Isometric hamstring testing has become increasingly common within applied practice, particularly with the accessibility of force plates. Among the available methods, the 90:90 isometric knee flexion test is perhaps the most widely adopted. It is simple to set up, quick to administer, and produces minimal fatigue; making it well suited for regular monitoring within both training and rehabilitation environments.
This is reflected in the literature. In our recent systematic review, the 90:90 position was the most frequently used force plate method for assessing isometric posterior chain force production, demonstrating acceptable within- and between-day reliability, alongside sensitivity to fatigue-related changes [1].
However, as with most tests, the challenge is not in collecting the data, but in interpreting what it represents.
The influence of joint position
The rationale for using different joint configurations in hamstring testing is not new. Positions such as 90:90 and 30:30 have been proposed to bias different regions of the hamstring complex, with the former thought to emphasise the medial hamstrings and the latter increasing contribution from the biceps femoris [2, 3]. This concept stems from early EMG work and has been reinforced in more recent studies demonstrating small, but consistent, differences in activation between positions.
Yet, the assumption that a given joint angle isolates a specific muscle is likely an oversimplification.
The hamstrings do not operate at a fixed length. Their force-producing capacity is influenced by the interaction of muscle architecture, activation, and joint moment arms across both the hip and knee [4]. As highlighted in the work of Kellis and colleagues, each hamstring muscle operates over different regions of its force–length relationship, with semimembranosus and biceps femoris long head contributing greater force across much of the functional range, while semitendinosus demonstrates a flatter force–length profile with lower maximal force capacity.
Importantly, these relationships are not solely dictated by knee angle. Changes in hip position appear to have a greater influence on muscle length than changes in knee angle alone, meaning that two tests performed at similar knee positions may still reflect very different operating conditions of the muscle–tendon unit.
This raises an important point. When we assess isometric force at a given joint angle, we are not simply measuring “hamstring strength”, but rather the capacity of the muscle group as a whole to generate force at that specific position.
Case 1: The value of context
The practical implications of this become clear when considering longitudinal athlete data.
Figure 1 presents 90:90 isometric knee flexion data from a 200 m sprinter collected across multiple timepoints throughout the season, including pre- and post-injury. At baseline, the athlete demonstrates an approximately 18% asymmetry between limbs. While this may initially be interpreted as a deficit, it is important to consider the context in which this athlete operates.
The curved running demands of the 200 m introduce asymmetrical loading patterns. Indeed, asymmetries in eccentric hamstring strength have been reported in elite sprint populations, particularly in longer sprint events, and may reflect normal sport-specific adaptation rather than dysfunction .
Following a 2b biceps femoris injury, a clear reduction in force is observed on the injured limb, exceeding the expected variability of the test. Using a conservative threshold based on reported between-day coefficient of variation values, this reduction provided a meaningful marker to guide return-to-running decision making. As force values recovered towards baseline, this information was used alongside other clinical and biomechanical measures to progress rehabilitation.
The key point here is that baseline data provided context. Without it, interpretation would have been limited to asymmetry alone, which in this case would have been misleading.
Case 2: A single test, a partial picture
Despite its utility, the limitations of relying on a single test become apparent when viewed alongside broader profiling.
In a second case example, 90:90 testing suggested that the athlete had returned to pre-injury levels of force production. However, when assessed using alternative joint positions, deficits remained evident. Whether this reflects differences in muscle contribution, alterations in force–length capacity, or simply position-specific strength is difficult to determine. What is clear, however, is that no single test captures the full functional capacity of the hamstring complex.
This aligns with the underlying biomechanics. The hamstrings function across a wide range of lengths and velocities, with sprinting in particular placing the muscle–tendon unit under high force at long lengths. Testing at a single joint configuration is therefore unlikely to reflect the full spectrum of demands placed upon the tissue.
Moving beyond reductionism
It has been suggested that the 90:90 position may bias the medial hamstrings, raising questions about its relevance given the high prevalence of biceps femoris long head
injury. While there is some basis to this argument, it again reflects a reductionist view of hamstring function.
The hamstrings do not operate in isolation. During sprinting, different components of the muscle group contribute at different phases of the gait cycle, with coordinated interaction required to manage both force production and limb control. Developing capacity in one region of the system may therefore influence the demands placed on another.
From this perspective, the question is not whether a test isolates a specific muscle, but whether it provides useful information within the broader context of the athlete.
Practical implications
Isometric testing, particularly in the 90:90 position, offers a practical and reliable method for monitoring changes in force production with minimal disruption to training. It can provide valuable information regarding fatigue, recovery, and rehabilitation progress.
However, it should not be interpreted in isolation.
A more complete understanding of hamstring function is likely achieved through a combination of tests performed across different joint positions, capturing variation in muscle length, leverage, and contribution. This approach better reflects the complexity of the muscle–tendon unit and its role during high-speed running.
Take home
The value of isometric hamstring testing lies not in any single protocol, but in how the information is interpreted. Joint angle matters, but not in the simplistic sense of isolating individual muscles. Rather, it reflects the conditions under which the system is asked to produce force.
Baseline data, repeated measures, and an appreciation of variability are essential to making this information meaningful. When combined with a broader profiling approach, isometric testing can provide useful insight into the evolving capacity of the hamstrings during both performance and rehabilitation.
1. Fahey, J.T., et al., Single joint posterior chain isometric testing using force plates: A systematic review of the methodologies and reliability of testing methods. J Sports Sci, 2026. 44(6): p. 797-819.
2. McCall, A., et al., Reliability and sensitivity of a simple isometric posterior lower limb muscle test in professional football players. J Sports Sci, 2015. 33(12): p. 1298-304.
3. Read, P.J., et al., Knee Angle Affects Posterior Chain Muscle Activation During an Isometric Test Used in Soccer Players. Sports (Basel), 2019. 7(1).
4. Kellis, E. and A.J. Blazevich, Hamstrings force-length relationships and their implications for angle-specific joint torques: a narrative review. BMC Sports Science, Medicine and Rehabilitation, 2022. 14(1): p. 166.