstrings injury. Risk factor assessment The dominant thigh was scanned (1.5T MRI scanner) and axial plane images (slice thickness: 5 mm) were used to measure BFlh muscle ACSAmax and volume. PA area was calculated as the sum of aponeurosis to muscle contact distance within each image (external and internal) multiplied by the slice thickness. MIS was measured in a prone position (hip and knee joint angles of 150°;full extension=180°). Main outcome measurements BFlh muscle volume, BFlh ACSAmax, BFlh PA area, MIS. Results BFlh PA area varied considerably across participants (mean=20.4±5.4 cm2, range=7.5-33.5 cm2, CV=26.6%) and it was not related to BFlh ACSAmax (r=0.041, P=.830) or volume (r=0.354, P=.055). Also, BFlh PA area was not related to MIS (r=0.178, P=.347). BFlh PA relative size (ratio to ACSAmax) was >4 times smaller in some individuals than others (CV=38%). Conclusions BFlh PA area was not related to BFlh muscle size or hamstrings strength. Our results confirm the large aponeurosis size variability previously reported (Handsfield et al., 2010) and support the notion that this could be a risk factor for hamstrings strain injury.

Tendinous tissue adaptation to explosive-vs. sustained-contraction strength training

The effect of different strength training regimes, and in particular training utilizing brief explosive contractions, on tendinous tissue properties is poorly understood. This study compared the efficacy of 12 weeks of knee extensor explosive-contraction (ECT; n = 14) vs. sustained-contraction (SCT; n = 15) strength training vs. a non-training control (n = 13) to induce changes in patellar tendon and knee extensor tendon–aponeurosis stiffness and size (patellar tendon, vastus-lateralis aponeurosis, quadriceps femoris muscle) in healthy young men. Training involved 40 isometric knee extension contractions (three times/week): gradually increasing to 75% of maximum voluntary torque (MVT) before holding for 3 s (SCT), or briefly contracting as fast as possible to ∼80% MVT (ECT). Changes in patellar tendon stiffness and Young’s modulus, tendon–aponeurosis complex stiffness, as well as quadriceps femoris muscle volume, vastus-lateralis aponeurosis area and patellar tendon cross-sectional area were quantified with ultrasonography, dynamometry, and magnetic resonance imaging. ECT and SCT similarly increased patellar tendon stiffness (20% vs. 16%, both p < 0.05 vs. control) and Young’s modulus (22% vs. 16%, both p < 0.05 vs. control). Tendon–aponeurosis complex high-force stiffness increased only after SCT (21%; p < 0.02), while ECT resulted in greater overall elongation of the tendon–aponeurosis complex. Quadriceps muscle volume only increased after sustained-contraction training (8%; p = 0.001), with unclear effects of strength training on aponeurosis area. The changes in patellar tendon cross-sectional area after strength training were not appreciably different to control. Our results suggest brief high force muscle contractions can induce increased free tendon stiffness, though SCT is needed to increase tendon–aponeurosis complex stiffness and muscle hypertrophy.